Surge Currents

If a motor is connected straight to a battery with no controller, it will draw a very large surge current.This surge current will generate a large torque pulse from the motor, we have seen relay switched motors from the Greenpower series where the motor case has been torn from the motor mount by this torque.

This does not happen when the motor is connected to a motor speed controller.

The surge is caused because the motor, when it is turning, acts as a generator. The generated voltage is directly proportional to the speed of the motor. The current through the motor is controlled by the difference between the battery voltage and the motor’s generated voltage (otherwise called back EMF). When the motor is first connected up to the battery (with no motor speed controller) there is no back EMF. So the current is controlled only by the battery voltage, motor resistance (and inductance) and the battery leads. Without any back emf the motor, before it starts to turn, therefore draws the large surge current.

When a motor speed controller is used, it varies the voltage fed to the motor. Initially, at zero speed, the controller will feed no voltage to the motor, so no current flows. As the motor speed controller’s output voltage increases, the motor will start to turn. At first the voltage fed to the motor is small, so the current is also small, and as the motor speed controller’s voltage rises, so too does the motor’s back EMF. The result is that the initial current surge is removed, acceleration is smooth and fully under control. Ever if the motor speed controller’s input is increased very suddenly it has a built in time constant which winds the output up slowly, so still there is no excessive surge and acceleration is smooth.

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