GM Spark 400 ftlb Torque

[SparkevBlogspot]

I don't have my Spark anymore but ...

When I have tried balancing the car on a local extremely steep hill (steep enough so I fall off the back of my bike when trying to ride up it!) the indicated power was in the 10-20kW region when the car was held stationery (admittedly not at full throttle but close)

There is a graph in the paper by Steven Tarnowsky (page 9) showing the power vs speed. - full power is not reached until about 40mph.

As I said the PWM control of the motor current means that the motor current can be a multiple of the battery current by a large factor, probably 10 times at low speed.

I would expect that the controller performs cycle by cycle current limiting so there should not be any perceptible time delay in the current limiting.

kevin

Since it takes about 30 kW for 8% grade at 50 MPH, about 20 kW would be needed to make it hold. SparkEV is capable of accelerating 0.4G, about 23 degrees. 23 degrees is about 42% grade. That steep of a road doesn't exist. I doubt you were even remotely close to full throttle on the road you were on.

Not sure if you're talking abut the wheel torque/power curve graph, but if you saw 0 MPH power, it shows 0 HP while the torque is over 90% of peak (and peak at 5 MPH). But at the motor, as you mentioned, torque is proportional to current. Unless you think torque does not relate to current, motor would be sucking close to 140 HP at 0 MPH.

PWM means nothing if the duty cycle has to be practically 100% to deliver the current.

 

[kevin]

I don't have my Spark anymore but ...

When I have tried balancing the car on a local extremely steep hill (steep enough so I fall off the back of my bike when trying to ride up it!) the indicated power was in the 10-20kW region when the car was held stationery (admittedly not at full throttle but close)

There is a graph in the paper by Steven Tarnowsky (page 9) showing the power vs speed. - full power is not reached until about 40mph.

As I said the PWM control of the motor current means that the motor current can be a multiple of the battery current by a large factor, probably 10 times at low speed.

I would expect that the controller performs cycle by cycle current limiting so there should not be any perceptible time delay in the current limiting.

kevin

Since it takes about 30 kW for 8% grade at 50 MPH, about 20 kW would be needed to make it hold. SparkEV is capable of accelerating 0.4G, about 23 degrees. 23 degrees is about 42% grade. That steep of a road doesn't exist. I doubt you were even remotely close to full throttle on the road you were on.

Not sure if you're talking abut the wheel torque/power curve graph, but if you saw 0 MPH power, it shows 0 HP while the torque is over 90% of peak (and peak at 5 MPH). But at the motor, as you mentioned, torque is proportional to current. Unless you think torque does not relate to current, motor would be sucking close to 140 HP at 0 MPH.

PWM means nothing if the duty cycle has to be practically 100% to deliver the current.

"PWM means nothing if the duty cycle has to be practically 100% to deliver the current".

I'm afraid you are not understanding some of the basics of the motor controller - it will never get to 100% duty cycle at low speed - the current limit will come into effect with the result that the duty cycle for maximum torque will be approximately proportional to the speed (up to the base speed of 40mph).

When the motor rotates it produces a back-emf proportional to the speed. When the motor controller turns the IGBT on the battery voltage is applied to the winding. The inductance of the motor windings causes the current to rise linearly with respect to time. This will continue until one of two things happens, either the desired ON-time is reached or the current reaches the predefined maximum for the windings. (this will only take 10's of microseconds).

The inverter switch (IGBT) is then turned off but the current in the motor inductance keeps flowing through the flywheel diodes as the magnetic field that has built up starts collapsing. The energy in the magnetic field now is providing torque but the battery current is essentially zero.

This will continue until it is time to pulse the winding again - if the motor is rotating it may be the next winding that is energized but in the case of a locked rotor the same winding may continually be used (I expect the motor controller has provision for the locked motor case to avoid overheating but it may take many seconds for this to take effect).

Depending upon the resistance of the motor winding it may only require a few percent duty cycle to achieve the maximum allowed motor current in the locked rotor case. The battery current will be the average of the ON and OFF times - as a result the battery current may only be a few percent of the winding current for this scenario.

When the motor is rotating faster the back-emf will be larger that will mean that the voltage across the winding inductance is lower and so a larger duty cycle is required to reach the maximum winding current. When the Spark is at about 40mph (base speed) the back-emf is about equal to the battery voltage and more complex means have to be used to get the motor to go any faster (field weakening).

The operation is very similar to that of a buck converter (https://en.wikipedia.org/wiki/Buck_converter) but obviously more complex as there are multiple switches and windings.

"23 degrees is about 42% grade. That steep of a road doesn't exist. I doubt you were even remotely close to full throttle on the road you were on."

True it is about 23% - (http://www.catshill.org/2017info.html).

kevin

 

[PhilPen]

According to multiple discussions from the experts on the DIY electric car forum over the years. Motor current and battery current are not the same, the motor current can be many fold higher than the battery current. This is courtesy of efficient sophisticated modern controllers. At very low motor speeds around 0 rpm the motor offers very little resistance to current flow due to absence of any significant opposing voltage due to an almost nonexistent generator effect at these speeds. Under these condition it requires very little voltage to push the amount of current required to generate the peak designated torque, and this is what the controller essentially provides. Thus current, torque and acceleration are essentially maximal, but at very low speeds the power drawn from the battery is not anywhere close to maximal. The controller in essence takes high voltage low current from the battery and supplies the motor with low voltage high current. As the motor and car pick up speed the generator effect produces an ever increasing opposing voltage and the controller has to supply a progressively increasing net voltage (and hence power output) to the motor to maintain the motor current/torque. At the peak, probably controller designated power (120 kw) output, the battery current can be close to the value the motor sees. This probably occurs in a Spark EV 3-4 seconds after a wide open throttle launch at ~40 mph.

 

[SparkevBlogspot]

When the motor rotates it produces a back-emf proportional to the speed. When the motor controller turns the IGBT on the battery voltage is applied to the winding.

Again, regardless of PWM, the basic Physics remains that the torque is directly related to current. You can test this with any motor; prevent it from moving and measure the current. It will pass peak current through it with full battery voltage at the terminals. If you let it move very slowly (large load), it will pass bit less current with full battery voltage.

In effect, peak power drawn from the battery is at 0 RPM while the motor provides zero power except heat. There's no magic just because PWM is used. The only "PWM magic" is that SparkEV peak torque does not behave like typical motor that taper from 0 RPM due to the controller limiting it, which was my original point.

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).

Below is torque curve graph I got from somewhere. Maybe this is the graph you were talking about. Note the power at 0 MPH is zero, but the torque is practically the maximum. If you assume torque is directly related to current, peak current would be passing through the motor at 0 MPH. Motor would have battery voltage applied to it, then the power from the battery would be peak power, more than 140 HP.

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.

http://www.huffingtonpost.com/2014/02/28/steepest-streets-america_n_4871559.html

0 rpm the motor offers very little resistance to current flow

This is exactly what I'm arguing. 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.

In another post, I suspected that SparkEV motor at 0 RPM runs at lower voltage (same peak current), and that motor controller is combating back EMF when moving by allowing higher and higher voltage at the motor terminals until the back EMF cannot be compensated with full battery voltage. Somehow, Kevin convinced me that isn't the case. But with this discussion, I'm more convinced that's what's happening: motor runs at lower voltage at its terminals than the battery until back EMF plus voltage to maintain the current equals battery voltage. In effect, motor controller is constant current regulator until available voltage runs out.

 

[PhilPen]

I was under the impression that the motor controller/motor function as a 'buck converter' which uses the motor windings themselves as the required inductor. That is they function as a power converter which steps down voltage (while stepping up current) from its battery input to its output the motor at a relatively high energy efficiency. This results in much lower current (and power) draw from the battery to generate peak torque at low motor rpm.

 

[SparkevBlogspot]

Regardless how you do it, torque is proportional to current. Whatever the voltage, look at the torque to determine current. And if the battery voltage is impressed on the motor terminals at 0 RPM, the peak power will be drawn from the battery at 0 RPM.

But as you suggest (and as I thought before Kevin convinced me otherwise), if the controller is moderating the voltage, probably to compensate for back EMF while maintaining the current (aka, torque), that would result in much less than peak power out of the battery at 0 RPM. As you say, sort of like buck regulator, though with current.

But if the controller is applying less than full voltage to the motor, what would happen if it did apply the full voltage at 0 RPM? Current will skyrocket. How much can be roughly extrapolated from the torque curve, and that's about 2X to 4X. If the motor didn't burn up (or kill the magnets), torque would also be 2X to 4X simply because of higher current.

 

[PhilPen]

I’m sure with a different controller or just a change in the current controller’s set parameters the Spark EV motor could be made to produce considerably more low rpm talk than it currently does. I think Spark EVs have been made with 3 peak motor torque levels, 450 lb-ft (for a SEMA show https://www.autoblog.com/2013/11/06/chevy-spark-ev-tech-performance-sema-2013/ ), the 400 lb-ft early models, and the 327 lb-ft later models. These differences in motor torque could easily be due to just setting different values for the permitted peak current in the controller. Thicker axles were added in the 450 lb-ft version. However, the SEMA vehicle was still intended to be a practical street rather than drag race car.
In EV drag racing motors are often operated at >10 times their designed continuous power levels, with 1000s of amps of current going through small 6” to 9” motors producing starting torques of 1000s of lb-ft. These currents would normally result in motor failure due to heat build up, however these events are usually so short that the motors don’t reach problematic temperatures before the run ends. I would expect the Spark EV motor (and possibly the battery pack) to also tolerate considerable very short term abuse.
In the days before sophisticated solid state controllers the massive destructive starting torques in DC series motors (fully charged battery packs can produce very large momentary discharge currents and 0 rpm motors were essentially a dead short on the battery pack) were circumvented by using a soft start resistor in series with the motor which was switched out as the speed increased. Without these resistors loaded motors could break drive shafts and unloaded motors mangle their armatures from the massive initial rotational acceleration as a consequence of the initial torque.
It might be nice if we could tweak the controller parameters a ‘little’ to make the Sport mode truly sportier but it would probably come at the cost of reduced battery and other component life and a voided warranty.

 

[SparkevBlogspot]

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+.

As for drag race with actual SparkEV, I don't think higher torque will benefit SparkEV without extensive modifications (ie, make it RWD and racing slicks). With any FWD of 50/50 weight distribution and "sane" tires, peak acceleration is about 0.4g (see engineering explained youtube video on tire traction), which SparkEV already achieves more to 35 MPH. Having more torque without extensive mods will simply spin the tires. You can tell from almost a guarantee tire squeal on full throttle.

The torque scenario I was talking about is purely academic. However, if we know SparkEV motor is capable of more, some nut job could adapt the motor to use in some insane EV application. I'd love to see AWD EV with Cd of 0.22 having combined 1000 ft-lb torque (400 front, 600 rear by over driving at low RPM). SparkEV motor could be capable of that. If Chevy actually made such EV, wouldn't that be fun? They might even call it "Tesla killer".

 

[PhilPen]

At close to full charge the LG battery pack is capable of delivering (~240 kW) about twice the maximum power that is ever drawn from it during use in the vehicle (see last graph in this link) https://avt.inl.gov/sites/default/files/pdf/fsev/batterySpark4878.pdf so the controller is definitely the limiting factor (either physically or via software restrictions) in terms of power delivery (the motor at least for a short duration will almost certainly deliver far more power than its current controller limited level). I couldn’t find the directly equivalent data for the earlier pack using a123 cells but the pack is listed as containing 336 pouch cells (if these are the same ones a123 advertises) they weigh 496 grams each and have a power density of 2.4 kW/kg (https://www.buya123products.com/uploads/vipcase/468623916e3ecc5b8a5f3d20825eb98d.pdf) that is 400 kW (536 hp) !!!! Therefore neither pack ever challenges its output limits in the Spark EV. The LG pack has a 10% higher voltage despite having fewer cells in series due to a greater per cell voltage.
The gear change is interesting GM said it was to put the motor in a slightly more efficient rpm range during city driving in one report I read (supposedly to partly compensate for the smaller pack, but that may not be the reason). If the motor was equally efficient over most of its rpm range increasing the rotational inertia with a higher gear ratio would decrease performance and the later Spark would perform slightly more poorly than the earlier Spark in that regard. It is interesting that in the Bolt they went for an even higher numerical gear ratio and higher rotational speed still. The higher gear ratio results in the need for more voltage and less current for the equivalent speed and likely fits the LG pack better, it also results in better motor rotor cooling at low vehicle speeds and decreased I ²r losses. By controlling the current delivery algorithm the motor can be matched to a wide range of gear ratios but some combinations may not be best suited for the battery life, motor cooling, or maximum range.
A Spark EV power train placed in the back of a Pontiac Fiero or similar (rwd and ass heavy) would make a very fun to drive EV especially with a little controller reprogramming which that vehicle could take advantage of (you might also be able to fit a second motor/controller in the front, and a single battery pack could possibly manage both?). BTW if you look at the layout of the Bolt they appear to have left a significant amount of space in the rear http://st.motortrend.com/uploads/sites/5/2016/01/2017-Chevrolet-Bolt-EV-powertrain-diagram-03.jpg possibly for a future AWD version?

 

[SparkevBlogspot]

Very interesting find!

I find 1986 Fiero GT to be the best looking 2 seater from any car company, ever. Wouldn't it be something to have ~175 HP FieroEV using SparkEV bits at about $25K? That might turn out to be "Miata killer". Maybe some crazy people out there may do that as custom project.

 

[kevin]

"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).

"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:

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

That's correct.

kevin

 

[kevin]

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.

kevin