mschwett
Well-Known Member
- Region
- USA
Here's what a DC permanent magnet motor would do without a controller, although a hub motor isn't wound to turn nearly so fast. At the low end, torque ( current ) would be so high that the motor would be wrecked in seconds.
Here are charts of a hub motor with two controllers. In both cases, maximum torque is about 3 times higher at 0 rpm than at max power; I've seen similar results with the two bikes I measured at different speeds. Note the knee in the torque at max power. That's where the controller quits restricting torque (amps) and lets back emf take over. If the controller didn't restrict current to the left of the knee, it would rise as steeply as it falls to the right of the knee, burning out the motor very quickly at low rpms.Futility of Motor Power Ratings
"There's no such thing as a rated watt!" Read here for why we don't advertise a simple power rating for the motors we sell. Also, have a look at our Youtube video where Justin talks about the power to propel bicycles in <a href="https://www.youtube.com/watch?v=ALde6zhLPs0">"School of Watts"</a>.ebikes.ca
A flat torque curve seems to be a feature of mid drive controllers. With a gearshift, a motor doesn't need extra torque to reach an efficient operating speed.
that’s interesting. essentially, the controller can be tweaked to produce different amounts of torque at different speeds, resulting in power peaking at a different speed than torque except by coincidence.
the torque curve also explains why electric motors are so good for accelerating from a stop, although i am not sure the torque continually
decreases from a peak at 0 rpm as shown. probably an oversimplification or programming intended to illustrate a particular result.