Regenerative Braking of Electric Motors

In the regenerative braking operation, the motor operates as a generator, where the speed of the motor exceeds the synchronous speed. During braking, some of the mechanical energy is converted into electrical energy and other is dissipated in the form of heat in winding and bearings of the electrical machine. Most of the electrical machines pass smoothly from motoring region to generating region, when over driven by the loads.

Regenerative braking with example

In the above diagram, the electric motor is driving a trolley car in the uphill direction. Now the gravitational force can be resolved into two components. One is perpendicular to the load force F and another one is parallel to the road surface F l, the parallel force pulls the motor towards the bottom of the hill. If we neglect the rotational losses, the motor must produce a force F m opposite to Fl to move the bus in the uphill direction. Here, the motor torque and speed are in the same direction but the load torque Tl is opposite to the motor torque Tm. Thus the power flow from the motor to the mechanical load.

In the above diagram, the electric motor is driving a trolley car in the downhill direction. The gravitational force does not change its direction but the load torque pushes the motor toward the bottom of the hill. The motor produces a torque in the reverse direction because of the direction of the motor torque is always opposite to the direction of the load torque. Here the motor torque and speed are in the opposite direction. The load torque Tl is opposite to the motor torque Tm. Hence, the load is driving the machine and the machine is generating electric power that is returned to the supply.