# Direct Drive,



## Jordysport (Mar 22, 2009)

As space is very limited in my project (see build) i was wondering if I could use a Motorcycle drive system ie chain, two cogs and the motor to still be able to pull away from the line. and obiously being able to alter the drive ratio's. 

Also many electric golf carts are geared 25:1 could it pull away if it was geared eg 10:1 ?? 

Many thanks.


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## order99 (Sep 8, 2008)

With enough motor, probably. The EV-1 was a direct drive from what I hear, and achieved highway speeds just fine. The late Citycar and Commutacar brands revved up to 40 MPH on motors weaker than today's golf carts on a Direct drive-only 48V too, though I don't know the fixed-gear ratio...


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## david85 (Nov 12, 2007)

The EV1 had a fixed ratio gearbox, I don't remember the gearing, but it wasn't direct drive. I want to say 5:1, but I'm really not sure.


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## major (Apr 4, 2008)

david85 said:


> The EV1 had a fixed ratio gearbox, I don't remember the gearing, but it wasn't direct drive. I want to say 5:1


Hi david,

This has been a point for argument, like forever. But *direct drive* can, and most times does, include a gear reduction between the motor and the wheels. *Direct drive* means that there is no gear ratio change mechanism in the drive line, like a multiple ratio transmission. *Direct drive* can have a single ratio gearbox and the rear end, which likely has an additional ratio, or have the motor connected to the rear end directly. This has been the standard terminology in the EV industry for as long as I can remember. And that's a long time.

And the EV-1 had a ratio of 10.946 to 1. It was *direct drive*.

Regards,

major


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## ftaffy (Mar 13, 2009)

I am looking at running direct drive in my lotus 7 (locost) EV. I am looking at using a 1:3.7 Diff (Ford Cortina TE), connected to a 9' DC Motor (ADC or Warp depending on price and availability when i go to purchase) using a 144V pack. If the controller is capable of 750 Amps for 5 sec then 0-100kph is possible (in 5 sec...) all in theory of course . 

Some numbers:
0-100 in 5 sec requires: 64.3KW to accelerate and 24.5KW to maintain 100kph
144V @ 750Amps gives 90KW
144V @ 250Amps gives 30KW

Power wise it looks ok, the problem is in the speed to motor rpm:
10kph = 479rpm
50kph = 2394rpm
75kph = 3591rpm
100kph = 4788rpm
110kph = 5266rpm ~ from what i understand i am starting to fall off the end hear
120kph = 5745rpm
130kph = 6244rpm

I am thinking of needing a ~0.75:1 2nd gear for higher speed running but dont want to add the extra ~30kg of weight. Looking for a small compact CVT or similar clutchless transmission. 


On the chain connection, we had a chain drive on the EV SAE i have previously mentioned with a 11:34 ratio. Max current we allowed for 200A, weight 188kg and it would pull off but not extremely quick. We believe it was due to the controller limiting current ramp up.


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## david85 (Nov 12, 2007)

major said:


> Hi david,
> 
> This has been a point for argument, like forever. But *direct drive* can, and most times does, include a gear reduction between the motor and the wheels. *Direct drive* means that there is no gear ratio change mechanism in the drive line, like a multiple ratio transmission. *Direct drive* can have a single ratio gearbox and the rear end, which likely has an additional ratio, or have the motor connected to the rear end directly. This has been the standard terminology in the EV industry for as long as I can remember. And that's a long time.
> 
> ...


LOL, I didn't realize I walked into an age old arguement. I always found it simpler to refrer to direct drive as input engine RPM = output shaft RPM. Old 3 speed automatics for example have 3rd gear called "direct drive" because there is no gearing in play once the planetary is rotating as one fixed assembly.

I don't want to start of on a tangent though.


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## Jordysport (Mar 22, 2009)

Ok, this is what i mean: 










So from what major said this is a direct drive system, going back on track, i have a bike in mind which has variable cogs from gear ratio of 5.2 : 1 to 3 : 1 

The motor i am thinking of atm is this one: 

http://www.agnimotors.com/home/index.php?option=com_content&task=view&id=5&Itemid=60 . the 95 series. 

would that be enough umph to get it to 60 - 70mph at 70V???? please could someone enlighten me on the risk of pancake motors (PM).


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## major (Apr 4, 2008)

Jordysport said:


> please could someone enlighten me on the risk of pancake motors (PM).


Hi Jordy,

My opinion is that the available PM motors (commutator type), whether axial or radial, lack the durability needed for EVs. They look good on paper, but often end up being big problems. The axial PM (pancake) motors are expensive. Yeah, the power density and efficiency look real nice, on paper. And they have worked for some specialized EV applications. But for a first timer, beware. I advise to use a wound field motor. Check out EValbum and find other projects of similar size and performance as yours using the motor. Then contact the builder or owner and see if he is pleased with it. And how many miles he has on it.

Now brushless DC is a different story. Also PM, in axial or radial packages. Basically AC drives. And the controller is in charge of keeping the motor within limits. And no brushes. These can work nicely for EVs, but expensive.

Regards,

major


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## major (Apr 4, 2008)

david85 said:


> Old 3 speed automatics for example have 3rd gear called "direct drive" because there is no gearing in play once the planetary is rotating as one fixed assembly.


Yeah, david,

Somewhat of a different context for direct drive. And, there, they did still have the reduction in the rear end. But for *direct drive*, in the EV context, it is assumed that there will be a reduction of some sort between the motor and wheel. And that it is a fixed reduction. Not variable.

About the only time you see a non-reduced direct drive EV is those idiots attempting wheel motors. And, many times, when they figure out the physics of the thing, they put a reducer in the wheel hub with the motor.

Regards,

major


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## Jordysport (Mar 22, 2009)

major said:


> Hi Jordy,
> 
> My opinion is that the available PM motors (commutator type), whether axial or radial, lack the durability needed for EVs. They look good on paper, but often end up being big problems. The axial PM (pancake) motors are expensive. Yeah, the power density and efficiency look real nice, on paper. And they have worked for some specialized EV applications. But for a first timer, beware. I advise to use a wound field motor. Check out EValbum and find other projects of similar size and performance as yours using the motor. Then contact the builder or owner and see if he is pleased with it. And how many miles he has on it.
> 
> ...


ok, sure but the only problem is my project is planned to weigh no more than 300kg !!. will have a look but i doubt i will find a vehicle within that.


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## ftaffy (Mar 13, 2009)

You will want a 30kg motor if you go DC.

Look at your speeds from that get the RPM of ur wheels.
Calculate the forces against you at those speeds + acceleration (as i did above).
Then look at your motor/rpm/torque graph and find the area which you want to operate in to maintain power, effiency etc. You might have high power low down but crap efficiency and you will find your motor gets a little too warm for comfort (we found this).
And thus you can find if you need reduction and if so what the amount.

Finding the forces:
You can calculate your frontal area easy enough. cd (drag) do some research of cd's of other vehicles and find 1 close to yours in shape. Drive train efficiency will be in the order 0.8-0.9, most people assume 0.9.

Nothing in the EV conversion/build process is a 100% answer because everything is still be tried, so find each component in EV album and look at those builds to see if they have any major failings.


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## Jordysport (Mar 22, 2009)

Ratio: 7.5 : 1
Voltage: 48V
Controller Amps: 400A 
Cd : 0.15
RR : 0.01

Don't need good accelaration in the region of 1 m/s^-2

Could you show you working plse as it is impossible to follow. (show equations plse.) 

thanx.


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## ftaffy (Mar 13, 2009)

Try this speadsheet, units in metric.
http://www.cameronsoftware.com/ev/data/EVDrivePower.xls (i didnt make it).

You have 48v x 400A = 19,200W or 19.2KW power, instant = acceleration. This should not be confused with energy (range available) which is calculated by V x Ah.

From what you said above and assuming a 1.5m^2 frontal area and assuming you want 60mph (~100kph) with a range of 30km.

To maintain 100kph you will need 6.211KW or 8.3HP. To accelerate to 100kph at around 1m/s you need 10.9KW over 27 secounds ~ from the spreadsheet. So not going to go into those calcs.

Since you are traveling at 100kph (27.8m/s) and want to travel 30km you can find the run time, that is the time taken to cover that 30km.
Run time = Distance/Speed or in unit terms Sec= (m)/(m/s) ~ this is in base SI units to keep with engineering basics.

Run time = 30,000/27.8 = 1079sec or 17min or 0.3 hr (seems obvious but for those who want something a little funkier and use this)

The next step is to determine your power usuage or Wh rating.

6.211 (kw)*0.3 (hr) = 1.86 kwhr = 1,860 Wh

Since you have traveled 30km we can find the Wh/km rating. (not required but still nice to know if you want a larger range)
Wh/km = 1,860/30 = 62 Wh/km

Now to find the Amp hour (Ah) rating of what your 48V pack will need:
_(1) Power (w) = Volt (V) x Current (A)
(2) A Watt Hour (Wh) = Volt (V) x Amp-hour (Ah)_

So we use the 2nd equation to get:
1,860 = 48 x Ah
Ah rating of pack = 38.75 Ah

You will never fully discharge your pack, so saying you use deep cycles which can go to 80% DoD you will need an extra 20% on that figure
1.25 x 38.75 = 48.3Ah The 1.25 comes from the need to add 20% + safety to the pack, so you need 120% of your calculated power in your pack.

The next issue is that battery discharge is not perfect. The faster you discharge a battery the less energy you get, same as if try to run a water mill by pouring a glass over it, pour slowly and you will get power over a long period of time, Tip the glass on it and it will spin fast for a moment but less water hits the wheel. - If that makes sense... 
All that simple means you need to multiple by around 1.8 due to the discharge nature of batteries:
1.8 x 48.43Ah = 87.18Ah

Noting that this last x 1.8 applies to lead acids and not lithium packs!

Acceleration current requirements:
Since we know you need 10.9KW to get to 100kph in 27 sec its a fairly easy calc to get the current.
Using equation (1) from before: 
_(1) A Power (W) = Volt (V) x Current(A)
_10,900=48xA
A= 227.08AmpsYour 400Amps exceeds this by abit, 19,200W. So that means your 0-100 time if can use all of that would leave you with a 0-100 time of around 10.1sec. ~ Use the spreadsheet.
So concluding:
Pack: 48V 90Ah
Current limit: 400A Should be possible with 4 x 12V batteries.


Notes: Not totally correct notation used in all the formulas, so bear with me please.


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## ftaffy (Mar 13, 2009)

Next you need to determine you motor rpm speed:

Using speed (m/s) say 100kph ~ 27m/s for simplicity sake.
And assuming wheel diameter of 0.3m

You need to convert you linear speed of 27m/s in to RPM.
So x 60sec to get into min.

27m/s x 60sec = 1620m/min

Next we need to covert this into a rotational speed:
_This is hard to explain in words, but think of a hammer thrower in the olympics. The guy in centre turns but does not move while the hammer moves at the outside of the circle. The man lets go of the hammer and it flies away. _
_The man has rotational speed in that he spins on the spot but travels no distance linearly. While the hammer when released flies away with linear speed. _
_Gets more fun now with that you can take a point on a circle of motion (hammer circular path) and draw an a line at 90 degrees and that is the direction if realsed the hammer will have linear (straight line) speed. _
If you dont get it, dont worry just use the formular.

Tire radius x PI x 2
Pi is 3.14 and is a constant
2 is do with that radius = 1/2 diameter

We have 
0.3 x 3.14 x 2 = 1.88

So 
1620/1.88 = 859 rpm
That is with 1:1 ratio, using that you will have a 1:7.5 ratio you need multiple by 7.5 to get engine speed.
859 x 7.5 = 6631 rpm

We know that at 6631 rpm you need to have at least 6.2KW or 8.3HP you need to look at your motor and check that this is so, else gear lower.
Ratio of 3 puts u around 2650 motor rpm. When looking at these ratings they are voltage/current draw dependent so be sure to check this, dont read off 144V values! or 300V :-(.

**** Please only take these possible and check all calcualtions before using them to put out any $$$. ****


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## Jordysport (Mar 22, 2009)

that is ace, many thanks, much easier and better to understand than Uve.

umm, btw last time i checked 27/1.88 was not 800 odd, sure u mean 1620 rather than 27??


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## ftaffy (Mar 13, 2009)

ahhh yes, typo. I do mean 1620 .


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