Electrical Engineering ⇒ Topic : Dynamic (or rheostatic) braking

Dynamic (or rheostatic) braking :

• The dynamic or rheostatic braking in case of a polyphase induction motor can be realised by disconnecting the stator winding from the A.C. supply and exciting it from a D.C. source to produce a stationary D.C. field.

In dynamic or rheostatic braking, the stator winding is employed on a D.C. field winding and the rotor winding as an armature winding.

• With a wound rotor machine, external resistors can be inserted into the rotor circuit to provide a load.
• With squirrel cage machines, however, the rotor winding itself has to form the load.

The source of excitation may be provided by :

• an independent D.C. source, or
• from the A. C. mains through a transformer-rectifier set.

The magnitude of the braking torque developed by the motor depends upon

• The excitation (strength of the field developed by the stator winding);
• The rotor circuit resistance;
• The speed of the motor

Braking torque can be controlled by any or both of the following methods

• By controlling the D.C. excitation.
• By varying the rotor resistance.

(1)A. C.dynamic braking is not popular owing to relatively high cost of capacitor banks.

(2)D.C. dynamic braking takes little power from the supply and prouides smooth braking torque, useful for mine winders and high inertia loads.

It entails the following advantages over plugging

• The absence of the reverse-rotation air-gap field (and, therefore, no tendency for the machine to run backwards).
• Lower rotor I²R loss.

The advent of automatic control of dynamic braking of induction motors employing closed loop systems has made induction motors more popular than D.C. motors, especially for the drives employed in mine hoists