Industrial control: Dynamic braking of a D-C Shunt motor and load

WITH high-production machines requiring rapid-duty cycling and accurate positioning, the speed-time characteristics of accelerating and decelerating motors are very important. D-c shunt machines are often employed for these applications because of their flexibility both in starting and stopping. One method commonly employed to bring the rotating system to rest is to disconnect the motor from the line and reconnect it across a resistor. The shunt field remains connected to the line so that the motor acts as a generator driven by the stored energy in the load. This method is known as dynamic braking. The elementary connection of the dynamic-braking circuit is shown in Figure 1. Since the torque developed is proportional to the product of field flux and armature current, and the latter depends on the speed, the braking effect obtained decreases as the motor loses speed. A friction-type load, independent of the speed, must be present in order to bring the motor to rest.