Chevy Bolt EV Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
508 Posts
Discussion Starter #1
I was surprised to read the Bolt uses a synchronous electric motor technology, quite different from Tesla. We know that the battery runs at 400 volts DC, so this implies there is a variable frequency DC to AC converter that can run from over a big frequency range as as the motor turns from 0 rpm to about 8000. Any electrical engineers on the forum? How are they doing this? If there is a maximum frequency for the converter, this would limit the top speed, which I read is 150 km/hr or about 92 mph.
 

·
Registered
Joined
·
118 Posts
What I know so far.
Nominally, the motor is supplied from a 355 volts DC supply, then converted (inverted) to an unknown AC voltage.
I was surprised to read the Bolt uses a synchronous electric motor technology, quite different from Tesla. We know that the battery runs at 400 volts DC, so this implies there is a variable frequency DC to AC converter that can run from over a big frequency range as as the motor turns from 0 rpm to about 8000. Any electrical engineers on the forum? How are they doing this? If there is a maximum frequency for the converter, this would limit the top speed, which I read is 150 km/hr or about 92 mph.
 

·
Registered
Joined
·
4,815 Posts
If there is a maximum frequency for the converter, this would limit the top speed, which I read is 150 km/hr or about 92 mph.
The maximum frequency is much more likely to be software limited so as to not overspeed the motor. With fixed gearing it's spinning very fast at high speeds, and electric motors have a redline just like internal combustion engines do.
 

·
Registered
Joined
·
735 Posts
Good point, often car makers over build components and then fine tune them to operate at really safe levels. Only thing I would question at that point is what's the risk of pushing it further?
 

·
Registered
Joined
·
9 Posts
I was surprised to read the Bolt uses a synchronous electric motor technology, quite different from Tesla. We know that the battery runs at 400 volts DC, so this implies there is a variable frequency DC to AC converter that can run from over a big frequency range as as the motor turns from 0 rpm to about 8000. Any electrical engineers on the forum? How are they doing this? If there is a maximum frequency for the converter, this would limit the top speed, which I read is 150 km/hr or about 92 mph.
It uses a VVVF (variable voltage variable frequency) converter of some sort. Almost certainly the ones for vehicles are specially designed, but general purpose VVVFs of various capacities are readily available. They have a built-in rectifier because the input is normally AC. You could find one on the Internet if you wanted to. Generally they are 3 phase input and output although smaller ones are available which will accept single phase input. They are commonly used for home elevators and other situations where variable speed control of 3 phase motors is required. My home elevator has one to ensure smooth starts and stops. Whether the same VVVF will work for either an induction motor or synchronous motor I'm not sure but it would be easy to find out. A VVVF for a 2 horsepower 3-phase induction motor would cost approximately $500 but obviously one for a 200 horsepower motor would be much more expensive.

In general, the voltage has to vary in direct proportion to the frequency, at least at maximum torque.

Historically speaking, until fairly recently designing a VVVF was very difficult because solid state switches were not ideally suited for the job. Now, however, there are insulated gate transistors which will operate at higher voltages and higher currents.

For more information, google "VVVF". It really doesn't take an electrical engineer to understand it although it would take an engineer to design it. My degree is in business administration.
 

·
Registered
Joined
·
9 Posts
The maximum frequency is much more likely to be software limited so as to not overspeed the motor. With fixed gearing it's spinning very fast at high speeds, and electric motors have a redline just like internal combustion engines do.
The permanent magnets of the Bolt motor are in the rotor. Most likely it uses alloy magnets containing a rare earth element. Unfortunately, although that type of magnet has an extremely strong field (10 to 20 times stronger than non-rare earth magnets), they are not very strong mechanically and, as I understand it, that factor limits the maximum speed. Also, the maximum permissible temperature is limited because at high temperatures, the magnets cease to be magnets. Another factor is that China has a virtual monopoly on rare earth elements because the refining method uses dangerous chemicals which are very environmentally unfriendly. Those elements are chemically very similar which makes them difficult to separate. Also, they occur with thorium which is slightly radioactive. It may be that more enlightened environmental regulations would make it possible to mine and refine rare earth elements here in the U.S.
 

·
Registered
Joined
·
4,815 Posts
It may be that more enlightened environmental regulations would make it possible to mine and refine rare earth elements here in the U.S.
If, by "enlightened" you mean "lax" in the same sense that the current administration seems to, then no thanks. Better to try to find some way to do it less harmfully, if possible.
 

·
Premium Member
Joined
·
787 Posts
The maximum speed of the Bolt is likely due to not wanting to physically spin the electric motor beyond a certain RPM, or the reduction gear beyond a certain RPM, likely due to mechanical concerns, not electrical...
 
1 - 8 of 8 Posts
Top