4 ms·

I wonder what torque looks like at 0 rpm. I thought that was peak? But If it’s bad maybe a torque converter would be useful?

Cerium 6y ago
Peak torque is at zero, but peak power is not.
mywittyname 6y ago
Electric motors "torque ripple" at low RPMs (well, at all RPMs, but it is most noticeable at lower RPMs), which can cause motor "cogging". Cogging is either a complete stall, or really jerky behavior that feels pretty close to what an ICE is like at very low RPMs. * There are different motor designs out there that alter power delivery for situations where very low-RPM torque is required. As with most design decisions, there are trade-offs involved. And yeah, I'm sure a torque converter would be helpful in that situation. But I think that generally, passenger cars don't need to go very fast, so most EVs can get away with short enough gearing that cogging isn't an issue, at the expense of top seed. * hold some low-power electric motor in your hands, turn it on then gradually clap the output with your fingers. You'll eventually feel the power output go to nothing and the motor will wind. That's cogging. Also, the whole "peak torque at 0 RPMs" thing is really a side effect of the fact that electric motors produce consistent power across the rev band (except at really low, and really high RPMs). And the formula for power is power = torque * RPM. Since power is essentially fixed with an electric motor, you rearrange that equation to be torque = power / RPM, and peak torque is at the limit as RPM goes to 0. In real world applications, torque output is limited by physics and current delivery, so peak torque is truncated at low RPMs.
- deleted 6y ago
  [deleted]
- rcxdude 6y ago
  Cogging is only really a problem if you either have a particular motor design or not very sophisticated control. EVs are not generally going to suffer from it.

Cerium 6y ago

Peak torque is at zero, but peak power is not.

mywittyname 6y ago

Electric motors "torque ripple" at low RPMs (well, at all RPMs, but it is most noticeable at lower RPMs), which can cause motor "cogging". Cogging is either a complete stall, or really jerky behavior that feels pretty close to what an ICE is like at very low RPMs. * There are different motor designs out there that alter power delivery for situations where very low-RPM torque is required. As with most design decisions, there are trade-offs involved. And yeah, I'm sure a torque converter would be helpful in that situation. But I think that generally, passenger cars don't need to go very fast, so most EVs can get away with short enough gearing that cogging isn't an issue, at the expense of top seed. * hold some low-power electric motor in your hands, turn it on then gradually clap the output with your fingers. You'll eventually feel the power output go to nothing and the motor will wind. That's cogging. Also, the whole "peak torque at 0 RPMs" thing is really a side effect of the fact that electric motors produce consistent power across the rev band (except at really low, and really high RPMs). And the formula for power is power = torque * RPM. Since power is essentially fixed with an electric motor, you rearrange that equation to be torque = power / RPM, and peak torque is at the limit as RPM goes to 0. In real world applications, torque output is limited by physics and current delivery, so peak torque is truncated at low RPMs.

deleted 6y ago
[deleted]
rcxdude 6y ago
Cogging is only really a problem if you either have a particular motor design or not very sophisticated control. EVs are not generally going to suffer from it.