# My EV build plans



## iti_uk (Oct 24, 2011)

Hello everybody!

I'm dreaming up an EV for commuting, and since I have time and patience, I'd like to try something interesting.

I'd like to use AC Induction motors, one for each wheel mounted inboard using driveshafts to try and keep unsprung mass at a minimum and to attempt to isolate the motors from road-surface vibrations.

I'm not looking to create a drag-strip monster, just something fairly respectable. I'm thinking maybe 10-20kW constant per wheel, possibly a peak of 40kW per wheel, with each motor either directly driving its wheel, or with a reduction gearbox as required (with a wheel diameter of 585mm and a Vmax of 160km/h, wheel speed is ~1450RPM). The vehicle itself will be a lightweight spaceframe with minimal dimensions to seat two people side by side. I would aim at a design weight of 600kg (similar to a fat Caterham).

My first question is; Where could I find electric motors suitable for the task? I would prefer water-cooled and physically small, with motor length being important as they will be mounted back-to-back in pairs. I don't have a planned budget yet, I'm just putting feelers out there to see what would be possible.

Chris


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## iti_uk (Oct 24, 2011)

No takers?

Edit:

I've found this motor:
http://shop.electro-vehicles.eu/shop/details.asp?prodid=1101015&cat=0&path=1100,1101

Rated battery voltage: 84VDC
Rated power (1h): 8kW
Peak Power (60 s) 10kW
Peak torque (60 s): 60 Nm
Rated current (1 h): 90 A
Max current (60 s): 180 A
Rated frequency: 140 Hz
Rated speed: 4100 rpm
Max. mechanical speed: 9000 rpm
Max. operative temperature: +120 °C
Cooling: Air
Construction type: Alu anticorodal, slick case
Weight: 24 kgs
Encoder: Magnetic
Thermisistor: PTC
IP protection class: IP 66
Power Cables: rubber flexible, 35mm2

It's air cooled, but that shouldn't be a problem - it's more of a luxury at this stage.

One of these per wheel with a 4-way controller would be nice - and at 700 Euros a pop, they're not mind-bogglingly expensive either! Maybe a 3:1 reduction gear to directly drive each wheel...

Anyone had any experience with these?

Chris


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## muffildy (Oct 11, 2011)

dont forget you need a controller for each motor gona cost ya almost as much as the motor if not more per controller.
the HPEV motor series is also popular (ac-12 etc)
theyre not as efficient as bldc though.


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## steven4601 (Nov 11, 2010)

Hi,

I am also using an EVE motor (M3AC60) in my conversion in a z3 roadster.

As far it goes with your EV conversion, it is your party  But here is some constructive criticism. 
Going with 4 motors will certainly require some extra in the budget. 4 motors, 4 (custom?) reduction gearboxes and likely also 4 controllers . It will add up. Using four smaller induction motors mean they will be less efficient as a large single units. But using four motor will be more unique. 


Using the motor specifications for the M2 8kw motor, if you accept 60nmx3:1 * 4 = 720nm total wheel torque for commuting I guess you should do it. It will keep up with regular traffic no problem, but you'll notice requiring full controller current/torque on most of the starts. 


What is the required minimum distance? Most conversions are in the order of 100watt/km for very aerodynamic bodies to 170 for square ish vehicles. FYI: the Z3 does ~ 125watt-h/km @ 80km/h. (9.5 .. 10kw to maintain 80km/h)


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## muffildy (Oct 11, 2011)

oh, also your peak draw would be 180x4 = 720 amps
you would need a battery capable of doing that.
headways are capable (supposedly) of 10-20C.
So if you went headway, you would need:
84/3.2 = 26 cells 10ah each; 720/10C = 72ah; 72/10ah/cell = about 7. So you would need about for peak draw 7*26 = 182 cells.
182 cells x 18.75 = 3412 that does not include the necessary BMS which would likely cost an additional 1-2k.
If you went with a battery with a lower max C draw, say 3c you would need a 240ah system. most likely run around 9k.


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## MalcolmB (Jun 10, 2008)

Hi Chris,
I'm trying a similar idea myself, but just driving two wheels and using cheaper, low-tech series wound motors. Do the motors have to be AC induction? There's a much wider choice of compact, lightweight motors if you include brushless DC/PMSM motors:

Yasa http://www.yasamotors.com/
Perm http://www.perm-motor.de/en/products
Enstroj http://www.enstroj.si/
Motenergy http://www.motenergy.com/brdcmo2.html


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## iti_uk (Oct 24, 2011)

Thanks for the replies, guys, this is all useful stuff and this will be a major learning experience for me.

The only reason I have specified induction motors is that I understand the electronics involved better than other systems, so far, and I understand that, other than bearings requiring replacement, induction motors are maintenance-free, whereas the magnets in PM motors weaken with time, although I admit that I may well be thinking unrealistically with that point - it may end up as irrelevent as "glasses frames which can withstand a nuclear blast" (as a mate once boasted). I'll definitely read up on the other types you mentioned, MalcolmB.

Interesting calculations, muffildy. I had assumed that 4 10kW (peak) motors would equal (minus efficiency) one 40kW (peak) motor. Is this not the case? Could I run a higher voltage and lower current to reduce the current-handling requirements of the battery/controller?

As for "requiring full controller current/torque on most of the starts" (steven4601), is this due to the nature of induction motors, and would a PM (or other) motor remove this requirement? I assume that the outcome of what you have described would be horrific efficiency in stop-go traffic - is this correct?

I'm not worried about cost regarding the gear reduction boxes. They will indeed be custom, but since this is a long-term, flexible (immature) project, either the design will change and the custom parts will not be required, or time will be spent finding a willing machine shop to carry out the work for a reasonable price. We'll see.

Chris


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## muffildy (Oct 11, 2011)

no, using 1 or 2 motors wouldnt change the battery requirements for a full 40kw burst. I was just letting you know that unless you are willing to pony up the dough for the amount of AH required you might decide instead to use 2 or 3 motors for a lower total peak power, but also lower cost. That being said, using a higher voltage motor generally does reduce the amp requirements but you might end up buying the same amount of batteries - they just get configured differently. Also, the higher the voltage, the less scalable it is cost wise.
For example,
200v with 300amp would need 200/3.2 = 62.5 cells, at a 10C draw capability style cell then you would need 3x that to meet the amps 187.5 cells.
100v 600amp would need 100/3.2 = 31.25 cells at the same 10C draw it would need 6x the cells for 187.5 cells again.
But this is simplified by using a 10C draw cell, lets say you use a 3C draw cell which a lot of them out there are - eg a 40ah cell style.
so you would need 300/40 = 7.5c, so to get over 7.5c you would need 7.5/3 = 3x cells. so again 187.5 cells, but its actually 60 total amps more than required - good for the life of the cells bad for your pocket.
now using the 600 amp again you need 600/40 = 15c to get that you would need 15/3 = 5x cells for only 156.25 cells. so you saved a little cash on batteries using the lower volt motor if you were using 40ah cells atleast.



It all depends on what you find is reasonable for acceleration time 0-60.


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## iti_uk (Oct 24, 2011)

muffildy said:


> no, using 1 or 2 motors wouldnt change the battery requirements for a full 40kw burst. I was just letting you know that unless you are willing to pony up the dough for the amount of AH required you might decide instead to use 2 or 3 motors for a lower total peak power, but also lower cost. That being said, using a higher voltage motor generally does reduce the amp requirements but you might end up buying the same amount of batteries - they just get configured differently. Also, the higher the voltage, the less scalable it is cost wise.
> For example,
> 200v with 300amp would need 200/3.2 = 62.5 cells, at a 10C draw capability style cell then you would need 3x that to meet the amps 187.5 cells.
> 100v 600amp would need 100/3.2 = 31.25 cells at the same 10C draw it would need 6x the cells for 187.5 cells again.
> ...


Ahh, thanks. As far as 0-60 is concerned, between the 9-10s bracket would be fine. Faster is always nice, but I'm not after a rocket-ship. Also, concerning power, I would like it to cruise happily and without too much efficiency drop at 130kph, with my above-stated top speed of 160kph (I'm working in kph only so that I am consistantly working in SI units - I my brain actually works in mph...)

What sort of performance numbers (assuming the car will resemble a slightly overweight Caterham) should I expect with four of the EVE M2-AC08/4-ASP (the one I listed previously) running at 100% load? Would there be any efficiency loss or problems with under-powering the motors (limiting the maximum battery load) to drop (?) the performance levels to my minimum targets? Then, later if I want I could scale up the power supply/controller capability without changing the motors to make them work at 100% design load for, say, a track day or some other hyjinx?

Another can of worms if I may - anyone played with axial flux motors? Worth experimenting with? (To be honest, I'm sorely tempted to try this idea even without an EV in mind, I'm just plain old intrigued...)

Chris


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## MalcolmB (Jun 10, 2008)

iti_uk said:


> anyone played with axial flux motors? Worth experimenting with?


Yes. Definitely. I guess you're talking about brushless axial flux, like the Yasa and Enstroj motors? These are still relatively new and expensive, and finding a suitable controller can be difficult (and expensive). Their power density is fantastic though.

Many of us lesser mortals have used the more affordable brushed DC axial flux motors, such as the Etek, Perm and Agni, which also have very good power density. The Agni motors are used on a lot of race bikes. One of the guys here – Jozzer – recently said that Agni will be launching a couple of larger brushed motors later this year. They have better than 90 per cent efficiency over a wide load range.


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## iti_uk (Oct 24, 2011)

MalcolmB said:


> Yes. Definitely. I guess you're talking about brushless axial flux, like the Yasa and Enstroj motors? These are still relatively new and expensive, and finding a suitable controller can be difficult (and expensive). Their power density is fantastic though.
> 
> Many of us lesser mortals have used the more affordable brushed DC axial flux motors, such as the Etek, Perm and Agni, which also have very good power density. The Agni motors are used on a lot of race bikes. One of the guys here – Jozzer – recently said that Agni will be launching a couple of larger brushed motors later this year. They have better than 90 per cent efficiency over a wide load range.


Wow, is that so?!? Well then that might be an avenue I can explore a bit myself over the next few months. Since I posted my last reply, I have been waiting for a process to finish (at work) and had a quick go at designing an axial flux motor on Catia, based on the DIY examples on youtube. Brushless, permanent magnet rotor. I've designed it to be stackable, just like the good old rotaries in RX7/8s. As many layers of pancake as desired... hmm this could be an interesting bench experiment...

So far I've designed the rotors/stators to be completely water-jettable from various thicknesses (min 3mm, max 14mm so far - in fact, those are the only two thicknesses so far) aluminium sheet - no complex machining required. I'll post up a piccy when I have something assembled...

Chris


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## steven4601 (Nov 11, 2010)

Regarding efficiency, magnetising 4 induction motors will be less efficient than magnetising one bigger motor. Also I^2R losses are massive with low voltage systems. Also from a battery point of view. Less current per battery juction/bolt is less loss per junction/bolt.


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## iti_uk (Oct 24, 2011)

steven4601 said:


> Regarding efficiency, magnetising 4 induction motors will be less efficient than magnetising one bigger motor. Also I^2R losses are massive with low voltage systems. Also from a battery point of view. Less current per battery juction/bolt is less loss per junction/bolt.


Makes sense. Maybe a high voltage system is the way to go for me.

Chris


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## MalcolmB (Jun 10, 2008)

You might find this blog interesting if you've not come across it before: http://scolton.blogspot.com/search/label/axial motor


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## muffildy (Oct 11, 2011)

A caterham....not the most aerodynamic choice.
according to:
http://www.mayfco.com/caterham.htm
the super 7 caterham has a Cd x ft2 of *9.81*
by comparison the 1999 honda insight is *5.1*
I would suspect using a caterham will give you around 500 wh/mi.
The efficiency of the system isnt going to change a whole lot at highway speeds, but because of the terrible coef of drag that car has it means going faster than say 45 mph will dramatically increase the energy required to maintain speed.
40kw should be sufficient to get a 10 second 0-60 mph, less kw than that and you would need to lose weight to keep the 10 seconds. 
Many motors have a minimum load they need to achieve good efficiency. usually its around 20-30% of the motors continuous rating. If you try to run a 300kw motor at 2kw it would have horrible efficiency.


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## MalcolmB (Jun 10, 2008)

Caterham's aren't the most aerodynamic, but still not too bad. According to Ecomodder, power consumption at a steady 45 mph is roughly 7 kW, rising to around 20 kW at a steady 70 mph: That's assuming drivetrain efficiency of 90% and motor/controller efficiency of 75%.

http://ecomodder.com/forum/tool-aero-rolling-resistance.php


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## muffildy (Oct 11, 2011)

Nissan leaf:
3rd party testing range is between 62mi and 138 mi, the battery pack is 
24 kwh so according to real world testing it has 387 wh/mi on the heavy side, and 174 wh/mi on the light side, averaged its 240 wh/mi.

using that calculator with nissan leaf stats...
3354# 
CdA 6.94
40 (test actually at 38) mph @ 4385 wh
70 mph @ 15795 wh (dont have test results for it)
Which would mean that the calculator is giving it a theoretical:
40 mph @ 110 wh/mi
70 mph @ 225 wh/mi
we get a possible variance between the actual wh/mi and the theoretical wh/mi of between 58% and 70% more wh/mi.

That all being said, inputting the caterman car:
1300#
9.81 CdA
you get:
45 mph @ 5507 wh or 122 wh/mi
70 mph @ 18674 wh or 266 wh/mi
Which is theoretically good, but if we get the same percentage increases as the nissan leaf then its: 193 wh/mi and 455 wh/mi.

*edited for @ instead of =*


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## major (Apr 4, 2008)

muffildy said:


> 40 (test actually at 38) mph = 4385 wh
> 70 mph = 15795 wh
> 45 mph = 5507 wh
> 70 mph = 18674 wh


How can speed (mph) equal energy (wh)? Please get your units straight.


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## muffildy (Oct 11, 2011)

perhaps i am misunderstanding it...
but isnt:
http://ecomodder.com/forum/tool-aero-rolling-resistance.php
telling you that at X mph you use Y wh?
If you take 1 hour of time
you would use the full amount of Y wh and go X miles?
So 70 mph = 70 miles
Y wh = 18674 wh
So in 1 hour, i would go 70 miles, using 18674 wh, or 18674/70 = 266 wh per mi?


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## major (Apr 4, 2008)

muffildy said:


> but isnt:
> http://ecomodder.com/forum/tool-aero-rolling-resistance.php
> telling you that at X mph you use Y wh?


I don't know what you think some web site tells you. But the power can be calculated or measured required to propel a vehicle at a given speed in certain conditions. Then one could say that a power of Y watts is required to propel it at a speed of X mph. There is no energy associated with that statement because both speed and power are instantaneous quantities. 

Once you have defined a time period, you can calculate the distance from the speed and the energy from the power.

Words and symbols have meanings and you are incorrect to type something which equates speed to energy such as this:


> 70 mph = 15795 wh


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## muffildy (Oct 11, 2011)

So what is that webpage saying? that the car uses 18674 w/s when at 70 mph?
Hmm, that would mean its absolutely awful wouldnt it?
(18674 w * 3600seconds)/1000 = 67226.4 kwh?
traveling 70 miles, so 67226.4/70 = 960kwh per mile? doesnt seem like it makes any sense if this is true? can you correct the conversions?


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## major (Apr 4, 2008)

muffildy said:


> So what is that webpage saying? that the car uses 18674 *w/s* when at 70 mph?
> Hmm, that would mean its absolutely awful wouldnt it?
> (18674 w * 3600seconds)/1000 = 67226.4 kwh?
> traveling 70 miles, so 67226.4/70 = 960kwh per mile? doesnt seem like it makes any sense if this is true? can you correct the conversions?


I don't know what that webpage is saying and don't care. Tell me what physical quantity is represented by a watt per second. Nevermind.



> "Watts per second" would be the same as "Joules per second per second".
> You'd use that monstrosity to describe how fast the rate of energy consumption
> or dissipation is growing or shrinking. ​
> We really have to twist our arm behind our own back to come up with that, and
> ...


I suggest you brush up on the physics of power and energy.


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## muffildy (Oct 11, 2011)

it matters what the website is saying because i think its saying watt hours, and it sounds like you think its saying watt seconds.


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## major (Apr 4, 2008)

muffildy said:


> it matters what the website is saying because i think its saying watt hours, and it sounds like you think its saying watt seconds.


I type exactly what I mean. I quote what you type and tell you it is incorrect. I don't "think its saying" anything. I am just responding to what you post.


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## iti_uk (Oct 24, 2011)

Whoa whoa whoa there.... Let's not go too off-topic on this.

I'm not using a Caterham, I was being a touch lazy in my description of the car I'll be using. The car will be a spaceframe-chassis 2-seater, small and as lightweight as possible. When I said "like a Caterham", I was referring to footprint and mass, not aerodynamic performance, sorry for the confusion.

The fact is that the chassis/body will be of my own design and will not look like a Caterham - so far I'm envisioning something more akin to a Fisher Fury or Ginetta G4, but with my own twist and tweaks from impirical data when I get a chance to do some aerodynamic testing (basic, not with a wind-tunnel, maybe simulation). However, at this point of the design process, other then a general requirement for "good" aerodynamic performance, body details are still up in the air. I'm starting with the powerplant/drivetrain and want to design the car around that.

Let's get back on topic with this; How much of a car's braking can regenerative braking reliably take over? I keep hearing that in F1, once the batteries are fully charged, the regenerative braking drops right off, leaving the mechanical brakes to do all of the work. Would this be an issue with a freshly-charged car, leaving from a house on the top of a steep hill? Is there a charging regimen which could negate this problem (only cahrge to 95% for example)? I'm asking so that I can size the mechanical brakes accordingly - the smaller the better, and of course the car needs a cable-operated handbrake.

Chris


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## muffildy (Oct 11, 2011)

so you have a problem with the short hand i used and there is no problem with the actual data? i wrote = simply because its faster than writing at 45 mph for this particular vehicle this website states it uses this energy.

The limit on how much your regenerative braking can do is limited by your batteries SoC like you said. In order to find the optimum SoC to charge to you would need to determine exactly how much energy is being recharged by that hill. Also keep in mind that the faster the battery gets charged the worse the lifecycle. Usually the max charging current is equal to the max discharging current. So GBS cells for example can charge and discharge at 3C. That all being said, regenerative braking will not stop a car in time to avoid a collision and so i believe cars like the prius use a mechanical brake as soon as the cars speed reaches 20 mph or if the user presses the brake down all the way. You should size your mechanical brakes based on if you didnt have regenerative braking because of the emergency braking situations.


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## iti_uk (Oct 24, 2011)

muffildy said:


> The limit on how much your regenerative braking can do is limited by your batteries SoC like you said. In order to find the optimum SoC to charge to you would need to determine exactly how much energy is being recharged by that hill. Also keep in mind that the faster the battery gets charged the worse the lifecycle. Usually the max charging current is equal to the max discharging current. So GBS cells for example can charge and discharge at 3C. That all being said, regenerative braking will not stop a car in time to avoid a collision and so i believe cars like the prius use a mechanical brake as soon as the cars speed reaches 20 mph or if the user presses the brake down all the way. You should size your mechanical brakes based on if you didnt have regenerative braking because of the emergency braking situations.


I see, thanks for that info. How are the Prius's mechanical brakes controlled to allow room for regenerative braking? Are they drive-by-wire or are they hydraulic?

If the brakes are only for use in an emergency, they can be small and un-ventilated - since an emergency stop is a single deceleration event, brake fade calculation only needs to take the single stop into account. Pre-emting the reply to my previous question regarding the Prius's brakes, I would expect that I would not be comfortable relying on drive-by-wire mechanical brakes in an emergency, and would prefer a hydraulic setup. Here is how I would imagine the brake system would work;

===============================================
In order of brake pedal travel;
0-5% - No braking
5-75% - Regenerative braking only, linear increase in retardation
75%100% - (brake pedal's pushrod adjusted to contact the brake master cylinder piston at 75%) regenerative braking intensity constant, increase brought about by mechanical brakes.

At 95-100% pedal travel, mechanical brakes should be capable of stopping the car in a specified distance.
===============================================

(the above numbers are for illustrative purposes only)

Chris


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## muffildy (Oct 11, 2011)

in the prius they are electronically controlled, and the system isnt perfect - recall toyota had recent problems with its braking software.

Im not sure if there are any production vehicles that use hydraulic braking yet.

Another option you could go with is flywheel energy storage, though it would be very expensive and hard to obtain.


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## iti_uk (Oct 24, 2011)

muffildy said:


> in the prius they are electronically controlled, and the system isnt perfect - recall toyota had recent problems with its braking software.
> 
> Im not sure if there are any production vehicles that use hydraulic braking yet.
> 
> Another option you could go with is flywheel energy storage, though it would be very expensive and hard to obtain.


True, although I thought the problems Toyota had were with the drive-by-wire throttle sticking open.

No production vehicles using hydraulic braking? Every car I know of has hydraulic braking... Do we have wires crossed on this one? I'm talking about a hydraulically operated (via master cylinder/slave cylinder) disk (or drum) brakes... Or do you mean "in conjuction with regenerative braking"?

I'd want to avoid flywheel energy storage - I'd want to keep all the stored energy in one system - spreading it over an electronic drive system and an angular momentum system would surely complicate things and lose efficiency.

Chris


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## muffildy (Oct 11, 2011)

oh, yes cars use that kind of hydraulic brakes.
i thought you were talking about the new kind by ford and the eaton corportation; braking turns a reversable pump hydraulic fluid from a low pressure accumulator into a high pressure accumulator.

and yes, it was a problem with the throttle for toyotas software; some people were pressing on the brake and the gas at the same time and it confused the braking software.


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## major (Apr 4, 2008)

muffildy said:


> so you have a problem with the short hand i used and there is no problem with the actual data? i wrote = simply because its faster than writing at 45 mph for this particular vehicle this website states it uses this *energy*.


muffildy,

I don't like the way you used = . But my main concern is your continual use of incorrect units. And I sense a deeper problem with you as demonstrated with the highlighted word in the quote above. It is power, not energy, which is needed to propel the vehicle at 45 mph. 

My apologies to iti_uk, but I feel it is important to use proper nomenclature and units so we are clear as to what we are discussing.



iti_uk said:


> I would prefer a hydraulic setup. Here is how I would imagine the brake system would work;
> 
> ===============================================
> In order of brake pedal travel;
> ...


I have installed and used regenerative brakes in a similar manner on several vehicles. With the hydraulic brake system, I have found that there is enough free play in the standard pedal adjustment to utilize that first inch or so of pedal travel to actuate a sensor (potentiometer) which commands negative torque from the electric drive. This allows full proportional use of regen before engaging the friction brakes. It also allows for a mix of electric (regen) and mechanical (friction) brakes when quicker deceleration is required. I find this method very intuitive and easy to use.

Regards,

major


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## iti_uk (Oct 24, 2011)

muffildy said:


> oh, yes cars use that kind of hydraulic brakes.
> i thought you were talking about the new kind by ford and the eaton corportation; braking turns a reversable pump hydraulic fluid from a low pressure accumulator into a high pressure accumulator.
> 
> and yes, it was a problem with the throttle for toyotas software; some people were pressing on the brake and the gas at the same time and it confused the braking software.


Ahh, fair enough. I remember considering that sort of hydraulic energy recovery system when I was doing the Formula Student thing at Uni. Interesting stuff.

So, what do you think of my scheme for mixing regen with mechanical braking? I need to do some more reading on how other hybrids/EVs do this...

Chris


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## muffildy (Oct 11, 2011)

power is energy over a period of time.
the period of time specified is 1 hour.
so unless the website is specifying that the watt use is on a per second basis, then i dont see what the problem is.
perhaps i should rewrite:
"so you have a problem with the short hand i used and there is no problem with the actual data? i wrote = simply because its faster than writing at 45 mph for this particular vehicle this website states it uses this *energy* *in this specific time*"

If the numbers are really incorrect then please enlighten me and correct them instead of just whining about short hand writing style.

iti_uk
if you intend to use both the mechanical brake and the regenerative brake at the same time all the time it would work, but the amount of energy reclaimed by the regenerative braking would be quite a lot less than if you did it how the prius does it.


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## major (Apr 4, 2008)

muffildy said:


> power is energy over a period of time.


That is incorrect. Energy is work and power is the rate at which work is done.



> the period of time specified is 1 hour.
> so unless the website is specifying that the watt use is on a per second basis, then i dont see what the problem is.


Speed is also a rate. So a speed value expressed in mph implies no time period per se. It is an instantaneous value. A rate.



> If the numbers are really incorrect then please enlighten me and correct them instead of just whining about short hand writing style.


How can one know what numbers represent when they are mislabeled? And if you'd consider what I am saying as an attempt to help you (which it is), you might learn something.



> iti_uk
> if you intend to use both the mechanical brake and the regenerative brake at the same time all the time it would work, but the amount of energy reclaimed by the regenerative braking would be quite a lot less than if you did it how the prius does it.


Back on topic, I don't see why the method expressed by iti_uk would be less effective than the Prius. The way I read it, and the way I have implemented it, the system only mixes regenerative with friction braking when faster deceleration rates are required over and above the maximum obtainable from the regeneration by itself. The Prius undoubtedly does the same thing.

Regards,

major


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## muffildy (Oct 11, 2011)

they are rates correct, the time specified is 1 hour though; assuming the rate is kept the same in that period of 1 hour time i want to know the wh/mi.
Simply input the data i did into the website, get the resulting data, and tell me how many wh/mi the nissan leaf uses according to the website, and then compare that figured to the actual figure that real world testers obtained.
i might learn something if you actually corrected it.

Iti_uk said:
"In order of brake pedal travel;
0-5% - No braking
5-75% - Regenerative braking only, linear increase in retardation
75%100% - (brake pedal's pushrod adjusted to contact the brake master cylinder piston at 75%) regenerative braking intensity constant, increase brought about by mechanical brakes.

At 95-100% pedal travel, mechanical brakes should be capable of stopping the car in a specified distance."

This is all fine, so long as you specify the 95-100% pedal travel to be using only the mechanical brakes stopping the car in a specified distance with no help from the regenerative brakes which may not be functional because of batteries being too full. I suppose if you could reroute the electricity to something other than the batteries when they are full then it wouldnt be an issue.


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## iti_uk (Oct 24, 2011)

major said:


> My apologies to iti_uk, but I feel it is important to use proper nomenclature and units so we are clear as to what we are discussing.


No problem, I have an engineering background and I am aware of the dangers of mixed/incorrect units. Let's keep this civil and carry on productively, this thread has been very informative to me so far.



major said:


> I have installed and used regenerative brakes in a similar manner on several vehicles. With the hydraulic brake system, I have found that there is enough free play in the standard pedal adjustment to utilize that first inch or so of pedal travel to actuate a sensor (potentiometer) which commands negative torque from the electric drive. This allows full proportional use of regen before engaging the friction brakes. It also allows for a mix of electric (regen) and mechanical (friction) brakes when quicker deceleration is required. I find this method very intuitive and easy to use.
> 
> Regards,
> 
> major


Interesting point. Yes, maybe 75% of the travel being regen only is excessive...

Going back to the axial flux motor (AxFM if I may, to differentiate from an airflow meter) idea, what is the consensus on PM vs series wound? I'm not sure which way to go with the CAD model I've got in front of me - to draw in slip-rings or not to draw in slip-rings...?

Chris


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## iti_uk (Oct 24, 2011)

muffildy said:


> This is all fine, so long as you specify the 95-100% pedal travel to be using only the mechanical brakes stopping the car in a specified distance with no help from the regenerative brakes which may not be functional because of batteries being too full. I suppose if you could reroute the electricity to something other than the batteries when they are full then it wouldnt be an issue.


That was what I meant - I re-wrote that last sentence a couple of times and lost that point in the process. I meant to say that the mechanical system _alone_ should be able to stop the car blah blah blah.

Chris


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## iti_uk (Oct 24, 2011)

muffildy said:


> power is energy over a period of time.
> the period of time specified is 1 hour.
> so unless the website is specifying that the watt use is on a per second basis, then i dont see what the problem is.


The definition of a Watt _is_ a Joule per second. If you want to use hours instead of seconds, you would need to specify Joules per hour (J/h).

Chris

edit:
Just re-read the beginning of the argument, this is slightly petty, isn't it major? Yes, it's true that mph does not equal wh/mi, but I can understand what is implied. If we're going to get _that_ fussy, you could argue that, because velocity requires a vector and a speed, that the interchangeability of the terms "speed" and "velocity" is also erroneous.

muffildy, maybe use "@" instead of "=" to make it clear.

Rate @ Speed


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## muffildy (Oct 11, 2011)

PM is better than series wound for regenerative purposes.
they will also have less maintinance. downside is extra expensive controller.


This is the best explanation i found on the web of how the prius brakes work:
http://priuschat.com/forums/gen-iii...300-how-does-brake-pedal-work-brake-wire.html

When you press the brake pedal, hydraulic fluid is moved in a piston just as in a regular car. However, in normal use, that piston doesn't then move fluid in the brake cylinders. Instead the pressure change is monitored by a pressure sensor connected to the Skid Control ECU. (The fluid enters a device called the Stroke Simulator, which provides the same sort of feel you expect from the brake cylinders.)

From there, the Skid Control ECU works out how much braking force you want, and asks the Hybrid Vehicle ECU to provide up to that amount through regeneration. The HV ECU replies saying how much it can provide. The Skid Control ECU then makes up the difference, if any, by controlling valves from a high-pressure reserve (the Brake Accumulator) to the brake cylinders, which increase the pressure in the brake cylinders, and other valves from the cylinders to the fluid reserve tank, which reduce the pressure.

The rear wheels are always braked with friction brakes as regeneration only operates on the front wheels (the motors are only connected to the front wheels).

If the wheels lose traction, the Skid Control ECU asks the HV ECU to stop providing regeneration and it modulates the pressure on the wheel cylinders to try to keep the wheels just on the verge of locking up. This is ABS.

There is a variable resistor - like a classic volume control on an amplifier - which monitors how fast you press the brake pedal. If you press the brake pedal quickly, it skips asking for regeneration (the HV ECU can't react quickly) and just applies the brakes. It also applies more force than you asked for - this is called Brake Assist - as it's known that drivers frequently don't brake hard enough in emergencies.

If there's a problem with regeneration, the HV ECU will report no braking effort and the Skid Control ECU does it all with the friction brakes. If the Skid Control ECU doesn't see a response from the HV ECU it assumes no regen is available and does all braking with friction brakes.

If there's a problem with the electronically-controlled braking system, the Gen 2 opened and closed valves to isolate the Stroke Simulator and send the fluid moved by the driver's foot directly to the front brakes _only_, with no power assistance. There are actually two pistons in the Master Cylinder, both of which are pressed by the rod that the brake pedal connects to, which gives two independent brake lines. One connects to the front left wheel and the other to the front right.

Gen 3 (2010 Prius) improves on this by using the accumulator as a booster and sending fluid from one Master Cylinder piston to the front wheels, the other to the rear. I believe it can keep electronic control of one pair and use driver's effort for the other.


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## major (Apr 4, 2008)

iti_uk said:


> Going back to the axial flux motor (AxFM if I may, to differentiate from an airflow meter) idea, what is the consensus on PM vs series wound? I'm not sure which way to go with the CAD model I've got in front of me - to draw in slip-rings or not to draw in slip-rings...?


Hi Chris,

Consensus? I see none here 

For anything larger than a motorcycle, a PM motor (axial or radial) would have to be brushless in my opinion. This puts you into an AC drive. That is a great way to go and gets you regen capability, but is expensive. Going the AC route, don't discount the induction motor. That is what I have used, mostly.

Series wound infers a DC commutator motor. Regeneration is not available with this type of drive. However the DC series motor and controller package is the affordable choice of most DIY EVcar conversions.

Cheers,

major


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## major (Apr 4, 2008)

muffildy said:


> they are rates correct, the time specified is 1 hour though;


I took a look at the website. Interesting calculator. But I see nothing there for a "time" input field or where it specifies a time period or where you specified a time period in post #17 or why a time period is needed for the calculations or results


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## iti_uk (Oct 24, 2011)

major said:


> Hi Chris,
> 
> Consensus? I see none here
> 
> ...


The design I'm trying out is similar to this one, which has permanent magnets on the rotor. I'm wondering whether it would be advantageous to replace the permanent magnets with coils supplied by slip-rings (not a commutator). What would you foresee as disadvantages of this design? Why would you avoid brushes on anything larger than a motorcycle?

When I started this thread I was considering induction motors first and foremost. Due to cost, weight, size and potentially high starting currents I'm not so sure that this is the way to go.

What do you think?

Chris


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## GerhardRP (Nov 17, 2009)

muffildy said:


> so you have a problem with the short hand i used and there is no problem with the actual data? i wrote = simply because its faster than writing at 45 mph for this particular vehicle this website states it uses this energy.


Mathematical equations are shorthand. 
The website calculates power (in watts) at various speeds, so we should write:
P(70 MPH) = 18674 W
And Energy converted in an hour as power times time:
E(70 MPH)=P(70 MPH)*1 hr = 18674 Whr
Distance traveled in one hour is 70 miles, so the energy per mile is:
E(70 MPH)/70 miles= 18674/70 Whr/mile= 268 Whr/mile.
Gerhard


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## muffildy (Oct 11, 2011)

your going to design and build your own motor? I dont know anything about that.

induction motors are good, but less efficient than pmdc motors

I think the reason you want to avoid brushes on a motor seeing high loads often is because it would cost time and money to be replacing those brushes every 2-3 years.

Yes that calculator is interesting, and as you say it has no input for time. I was assuming that the watts predicted by the calculator was based on 1 hour simply because it wouldnt make much sense for it to be based on a 1 second time. If it was watts per second at that speed then as i calculated in another post the watts/hour would be quite a ridiculously high number.


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## GerhardRP (Nov 17, 2009)

muffildy said:


> If it was watts per second.


Come on, guys ... watts are Joules/second, for DC...Volt*Amps.
Gerhard


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## major (Apr 4, 2008)

iti_uk said:


> The design I'm trying out is similar to this one, which has permanent magnets on the rotor. I'm wondering whether it would be advantageous to replace the permanent magnets with coils supplied by slip-rings (not a commutator). What would you foresee as disadvantages of this design?


That would a synchronous machine then with a wound field. I'm not sure why you'd put the field in series with the armature but you could. Slip ring/sliding brush would carry a lot of current, be a loss and durability issue. Likely better to separately excite the field like done in an alternator.



> Why would you avoid brushes on anything larger than a motorcycle?


Nothing available on the market in PM or axial of which I am aware.



> When I started this thread I was considering induction motors first and foremost. Due to cost, weight, size and potentially high starting currents I'm not so sure that this is the way to go.
> 
> What do you think?


I like induction motors for EV propulsion. I don't see a downside with them. The efficiency argument is mostly hype. Tell me you wouldn't be happy with the efficiency numbers from the Tesla motor 

major


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## muffildy (Oct 11, 2011)

GerhardRP said:


> Come on, guys ... watts are Joules/second, for DC...Volt*Amps.
> Gerhard


what i mean is its a question of if the site is saying w or wh. I think its saying wh because w would make numbers ridiculous.

there are several PM brushed motors i am aware of, they are all low HP motors though. For example: 
http://www.motenergy.com/me0709.html
you would of course need more than 1 to get acceptable acceleration.


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## Salty9 (Jul 13, 2009)

muffildy said:


> what i mean is its a question of if the site is saying w or wh. I think its saying wh because w would make numbers ridiculous.
> 
> there are several PM brushed motors i am aware of, they are all low HP motors though. For example:
> http://www.motenergy.com/me0709.html
> you would of course need more than 1 to get acceptable acceleration.


It's not too clear but I assume that $99.00 is only for the brush assembly.


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## muffildy (Oct 11, 2011)

thats right, hes building a motor though so i dont know if he wants the brushes or a motor...
heres a link to the actual motor:
http://www.cloudelectric.com/product_p/mo-ptx-0709.htm
at 549$ a piece


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## major (Apr 4, 2008)

muffildy said:


> GerhardRP said:
> 
> 
> > muffildy said:
> ...


It is clear to Gerhard and myself that the website calculates power in watts. It also includes the conversion of that power in watts to units of horsepower. wh (watt hours) makes no sense in this context because it is a unit of energy, not power. And w/s (watts per second) just doesn't make sense at all in any context.



GerhardRP said:


> The website calculates power (in watts) at various speeds,


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## muffildy (Oct 11, 2011)

hmm ok researched a little
W/s is the rate at which power increases.

I dont know why your making such a fuss over it though, granted i learned a little bit, but the equations were correct.
i feel like ive been hit by the equivalent of the grammar police.


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## major (Apr 4, 2008)

muffildy said:


> I dont know why your making such a fuss over it though


This is why:



> granted i learned a little bit


And others read these posts. We put up with grammar and spelling, but do make a fuss about bad physics.


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## GerhardRP (Nov 17, 2009)

muffildy said:


> hmm ok researched a little
> W/s is the rate at which power increases.
> 
> I dont know why your making such a fuss over it though, granted i learned a little bit, but the equations were correct.
> i feel like ive been hit by the equivalent of the grammar police.


The grammar police would point out that you meant "...why you're making such a fuss..."
Your equations were NOT correct because the units did not match. Freshman physics [and I mean high school].
Gerhard


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## muffildy (Oct 11, 2011)

you rewrote my equations and got the same answers so how are the units wrong?


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## major (Apr 4, 2008)

muffildy said:


> you rewrote my equations and got the same answers so how are the units wrong?


You have been massacring units since you first posted. 



muffildy said:


> *edited for h's...*


I helped you out on that one hoping you would be more careful. And I've pointed out to you multiple times on this thread where your units are incorrect. Yet you do not understand.  The unit "wh" is not a unit of power. I don't know how we can make it any clearer.


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## muffildy (Oct 11, 2011)

GerhardRP said:


> Mathematical equations are shorthand.
> The website calculates power (in watts) at various speeds, so we should write:
> P(70 MPH) = 18674 W
> And Energy converted in an hour as power times time:
> ...


yes, its not a unit of power as you say, but GerhardRP got the same answers as i did with the same units. the only difference is he wrote a lot more details.


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## major (Apr 4, 2008)

iti_uk said:


> Just re-read the beginning of the argument, this is slightly petty, isn't it major? Yes, it's true that mph does not equal wh/mi, but I can understand what is implied. If we're going to get _that_ fussy, you could argue that, because velocity requires a vector and a speed, that the interchangeability of the terms "speed" and "velocity" is also erroneous.


Yes iti,

I had a funny feeling when I was typing velocity. Got busted by the vector police  I edited my prior posts to read speed instead of velocity. And it may be picky, but I think too often folks don't know the difference between power and energy and that is the reason for the incorrect units, not just typos. If we let it slide, do we not invite more confusion? Now it does seem to me you would have known the difference, but back early in the thread that was not apparent.

I'll be more careful with my vectors,

major


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## iti_uk (Oct 24, 2011)

major said:


> Yes iti,
> 
> I had a funny feeling when I was typing velocity. Got busted by the vector police  I edited my prior posts to read speed instead of velocity. And it may be picky, but I think too often folks don't know the difference between power and energy and that is the reason for the incorrect units, not just typos. If we let it slide, do we not invite more confusion? Now it does seem to me you would have known the difference, but back early in the thread that was not apparent.
> 
> ...


 No probs, I can understand that. I agree, I cringe when the units are misunderstood or just plain incorrect.



muffildy said:


> hmm ok researched a little
> W/s is the rate at which power increases.
> 
> I dont know why your making such a fuss over it though, granted i learned a little bit, but the equations were correct.
> i feel like ive been hit by the equivalent of the grammar police.


The reason for the fuss regarding units is that the difference between Watts (J/s) and (rate of increase of watts) (W/s, or J/(s^2)) is the same as the difference between speed (m/s) and acceleration (m/(s^2)). Now, you wouldn't say that speed and acceleration are the same thing, so why would you be surprised that there is fuss when a bad reaction to the interchanging between W and W/s?

Also, Watt-hours is a measurement of energy, not power or rate of increase of power.

The equations you used may well be correct, but if you are using the incorrect units in the answer it makes a nonsense of the work you have done. If I were to tell you that my car had travelled at 100mph for thirty minutes, and then went on to deduce that my car had travelled at the speed of 50cd, you would be right to be confused as to why I had resulted in light intensity and counted my working as erroneous.

Repeat the equation you did originally (or was found on that site), but instead of plugging in numbers, plug in units. Make sure you understand where the answer's units come from and how they are manipulated in the equations.

Yes, I am contemplating making my own motor. I have contacts who have CNC waterjets and mills, and I'm an engineer (albeit, mechanical) with an understanding of some basic fundamentals of electronics and a hunger to learn more.



major said:


> That would a synchronous machine then with a wound field. I'm not sure why you'd put the field in series with the armature but you could. Slip ring/sliding brush would carry a lot of current, be a loss and durability issue. Likely better to separately excite the field like done in an alternator.


That is correct. The "series" thing i mentioned yesterday was a mistake, of course it would not be series. By your last sentence in that paragraph, do you mean that I should set it up as an induction motor? Then it wouldn't need to be synchronous, like a squirrel cage motor isn't synchronous? I'd like to avoid the high starting current draw of the squirrel-cage, so would PMs be a better way to go? Would brushes on slip rings be that detrimental to efficiency and durability? I realise that the brushes would then become a "service item", but as long as their lifespan can be measured in thousands of miles rather than tens of miles I could live with them.

Chris


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## major (Apr 4, 2008)

iti_uk said:


> By your last sentence in that paragraph, do you mean that I should set it up as an induction motor?


When I said "Likely better to separately excite the field like done in an alternator.", there was no reference to induced current to the rotor as done in the induction motor. Rather that if you want to conduct current into the rotor to use a low current winding instead of a high current winding as would be required if that field winding (on the rotor) was to be put in series with the armature (stator).



> Then it wouldn't need to be synchronous, like a squirrel cage motor isn't synchronous?


An alternator is a synchronous machine (typically) and the induction motors are asynchronous (always). Technically you can use an IM as an alternator (AC generator) but it would still be asynchronous. Most (almost all) alternators are synchronous machines so that they can have field control (control of the current thru the slip rings).



> I'd like to avoid the high starting current draw of the squirrel-cage, so would PMs be a better way to go?


The high starting currents of which you speak are a relic of mains connected induction motors started via a contactor across the mains. Once you have an inverter driven IM, as required in an EV, it is no longer an issue. Starting currents with an IM drive in the EV would be close to the same as for a PMSM drive.



> Would brushes on slip rings be that detrimental to efficiency and durability? I realise that the brushes would then become a "service item", but as long as their lifespan can be measured in thousands of miles rather than tens of miles I could live with them.


Sure. You don't get something for nothing. There will be a loss associated with the contact drop and friction from brush/slip ring. And always a possibility of "trouble" with any subsystem. But if your field current is low and mechanical assembly done proper, losses will be low (tolerable) and life long, as with the alternator in your ICE car.

I think you're making a mistake trying to design and build your own motor for your EV project. You seem like a smart guy. And linked to a fellow who made a plywood motor attempting suicide on his porch. I have seen quite a few try to do this and the only one actually able to propel a car with his own design and built motor is Alan Cocconi (aka ACPropulsion).

Regards,

major


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## iti_uk (Oct 24, 2011)

major said:


> I think you're making a mistake trying to design and build your own motor for your EV project. You seem like a smart guy. And linked to a fellow who made a plywood motor attempting suicide on his porch. I have seen quite a few try to do this and the only one actually able to propel a car with his own design and built motor is Alan Cocconi (aka ACPropulsion).
> 
> Regards,
> 
> major


Yeah, that guy was a nutcase. That link was only to show the nature of the motor, not the method of testing or construction (or suicide). I would be making a small scale prototype with very low power input to develop the controller, then any prototypes built after that would be of properly-specced materials and construction. As a note, I'm not looking to spin the motor beyond 2000RPM and will not require it to handle more than 20kW peak, 10kW constant (there'll be four of them on the vehicle).

Even if this doesn't end up in an EV, I'm intrigued enough to give it a go on the bench. I'm not looking to die just yet, so I'll be taking proper precautions. I'm aware of the dangers of working with high energy systems, but I'll be running no more than 9V (PP3 cell) at first as a proof on concept, so risk will be low at first.

Chris


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## steven4601 (Nov 11, 2010)

Don't get sidetracked Chris if you want to finish the realization of your initial plan.
Making a motor of your own will be adventurous, but also a great time consumer. 

Probably like many of the DIY'ers having great plans, I also want to build an 3 phase controller, powerful charger etc too. But I cant do that at the same time when I have work & a conversion project still in progress. Time is in shortage, make decisions where you want to invest time in. 

Steven


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## DavidDymaxion (Dec 1, 2008)

Amen to that!

It has been great fun and a great learning experience machining my own adapter, making my own controller, etc., but it sure has cost in the time department!


steven4601 said:


> Don't get sidetracked Chris if you want to finish the realization of your initial plan.
> Making a motor of your own will be adventurous, but also a great time consumer.
> 
> Probably like many of the DIY'ers having great plans, I also want to build an 3 phase controller, powerful charger etc too. But I cant do that at the same time when I have work & a conversion project still in progress. Time is in shortage, make decisions where you want to invest time in.
> ...


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## iti_uk (Oct 24, 2011)

True. I'm a (sort of) CADdy and currently have pleanty of spare time at home (I would never dream of doing this at work, of course! Especially while long drawn-out processes are running and my hands are tied regarding proper work) and I've already got a quick model of the rotor/stator assembly for a full-scale, as well as a CAD model of an MDF-based super-low-power prototype (as proof of concept and for controller initial development) almost complete. I'm designing everything so that it can either be water-jetted or, for more precisely toleranced parts, machined from one side - such is the beauty of AxFMs (my new acronym). I'm hoping to send of a .dxf to be machined/jetted in the next week or so.

While I'm at it, I've got a couple of things to run by more knowledgeable people than myself;

I am thinking that liquid cooling with this type of motor (spinning disk, effectively), and that air cooling will be adequate. I have designed the rotor(s) to be hollow except for the PMs (or EMs, depending on which concept I am going with), with radial vanes which would act as a centrifugal air pump when the motor is spinning. The stator is similarly hollow, but with no vanes, save any stiffening structures. Imagine cotton reels lined up on a common axis and you'll get the basic idea...

In this setup, air is drawn into the motor from around the circumference of the stator, past the EMs, into the centre (around the axle/bearing structure) of the stator, where vents allow it to pass into the centre of the spinning rotor, then out past the E/PMs in the rotor and exhausted radially from the motor. I am considering using manifolds to separate the intakes and exhausts but I'm not sure if this will be necessary at all.

What do you think? I know, a picture speaks a thousand words, but that will have to wait for the moment, I'll sort out a screen cap when I have some minor details cleaned up.

Chris


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## muffildy (Oct 11, 2011)

how much power a bldc motor can produce is primarily determined by how effective the cooling on the motor is.
For example,
a 12 kw continuous rated air cooled motor vs
a 30 kw continuous liquid cooled motor. they would be identical except for the cooling method. So if your goal is the greatest HP per pound of motor then liquid cooled is the way to go.
Im not sure if the same applies to induction or series wound.


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## iti_uk (Oct 24, 2011)

muffildy said:


> how much power a bldc motor can produce is primarily determined by how effective the cooling on the motor is.
> For example,
> a 12 kw continuous rated air cooled motor vs
> a 30 kw continuous liquid cooled motor. they would be identical except for the cooling method. So if your goal is the greatest HP per pound of motor then liquid cooled is the way to go.
> Im not sure if the same applies to induction or series wound.


Ahh, excellent point. I wonder how liquid cooling could be achieved... I imagine that it would be better to try and cool the rotor as well as the stator if possible. I wonder whether through-main-shaft coolant passages would be possible or even effective, and how difficult to implement would they be... Back to Catia for some messing about with this idea!

Chris


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## georgejlee (Nov 20, 2011)

Chris,

I like your idea of using separate motors for each wheel. Our company is developing a high efficiency digitally controlled brushless DC motor that might fit your needs. It's currently still in the prototype stage but this design can be made cheaper, lighter, more efficient, and more responsive than anything else in the market. Right now we have a 6" by 5" prototype 20 pound motor that produces 1.3 kW at 3000 rpm with 85% efficiency (much more efficient with lower load). We are currently testing a prototype that delivers 3.8 kW, and we are working on larger, more powerful, and more efficient prototypes as well.

See http://digital-motor.com/Prototype_Systems.html

Take a look and let us know if you're interested or if you have suggestions on how to improve it. Thanks!

George Lee
ACT Machines


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## iti_uk (Oct 24, 2011)

georgejlee said:


> Chris,
> 
> I like your idea of using separate motors for each wheel. Our company is developing a high efficiency digitally controlled brushless DC motor that might fit your needs. It's currently still in the prototype stage but this design can be made cheaper, lighter, more efficient, and more responsive than anything else in the market. Right now we have a 6" by 5" prototype 20 pound motor that produces 1.3 kW at 3000 rpm with 85% efficiency (much more efficient with lower load). We are currently testing a prototype that delivers 3.8 kW, and we are working on larger, more powerful, and more efficient prototypes as well.
> 
> ...


Cool, I'll have a good look through your site today and get back to you. Sounds like we're talking along the same lines.

Chris

Edit: I've had a quick look over your site - what do you mean by: "In contrast, the wire winding for the digital motor is in longitudinal direction with S pattern on a non conductive material as shown in the patent."? Do you have any diagrams or pictures explaining the nature of the "s-windings" and the non-conductive cores?


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## dtbaker (Jan 5, 2008)

a decent EV is going to need 10kW per wheel..... meaning significant weight and cooling. Unsprung weight in the wheel itself will be highly problematic, and you're limited to one 'gear'...

I see wheel motors as having some shot at being good regen brakes, with burst acceleration, but not suitable for continuous drive over a full range of speeds and/or rough road conditions.


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## iti_uk (Oct 24, 2011)

dtbaker said:


> a decent EV is going to need 10kW per wheel..... meaning significant weight and cooling. Unsprung weight in the wheel itself will be highly problematic, and you're limited to one 'gear'...
> 
> I see wheel motors as having some shot at being good regen brakes, with burst acceleration, but not suitable for continuous drive over a full range of speeds and/or rough road conditions.


Would 4 small water-cooled 10kW (or slightly greater?) motors weigh much more than one 40kW (or more) motor plus differential+casing?

Also, I'm not looking at doing in-wheel (unsprung) motors, I would be mounting the motors inboard, back-to-back, and using driveshafts with UJs. This will keep the unsprung mass down and provide better protection for the motors, cabling and coolant lines.

Will limiting myself to one gear matter if I spec the motor properly to match the requirements of the vehicle? I know I will be losing the mechanical advantage of having lower gears, but if a motor can be designed to have good torque from 0rpm, as long as it doesn't run out of headroom (rev beyond a point where it loses efficiency significantly), it should be alright, right? I'm only looking to spin it up to 2krpm maximum...

Chris


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## muffildy (Oct 11, 2011)

youll lose much efficiency at low speeds with a low rpm like that.
For example,
100v 100 amps = 10kw input; 90% efficiency of the motor = 9kw
(kW*9549)/Nm = RPM
Lets say the absolute maximum Nm your motor can put out is 30 Nm
Solving this equation gives you: 2864.7 rpm for that 90% efficiency, and anything below that rpm is going to be less efficient.
Lets say for example you limit it to 2000 rpm, to get the same efficiency at that rpm the motor would need to produce 42.97 Nm of torque. But your motor isnt likely to be running at 2000 rpm all the time, at what rpm do you want to be most efficient is the question then? i would prefer obviously to be most efficient at the lowest possible rpm, but no motors on the market can produce that amount of torque.


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## iti_uk (Oct 24, 2011)

muffildy said:


> youll lose much efficiency at low speeds with a low rpm like that.
> For example,
> 100v 100 amps = 10kw input; 90% efficiency of the motor = 9kw
> (kW*9549)/Nm = RPM
> ...


Isn't that just telling us where maximum power/torque is produced and not saying anything about efficiency...

Rather than using peak numbers, I would prefer to use a dyno plot with an efficiency curve to work out the suitability of the motor.

Also, my understanding is that DC PM motors produce maximum torque at 0RPM...

Chris


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## muffildy (Oct 11, 2011)

not exactly
its not telling you the maximum torque - thats a data point which will remain constant based on the motors design. It is telling you what rpm at that point that the motor needs to run at in order to produce the correct amount of power, the efficiency is assumed at 90%.
So, if your DIY motor can only produce 30Nm it had better be capable of higher rpm than 2000 if you want it to be producing 9kw of power.


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## dtbaker (Jan 5, 2008)

iti_uk said:


> Also, I'm not looking at doing in-wheel (unsprung) motors, I would be mounting the motors inboard, back-to-back, and using driveshafts with UJs. This will keep the unsprung mass down and provide better protection for the motors, cabling and coolant lines.


ok, this is better.... 
but you still need some gears.




iti_uk said:


> Will limiting myself to one gear matter if I spec the motor properly to match the requirements of the vehicle? I know I will be losing the mechanical advantage of having lower gears, but if a motor can be designed to have good torque from 0rpm, as long as it doesn't run out of headroom (rev beyond a point where it loses efficiency significantly), it should be alright, right? I'm only looking to spin it up to 2krpm maximum...
> Chris


motors have much wider power/efficiency band than ICE, but not infinitely so... if you wanted to operate a vehicle at a steady state speed you could get away with a fixed gear such that peak motor eff matched needed rpm for driveshaft. but if you want a 'real' vehicle with decent accel from 0 AND highway speed, you'll need gears; perhaps just two, but you'll need some.


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## iti_uk (Oct 24, 2011)

muffildy said:


> not exactly
> its not telling you the maximum torque - thats a data point which will remain constant based on the motors design. It is telling you what rpm at that point that the motor needs to run at in order to produce the correct amount of power, the efficiency is assumed at 90%.
> So, if your DIY motor can only produce 30Nm it had better be capable of higher rpm than 2000 if you want it to be producing 9kw of power.


I do understand the power-torque equation, and what you have just said reflects my question's point, but earlier you said;



muffildy said:


> youll lose much efficiency at low speeds with a low rpm like that.


The two points seem unrelated in my mind. You're saying that I'll be losing efficiency but then use a hypothetical constant efficiency to tell me the relationship between rpm, torque and power. The one thing you have turned into a constant is the one thing you stated would be changing due to low rpm, so I am left wondering what point you are trying to get across to me. I'm sure you have a point, but I'm confused so far as to what it is; are you saying that electric motors are less efficient at low rpm?



dtbaker said:


> motors have much wider power/efficiency band than ICE, but not infinitely so... if you wanted to operate a vehicle at a steady state speed you could get away with a fixed gear such that peak motor eff matched needed rpm for driveshaft. but if you want a 'real' vehicle with decent accel from 0 AND highway speed, you'll need gears; perhaps just two, but you'll need some.


I suppose we need to define the required acceleration. "Highway speed" would, of course, be 70mph, so I would need to crunch some conservative numbers to estimate the drag force at that speed.

After further jumping around between drive solutions, I have come across this motor from Green MotorSport (GMS M1) which looks like an interesting proposition.



> Motor Continuous Rating @ 45 Deg *18 **KW*
> Peak 5 minute rating *27.5KW*
> Torque *45.5 NM* @ 4000 rpm or *50NM* at 3800 rpm (_cal_)
> Weight of motor unit without coolant. *15.54KG*
> ...


Seems like four of them would be okay, with a reduction box on each (apparently also available from them).

I'm not looking for an infintiely wide powerband, just enough to decently cover a 2000rpm range, starting from zero (or just above zero). Of course, peak efficiency should ideally come at or around cruise rpm, the 0-to-low speed nature of the motor just needs to be able to provide decent enough torque to get the vehicle shifting without being hideously (ICE ) inefficient. Of course I would expect to dump a whole load of charge if I floored it from a standing start, that's inescapable.

Chris


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## MalcolmB (Jun 10, 2008)

iti_uk said:


> Would 4 small water-cooled 10kW (or slightly greater?) motors weigh much more than one 40kW (or more) motor plus differential+casing?


I think that depends on how good your motor/controller design is 

There are too many factors to weigh up easily. If you power the drive shafts directly without a reduction your motors will obviously need to produce enough torque to give your desired acceleration. Since continuous torque is directly related to copper fill, your motors will need to be bigger than they would be if you had the torque multiplication of a gear reduction. On the other hand if you water-cool the motors you could improve the continuous torque rating without adding copper, but then your efficiency starts to drop.

The elegance of driving each shaft directly with individual motors really appeals to me, but the weight saving you achieve by eliminating a gear reduction could be lost in adding more mass in copper and/or cooling.

The simplest and quickest solution would be to use a single motor to drive an existing gearbox or diff, which is why so many people choose this route. This could well also be the lightest solution when you figure in the added mass of four motor casings and mountings plus four controllers and cooling systems. It's really a question of how you want to spend your time (and money). Personally, I'd love to see a four-motor direct drive conversion.


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## muffildy (Oct 11, 2011)

iti_uk said:


> The two points seem unrelated in my mind. You're saying that I'll be losing efficiency but then use a hypothetical constant efficiency to tell me the relationship between rpm, torque and power. The one thing you have turned into a constant is the one thing you stated would be changing due to low rpm, so I am left wondering what point you are trying to get across to me. I'm sure you have a point, but I'm confused so far as to what it is; are you saying that electric motors are less efficient at low rpm?
> 
> Chris


In order for the motor to produce 9kw at say 50 rpm, the motor would need to be designed to be able to produce 1719 Nm of force. since it cannot produce that amount if its receiving 10kw then not producing 9kw its not efficient, or its not receiving 10kw input. either way its not producing the power desired.


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## iti_uk (Oct 24, 2011)

muffildy said:


> In order for the motor to produce 9kw at say 50 rpm, the motor would need to be designed to be able to produce 1719 Nm of force. since it cannot produce that amount if its receiving 10kw then not producing 9kw its not efficient, or its not receiving 10kw input. either way its not producing the power desired.


I can see where the confusion lies now - I'm not looking to produce 10kW over the whole power range. I know that 10kW at zero rpm requires infinite torque, but I'm not seeking the impossible. When I say "10kW", I'm referring to the general size of the motor as a guide. Constant torque, on the other hand, would be nice.

Chris


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## iti_uk (Oct 24, 2011)

MalcolmB said:


> I think that depends on how good your motor/controller design is
> 
> There are too many factors to weigh up easily. If you power the drive shafts directly without a reduction your motors will obviously need to produce enough torque to give your desired acceleration. Since continuous torque is directly related to copper fill, your motors will need to be bigger than they would be if you had the torque multiplication of a gear reduction. On the other hand if you water-cool the motors you could improve the continuous torque rating without adding copper, but then your efficiency starts to drop.
> 
> ...


Interesting point about copper fill - I hadn't thought about that one and it makes sense. I really need to get my hands on a dyno plot or two for some example motors.

Chris


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