DIY Electric Car Forums banner

Why do electric motors have a constant torque?

18782 Views 4 Replies 4 Participants Last post by  cyclops2
Re: Why do electric car motors have constant torque?

why do electric motors have a constant torque over all RPM ranges?
They do not.


I just had another coff of cuppee this morning :) so I'll elaborate a bit. The torque output of the electric motor is dependent on the load presented to the shaft opposing rotation. When that electric motor is mechanically connected to a large mass, the torque output tries to accelerate against that mass and then the magnitude of the motor torque is limited by the controller. Most controllers have a current limit.

So when you jump into your EV and mash the throttle pedal to the floor, the controller goes into current limit and allows that amount of current to flow to the motor. As the car accelerates, that current stays at this current limit until the motor reaches base speed. So for that portion of the acceleration profile, there is a constant current to the motor. Since the motor torque is proportional to current, the motor torque is constant (at its maximum) for that portion of the acceleration event.

But after base speed is reached and exceeded, the motor torque reduces. And if you lift the throttle and accelerate at less than maximum, torque will be less.

So electric motor torque is not constant. There is a region of operation in conjunction with the controller and for certain load conditions which will give the appearance of constant torque.


1 - 5 of 5 Posts
Re: Why do electric car motors have constant torque?

Excellent explanation, major. This should go in the Wiki.
Re: Why do electric car motors have constant torque?

Excellent explanation, major. This should go in the Wiki.
Thanks Giz. Should have had that coffee earlier. Looks like I explained that to a spam :eek: Oh well, WTF.
Actually the motor controllers can do constant torque to a certain rpm. Then start on the constant horsepower portion. You can shift the change over point a little bit by doing some reprograming in the computer.

If you just took a big switch and closed it . The big switch car would reach top speed first.
But there needs to be a balance of current flow & total output power over a short time to prevent damage to the power parts.

I remember testing the hoist motors used to lift the cargo containers from the ships at the Newark, N J piers. Suprise.

They were tripping the largest fuses in our largest overhead Buss Duct switches.

Found out that they are wound to a electrical code of ....UNLIMITED POWER. :) There were only 3' long by 2.5' high. Not really big.
So I had to go to a US Navy scrap yard and buy a MASSIVE 480 vac 3 phase contactor used to control a naval tenders generator. When I closed the contactor we could hear the cables in the electrical pipes slap.
I assume the motors for EVs are or could be wound to the same code.
See less See more
1 - 5 of 5 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.