# Matching a motor to a donor car



## major (Apr 4, 2008)

puddleglum said:


> In a DC motor, the HP and torque curves seem linear but the speed curve is not. I'm not sure how that fits with the above statements. Could someone please explain that, or show me where I am mistaken above.


You're mistaken about this. HP is not linear w/r/t torque for a DC motor.


puddleglum said:


> In a DC motor, the HP and torque curves seem linear.....


Examine the performance characteristic curve for the DC motor and it will be apparent.


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## major (Apr 4, 2008)

puddleglum said:


> It is power (Kw/HP) through the appropriate gear reduction that will determine acceleration and speed, not torque.


Acceleration is proportional to torque, not power.


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## puddleglum (Oct 22, 2008)

major said:


> You're mistaken about this. HP is not linear w/r/t torque for a DC motor.
> 
> Examine the performance characteristic curve for the DC motor and it will be apparent.


So the formula is right and I am reading the performance charts incorrectly? If that is the case, that is good and I can work with that. Can you please define "w/r/t"


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## puddleglum (Oct 22, 2008)

major said:


> Acceleration is proportional to torque, not power.


Can you please explain that more as I am having trouble understanding why that would be the case? I'm not arguing, just trying to understand. 
So, as an example, are you saying that a motor that makes 200 ft/lb of torque up to 2000 rpm with a direct drive is going to accelerate better than a motor that makes 100ft/lb of torque up to 4000 rpm and has a 2:1 gear reduction? Or, are you saying that all other factors being equal, a motor with 200ft/lb of torque will accelerate twice as fast as one with 100ft/lb, or both?

On to specific questions.
I have 2 forklift motors, one is an 11 inch (10kw) rated at 55ft/lb at 280 amps at 44.3v and 1300 rpm. I think that is pretty close to what a warp 9 would be at that low voltage so I'm guessing performance will be similar to that. If my above formulas are correct, I would think it could produce up to 200+ft/lb with a 1000 amp controller, but speed needs to be limited to maybe 4000 max rpm. Does that sound about right? 
I also have a 10 inch pump motor with a short frame. I don't have any specs on it other than it was drawing a little over 300 amps at 45v in operation and is rated at 11kw (15min). I assume it spins faster at a lower torque, possibly similar to a Impulse 9, but I can't confirm that. I'm thinking about trying to hook it back up to the pump with a flowmeter so I can run some tests. 
If I put the big motor in a small car (I'm considering an Aveo) it could take out the transmission unless I limit the current, right? If I were to use that motor, would it be better suited to a larger car with a higher torque ICE and a more robust trans. or would it still be suitable to a small car? 
If I were to use the smaller motor, it would have less torque but spin faster (5000 rpm maybe?). I'm wondering if that might be a better choice for a 2500lb car, as long as the motor is not too small. Thoughts?


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## major (Apr 4, 2008)

puddleglum said:


> So the formula is right and I am reading the performance charts incorrectly? If that is the case, that is good and I can work with that. Can you please define "w/r/t"


Sorry. w/r/t = with respect to, or with regards to.


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## major (Apr 4, 2008)

puddleglum said:


> Can you please explain that more as I am having trouble understanding why that would be the case?


The basic concept or formula describing acceleration and mass is F = M * a. Force = Mass times acceleration. Translating to the rotational system it becomes Torque = Inertia times angular acceleration. Torque is the fundamental property. Power is a consequence or resulting property or attribute of the system.

Perhaps I am over sensitive after this thread: http://www.diyelectriccar.com/forums/showthread.php/torque-irrelevant-relevanti-36904.html



> So, as an example, are you saying that a motor that makes 200 ft/lb of torque up to 2000 rpm with a direct drive is going to accelerate better than a motor that makes 100ft/lb of torque up to 4000 rpm and has a 2:1 gear reduction? Or, are you saying that all other factors being equal, a motor with 200ft/lb of torque will accelerate twice as fast as one with 100ft/lb, or both?


You haven't mentioned power at all in those questions. 



> So, as an example, are you saying that a motor that makes 200 ft/lb of torque up to 2000 rpm with a direct drive is going to accelerate better than a motor that makes 100ft/lb of torque up to 4000 rpm and has a 2:1 gear reduction?


Use lb.ft. for the units of torque please. In this case, the motor with 200 lb.ft. would accelerate the vehicle faster because it would have to accelerate its own rotor to only half the rotational velocity (assuming the motor with 100 lb.ft. had an equal size rotor). 



> Or, are you saying that all other factors being equal, a motor with 200ft/lb of torque will accelerate twice as fast as one with 100ft/lb, or both?


If the reflected moment of inertia at the motor shaft and the 2 rotors are equal size, then twice the torque accelerates twice as fast.


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## Tesseract (Sep 27, 2008)

Torque is force and force determines rate of change in velocity (ie - acceleration) from rearranging the familiar equation:

Force = Mass * Acceleration


It is velocity that depends on power. That is to say, a constant amount of torque will deliver a constant rate of acceleration for a given vehicle mass up until you run out of power... that's your top speed.

EDIT: seems major & me posted at the same time...


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## puddleglum (Oct 22, 2008)

Thanks for the replies and for the link. I read a few pages but I will read it through. I certainly do not want to rehash that debate and I highly respect the expertise of many of you here. I think I understand what you are saying and it makes sense when you use the terms torque and force interchangeably in the above equation. I'm trying to get this straight, but it seems to be harder than I thought. 
I was taught that torque was simply rotational pressure, a tendency to rotate, but no work (power) is produced until you include a distance and time component. I had thought I was talking about power in my examples because I had force (torque) through a distance (rotations) in a period of time(per minute). Is that incorrect? 
Please try and forgive my poor terminology (I knew better about the lb.ft. too) I'm working on it.

I seems like you're saying a big motor is always better.
So, I guess I'm still wondering how this works with choosing a motor for a car. If I choose a small car that had a low torque high rpm ICE, the transmission will only take so much torque. Is a big (low speed) motor still going to be the best choice, if I need to limit the peak torque to what the transmission can handle, or is it best to choose a motor that has a similar rpm and torque range to the original ICE? Which would likely give the lowest wh/mi. and best range?


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## major (Apr 4, 2008)

puddleglum said:


> I was taught that torque was simply rotational pressure, a tendency to rotate, but no work (power)* is produced until you include a distance and time component. I had thought I was talking about power in my examples because I had force (torque) through a distance (rotations) in a period of time(per minute). Is that incorrect?


No, that is not incorrect  How's that for a triple negative.

Work is force over a distance. Power is the rate at which the work is done. You get neither (work or power) without force. Also, you get no work done without motion. 

As you saw, there are differing opinions about it. But the basic equation is Power = force times speed (or Power = torque times angular velocity). Power is the result of force and motion.

* I hope you don't think work = power.



> I(t) seems like you're saying a big motor is always better.


No, I'm not saying that, necessarily. Although I seldom hear complaints about too much motor, or too much torque, or too much power. But I personally prefer the smallest motor which will get the job done. I say run 'em hot and fast. Like the advice from some famous racer: The equipment (racecar) should last one inch past the finish line. If it runs longer than that, it is over designed and weighs too much.

Well, I don't like to cut it that close. I want it hold together for the victory lap 

You have to weigh the options against the constraints you face and decide. You say the smaller motor is a pump drive. If it has an internal spline shaft, that would be the deciding factor for me. Go with the big guy and save yourself the hassle of reshafting the smaller. All the theory is good, and helps a bunch, but be practical and get the job done. If you can hook up the smaller motor and gear it down and it saves 100 lbs, go for it and throw in a couple more cells.

Regards,

major


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## Siwastaja (Aug 1, 2012)

puddleglum said:


> I'm trying to get this straight, but it seems to be harder than I thought.


Hi,

Torque and RPM are just two sides of power. You need both to create power. Power is the product of these two. Just like electrical power needs both current and voltage.

Power = rpm x torque
Power = voltage x current

You can exchange torque for rpm and vice versa by changing the gear ratio, but the product (power) remains the same.

At the wheels, torque means acceleration/deceleration and rpm means speed.

You can think torque as a force that is needed to cancel out the drag. If you have more than the drag, you accelerate. If you have less than the drag, you decelerate. Speed (~ RPM) is what follows.




> I seems like you're saying a big motor is always better.


Of course bigger is better 




> So, I guess I'm still wondering how this works with choosing a motor for a car. If I choose a small car that had a low torque high rpm ICE, the transmission will only take so much torque. Is a big (low speed) motor still going to be the best choice, if I need to limit the peak torque to what the transmission can handle, or is it best to choose a motor that has a similar rpm and torque range to the original ICE? Which would likely give the lowest wh/mi. and best range?


No need to make it so complex. Availability, price and usually simply the "size" are the most important parameters.

Most electric motors have similar efficiency of about 90% over most of the operating region, so no big difference in range. If you retain the transmission, just use the lowest gear that still gives you the top speed you want without over reving the motor. This applies in practically all cases.

If you undersize the motor like many people tend to do with easy-to-find cheap DC motors, you may need to change gears while driving like you do in an ICE car, to get decent acceleration and _maybe _increase efficiency in certain conditions. With a big enough motor, you can lock it in usually in third gear or so, and just drive with that. With an RWD, you may be able to skip the transmission but this requires enough reduction gearing in the differential. Some differentials have more than others. 1:5-1:6 would be optimal but you could live with 1:3.5 to 1:4. In this case, a large, high-torque motor is good.

If you can freely choose your gear ratio, you can do what OEMs do, go with a high-rpm (low-torque, for the same power!) AC motor, 10 000 rpm or more. The benefit is better power/weight ratio, so, a smaller motor for the same power (and same torque _at the wheels_). But usually DIYers don't go this way.


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## PStechPaul (May 1, 2012)

It seems to me that if a transmission and drive train can spin the tires, you don't need to worry about the motor's maximum torque. The maximum torque will be just at the point before the tires lose traction, and then it takes much less torque to burn rubber. But it might be a problem if you use a higher gear than you would for the ICE, since the electric motor has a lot more torque at the low end where the ICE would just stall.


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## Siwastaja (Aug 1, 2012)

major said:


> Hey Siwa,
> 
> Got a typo? power ---> RPM


Thanks! Fixed.


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## puddleglum (Oct 22, 2008)

Can you use the unloaded motor speed at 12 volts to tell anything useful about how fast the motor will run under load? I ran my two motors today. The larger one was about 10% slower than the smaller one. I expected more difference than that.


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## Duncan (Dec 8, 2008)

Hi Puddleglum

You want to choose between your two motors?

My tuppence worth

Maximum rpm - blow up rpm, this will probably be higher for the smaller motor
I was told (for my 11 inch unit) - 4,000rpm will be OK - 5,000rpm MAY be OK
A 9 inch would probably be OK at 7,000rpm

For a given current you will probably get more torque from the larger motor

If I had a 9 inch I could get the same torque by using more amps
I think about 20% more

A bigger motor will withstand an overload for longer,
My 11 inch weighs 102Kg - a 9 inch ~ 70Kg so about 50% more time to melt,

Combine that with the greater torque required and I think an 11 inch at the same torque (overload) will last twice as long

If you have ~150v available you will probably have enough voltage to power either of them to destruction

My car as an example
With the 500 amp controller
11 inch - more torque - top speed ~120kph
9 inch - less torque - top speed ~ 160kph
Bearing in mind the fastest I have been up to is 110kph

With a Soliton
I should be able to spin the wheels with either motor
So the higher top speed and 30Kg less mass of the 9 inch would be useful

BUT the chances of melting the thing would increase substantially!

If there had been a 9 inch sitting next my 11 inch I would have gone for it instead
BUT I am "Pushing the Envelope"


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## puddleglum (Oct 22, 2008)

Thanks Duncan, That's good info. I was just trying to determine if if that 10% rpm difference unloaded would be the same loaded. It's not super important, just curious. I could make a poor mans dyno with the hydraulic pump to test the pump motor if I wanted to. I may do that at some point, but it will take a lot of fiddling to set it up. I know my 11" ( same as yours) will work well and I have rated speed and load to calculate from. I don't have that with the 10" pump motor. I'm pretty sure it would still be quite suitable for a small car, but I 'd like to have an idea of its output. I guess I really should test it instead of guessing.
Duncan, how is your brake end adapter flange holding up as a drive? is it just floating, or did you lock it to the shaft somehow? I have mine, but I also saved the internal splined diff input gear. I see it is much harder than the brake flange, but would be harder to machine as well.


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## Duncan (Dec 8, 2008)

puddleglum said:


> Duncan, how is your brake end adapter flange holding up as a drive? is it just floating, or did you lock it to the shaft somehow? I have mine, but I also saved the internal splined diff input gear. I see it is much harder than the brake flange, but would be harder to machine as well.


Hi Puddleglum,
I left the spline floating 
You need a floating spline if you have a solid (two Hookes joints) prop shaft

I just found a rubber boot that would go the motor shaft and over the brake end adapter and packed it with grease

I don't know how it is working, I have done 500km - but I haven't taken it apart to see
No clonking or anything noticed

I don't think it will wear so long as it is well greased

I didn't have the internal splined diff input gear to play with - is the splined bit harder or just the gear surfaces?

I didn't do any machining on the brake end adapter,
I just made up a simple plate with eight holes and two registers to adapt the brake end adapter to the Hookes joint on the end of the propshaft

My propshaft is one part of a two piece Subaru propshaft with about three feet taken out of it

The NZ certification people did require me to get it balanced


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## puddleglum (Oct 22, 2008)

major said:


> You have to weigh the options against the constraints you face and decide. You say the smaller motor is a pump drive. If it has an internal spline shaft, that would be the deciding factor for me. Go with the big guy and save yourself the hassle of reshafting the smaller. All the theory is good, and helps a bunch, but be practical and get the job done. If you can hook up the smaller motor and gear it down and it saves 100 lbs, go for it and throw in a couple more cells.
> 
> Regards,
> 
> major


Here are some pics of the motors. The 11" has the best brushes and comm.


















The 10" pump motor would require some machining of the DE plate. One cable is attached directly to the field coils, not sure if that is bad, and has a short frame.

























Anyone know what that little wire on the brush is for?

Thanks, Duncan


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## major (Apr 4, 2008)

puddleglum said:


> Anyone know what that little wire on the brush is for?


My guess it is to connect a shunt field; so the motor is compound wound.


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## major (Apr 4, 2008)

puddleglum said:


> Here are some pics of the motors. The 11" has the best brushes and comm.
> 
> 
> 
> ...


Hitachi makes a very good motor. Notice the patina on the comm in the traction (11") motor vs pump (10") motor. That deep brown/gray uniform color is great. The copper color comm on the pump motor looks almost unused.


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## puddleglum (Oct 22, 2008)

major said:


> Hitachi makes a very good motor. Notice the patina on the comm in the traction (11") motor vs pump (10") motor. That deep brown/gray uniform color is great. The copper color comm on the pump motor looks almost unused.


The comm on the 11" motor is not as dark as it appears but it is very smooth. The 10" comm is actually scratched from running dirty. The forklift spent many years in a college wood shop. 
The compound winding makes sense as it would limit the speed of the pump motor. Would the motor still work okay as an EV motor? I've read there are cumulative or differential ones but only the cumulative is good. Is there a easy way to tell the difference?


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## major (Apr 4, 2008)

puddleglum said:


> The comm on the 11" motor is not as dark as it appears but it is very smooth. The 10" comm is actually scratched from running dirty. The forklift spent many years in a college wood shop.
> The compound winding makes sense as it would limit the speed of the pump motor. Would the motor still work okay as an EV motor? I've read there are cumulative or differential ones but only the cumulative is good. Is there a easy way to tell the difference?


Nobody in their right mind would use a differential wound motor on a fork truck so it is safe to assume it is cumulative. It'd probably work for you, but after seeing them, I like the 11" better


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## puddleglum (Oct 22, 2008)

Thanks major for all your help good advice.


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