Electrical Braking

• Regenerative Braking

  1. Regenerative braking implies operating the motor as a generator while it is still connected to the supply Network.
  2. The Mechanical Energy is converter to Electrical Energy.

   

   If E>V then Ia becomes negative    

  This is the condition for regenerative braking where the generated energy is supplied to the source.

  3. If we give attention to the equation (1) for increase in E Field flux φ cannot increase substantially beyond rated because of Saturation. Therefore for a source of fixed Voltage of rated Value regenerative braking is possible only for speed higher than the rated and with a variable Voltage source it is also possible below rated speed.
  4. In a series motor as speed increases armature current hence the flux decreases. Consequently the condition of (4) cannot be achieved. Thus regenerative braking is not possible in series Motors.
  5. In actual system when the machines regenerate its terminal Voltage rises. Consequently the regenerated Power flows into the loads connected to the supply and the source is relieved from supplying this much amount of Power. Regenerative braking is possible only when there are loads connected to the line and they are in need of power more are equal to regenerated power. If the capacity of the load is less than the regenerated power all the regenerated power will not be absorbed by the loads. The remaining power will be supplied to the line and line voltage will rise to dangerous value leading to insulation breakdown. Hence regenerative braking should only be used when there is load to absorb the regenerated Power.

• Dynamic braking

  1. In dynamic braking, motor armature is disconnected from the source and connected across a resistance (say RB).The generated energy is dissipated in RB and
  2. For series machine works as a self excited generator the field connection is reversed so that the field assists the residual magnetism.

• Plugging

  1. For plugging the supply Voltage of a separately excited motor is reversed so that it assists the back emf in forcing the armature current in reverse direction.
  2. A resistance RB is connected in series with armature to limit the current.
  3. For plugging of a series motor armature alone is reversed.
  4. Plugging involve reconnecting the power supply to the motor so that it tends to drive in opposite direction. It is obvious that the system will come to rest and then drive in opposite direction.
  5. In case it is required to bring the drive to rest it is required to include a relay which will disconnect the supply exactly at the instant when motor stops.
  6. This is the most inefficient method of braking as in addition to dissipating the Electrical energy converted from Mechanical Energy in the resistance of the circuit the electrical energy drawn from the supply is wasted.

• Induction Motor Braking Regenerative Plugging Dynamic Braking of Induction Motor

  Induction motors are used at various places. Speed control of induction motors is quite difficult and that’s why their use was restricted and DC motors had to be used as their speed regulation was possible. But when induction motor drives were invented and implemented, they were given preference because of many advantages over dc motors. Whenever controlling of motors is done, braking is the most important term, so as with induction motors. Induction motor braking can be done by different methods, which are 

  1. 
     Regenerative braking of induction motor
     
     ii. plugging Braking of induction motor
     
     iii. Dynamic braking of induction motor is further categorized as 
     
     a) AC Dynamic breaking 
     
     b) Self excited braking using capacitors 
     
     c) DC Dynamic braking 
     
     d) Zero Sequence braking

• We know the power (input) of an induction motor is given as. 
  
  P_in = 3VI_scosφ_s

• Here, φ_s the phase angle between stator phase voltage V and the stator phase current I_s. Now, for motoring operation φ_s < 90° and for braking operation φ_s > 90°. When the speed of the motor is more than the synchronous speed, relative speed between the motor conductors and air gap rotating field reverses, as a result the phase angle because greater than 90° and the power flow reverse and thus regenerative braking takes place. The nature of the speed torque curves are shown in the figure beside. It the source frequency is fixed then the regenerative braking of induction motor can only take place if the speed of the motor is greater than synchronous speed, but with a variable frequency source regenerative braking of induction motor can occur for speeds lower than synchronous speed. The main advantage of this kind of braking can be said that the generated power is use fully employed and the main disadvantage of this type of braking is that for fixed frequency sources, braking cannot happen below synchronous speeds.

  Plugging Braking of induction motor

  Plugging induction motor braking is done by reversing the phase sequence of the motor. Plugging Braking of induction motor is done by interchanging connections of any two phases of stator with respect of supply terminals. And with that the operation of motoring shifts to plugging braking. During plugging the slip is (2 - s), if the original slip of the running motor is s, then it can be shown in the following way.

From the figure beside we can see that the torque is not zero at zero speed. That’s why when the motor is needed to be stopped, it should be disconnected from the supply at near zero speed. The motor is connected to rotate in the reverse direction and the torque is not zero at zero or any other speed, and as a result the motor first decelerates to zero and then smoothly accelerates in the opposite direction.

Dynamic braking of induction motor

There are four type of dynamic braking of induction motor or Rheostatic braking , we will discuss about then

AC Dynamic Braking

- This type of induction motor braking is obtained when the motor is made to run on a single phase supply by disconnecting any one of the three phase from the source, and it is either left open or it is connected with another phase. When the disconnected phase is left open, it is called two lead connection and when the disconnected phase is connected to another machine phase it is known as three load connection. The braking operation can be understood easily. When the motor is running on 1-phase supply, the motor is fed by positive and negative sequence, net torque produced by the machine at that point of time is sum of torques due to positive and negative sequence voltage. At high resistance the net torque is found to be negative and braking occurs. From the figure below the two and three load connections can be understood.

The figures above shows the circuit diagram and various characteristics of self excited braking using capacitors. As we can see from the figure, in this method there capacitors are kept permanently connected across the source terminals of the motor. The value of the capacitors are chosen depending upon their capability to deliver enough reactive current to excite the motor and make it work as a generator. So, that when the motor terminals are disconnected from the source the motor works as a self excited generator and the produced torque and field is in the opposite direction and the induction motor braking operation occurs. In the figure (b) the curve A represents the no load magnetization curve and line B is the current through capacitors, which is given by

Here E is the stator induced voltage per phase 

The speed torque characteristics under self excited braking is shown in the figure (c). To increase the braking torque and to utilize the generated energy sometimes external resistance are connected across the stator terminals.

DC Dynamic braking To obtain this type of braking the stator of a running induction motor is connected to a dc supply. Two and three load connections are the two common type of connections for star and delta connected stators.

Two loads dc dynamic braking operation.
Now coming to the method of operation, the moment when ac supply is disconnected and dc supply is introduced across the terminals of the induction motor, there is a stationery magnetic field generated due to the dc current flow and as the rotor of the motor rotates in that field, there is a field induces in the rotor winding, and as a result the machine works as a generator and the generated energy dissipates in the rotor circuit resistance and dynamic braking of induction motor occurs.

Zero sequence braking In this type of braking all the three stator phases are connected in series and single phase ac or dc is connected across them (as shown in the figure). This type of connection is called zero-sequence connection, because current in all the stator windings are co-phasal. When the connected supply is ac, resultant field is stationery in space and pulsates at the frequency of supply, when the supply is dc, resultant field is stationery and is of constant magnitude. The main advantage of this induction motor braking is that all the stator phases are uniformly loaded. It does not require large rotor resistance like ac dynamic braking, it does not require large rotor resistance.