Choosing a Controller

When choosing a controller, both it and the motor should be matched to the mechanical task that is to be done.

So – you first of all need to understand the mechanics:

• What mass do you need to move?
• How fast do you want to move it?
• What acceleration do you want?
• What gradients must it climb?

Remember that a motor is no different to a human: if, for instance, you are going to replace pedal power by electric power on your cycle, the motor is going to have exactly the same problems you did starting the bike in a low gear on a steep hill. The force you need to push the pedals depends on the gear ratio and if that’s wrong you will not be able to push the pedals.

The same thing applies to an electric motor! If the gearing’s wrong the motor simply won’t be able to develop enough push (torque) to start the machine, also the motor will try to draw a high current.

We have this calculator into which you can plug all your vehicle weights and performance figures: it will calculate how much motor current you need.

The bicycle analogy is a good one to keep in mind. When you are pedalling, you do a lot of work uphill and little work downhill. The same applies to the motor. The ‘push’ on the pedals is the motor torque, controlled by the current. The speed your feet are moving is the same as the motor speed, controlled by the voltage fed to the motor. A motor speed controller controls the voltage fed to the motor.

Once you understand the mechanical task you are undertaking, then can you chose a suitable motor, and only then you can chose a suitable controller. The controller must be capable of powering the mechanical task. You have chosen the motor to match the task, so a controller to match the task will automatically match the motor! And it is the task, not the motor, which determines the controller features you require.

Existing motor

If you have an existing motor, and wish to source a controller for it, be warned: that is not normally the best way to go about things. However, motors are normally rated in terms of a ‘safe working load’ – the nameplate ratings are normally continuous allowable loads. Controllers are not so rated, but their current rating is normally that which they can give for a short time, maybe one minute.

Most motors will however take something like 300-400% overload for a minute. It follows that you probably need to use a controller rated at 3-4 times the motor’s continuous rating! So if the motor’s nameplate says 30 amps, a controller capable of giving around 100 amps or more should be considered.