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 Post subject: Re: GM Spark 400 ftlb Torque
PostPosted: Thu Aug 10, 2017 11:52 am 
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Joined: Sat Jul 27, 2013 9:52 am
Posts: 125
Location: Los Gatos
"Again, regardless of PWM, the basic Physics remains that the torque is directly related to current."

True, on motors of this type the torque is pretty linear with respect to current up to some limit.

"In effect, peak power drawn from the battery is at 0 RPM while the motor provides zero power except heat."

Peak power is not drawn from the battery in this condition - It is probably in the region of 10-20kW.

The current in the motor windings will be that limited by the controller and will be pretty much at the maximum. The motor controller and the motor inductance acts as a transformer allowing a significant multiplication of the current from the battery.

There is no back EMF at 0 MPH, yet the torque is close to peak torque. If you believe that the torque is directly related to current through the motor, then the motor MUST be using peak current. That current comes from the battery. As you mentioned, motor voltage is battery voltage. That would mean the peak power from the battery is drawn at 0 MPH (5 MPH, but it's close enough for this argument).

By the way, I was wrong there's no road with 42% grade. There's a road with 45% grade! Silly me, I did research steep roads for my blog post, yet I forgot about it.

I found a reference in an SAE paper about the Bolt that the design maximum grade of the Spark is 28%. My 23% slope was not far off the maximum design spec. (I don't know what GM defines as 'launch grade" - it may be that the car is able to go at say 10mph or similar).

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"Peak current occurs at 0 RPM in typical motor without the controller limiting it. That's why most motor curves taper torque starting at 0 RPM while SparkEV tapers at 35 MPH. But if SparkEV is allowed tapering like typical motor (no controller), it would pass 2X to 4X the current, and the torque at 0 RPM would be 2X to 4X."

Maybe - the maximum design current of the SparkEV motor is 450A (another SAE document). Above that current a few things can happen. The motor can overheat, the magnets could demagnetize losing torque or the steel in the laminations could saturate causing the slope of the torque vs current to reduce. Saturation is not permanent so the motor could be designed to run into slight saturation.

I found a reference to the Volt motor that the short term torque can be about 50% more than the continuous torque capability - the Spark is probably similar. This is probably the difference between demagnetization/saturation and overheating:

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"In effect, motor controller is constant current regulator until available voltage runs out."

That's correct.

kevin


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 Post subject: Re: GM Spark 400 ftlb Torque
PostPosted: Thu Aug 10, 2017 12:02 pm 
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Joined: Sat Jul 27, 2013 9:52 am
Posts: 125
Location: Los Gatos
SparkevBlogspot wrote:
450 ft-lb and maybe even higher may be possible with 2014 SparkEV. But seeing how they had to "detune" to 327 ft-lb for 2015+, it may be that the LG battery cannot deliver as high current as A123 battery. If LG battery is capable of 400 ft-lb of current, I doubt Chevy would've gone to the trouble of different gearing for 2015+.

I found this chart in the Steven Tarnowsky paper you quoted from about the Spark design. The choice of gearing ratio is a trade off of acceleration, highway range and city range.

It looks like in the 2015 (LG) version they aimed for more city range at the expense of acceleration and highway range. The LG battery is slightly lower capacity than the A123 so maybe they were just trying to keep the same combined EPA range.

The motor controller was set to a lower current limit so the axle torque is virtually the same with either gearing - maybe that was to keep the traction limit the same.

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kevin


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