At a basic level a motor speed controller just regulates the speed and direction of an electric motor by manipulating the voltage that is applied to it, – but it actually has to do quite a lot more than that;
- Provide a controlled start-up. A stalled motor can take up to 20 times its rated running current, if you suddenly connect the battery to the motor there can be a very high initial surge. We have seen motor cases ripped apart by the high torque generated by an uncontrolled switch on.
- Reversing; to do this safely it first has to stop the motor – reversing from full speed can get exciting!
- Reverse polarity protection, just in case someone connects positive to negative.
- Protection against operator misuse, regenerative braking, safe response in case of broken wires etc.
- And all the other features that various applications require.
How does a speed controller work?
The controller works by switching the battery connection to the motor on and off around 20,000 times a second. The motor averages this out, as this rate of switching is too fast for the motor to detect. If the battery is only connected for half the total time, then the motor sees the 24v battery as if it were only 12v, and goes at half speed. Also, because the switching is so fast, the motor’s inductance – which acts like an electrical flywheel – keeps the current in the motor flowing constantly. But that current is only flowing for half the time from the battery, so the battery current will be half the motor current.
Power is voltage times current, so the motor controller is actually working like a transformer: in the above motor voltage times motor current will be equal to battery voltage times battery current, so virtually all the power from the battery is fed to the motor. Losses in the controller are small as power is heat and the controller really cannot dissipate much heat – or it would explode!
If you want to learn more about how the controller does this, there’s a more technical description in our 4QD TEC sister site
You should also read our page on permant magnet motors.