How is DC-injection braking applied to induction motors?
Direct current is injected into the stator winding after this winding has been disconnected from the supply. This sets up a stationary field, inducing e.m.f.s. and hence currents in the rotor circuits. This method provides a high-braking torque with low losses, but the braking effect is small at high speeds for machines of normal slip and it is usually necessary to use heavy DC exciting currents to effect a quick-stop. Some improvement of the braking characteristic can be made by increasing the rotor-circuit resistance at high speeds and reducing it as the speed falls. Fig. 37- Basic connection diagram for capacitor and magnetic dynamic braking. Operation of stop button opens supply lines and inserts capacitors ‘C’ across motor terminals. Voltage relay ‘VR’ provides the necessary delay before operating to short-circuit the motor terminals. Instead of ‘VR’ a timer or limit switches maybe used R1 are resistors for controlling braking torque. R2 discharge resistors and R3 control resistor
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