Electric motor & electrical energy?

Question

I just wonder why the electric motor, like Brushless Dc motor or switched reluctance motor, cannot generate adequate electricity to charge the battery or other energy storages at the low speed? Someone told me that the charge efficiency of a motor is a function of motor speed and especially an electronic switch or circuit is used for controlling the output current. But I'm still totally off the board. Because I think since the rotor (armature) is still running, there must be output current to the energy storage, thus the motor can also charge the battery at low speed.
Oh! Please! Could somebody just explain what happened as the motor is at low speed?

Answer 1

A generator can not provide more energy to recharge a battery than the energy supplied to rotate the generator. In fact the process is not 100% efficient and some of the energy driving the generator will be lost (windage, bearing friction, winding resistance loses, heat loses, etc.). Rotating a generator causes its magnetic poles to pass (with resistance) through a magnetic field. Obviously the amount of work done depends on how often in a minute (or second) the poles are forced through the field. This is like doing more work if you shovel sand twice as fast. The faster the generator is rotating, the more energy is being converted from mechanical drive force to electrical flow to energy recharging the battery. At the extreme, if you stop the generator no work is done on the generator and no current flows to the battery.

Answer 2

All motors are generators, and vice versa. If the battery is driving the motor, then it is actually discharging the battery, no matter what the speed. If you want to charge the battery (assuming that it is a rechargeable battery in the first place), then you have to have some way of mechanically turning the armature. According the the First Law of Thermodynamics, one does not get energy from nothing.

I think that what you mean is that the motor should generate some back EMF from the armature's own inherent inertia when it is shut off and slowing down. If the battery is really weak, and/or you have some kind of diode circuit to prevent current from flowing back out of the battery, it might charge it .

Answer 3

I notice that you're talking about DC brushless motors here.

Basically, brushless DC motors are AC motors, but they use a circuit that generates some type of rotating field or something.

The point is that if you rotate a typical DC brushless motor, the current won't be passed backwards through the control circuit to charge a battery or generate electricity like a normal "brushed" DC motor.

Answer 4

Primarily the Electric Motor converts electrical energy to Mechanical energy - the motor turns. But the motor also produces heat - thermal energy - from the resistance to electrical flow in the wires. And this heat radiates out so electricity also produces thermal radiant energy.

Answer 5

I agree with NovaKarl. You need a brushed DC motor. The brushless motor doesn't work on simple DC current signal, therefore you can generate simple DC current with it.