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#1
06-04-2009, 09:54 PM
 roflwaffle Senior Member Join Date: Sep 2008 Posts: 109
Electric motor output wrt voltage

Does anyone know how motors like ADC's 9"er change the power curve, specifically the power curve before peak power, when the voltage increases? Based on what I've seen, peak power moves to a lower speed, but does power before that speed (and peak power) also increase proportionally?
#2
06-05-2009, 06:50 AM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 5,177
Re: Electric motor output wrt voltage

Quote:
 Originally Posted by roflwaffle Does anyone know how motors like ADC's 9"er change the power curve, specifically the power curve before peak power, when the voltage increases? Based on what I've seen, peak power moves to a lower speed, but does power before that speed (and peak power) also increase proportionally?
Hi rofl,

This is a series motor. When you have the RPM and power plotted against torque (on the x-axis, horizontal), then both the RPM and HP are proportional to motor voltage change. In other words, a 10% increase in motor voltage at a given torque value increases RPM and HP by 10%. The current (amps) does not change with voltage at that torque value. This is an approximation, but very accurate in the middle of the performance plot. Becomes less accurate at very low and very high loads (torque).

Regards,

major
#3
06-05-2009, 05:30 PM
 roflwaffle Senior Member Join Date: Sep 2008 Posts: 109
Re: Electric motor output wrt voltage

It also seems that an increase in voltage decreases the rpm at which peak power is reached, which is what I'm really interested in. I'd like to stick an 8-9" motor in series behind the transmission in an old diesel, so that I can get a good ~20-50+hp, maybe more if the diff can take it, in every gear. The lower I can make peak power with a motor, the better, since for instance 1500/2500rpm motor speed would be ~30/50mph, so if I could get an additional 40-70hp from a 10kWh pack at 5C rates during that time I would be one happy camper.
#4
06-05-2009, 06:59 PM
 Bowser330 Senior Member Join Date: Jun 2008 Posts: 1,648
Re: Electric motor output wrt voltage

From the sounds of things you are looking to get into something like this...

http://www.go-ev.com/EMIS.html

It may have worked for some but the EMIS didn't really work for this customer...

http://www.hrivnak.com/

Now my question is why cant we sandwich the e-motor between the engine and the transmission...
#5
06-05-2009, 07:13 PM
 roflwaffle Senior Member Join Date: Sep 2008 Posts: 109
Re: Electric motor output wrt voltage

Sticking the motor between the trans and the engine would be a total pain in the arse and would probably require a ton of unibody fabrication. Otoh if I stick it after the trans all that's required is a shorten driveshaft, the appropriate linkage on the motor, and another trans cross-member that's modded so the motor can bolt to it.

It'd be better in terms of power output, but I'm not going for tire squeeling performance, just a ~50hp boost, since the car only came w/ ~50hp from the factory, and the ability to go electric only when I want to during low load situations.

EMIS on conventional automatics is kind of a catch 22 situation, since adding electric power and throttling back the engine tends to reduce engine efficiency, and being in an automatic means most drivers can go electric only at low loads and shut off the gas engine, which is where most gains would be seen. Using LAs only makes the situation worse, since they don't pack a lot of energy, especially at higher discharge rate, and weight a ton. I was thinking about a 9" motor, w/ 40 100ah lfps, so range at 55mph should be about 40 miles, and in the lower load situations where I would want to use it, about 1.5-2 times that, so maybe 60-80 miles in the city or on a slow moving freeway. I should also be able to get about 60kW from the pack, so about 52-53kW/70hp peak at the motor, on top of the ~50hp peak from the engine, so in 2nd and 3rd I should have a good 100+hp, which would make for "modern" acceleration as opposed to the 0-60 in ~25-30s in the car's original fit. Granted, there will be some downsides, like dropping carrying capacity form ~1150lbs to ~750lbs, but it should do pretty good considering the motor power/gearing/vehicle specs (curb weight, CdA) are nearly identical to the RAV4 EVs, except there will be an extra 50hp coming from a nearly 50 year old diesel up front, so 0-60 will probably be around 14-15s instead of the stock 18s and while all electric range would only be about half what it is for a RAV EV, I can just flip on the diesel engine and drive for another 600 miles or so.

Last edited by roflwaffle; 06-05-2009 at 07:37 PM.
#6
06-05-2009, 07:29 PM
 Bowser330 Senior Member Join Date: Jun 2008 Posts: 1,648
Re: Electric motor output wrt voltage

you are right, it would require some expanding of the tunnel, extra mounts/braces, dual adapter plates and a custom driveshaft...

But the benefits are not only electric boost, since the electric power kicks in so low, it would essentially be powering the car through the city and you would not be required to use as much gas to get up to speed. Additionally with the regen activation, which could be setup to be triggered by the brakelight swith, could also help save on brakes...

I dont know I know it seems like a lot of fab work, But i see the benefits as very attractive...maybe Im wrong...
#7
06-05-2009, 07:50 PM
 roflwaffle Senior Member Join Date: Sep 2008 Posts: 109
Re: Electric motor output wrt voltage

Like I mentioned in my edited post, even w/ the motor behind the trans, I should have nearly identical gearing, curb weight, CdA, and peak power output to a RAV4 EV, so performance should be o.k. Regen should be doable no matter where the motor is, and I'll probably go w/ a modded alt off to the side if there's enough room. I think putting the motor in front of the trans would be a fun project on an old Subaru awd car, but IMO the extra work just isn't worth the extra power, especially since most of it wouldn't be usable (the only big power gain I would have is in first and second since third and fourth are at 1.29:1 and 1:1, which would be very limted since first goes to ~15mph and second goes to ~27mph IIRC) except to burn tires, which would certainly be fun, but not worth the extra cost. I could probably go buy a beater V8 mustang for the cost of enlarging the tunnel, moving the trans back and making new mounting points, putting in different clutch setup, and on and on, if I really wanna waste rubber that much.

edited for errors
#8
06-06-2009, 11:27 AM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 5,177
Re: Electric motor output wrt voltage

Quote:
 Originally Posted by roflwaffle It also seems that an increase in voltage decreases the rpm at which peak power is reached,
Hi rofl,

Undoubtedly, you will be current limited by the controller. For example, 500 amps. So peak power will occur at 500 amps when the motor controller reaches 100 % duty cycle, or passes full battery voltage to the motor. At RPM higher than that, the torque and HP decrease, as well as current.

So, peak power for you is at 500 amps and battery voltage. If you increase battery voltage, you increase motor RPM at 500 amps and also increase peak power.

Regards,

major
#9
06-07-2009, 01:29 PM
 roflwaffle Senior Member Join Date: Sep 2008 Posts: 109
Re: Electric motor output wrt voltage

It's probably that I don't know what I'm talking about.

In all the power/torque/amp/eff curves I see, the info is only from peak power and onward. Is performance symmetrical (I've read the electric motor power curves are hyperbolic) about the rpm at which peak power is made. For instance, if an adc 9" motor at 144V makes 90hp at 3k rpm, dropping to 30hp at 6k rpm, would it also climb from ~20-30hp at some very low rpm, say between 0-1000rpm, to 90hp at 3k rpm in the same manner, w/ a nice beefy power curve from ~2000-3000rpm going from ~50-90hp, and going from ~20-50hp from ~1000-2000rpm?

Last edited by roflwaffle; 06-07-2009 at 08:42 PM.
#10
06-08-2009, 08:44 AM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 5,177
Re: Electric motor output wrt voltage

Quote:
 Originally Posted by roflwaffle It's probably that I don't know what I'm talking about. In all the power/torque/amp/eff curves I see, the info is only from peak power and onward. Is performance symmetrical (I've read the electric motor power curves are hyperbolic) about the rpm at which peak power is made. For instance, if an adc 9" motor at 144V makes 90hp at 3k rpm, dropping to 30hp at 6k rpm, would it also climb from ~20-30hp at some very low rpm, say between 0-1000rpm, to 90hp at 3k rpm in the same manner, w/ a nice beefy power curve from ~2000-3000rpm going from ~50-90hp, and going from ~20-50hp from ~1000-2000rpm?
Hi rofl,

Plotted against torque, the power curve resembles a parabola with a constant motor voltage. The peak of the power curve will be in the neighborhood of half the stall torque (zero RPM) and half the stall current (voltage divided by motor resistance). So, in your 9 inch motor at 144V example, uncontrolled stall current would be like 5000 amps. So, uncontrolled peak power would at like 2500 amps. Obviously conditions any available controller will not tolerate. So operation of the motor on the high current side of peak power is a non-issue. Even if it was not, you would not want to do so because motor efficiency would be 40 % or lower.

You have to consider the motor and controller as a package deal when it comes to calculating your shaft power. When doing this, peak power will be at current limit (like 500 amps for a Curtis) and when the controller comes up to full voltage to the motor (144V). At that point, the motor RPM is set. At higher RPM, the motor torque decreases as well as power and current. At lower RPM, providing you have it floored and are still in current limit, motor current will be 500 amps, so you get the same torque, but the controller reduces the motor voltage and you get proportionally less power, and battery amps decrease.

So, the motor performance curves you see don't carry the plot out beyond peak power because the motor maker does not want you to operate the motor in that region, for good reason.

Hope you can follow me on this,

major

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