CHAPTER 9 - MOTOR STOPPING

MOTOR STOPPING

                The most common method of stopping a motor is to remove the supply voltage and allow the motor and load to coast to a stop. However the motor must be stopped more quickly or held in position by some sort braking device. Electric braking uses the windings of the motor to produce a retarding torque. There are two different means of electric braking:

a.       Plugging

             Plugging stops a poly phase motor quickly by momentarily connecting the motor for reverse rotation while the motor is still running in the forward direction.

             A zero-speed switch (plugging switch) is coupled to a moving shaft on the machinery whose motor is to be plugged.

             Anti-plugging protection, according to NEMA, is obtained when a device prevents the application of a counter torque until the motor speed is reduced to an acceptable value.

b.      Dynamic braking

            Dynamic braking is a method of braking that is used the motor as generator during the braking period immediately after the motor is turned off.

                

Electric braking can be achieved with a three-phase induction motor by removing the ac power supply from the motor and applying direct current to one of the stator phase.

           

 Electromechanical friction brake refers to a device external to the motor that provides retarding torque. It also has ability to hold a motor stationary and are used in application such as crane that require the load to be held.

            An advantage using dynamic braking is that motor can be stopped rapidly without causing brake linings or drums to wear. But dynamic braking cannot be used to hold a suspended load.

            The electric load brake (eddy current brake) is a simple, rugged device that consist of an iron rotor mounted inside a stationary field assembly.

References:
Petruzella, Frank. 1996. Industrial Electronics. Mc-Graw Hill. Singapore