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| Circuit diagram of DC Motor |
Regenerative Braking
- In this form of braking, an attempts is made to return the stored Kinetic Energy of rotor back to source as much as possible.
- Braking can be achieved by keeping Ea > V either by increasing excitation or by reducing terminal voltage.
- This would result in reversal of armature current and reversal of direction of torque and sends the machine to generator mode but machine cannot keep running as there is no Prime Mover to supply mechanical power.
- But armature current must be kept within limits and as soon as Ia = 0, machine must be switched off.
Plugging
- In this technique, supply to armature terminals is reversed while field is left undisturbed.
- The current reverses resulting in to negative torque that brings the rotor quickly to rest and the supply in the motor should then be switched off to prevents its rotation in reverse direction.
- During plugging armature current should be maintained within safe limits by inserting an external resistance in the armature circuit or by adjusting the value of supply voltage.
Armature Current under Plugging condition is given by, Ia' = (Vt + Eb) /(Ra + Rext)
The expression for Plugging Torque is given by, Tplugging = Eb Ia' /Ꞷ
The Load Torque under normal running conditions is given by, TLoad = Eb Ia /Ꞷ
where, Ia = (Vt - Eb) /Ra is the armature current before plugging.
The net Braking Torque on motor is given by, TBraking = TPlugging + TLoad
- During plugging, electrical power is taken from the mains and mechanical power is derived from shaft and sum of these 2 powers dissipates at heat in the rotor. Precaution must be taken so that developed heat may not destroy the rotor.
Dynamic (Rheostatic) Braking
- Armature supply is replaced by a resister.
- So machine goes into generator mode and extracts mechanical power from the shaft bringing the motor to stand still.
- Braking can be made faster by increasing RL or decreasing If or both.
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