Plugging Braking Method  (Part-3)

Plugging is the braking method in which a reverse torque is generated by applying reverse voltage or phase sequence to the motor. Plugging is the fastest braking method since it drives the motor to reverse no matter what the running speed of the motor is.

For DC motors, plugging is done by reversing the supply or reversing the driver H-bridge operation. For grid driven induction machines, the braking can be easily applied just by interchanging the connection of 2 phases. For brushless motors driven by a controller, plugging is applied electronically and the commutation sequence or phase voltage is altered.

As mentioned before, plugging is the fastest braking method since it is a “brute force” applied to motor. However, it creates a huge stress on both the motor and the controller in many ways.

For a motor running with a constant load and speed, the phase input voltage and phase back-EMF has same polarity and a small difference and a proper controller follows phase back-EMF and preserves the difference. When a plugging is started; the controller changes the polarity of at least one phase. When this occurs the polarities of applied phase voltage and back EMF is opposite and the current is limited only by phase resistance and inductance. The phase current, only limited by internal impedance of the immediately rises at a huge rate and a very high reverse torque is generated. This high current both stresses the motor windings and switching elements of the controller. If the motor is running under highly inertial load, prolonging plugging may even lead to melt down of motor structure due to excessive heat generated by windings.

Secondly, when the switching elements are switching for plugging, the instantaneous voltage appearing on the switching element may reach input voltage added to the back EMF, which may be twice the normal operating voltage. If switching elements are not rated, they may instantly fail.

For electronically controlled plugging, the controller should either track rotor position or phase currents to apply safe voltage so that the plugging current is always under defined limits of both the controller and the motor.

Plugging puts down all electrical stress to the motor itself, heating up the motor. As mentioned before, if braking period is long, high temperature rise may even damage to the motor. Therefore, the motor’s operating conditions should be closely monitored to apply a safe plugging.

So, analyzing 3 different braking techniques, the last part will explain which technique to use.

Breaking methodsDc motors