10K at 80MPH - Chevy Bolt EV Forum
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post #1 of 19 (permalink) Old 04-13-2019, 02:17 PM Thread Starter
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10K at 80MPH

Yesterday I had to do a sudden Seattle-Bellingham-Seattle run in our Bolt, which we obtained in July of last year, basically in truck mode w/a load of tools and boat parts.

A sunny Friday and folks were being very frisky with their speed. Normally I'm not inclined to speed but yesterday was all in a hurry so I joined in.

The car clicked over to 10,000 miles while whisking along in a train at 80MPH which as it happens is the fastest I've driven the Bolt, so far. I was somewhat surprised that the car still has decent punch when accelerating at 70 plus.

The entire trip except the brief city bits at ends was done in illegal mode, speed 64-80, 186 miles door-door, ambient 51 degrees, cabin at 72 w/no recirc. Got home w/just under a quarter battery left. Maybe because traffic was dense, so in a hurricane of air moving my way much of the time?

I'm still feeling like I'm 16 years old again, after 10,000 miles of driving this thing. It's just too good.
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post #2 of 19 (permalink) Old 04-13-2019, 02:34 PM
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On my recent trip up to Utah, I was able to finally have a good run on 80 mph freeways. Honestly, it felt not all that different than doing seventy *cough* *cough* *mumbles* miles per hour.
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post #3 of 19 (permalink) Old 04-13-2019, 03:24 PM
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Runs fine at 92 mph too. Today I had it to 90 going into a 20 mph wind just to see the power usage, 44kw. A little extrapolation supposing the bolt had a 2nd gear, 150 mph?
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post #4 of 19 (permalink) Old 04-13-2019, 03:40 PM Thread Starter
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Runs fine at 92 mph too. Today I had it to 90 going into a 20 mph wind just to see the power usage, 44kw. A little extrapolation supposing the bolt had a 2nd gear, 150 mph?
Interesting! That's a definitive indication that the speed limit is not about battery protection, or at least is not an issue for short durations. Supposing that the speed limit is about not having the motor disassemble itself, if the ultimate arbiter of top speed is motor RPM then there are maybe some simple possibilities for breaking that limit other than wheel size.

I wonder what the safety factor for the rotor is? If 1.5 then 12000 RPM would be OK, for most samples and for at least a little while.

Given the simplicity of the layer 1 signal emanating from an RPM sensor in any form, a selectable divider would be a fun thing to try. There may be more sophistication in the speed limit of the car but a divider for the raw RPM signal is so simple (off-the-shelf for some implementations) that it's tempting to try it.

Same applies if RPM is not measured directly at the motor but elsewhere. Somewhere there may be a chopper or tone ring or whatever providing feedback.

On the other hand if the limit is set by the motor drive electronics timings as opposed to mechanical RPM feedback, forget it.
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post #5 of 19 (permalink) Old 04-13-2019, 03:51 PM
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The limit is based on RPMs. Peak power to the motor is ~160 kW, though with losses, it's probably close to the rated 150 kW. That's more than enough power to sustain 150+ mile speeds. The motor just won't rotate that fast.
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post #6 of 19 (permalink) Old 04-13-2019, 04:51 PM
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Originally Posted by dbostrom View Post
I was somewhat surprised that the car still has decent punch when accelerating at 70 plus.
The Bolt will zip to 90 mph, faster than most, up a mountain. And down the mountain at 94 mph pulls about 15 kW...or so I have heard. :-)
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post #7 of 19 (permalink) Old 04-13-2019, 06:58 PM
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Got home w/just under a quarter battery left. Maybe because traffic was dense, so in a hurricane of air moving my way much of the time?
Yes, for efficiency driving “in traffic” is a big win.

You don't need to be tailgating either, just being behind another car at a reasonable following distance will be beneficial. The bigger the vehicle in front, the more it'll help. Interesting, aerodynamics is such that if you are benefiting from the person in front, they are also benefitting from you (not as much, but some).

I seem to follow a rule these days that I'll speed up to stay behind someone, but with nothing in front of me I'll ease up on speed. It leads to an interesting phenomenon that I'll putter along sedately, and then someone will pass me and I'll tuck in behind them.

Also, if the traffic snarls up and slows down, I may be reaching my destination more slowly, but the journey will be more efficient.

It does lead to the odd phenomenon that if I pull onto the freeway heading to work in the morning and for some reason the road is pretty clear, I'm disappointed. If it is filled with trucks and traffic is only going at 60 mph, I'm quite happy.
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post #8 of 19 (permalink) Old 04-13-2019, 08:46 PM Thread Starter
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Originally Posted by NewsCoulomb View Post
The limit is based on RPMs. Peak power to the motor is ~160 kW, though with losses, it's probably close to the rated 150 kW. That's more than enough power to sustain 150+ mile speeds. The motor just won't rotate that fast.
Definitely not via drive electronics timings? If so it should be a relative snap to make the car go faster. Way less effort than putting headers on an IC engine, for instance.

This excellent paper on the design of the motor "Electric Motor Design of General Motors’ Chevrolet Bolt Electric Vehicle" does indicate an RPM measurement as being involved in the inverter mappings. Presumably the system will indeed need a hard, physical indication of RPM.

Intriguing that the paper above models AC winding losses of the motor up to 15,000 RPM. As expected, the motor won't linearly track input power for output power as RPMs increase, so the ultimate speed of the car isn't simply a matter of extrapolating KW at 93MPH up to 150MPH.

The RPM sensor can certainly be made to lie. What's not clear is how the following system will behave when taken outside of its planned limits:

A six-step control [5] is implemented in order to provide the
maximum efficiency and performance under the voltage constraint at
high speed operating conditions. Based on the torque demand (Te*)
and the operating condition (motor speed Nr* and inverter input
voltage Vdc) as shown in Fig. 14, the control command look-up table
(LUT) determines the operating current command vector Is**. Also
the same information is used to look up the control mode (SVPWM
vs six-step) and the modulation index reference MI* from the table.
The outputs are fed to the voltage controller along with the feedback
modulation index MI from the voltage output of the current
controller. The voltage controller is to limit the fundamental output
voltage of the inverter, and adjusts the reference current vector for the
current control via ΔIs, which results in the final current command
vector Is* for the current controller core. The output of the current
controller is the output voltage vector Vs**, which is later limited by
the Voltage Limit, to output voltage vector Vs*. The difference
between the unlimited and limited voltage is multiplied by the
anti-windup gain Ka. For space vector pulse width modulation
(SVPWM) mode, Ka is set such way that the integrator windup is
avoided. When the control mode is set to the six-step control mode,
Ka is set to zero to induce the windup phenomenon of the integrators.
This allows the output voltage magnitude to be saturated in the
integrator, and provides the natural transition of the current control
from the normal field-weakening control to the six-step control
without a need for the separate controls. The output voltage vector
Vs* is fed to the pulse width modulation module, which selects the
actual modulation method based on the modulation index and the
control mode commands from the current and voltage control.
The behavior of the drive electronics timing could cause unexpected and quite possibly disappointing results even (or perhaps especially) when told a lie about RPM. But it's sure tempting to find out.
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post #9 of 19 (permalink) Old 04-14-2019, 01:00 AM
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The behavior of the drive electronics timing could cause unexpected and quite possibly disappointing results even (or perhaps especially) when told a lie about RPM. But it's sure tempting to find out.
This is not an 1970s ICE vehicle. You can't just fool a sensor.

The drive electronics in the Bolt uses lookup tables to work out the three-phase AC power to send to the motor. In particular, the frequency of the AC it sends depends on the current motor RPM and the desired torque.

Whether the motor can go beyond 93 mph depends on exactly how far those lookup tables are defined for, and what they actually say to do beyond 93 mph.

Also, you quote the paper on the design of the Bolt's electric motor, but you omit the one thing that matters, the paper lists the motor's maximum RPM, as 8810. If you do the math, you find that driving the Bolt faster than 94.8 mph pushes the motor past its maximum RPM.

Every electric motor has a max RPM. The Bolt's is 8810. I'm sure you can find plenty of videos on youtube of people pushing electric motors outside their design parameters. Usually these kinds of antics end with a fried electric motor.
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post #10 of 19 (permalink) Old 04-14-2019, 01:06 AM
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Incidentally, if you did want to hack the Bolt to go faster, you'd be better off changing the gearing. If you changed the final drive ratio from 7.05 to 6.0, you'd have a maximum speed of 110 mph. (But you'd have less torque at the wheels.)
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