Friday, November 11, 2016

Regenerative,Dynamic and Plugging braking

Brake obviously used to reduce the speed of any moving, rotating equipment’s. In day to day activity we use brakes in vehicles to stop them or reduce the speed of the same.

We come through braking of 2 types:-
(i)                  Electrical Braking
(ii)                 Mechanical braking

In mechanical braking speed is reduced by using mechanical process. But in electrical braking both electrical and mechanical systems are used for braking.
Electrical braking system works on the principle of reversing the direction of flux.

In most of systems motor is allowed to slow down and eventually stops after switching off the power supply of the motors. But for certain systems braking are needed to be provided. There are various types of motors such as Induction motors, DC motors, Single phase motors, And Synchronous motors etc. As these all types of motors are different from each other so type of braking method will also differ from each other.

There are following type of electrical braking systems:-
1.                   Regenerative braking
2.                   Dynamic braking
3.                   Plugging

Above stated types will be used for all types of motors, Now let’s discuss about them briefly in this article:-

1.   Regenerative Braking

This type of braking is used where speed of the motor exceeds the synchronous speed of the motor. As clear from its name in this braking motor will act like a generator and started generating power which will be used for supplying other loads.
Main principle of this type of braking system is that Rotor must rotate at a speed greater than synchronous speed, than motor starts operating like a generator which will leads to reversal in direction of flow of current and torque thus braking takes place.
Regenerative braking is used with DC and AC drives, now let’s see how regenerative works with these drives:-

(a)        DC drive:-

The increase in DC voltage for the DC drive occurs at the armature connection. In DC drives there are both forward as well as reverse bridges; by using this reverse bridge DC energy from the armature will get transferred to the source line. If there are only forward bridges in DC drives than a shunt generator is connected in parallel with armature so as to dissipate this energy into heat.

(b)         AC drives:-

As we know that in AC drives power supply connected is first converted to DC and then again into AC for feeding the power to motors. There circuitry is only able to handle power in motoring mode only so whenever motor is in regenerating mode than DC voltage get increased at DC link and drive may get trip so that it can’t damage whole Drive. So there are following ways to handle the excessive energy:-

(i)          By providing a bridge converter for regeneration.
(ii)         By connecting many drives DC bus bars so that regenerative energy from one motor can be used for other motor.
(iii)        By dissipating this energy through heat. By using a resistor.


In this braking motor has to run above synchronous speed that may cause damage to the motor mechanically and electrically.
2.   Dynamic  Braking:-
In this method resistors are used for braking the motor. In this method motor is disconnected from the power supply and connected across a resistance. Whenever motor is disconnected from the source than inertia will kept rotating the motor, such condition of motor is called self –excited generator and direction of flow of current and torque reverses. In this system there a unit which consists of controller, switching device. Resistors are separated components. This whole unit is rated in Amperes. HP rating of the unit is 20%. Resistors are sized according to applications.  In this system chopper is mounted in enclosure and resistor is mounted to 100 feet away.
3.   Plugging

In this type of braking supply at terminals is reversed which results in generator torque to also get reversed , thus it will leads to decrease in speed. In this method external resistance is used to limit the current. Disadvantage associated with this method is that here power is wasted during braking also.

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