# AC induction motor thoughts



## JimDanielson (Oct 19, 2008)

Im working on an induction motor and wondered what you guys thought. The past year I have spent working on 3 phase motors, I have built several running motors at a few hundred watts power, most synchronous machines. Currently, I am working on scaling up several of these ideas to a ~30kw motor.

All the parts for this larger prototype are nearly manufactured, waiting on electrical laminations. By the end of the month it should be running on the bench.

A major difference this motor will have is the max rpm. Prototype is a 12pole machine. If I remember the frequency on the Curtis Controller correctly, it will approach 3000rpm top speed. But, this final model may be designed as a 24 pole machine. The main reason I am doing this is for elimination of the transmission. The goal would be to have 2 of these motors directly coupled to the CV joints of a car. It would save on weight and inefficiency of gears.


I am also working on a synchronous rotor for 3 phase machine that will not have permanent magnets or brushes. This works with the small prototypes but I will be holding off for the large motor due to a number or reasons, mainly being time to get it working perfect.


Questions; what do you think of directly coupling to CV joints?
how do you feel about having to deal with 2 motors?
would you buy a motor with 1500rpm top speed?

Ill try to answer any questions, because I am filing provisional patents on some of these features I might not be able to say everything. It would be great to hear if it sounds interesting to you


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## somanywelps (Jan 25, 2012)

> Questions; what do you think of directly coupling to CV joints?

I don't like it, the torque's going to be too low. The Tesla and others use an 8:1 gearbox for a reason.

See if you can find an ~8:1 diff and link to that.

> how do you feel about having to deal with 2 motors?

up to the software. If they're inline on the same shaft you don't even have to worry about it.

> would you buy a motor with 1500rpm top speed?

Never. 0-1500rpm is a pathetic power band. I wouldn't even do 3000.

6000 is decent with a transmission or very high torque and a 3:1-4:1 diff.

12000 is great with an 8:1 gearing.

Also 35kw is far too low. You're fighting AC-50's and whatnot.

Now if you were making 100kw-200kw motors less than Remy and AC propulsion, well now you've got possible OEMs interested...


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## iti_uk (Oct 24, 2011)

I am very interested in your work. I have previously expressed my desire to have a motor-per-wheel vehicle (4 motors), using CV jointed half shafts to allow chassis-mounted motors. Since I was looking at motor-per-wheel, I was imagining 15kW or so per motor, which would fit nicely with your proposed two 30kW motors.

To answer your questions;

I can't see any problems coupling directly to CV joints. I have heard about high currents at low motor speed, suggesting that mechanical down-gearing of a faster-spinning motor might eliminate low-RPM current spikes (or at least narrow the range). Of course, having direct drive means low voltage and high current, so you'd have to use heavy cabling. In my head I have a motor driving a planetary reduction gearset, in turn driving the CVJ/halfshaft/wheel.

I think having motor-per-wheel is the way forward (as I see it so far), and opens up fantastic possibilities with traction control and active virtual-differential algorithms. Very exciting.

I certainly would buy a motor as spec'd, especially if it were liquid-cooled. I have been looking into refridgeration as a possible method of cooling while allowing a decent cabin heater - evaporators on the motors and controllers, and condensers in the vehicle's HVAC. I'm not aware of having seen this before.

Do you have any pictures of your prototypes? What are the projected power and torque levels, dimensions and masses?

Chris


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## jddcircuit (Mar 18, 2010)

I am interested.
If you have a motor that produces the necessary torque within your weight budget without a gear reduction then it would be worth consideration.

I was surprised to see how much losses were attributed to the gear reduction and differential in one of the Prius transaxle studies that I read on line. I can't remember what it was but IIRC it was significant percentage.

However I remember Major stating that torque multiplication using gearing vs. using extra copper can result in a much lighter package. I want to think it was a 4 to 1 weight ratio. But then I see some slick high pole outrunners flying RC airplanes where weight is a premium.

good luck
jeff


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## Nathan219 (May 18, 2010)

Please let me know how you motor turns out. You are building a torque motor, like the kind they use in elevators. I have been trying to source a motor like yours, but they are very expensive generally. 1500 RPM is good for well over 100mph depending on tire size. 
Good luck keep us posted!


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## subcooledheatpump (Mar 5, 2012)

Hey, some people aren't reading this correctly. He's making multipole motors to elminate the gearbox. More torque but less speed. That means his power band isn't "pathetic"

The Tesla Roadster has an 8.28:1 gearbox. That means 15,000 RPM goes in but only 1811 RPM comes out. 

So if he wants to make a motor with a maximum speed of 1500 RPM and it's 24 poles, That means he'd be getting a ton of torque in exchange for a ton of speed, enough torque to get the wheels turning without a gearbox/transmission 

But there are a few problems with that. Multipole motors are alot less efficient, They consume more reactive power. The frequency will also need to be high, more losses. The shafts will need to be extra hard and made with high precision since they are connecting right to the CV shafts. Other than that, sounds good to me.


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## peggus (Feb 18, 2008)

Torque is a function of fluxdensity in the airgap and the geometry of the airgap (radius, length). Polecount doesn't factor into it much, one might however be mislead by a commonly known equation into thinking that it does. 

You want a high torque motor, you end up with a large motor since flux density is limited by the materials used.


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## JimDanielson (Oct 19, 2008)

subcooledheatpump is right, I have run magnetic simulations on this 12 pole motor and am getting ~200ftlbs peak. The 24 pole motor would have the same slot design and winding pattern so the torque would be about double.

The AC50 peaks at 115 ftlbs of of torque. If you look at a civic ratio for example of about 4:1, you would have under 500ftlbs of wheel torque with the AC50. With the current prototype I would have 400 ftlbs of wheel torque and the 24pole could potentially have nearly 800ftlbs at the wheel.

Also, I have the ability to increase the height of the lamination stack with no change to manufacturing. Right now I am running all my simulations on a short stator stack at a rather large diameter (nearly 14inches on the 24pole). The lam stack can be lengthened to increase both power and torque. These simulations are being run right at the peak flux density of the silicon steel I am using.

Of course, with some inefficiencies and manufacturing tolerances, I do not expect to get the exact numbers in the real world, but thats whay I have a prototype that I will be able to test soon.


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## JimDanielson (Oct 19, 2008)

jddcircuit mentioned the extra copper requirement. I have designed this motor so the temperature limitations on all the materials are at least 350C (including protective coating on the wires); class H insulation is limited to 180C. Although it would be inefficient to run continuously at this temperature, it would allow bursts of a much higher peak torque and current. I can operate the copper closer to its fusing current.

I am not positive it will give the desired output for long enough, but I am giving it a shot.

The main goal would be to have extra power available for merging onto the highway quickly and then go back to a much lower average power


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## JRoque (Mar 9, 2010)

Hello Jim, will you be driving the two motors with Curtis controllers? IIRC they can synch both motors and account for speed differentials while turning. Otherwise, the controller coding will be involved, to say the least.

JR


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## mizlplix (May 1, 2011)

Single speed electric propulsion is A worthy ideal.

Tesla does it by ultra high motor RPMs and appropriate gearing.

The reverse can be done also. Low RPMs with high torque and appropriate gearing.

Anytime you can eliminate a component, it is a good thing.

In this case it is the transmission. 

(weight, heat, cost, friction). Gone.....

Miz


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## JimDanielson (Oct 19, 2008)

Curtis or Sevcon. Both have CAN communication so you can use multiple controllers. I know with sevcon it takes an additional potentiometer input from the steering wheel to apply torques independently to both right and left wheel. I have not checked this on curtis yet, I have a sevcon for testing.


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## bjfreeman (Dec 7, 2011)

JimDanielson said:


> Curtis or Sevcon. Both have CAN communication so you can use multiple controllers. I know with sevcon it takes an additional potentiometer input from the steering wheel to apply torques independently to both right and left wheel. I have not checked this on curtis yet, I have a sevcon for testing.


based on my direct drive system, you should design your own Vehicle Control Unit that takes into account each wheel speed, like a anti-skid module. you may be able to pick that off the current Canbus of your vehicle.
you also have to adjust for wheel speed when turning, since you don't have the gear box that takes care of that.. *
*


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## PStechPaul (May 1, 2012)

I am very interested in the details of this project. My original goal was to make a direct drive induction motor and use the maximum number of poles to get a low speed at 60 Hz, and then overclock by 6x (keeping the same V/F ratio), which I thought would produce 6 times the power for the same weight and size. 

But, I found that higher number of poles in the same size motor reduces the horsepower while it increases torque. For instance, I have the following motors:

1.0 HP 8 pole 850 RPM 6.2 lb-ft
1.5 HP 4 pole 1725 RPM 4.56 lb-ft
2.0 HP 2 pole 3450 RPM 3.04 lb-ft

I think this is due to the overlap of the windings for the poles. The motor I rewound had a 36 slot stator, and I wound it for the maximum number of poles (12). This means that each slot has two windings, which include phases which are 120 degrees apart, so the combination is at 60 degrees, but the vector magnitude is 1/2. When there are fewer poles, there is less reduction in amplitude because of variable overlap. 

It seems that larger motors have a more linear relationship of HP and torque vs RPM, so a 50 HP motor is the same size for 2, 4, 6, or 8 poles. I think this is because they have many more slots and can have a more efficient winding pattern. So, depending on the number of slots in your 12 and 24 pole motors, this phenomenon may come into play. I may not have a full understanding of this, and I may not be explaining it well, but it seems to account for the figures above. 

I was unable to measure the torque of my motor, but it seemed lower than I had expected and hoped. It was originally a 1/2 HP 120V single phase PSC motor, with 36 slots, and I rewound it with much heavier magnet wire for 8 VAC three phase and 12 poles, for sync speed of 600 RPM at 60 Hz. I overclocked it to 4x, I think, and verified that it ran about 2400 RPM (and about 32 VAC). I had intended to make an automotive direct-drive motor for the rear axles of a FWD car as a hybrid, but it was not really practical for even 5 HP per wheel. Using 48 VDC, the motor would need wires for 80 amps. It was even more problematic for overclocking a small 1.5 HP nominal rated motor.

I am now only working on a project for a small utility vehicle or tractor, which has a more limited range of speeds and torque than a road vehicle, but I could use a slow speed motor as you are designing. I think it would be best to design it with multiple windings so that it could be connected for 120/240/480 volts. And thus it should be possible to use the 120V connection on a 480 VAC controller to get 4x RPM and HP. So your 24 pole motor might run at 300 RPM on 60 Hz, but 1200 RPM at 240 Hz. Then it could possibly be overclockd beyond that to as much as 600 Hz and 3000 RPM, but keeping voltage at 480 which reduces torque. But you don't really need as much torque at the high end unless you have very high wind resistance or absolutely need to maintain maximum speed fully loaded on steep inclines.

The usual argument against overclocking is eddy current and magnetization losses due to the material and thickness of standard laminations. But if you are designing from scratch with custom laminations, you may be able to get them of higher grade material and much thinner. I look forward to more details and test data on your prototype.


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## subcooledheatpump (Mar 5, 2012)

Thing is about rewinding motors, it goes back to what peggus said, you can't just rewind a 2 pole motor into a 8 pole motor and make more torque.(well you can to a certain extent anyway but you lose power) For making more torque you actually need a physically bigger motor since you run out of magnetizeable material with a small one. 

For example, a factory rated 1 HP 8 pole motor is larger than a factory rated 1 HP 2 pole motor. Simply because to make more torque you do need more iron. Trying to get the same magnetic flux density out of a given size motor as a larger one will only cause it to saturate. (the magnetic fields inside a given piece of iron can only be so strong before the iron won't be magnetic anymore)

That doesn't mean I think that OP's idea won't work though, since he's making custom motors, he can design them with enough magnetizeable material for whatever specifications he wishes.


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## JimDanielson (Oct 19, 2008)

Hi Paul, it sounds interesting what you are doing. My project is a bit different. I have custom designed the laminations for this motor. The 12 pole machine has 72 slots and the 24 pole machine will have 144 slots. Both machines will have an equal flux density on the slots, and therefore, the diameter of the 24 pole motor is nearly double.

The prototype laminations are laser cut. But I will be blanking the final product on a die.

I will not be running this motor over 300 hz as that is the max on the Curtis/Sevcon. And running it higher would start to significantly increase the loses.

For voltage, this will only have one set off windings. I want to achieve maximum slot fill and it will have a very specific application, so there is no need for each motor to fit to multiple voltages. The exact voltage I am planning to use has not been completely decided, but it will most likely be near 100volts.


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## somanywelps (Jan 25, 2012)

If you're making your own motors, make some beefy ones and take on Remy and co. 

I see where you're going with this, but it's going to have to compete with AC-50's with a transmission.


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## Ryan800 (Apr 15, 2010)

200ft-lb isn't enough for 2wd - 400ft-lb total to the wheels will just barely pull a geo metro up a slight hill. 800 is much better but still on the low end. For reference, most compact cars, such as a Honda Civic make about 1500ft-lb peak in first gear.

I don't think anyone with an AC-50 starts out in 4th gear, which would be about 400ft-lb to the wheels. From what I've read even starting in 3rd is fairly slow.

On the other hand, I spent some time looking for a reduction gear sets appropriate for EVs and they're very hard to find. Diffs just don't have a high enough ratio unless you go with a really big motor. So eliminating it would be great, just make sure you can get a practical amount of torque otherwise there won't be a market. Speaking of which, the market might not be very big even if the motor does work unless you can convince OEMs to buy.


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## JimDanielson (Oct 19, 2008)

Thanks for the feedback. I guess I had misread the civic gear ratio chart. I will definitely look into increasing the torque for the 24 pole motor (too late to modify anything on the 12 pole, the stator gets in tomorrow). I am now thinking more like 6 or 7 hundred ftlbs per motor.

As for intended market, DIY is really just a short term plan while I work on finishing my other motor and talking to OEMs. I know that DIY will probably not offer a big enough market to survive.

The other motor is an inductively powered synchronous machine. So, it should have advantages of efficiency similar to PM BLDC machines. But a much lower cost because permanent magnets end up being about half the cost in high power bldc motors. Proof of concept prototypes work, but I am far away from being able to manufacture it.


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## PStechPaul (May 1, 2012)

What may be needed is a two-speed planetary drive, which can switch from a reduction drive to a straight-through 1:1 drive by means of electrical actuators. It should be possible to get all the torque needed in the reduction mode, and then all the speed needed in the direct mode, with very little mechanical losses. I had thought about using a system like the old Sturmey-Archer 3 speed bicycle hub, which has reduction, direct drive, and overdrive modes. Here are details of the 2-speed drive: http://www.freshpatents.com/-dt20091001ptan20090247346.php

Maybe something like these:
http://www.axiom.org.za/Torque-Hub-Product-Selection-Guide.pdf

They're probably expensive.

I wonder if a six-phase motor might be the way to go for a high torque application? It was just a thought, but it seems that someone has done research on this and it greatly improves torque density, among other benefits. I just found it and have not read much of it, but if you have not wound the stator yet, this might be something to try:
http://lipo.ece.wisc.edu/2002pubs/2002_12.pdf


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## subcooledheatpump (Mar 5, 2012)

Heads up on the civic gear ratio chart, you multiply the gear ratio by the final drive 

Most cars first gear are something like 11:1 overall ratio 

The van that I converted had original first gear of 3.06:1 and final drive 3.73:1. 

So 3.06*3.73= 11.4

Torque from engine, 340 ft lbs* 11.4 =3880 ft lbs at the wheels. 

old 90's civics got about 100 ft lbs at the engine, so just an estimate 100*11=1100 ft lbs at the wheels


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## iti_uk (Oct 24, 2011)

(Revisited and tidied up this post)

Just for a quick reference;

Assumptions:
- ignore rolling and aerodynamic resistances
- assume wheel diameter of 0.6m
- assume vehicle mass of 1000kg


1) Two direct drive 200lb.ft motors (400lb.ft total) would be capable of maintaining speed up a slope of just over 10 degrees incline.

2) One 100lb.ft motor with overall gearing of 11.4:1 would be able to maintain speed up a slope of just over 30 degrees. (As per civic example suggested by subcooledheatpump)

For vehicle 1) to be equivalent to vehicle 2), the 200lb.ft motors would need to be upgraded to 600lb.ft.

I've attached a cheap-'n'-cheerful zipped excel sheet with the calcs. Just for fun.


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## mizlplix (May 1, 2011)

OK, Just for reference:

My car- AC50/ powerglide(2 speeds), 6.14:1 rear gears, 3,300Lbs.

based 0n 6,500 RPMS.....1.75 low/ 1:1 in top

First gear= 50 MPH.

Top gear= 95 MPH

Decent acceleration but *needs* both gears.....

An AC50 is a converted 7.5HP motor and is 8 inches O.D.
A 10 HP motor is about 10 inches in O.D.
A 15 HP motor is about 16 inches in O.D.(this would work as a direct drive motor {I feel}) it is 3 times the weight of an AC50-unless a custom housing/end bells are made, then u need an encoder and thermister too.

Just my guesses at this point, nothing to back it up except opinion.

Miz


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## iti_uk (Oct 24, 2011)

mizlplix said:


> An AC50 is a converted 7.5HP motor and is 8 inches O.D.
> A 10 HP motor is about 10 inches in O.D.
> A 15 HP motor is about 16 inches in O.D.(this would work as a direct drive motor {I feel}) it is 3 times the weight of an AC50-unless a custom housing/end bells are made, then u need an encoder and thermister too.
> 
> ...


Interesting.

Bear in mind that we are talking about having multiple direct drive motors (if I read the OP correctly - this is certainly what I am talking about).

So the 15HP motor you mention is 16" diameter and the AC50 (7.5HP) is 8" diameter. The former is three times heavier than the latter. Would you say that two AC50s are equivalent to one (15HP) motor in terms of power and torque? If so, then two AC50s, mounted back to back should be okay...?

Chris


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## mizlplix (May 1, 2011)

Motor torque is largely determined by rotor diameter. Then to a lesser extent by pole count and winding pattern.

I have not done valid tests to determine if one 15" motor or two 8" motors has the most torque. Given one fixed constant (the curtis controller/inverter) .....

Having said that...Kevin in Topeka has a huge motor running in a full sized pick-up utilizing a curtis controller to run that behemoth motor. It drives like a normal truck (according to a friend who has ridden in it). It is air cooled too.

Leading me to believe that:

1-the curtis has more potential than an AC50 can utilize. and...

2-a properly designed and wired large motor will use the curtis controller more efficiently than a smaller motor.

miz


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## JimDanielson (Oct 19, 2008)

You are right iti, the plan is for at least 2 drive motors. Potentially even 4.

That is interesting how much the AC50 is being overpowered. I would guess the electrical steel is saturated. I am running my peak torque simulations to keep the steel under 1.4 Tesla. Any higher and the loses would start to go way up.

Also, motor torque is determined by length of motor linearly.


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## bjfreeman (Dec 7, 2011)

of all the subject on EV, this is the one I had to go to someone that really knows 3 phase motors.
I ended up with a 8 pole squirrel cage. other than they work, I really can't discuss the good and bad of this design. It is and area I am not that educated in.


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## PStechPaul (May 1, 2012)

JimDanielson said:


> You are right iti, the plan is for at least 2 drive motors. Potentially even 4.
> 
> That is interesting how much the AC50 is being overpowered. I would guess the electrical steel is saturated. I am running my peak torque simulations to keep the steel under 1.4 Tesla. Any higher and the loses would start to go way up.
> 
> Also, motor torque is determined by length of motor linearly.


Another thing to consider is that most AC induction motors can provide at least 3 times normal rated torque for short durations (such as acceleration), so if your motor is rated 200 lb-ft it may be able to give 600 lb-ft before stalling, so 1200 lb-ft combined, which is 1200 lb of force in the axis of linear motion. This would be enough for a 3600 lb vehicle with 24" diameter tires on a 30% slope (as long as it was a short section of road). But if your figure is for peak torque, then it would be undersized except for a very small, lightweight vehicle.

Have you looked into making your motor six phase? I added a paragraph to my previous post. Apparently you can increase torque as much as 40% by using two 3-phase windings spaced 30 degrees apart, and injecting harmonic content at the proper phase angle. I found other studies about this, but the one I cited seems to be the most informative:
http://lipo.ece.wisc.edu/2002pubs/2002_12.pdf

I also like your idea of making a BLDC motor or synchronous machine by using a second rotor and stator (rather than brushes) to magnetize the rotor by means of induction. This may be able to produce characteristics similar to a series wound DC motor, although current might be limited by magnetic coupling and electronic commutation losses. I'm not sure I understand all the details, and it would be very interesting to discuss this idea. It is probably something like this:
http://en.wikipedia.org/wiki/Brushl...hless_wound-rotor_doubly_fed_electric_machine

Here is a company that has patented such a machine which can be used as either a motor or generator:
http://www.bestelectricmachine.com/


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## Tesseract (Sep 27, 2008)

JimDanielson said:


> ...Also, motor torque is determined by length of motor linearly.


All else being equal, motor torque is proportional to length and the square of diameter.



PStechPaul said:


> Another thing to consider is that most AC induction motors can provide at least 3 times normal rated torque for short durations...


That's the theoretical maximum which is rarely achieved in the real world, and only then if the inverter has especially good control over the direct and quadrature currents. That generally requires a really fast DSP (or FPGA) with floating point unit and a high line count encoder. 



PStechPaul said:


> Have you looked into making your motor six phase? I added a paragraph to my previous posthttp://lipo.ece.wisc.edu/2002pubs/2002_12.pdf...


So... an inverter with six half-bridges to drive custom - literally one-off - 6ph. motors? That would make an AC Propulsion system look downright reasonably priced...


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## JimDanielson (Oct 19, 2008)

Potentially you could get more torque out of this motor, but I am already running this motors near saturation(~1.4 Tesla). And with the copper I am running it at 4 times the current capacity recommended for continuous use recommendation for motor design. But, I believe this will be fine because my insulation is much higher and I am still half the preece fusing current. I would guess the motor would only be able to run at this torque for under a minute. (edit: by running, I mean simulation, bench testing wont happen for 2-3 weeks, and dyno testing, once I find a dyno shop for cheap in N Cali)

6 phase sounds interesting, that would be great to get more torque out of the same size. Main reason I wouldn't consider it is because I want to use an off the shelf motor controller. Do you know if multiple phase share slots on their 6 phase design?


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## subcooledheatpump (Mar 5, 2012)

6 phase almost sounds like it would do more harm than good. The same stator would be sharing twice as many currents and twice as many magnetic field lines/flux. Plus the inverters semiconductor count would double. As was said before, pricey and difficult

Bigger motor=more constant torque. Makes sense. More magnetizeable material. But really, anyone could figure that out. a Factory rated 100 HP motor for example is quite a size. whereas a 1 HP factory rated motor is quite small. I haven't actually measured anything apart from the current and voltage readings, but if they are true to what the motor is actually doing, I am able to get about 4X torque out of a 15 HP 6 pole motor. Of course as I said though, thats not dyno proven and could well be a false assumption. Works well in my van though.


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## JimDanielson (Oct 19, 2008)

an update:

today the motor spun, nothing too exciting though. the curtis controller is still programmed for the ac50 and I am trying to mess with all the motor settings to get it to work better. wouldnt go over 15rpm


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## leong (Aug 22, 2012)

as someone mentioned before, beyond 4 poles you will experience a lot worse on the power factor (reactive power) thus your controller rating needs to go may be doubled or tripled to deliver the same power. Usually induction machine for traction purpose is 4 pole or 6 pole at most.


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## major (Apr 4, 2008)

leong said:


> as someone mentioned before, beyond 4 poles you will experience a lot worse on the power factor (reactive power) thus your *controller rating needs to go may be doubled or tripled* to deliver the same power. Usually induction machine for traction purpose is 4 pole or 6 pole at most.


Hi leo,

I think you exaggerate. I could see maybe 10 -20% higher rated, not 2 to 3 times. I recalled this study to support this http://www.reliance.com/pdf/motors/whitepapers/B7100.pdf See figure #1. Have you a support citation?

Regards,

major


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## leong (Aug 22, 2012)

Hi, major

I was referring to the 12 - 24 pole case. yes i have seen this whitepaper a while back. You might have noticed that the peak torque power factor is reducing at a faster pace when the pole pair number increases. The figure stops at 8 pole, for 24 pole, you may get well under 40%, comparing to 90% power factor, you need an inverter with over 2x KVA rating. From the machine efficiency point of view, higher pole count actually gets better machine efficiency, but from the system point of view, the efficiency saved on the machine would be shifted to the drive electronics due to higher KVA. I don't have a citation for the pole count vs. power factor. I did some numerical study a while back and reached the conclusion not to design an IM with more than 6 poles, then read the paper you mentioned as a verification. 



major said:


> Hi leo,
> 
> I think you exaggerate. I could see maybe 10 -20% higher rated, not 2 to 3 times. I recalled this study to support this http://www.reliance.com/pdf/motors/whitepapers/B7100.pdf See figure #1. Have you a support citation?
> 
> ...


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## JimDanielson (Oct 19, 2008)

My power factor is about 72% at no load right now and improves at higher load. At 200 amps draw it goes up to 75%. These tests were done on my 24 pole motor prototype at 1500rpm on Curtis controller at 300hz.



I am still working on getting a test dyno set up for measuring torque at that speed.

I did get a 0 speed torque output of 40ftlbs. It is not too high but this motor is much smaller than any production version would be. And the amount of power I can put into the machine is low because the current on the controller is limited. If anyone knows of an affordable 1500amp controller, I would be interested to test with it.


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## leong (Aug 22, 2012)

Jim, how did you measure the power factor? If you are running the machine with an inverter, the motor terminals would be driven by switching PWM voltages. You would need some sophisticated instrument to filter, compensate in order to measure the power directly going into the motor. If you are measuring the power factor going into the drive, that is totally different than the power factor on the motor. The drive itself may have a PFC stage and the power factor would approach unity. 





JimDanielson said:


> My power factor is about 72% at no load right now and improves at higher load. At 200 amps draw it goes up to 75%. These tests were done on my 24 pole motor prototype at 1500rpm on Curtis controller at 300hz.
> 
> 
> 
> ...


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## JimDanielson (Oct 19, 2008)

Yes, I understand how to measure power factor... I measured on the motor. My oscilloscope can filter/average pwm signals.

The inverter is powered by a DC battery.


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## leong (Aug 22, 2012)

did you filter both the voltage and current with the same settings? did you measure the line to line voltage and line to neutral current? is your motor Y connected? did you use VIsin(theta) formula to calculate power/reactive power?




JimDanielson said:


> Yes, I understand how to measure power factor... I measured on the motor. My oscilloscope can filter/average pwm signals.
> 
> The inverter is powered by a DC battery.


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## PStechPaul (May 1, 2012)

It's really important to use a proper wattmeter (or simultaneous samplng A/D converters) if the voltage and current waveforms are at all distorted. I can believe the 72% PF at no load or light load but at full load I would think it would be much higher than 75%. I'm not sure how much this will affect efficiency. A low PF essentially means that the current will lag the applied voltage because of inductancebut the losses are mostly determined by the current. It may mean that a higher voltage is required for the same amount of power because of the phase angle, but I think the motor will still draw the same current for the same torque. Of course a motor with more poles will have more torque at slower speed for the same applied voltage and frequency. It would be good to see details of the meaurement and actual data and graphs,


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## leong (Aug 22, 2012)

good call... 

essentially reactive power is the part of energy to establish the magnetic field in the induction machine. At no load the power factor should be very very low since all energy are used to generate the field, not to generate any real power. 

when measure the voltage, just remember there is a 30degree shift between the line-line voltage and line-neutral voltage, while the current is usually measured as line-neutral.



PStechPaul said:


> It's really important to use a proper wattmeter (or simultaneous samplng A/D converters) if the voltage and current waveforms are at all distorted. I can believe the 72% PF at no load or light load but at full load I would think it would be much higher than 75%. I'm not sure how much this will affect efficiency. A low PF essentially means that the current will lag the applied voltage because of inductancebut the losses are mostly determined by the current. It may mean that a higher voltage is required for the same amount of power because of the phase angle, but I think the motor will still draw the same current for the same torque. Of course a motor with more poles will have more torque at slower speed for the same applied voltage and frequency. It would be good to see details of the meaurement and actual data and graphs,


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## Ivansgarage (Sep 3, 2011)

On a 3 phase motor witch leg is the line-neutral?

Just had to ask.


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## leong (Aug 22, 2012)

traction motor usually do not have the neutral exposed, so you can only indirectly get the neutral by doing the sqrt(3) scaling and 30deg rotation from the line-line voltage. for industrial motors, some time 9 wire some time 12 wire, you can choose the Y or Delta connection and pull out the neutral, just do not ground it..




Ivansgarage said:


> On a 3 phase motor witch leg is the line-neutral?
> 
> Just had to ask.


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## mizlplix (May 1, 2011)

Yah, Ivan. 

It is an internal motor tap and does not have an external connection.

Read up on transformer theory and mentally reverse it for motor duty.

(Or have your wife read it to you)


Miz


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## Stiive (Nov 22, 2008)

JimDanielson said:


> today the motor spun


Pics? Vid?


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## JimDanielson (Oct 19, 2008)

Leong, thanks for the feedback, it sounds like you have a lot of experience measuring power factor. I haven't been doing it long but think I have it right here. 

For my setup, the motor is Y winding with a ground. Since I wound it myself, I tapped off the ground point. In this picture, CH2 is the motor voltage, CH1 is current. I fried the scaling circuitry on my current clamping probe, so ended up connecting directly to the winding inside the device.

In this waveform, the controller is at 300hz. The motor is spinning at 1500rpm.

I used the scope to filter anything over 500hz for both signals and averaged 16 samples. The scope has a tool for measuring phase shift. As you can see, current is lagging 89 degrees.

(360-89)/360=0.753

Since both measurements were taken line to neutral I did not think I had to do any adjustments to the number. Please let me know what you think

Currently uploading a video to vimeo and will share when it is done


EDIT: Also should note, the motor was running unloaded. I think the curtis controller said ~24amps


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## PStechPaul (May 1, 2012)

Sorry, but your calculation of power factor is totally wrong. First, any calculation based on phase angle requires sinusoidal waveforms, and yours are far from that. But if they were, and the phase angle is 89 degrees, then the power factor would be the cosine of 89 degrees, or 0.017 (1.7%).

http://en.wikipedia.org/wiki/Power_factor

But the definition of power factor is the ratio of actual power to APPARENT power based on volts * amps (or VARS, sometimes described as volt amps reactive). So what you really need is a voltmeter, ammeter, and wattmeter. And make sure the meters are true-RMS.

One problem with using phase angle for the calculation is determining the exact point of zero crossing. It is well defined for sine waves as well as triangle and square waves, but for PWM and rectangular waves (modified sine waves), there is a significant portion of the wave where the amplitude is zero, so the zero crossing is undefined.


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## leong (Aug 22, 2012)

Jim, the waveforms look ok. I'm glad you are able to access the neutral. but as Paul pointed out, your formula is not the right one to compute the power factor.

don't get too frustrated yet, as I mentioned, if you are using a field oriented control you will see the power factor to be very low at no load because all the apparent power are only used to establish the magnetic field.

one thing you would need to do is to try to reduce the field intensity (don't know how the controller you are using can do that) by reducing the voltage but maintaining the frequency (reduce the volt-second).

unfortunately, as my first post mentions, your best power factor after all tuning may still be very low comparing to 4 pole machine. most likely under 50%...

the other thing is about safety. please make sure you don't have a ground loop now that you are tapping the neutral and have the neutral tied to ground via your scope (unless u r using an isolated probe). any dc leakage (hope you have hipot'ed dc to gnd) may form a ground current that may damage your instruments...






JimDanielson said:


> Leong, thanks for the feedback, it sounds like you have a lot of experience measuring power factor. I haven't been doing it long but think I have it right here.
> 
> For my setup, the motor is Y winding with a ground. Since I wound it myself, I tapped off the ground point. In this picture, CH2 is the motor voltage, CH1 is current. I fried the scaling circuitry on my current clamping probe, so ended up connecting directly to the winding inside the device.
> 
> ...


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## JimDanielson (Oct 19, 2008)

Well that's embarrassing, thanks for the correction. I guess I had expected the power factor to be mid 70s when I was researching other 3 phase machines, so once I got that number I was pretty satisfied and didn't check my equation.

The grounds on my scope are isolated from the inverter/motor. The inverter is powered by a battery that I only charge when not testing.

I will look into reducing the volt-second

as promised video: http://vimeo.com/48116123
password:"diyec"


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## major (Apr 4, 2008)

JimDanielson said:


> as promised video: http://vimeo.com/48116123
> password:"diyec"


Hi Jim,

I'd take that key out of the motor shaft or you'll be taking it out of your eye 

Be safe,

major


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