Motors - Operation and Issues - 4

Braking methods in Induction motors

Braking in induction motors refers to quickly bringing the speed of the motor to zero. Braking can be categorized into two broad categories viz. mechanical braking and electrical braking.

Mechanical braking involves stopping the shaft by means of a braking shoe. When the braking is to be done, the supply to the motor is cut off and the brake is applied to bring the motor to a halt.

Mechanical braking used in cranes and hoists. It is also used in elevators when the elevator has to stop at a specific floor of the building.

Electrical braking involves stopping the motor using electrical means. Most electrical braking systems have a mechanical brake to hold the shaft in position once the machine has been stopped.


There are two main types of Electrical braking.

Plugging
Dynamic braking
Regenerative braking


Plugging

Plugging involves reversing the supply in two of the phases. For instance, R and Y can be interchanged. This leads to a torque being developed in the opposite direction to the rotation of the motor. This causes the motor to stop at once. Once the motor stops, the reverse supply is cut off (to prevent the motor from running in the opposite direction). The rotor is secured by a mechanical brake.

Dynamic Braking can be classified into DC injection braking, AC dynamic Braking and Capacitor Braking.


AC dynamic Braking

In AC dynamic braking, the supply to one of the phases is cut off. Thus the motor runs as a single phase motor. This induces negative phase sequence components in the supply and the motor stops. Another method is to give the remove one phase and give the same phase to two terminals. For instance, two terminals will have 'Y' phase and one will have 'B' phase.


DC injection braking

In DC injection braking, a separate rectifier circuit produces a dc supply. When the brake is to be applied, the ac supply to the stator is disconnected and a dc supply is given to two of the phases. The dc voltage in the stator sets up its own magnetic field. The conductors of the rotor which is rotating will cut the magnetic field. As the conductors are short circuited, a high current is produced. This causes a braking torque to be produced in the rotor. The current produced in the rotor is dissipated as heat. This system can be used only when the rotor can withstand the heat which will be produced when the brake is applied.


Capacitive Braking

Here the AC supply to the stator terminals is cut off and the terminals are connected to a three phase capacitor bank. The capacitors will excite the induction generator. This sets up a magnetic field which will cut the rotor bars. The rotor energy is thus converted into heat and the motor is stopped.


Regenerative Braking

In Regenerative braking, the supply frequency to the stator is reduced. This is possible with VFDs where the frequency can be varied. When the supply frequency is reduced, the synchronous speed of the motor is reduced. When the synchronous speed falls below the rotor speed, the induction motor works as an induction generator and power is supplied back to the terminals. The energy in the rotor is thus recovered. Due to the loss of energy, the rotor slows down and stops.



Efficiency of the Electric Motor 

The efficiency of the Electric Motor is about 92 %.

The efficiency is lower for smaller size motors. The efficiency for smaller motors can drop to around 80%

The losses in the motor are the iron losses due to the magnetic field, the copper losses due to the current, the mechanical losses (friction and windage losses) and the stray losses such as the harmonic losses.


Why should the dc series motor never be run without any load ?

The dc series motor develops a very heavy torque during start-up. The motor relies on the connected load to restrain the speed.

Hence, if the dc series motor is run without any load. it may result in overspeeding which can cause serious damage to the motor .



What happens when an induction motor is run above the rated speed ?

When the induction motor is run above the synchronous speed which is the speed of the rotating magnetic field, it works as an induction generator. That is, it generates active power kW while it still consumes reactive power kVAr in order to establish the magnetic field.  This usually happens when another prime mover such as a wind turbine is coupled to the motor shaft.

The slip (Ns-Nr) then becomes negative.

Another scenario where the motor can overspeed is when the frequency of the input power is itself increased by means of a Variable frequency drive. Then the motor is said to be running like an induction motor but at a higher speed.

The torque characteristics may vary with the varied speed. The rotor, gears and the coupling may experience increased centrifugal force which can cause damage. Hence, the overspeed limits need to be ascertained from the manufacturer.