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What Does a Motor Speed Controller Do?

Fundamentally a motor speed controller (aka a motor driver) 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;

We’ve got a short video here that explains the basics….

YouTube video

But they can do some or all of the following….

  • Provide a controlled start-up [or soft start]. A stalled motor can take up to 20 times its normal operating current, if you suddenly connect a battery to a motor there can be a very high initial current surge. We have seen motor cases ripped apart, and gear teeth stripped by the high torque generated by an uncontrolled switch on. A motor with a speed controller can limit this initial torque spike to give a smooth [soft] start-up.
  • Reversing; to do this safely the controller first has to stop the motor – reversing from full speed can get exciting if not done properly!
  • Reverse polarity protection, just in case someone connects positive to negative.
  • Protection against circuit faults, the controller should give a safe response in case of broken control wires etc.
  • Provide all the other  features that various applications require such as smooth acceleration and deceleration, setting top speed, limiting current, proportional control etc.

How does a motor speed controller work?

All 4QD controllers work by switching the battery connection to the motor on and off around 20,000 times a second using a technique called pulse width modulation [PWM].The voltage at the motor looks like this…..

motor speed controller PWM waveform

The motor averages these pulses 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 [B], then the motor sees a 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 example 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.

If you want to learn more about how the controller does this, there’s a more detailed description of PWM here.

Now that you know what they do, come and have a look at our range of motor speed controllers and the sort of projects that they have been fitted into.

We also have a page that explains more about the different types of electric motors such as permanent magnet, series wound, shunt wound etc.

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