Speed control of d.c. Motors:Electric Braking of Series Motor

Electric Braking of Series Motor

The above-discussed three methods as applied to series motors are as follows :

(a) Rheostatic (or Dynamic) Braking

The motor is disconnected from the supply, the field connections are reversed and the motor is connected in series with a variable resistance R as shown in Fig. 30.24. Obviously, now, the ma- chine is running as a generator.

The field connections are re- versed to make sure that cur- rent through field winding flows in the same direction as before (i.e., from M to N ) in order to assist residual magnetism. In practice, the variable resistance employed for starting purpose is itself used for braking purposes. As in the case of shunt motors,

Plugging or Reverse Current Braking

As in the case of shunt motors, in this case also the connections of the armature are reversed and a variable resistance R is put in series with the armature as shown in Fig. 30.25. As found in Art. 30.7 (b),

Regenerative Braking

This type of braking of a sereis motor is not possible without modification because reversal of Ia would also mean reversal of the field and hence of Eb. However, this method is sometimes used with traction motors, special arrangements being necessary for the purpose.

Example 30.47. A 400 V, 25 h.p. (18.65 kW), 45 r.p.m., d.c. shunt motor is braked by plugging when running on full load. Determine the braking resistance necessary if the maximum braking current is not to exceed twice the full-load current. Determine also the maximum braking torque and the braking torque when the motor is just reaching zero speed. The efficiency of the motor is 74.6% and the armature resistance is 0.2 W. (Electrical Technology, M.S. Univ. Baroda, 1988)