Dynamic braking is used to brake non-reversing drives. The stator is transferred from AC mains to DC mains, as shown in Figure 4.18.
The DC flowing through the stator sets up a stationary field. This induces rotor currents that produce a torque, to bring the rotor to rest quickly. The developed torque and retardation during braking may be controlled by the amount of the DC power. Additional resistances r1 and r2e in the stator and rotor circuits control the DC excitation and braking torques, respectively.
An equivalent circuit and phasor diagram of the motor during dynamic braking are shown in Figure 4.19.
When the stator is fed from the DC, the MMF produced is stationary. The MMF depends upon the stator connections for feeding the DC, the number of turns, and the current. The possible connections of the stator for feeding the DC are shown in Figure 4.20.
Dynamic braking is employed in conjunction with automatic control. Induction motors are more popular in hoists than in DC motors due to this reason. Methods of feeding the DC supply to the stator are shown in Figure 4.20. The limiting resistor R controls DC excitation.
Torque control is achieved by a rotor resistance variation. Alternately, an AC supply with a bridge rectifier may be used to feed the motor.
In AC dynamic braking, the stator is switched to a capacitance bank. The machine runs as a self-excited induction generator. All the mechanical energy is dissipated as electrical energy in the rotor resistance. This method is un-economical due to a high cost of capacitors.