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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 is one of the primary reasons for using a motor speed controller.

When a motor is turning it also acts as a generator and produces its own internal voltage known as the back EMF, this back EMF is opposite to the battery voltage, and rises as the motor speeds up.

But at the instant a motor is first connected to a battery (with no controller in the circuit) there is no back EMF. So the surge current is determined only by the battery voltage, the motor resistance (and inductance), and the battery leads. Depending on the motor design, this surge [or stall] current can be up to five times the normal operating current.

As the motor starts to speed up, the back EMF increases and eventually will almost balance the battery voltage and the motor will be at its steady state operating speed. We say “almost” because it is the difference between the two that causes the motors operating current to flow.

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 limited, acceleration is smooth and fully under control. Ever if the 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.