Question

With reference to deck machinery driven by A.C induction motor describe each of the following types of braking.

Regenerative. Plugging. Dynamic.

Regenerative. It is used with 3 phase induction motor to prevent over speeding i.e. with crane

When lowering

If motor is a pole change multi speed type the Regenerative. Braking can be used to lower the speed before applying frictional brake.

Plugging.

When 2 supply leads are reversed this reverses the stator rotating felid and so applies a reverse braking torque. The supply must be disconnected when the motor stops or else the motor will restart in the reverse direction.

Reverse torque is about half full load torque and motor current will be large.

Dynamic.

While running the motor can be switched off the main a.c. supply and then a d.c. current can be injected into the motor phase winding.

D.C injection causes fixed field in the motor which gives retarding torque. Dynamic braking is used with commutator motor such as universal motor but not with 3 phase induction motor.