Perhaps Chris and Gerry are thinking of the energy supplied by the motor while braking?
Look at it this way: while the motor is driven at a certain RPM by the drive, the drive supplies a voltage to the motor that is greater than the back EMF voltage generated by the motor. This allows a current to flow in the motor which creates torque and thus keeps the motor up to speed despite friction and other "opposing" forces.
When the drive wants the motor to stop, it lowers the voltage to the motor. Until the motor has had time to slow down, the motor's back EMF will be greater then the voltage supplied by the drive. Since there's a voltage difference, current will flow, but in the opposite direction than before - the motor now works as a generator.
So a more massive load or a higher RPM, or a faster ramp down of speed, will give you more energy fed back into the drive from the motor. If you ramp down slowly most of the braking will be taken care of by friction instead. (Note the difference between back EMF and braking energy - back EMF depends only on motor speed; braking energy depends on load, motor speed, and from an electrical point of view, rate of deceleration.)
There are different ways to brake an electrical motor:
* Let it coast to a stop (just disconnect it and let friction do its work).
* Dynamic braking (this is what Einar did by connecting the motor leads. Usually they are connected through a resistor to limit current). The faster the motor spins the stronger the braking effect - as it must be since back EMF (= voltage between motor leads) is proportional to speed, current is proportional to voltage, and torque is proportional to current. So the higher the RPM the more torque is generated to brake the motor.
* "Controlled" braking. Here the drive controls current to give desired braking torque, just like it controls the motor to give desired acceleration torque - the drive doesn't care if it's braking or accelerating or keeping a constant speed. This is what's going on in a servo drive.
Arvid  |