I have an active but paraplegic friend who is building an electric off-road scooter using DC motors. Those motors will have to reverse directions frequently while under load. Will they tolerate immediate reversals, or must there be a delay? — JO, Valley Springs, California
Modern brushless DC motors are amazing devices that can handle torque reversals instantly. In fact, they can even generate electricity during those reversals!
Instant reversals of direction, however, aren’t physically possible (because of inertia) and aren’t actually what your friend wants anyway. I’ll say more about the distinction between torque reversals and direction reversals in a minute.
In general, a motor has a spinning component called the rotor that is surrounded by a stationary component called the stator. The simplest brushless DC motor has a rotor that contains permanent magnets and a stator that consists of electromagnets. The magnetic poles on the stator and rotor can attract or repel one another, depending on whether they like or opposite poles—like poles repel; opposite poles attract.
Since the electronics powering the stator’s electromagnets can choose which of the stator’s poles are north and which are south, those electronics determine the forces acting on the rotor’s poles and therefore the direction of torque on the rotor. To twist the rotor forward, the electronics make sure that the stator’s poles are always acting to pull or push the rotor’s poles in the forward direction so that the rotor experiences forward torque. To twist the rotor backward, the electronics reverses all those forces.
Just because you reverse the direction of torque on the rotor doesn’t mean that the rotor will instantly reverse its direction of rotation. The rotor (along with the rider of the scooter) has inertia and it takes time for the rotor to slow to a stop and then pick up speed in the opposite direction. More specifically, a torque causes angular acceleration; it doesn’t cause angular velocity. During that reversal process, the rotor is turning in one direction while it is being twisted in the other direction. The rotor is slowing down and it is losing energy, so where is that energy going? It’s actually going into the electronics which can use this electricity to recharge the batteries. The “motor” is acting as a “generator” during the slowing half of the reversal!
That brushless DC motors are actually motor/generators makes them fabulous for electric vehicles of all types. They consume electric power while they are making a vehicle speed up, but they generate electric power while they are slowing a vehicle down. That’s the principle behind regenerative braking—the vehicle’s kinetic energy is used to recharge the batteries during braking.
With suitable electronics, your friend’s electric scooter can
take advantage of the elegant interplay between electric power and mechanical power that brushless DC motors make possible. Those motors can handle torque reversals easily and they can even save energy in the process. There are limits, however, to the suddenness of some of the processes because huge flows of energy necessitate large voltages and powers in the motor/generators and their electronics. The peak power and voltage ratings of all the devices come into play during the most abrupt and strenuous changes in the motion of the scooter. If your friend wants to be able to go from 0 to 60 or from 60 to 0 in the blink of eye, the motor/generators and their electronics will have to handle big voltages and powers.