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 

i. 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 

Regenerative Braking of Induction Motor

                           We know the power (input) of an induction motor is given as. 

                                                                               Pin = 3VIscosφs

Here, φs the phase angle between stator phase voltage V and the stator phase current Is. Now, for motoring operation φs< 90° and for braking operation φs > 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.

Self excited braking using capacitors 

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 electrical 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.

Another diagram is shown below to illustrate how by diode bridge two load coonection can be obtained within a circuit.

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. The circuit diagram and the speed torque characteristics are shown below.

The above discussion easily describes the concept of induction motor breaking.

Speed Control of Three Phase Induction Motor

A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to control its speed. The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor. Before discussing the methods to control the speed of three phase induction motor one should know the basic formulas of speed and torque of three phase induction motor as the methods of speed control depends upon these formulas.

Synchronous speed 

Where f = frequency and P is the number of poles

The speed of induction motor is given by,

Where N is the speed of rotor of induction motor,
Ns is the synchronous speed,
S is the slip. 

The torque produced by three phase induction motor is given by,

When rotor is at sandstill slip , s is one.

So the equation of torque is,

Where E2 is the rotor emf
Ns is the synchronous speed
R2 is the rotor resistance
X2 is the rotor inductive reactance

The Speed of Induction Motor is changed from Both Stator and Rotor Side

The speed control of three phase induction motor from stator side are further classified as :

1. V / f control or frequency control.
2. Changing the number of stator poles.
3. Controlling supply voltage.
4. Adding rheostat in the stator circuit.

The speed controls of three phase induction motor from rotor side are further classified as:

1. Adding external resistance on rotor side.
2. Cascade control method.
3. Injecting slip frequency emf into rotor side.

Speed Control from Stator Side

1. V / f control or frequency control - Whenever three phase supply is given to three phase induction motor rotating magnetic field is produced which rotates at synchronous speed given by
   
   In three phase induction motor emf is induced by induction similar to that of transformer which is given by
   
   Where K is the winding constant, T is the number of turns per phase and f is frequency. Now if we change frequency synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor . So, its important to maintain flux , φ constant and it is only possible if we change voltage . i.e if we decrease frequency flux increases but at the same time if we decrease voltage flux will also decease causing no change in flux and hence it remains constant. So, here we are keeping the ratio of V/ f as constant. Hence its name is V/ f method. For controlling the speed of three phase induction motor by V/ f method we have to supply variable voltage and frequency which is easily obtained by using converter and inverter set.
2. Controlling supply voltage: The torque produced by running three phase induction motor is given by
   
   In low slip region (sX)2 is very very small as compared to R2 . So, it can be neglected. So torque becomes
   
   Since rotor resistance, R2 is constant so the equation of torque further reduces to
   
   We know that rotor induced emf E2 ∝ V. So, T ∝ sV2.
   From the equation above it is clear that if we decrease supply voltage torque will also decrease. But for supplying the same load, the torque must remains the same and it is only possible if we increase the slip and if the slip increases the motor will run at reduced speed . This method of speed control is rarely used because small change in speed requires large reduction in voltage, and hence the current drawn by motor increases, which cause over heating of induction motor.
3. Changing the number of stator poles : The stator poles can be changed by two methods
4. Multiple stator winding method.
5. Pole amplitude modulation method (PAM)
6. Multiple stator winding method – In this method of speed control of three phase induction motor , the stator is provided by two separate winding. These two stator windings are electrically isolated from each other and are wound for two different pole numbers. Using switching arrangement, at a time , supply is given to one winding only and hence speed control is possible. Disadvantages of this method is that the smooth speed control is not possible . This method is more costly and less efficient as two different stator winding are required. This method of speed control can only be applied for squirrel cage motor.
7. Pole amplitude modulation method (PAM) – In this method of speed control of three phase induction motor the original sinusoidal mmf wave is modulated by another sinusoidal mmf wave having different number of poles.
Let f1(θ) be the original mmf wave of induction motor whose speed is to be controlled.
f2(θ) be the modulation mmf wave. 
P1 be the number of poles of induction motor whose speed is to be controlled.
P2 be the number of poles of modulation wave.

After modulation resultant mmf wave

So we get, resultant mmf wave

Therefore the resultant mmf wave will have two different number of poles


Therefore by changing the number of poles we can easily change the speed of three phase induction motor.
8. Adding rheostat in the stator circuit - In this method of speed control of three phase induction motor rheostat is added in the stator circuit due to this voltage gets dropped .In case of three phase induction motor torque produced is given by T ∝ sV22. If we decrease supply voltage torque will also decrease. But for supplying the same load , the torque must remains the same and it is only possible if we increase the slip and if the slip increase motor will run reduced speed.

Speed Control from Rotor Side

1. Adding external resistance on rotor side – In this method of speed control ofthree phase induction motor external resistance are added on rotor side. The equation of torque for three phase induction motor is
   
   
   The three phase induction motor operates in low slip region .In low slip region term (sX)2 becomes very very small as compared to R2. So, it can be neglected . and also E2 is constant. So the equation of torque after simplification becomes,
   
   Now if we increase rotor resistance, R2 torque decreases but to supply the same load torque must remains constant. So, we increase slip, which will further results in decrease in rotor speed. Thus by adding additional resistance in rotor circuit we can decrease the speed of three phase induction motor. The main advantage of this method is that with addition of external resistance starting torque increases but this method of speed control of three phase induction motor also suffers from some disadvantages :
   
   1. The speed above the normal value is not possible.
   2. Large speed change requires large value of resistance and if such large value of resistance is added in the circuit it will cause large copper loss and hence reduction in efficiency.
   3. Presence of resistance causes more losses.
   4. This method cannot be used for squirrel cage induction motor.
2. Cascade control method – In this method of speed control of three phase induction motor, the two three phase induction motor are connected on common shaft and hence called cascaded motor. One motor is the called the main motor and another motor is called the auxiliary motor. The three phase supply is given to the stator of the main motor while the auxiliary motor is derived at a slip frequency from the slip ring of main motor.
   Let NS1 be the synchronous speed of main motor. 
   NS2 be the synchronous speed of auxiliary motor.
   P1 be the number of poles of the main motor.
   P2 be the number of poles of the auxiliary motor.
   F is the supply frequency.
   F1 is the frequency of rotor induced emf of main motor.
   N is the speed of set and it remains same for both the main and auxiliary motor as both the motors are mounted on common shaft.
   S1 is the slip of main motor.
   
   The auxiliary motor is supplied with same frequency as the main motor i.e
   
   Now put the value of
   
   Now at no load , the speed of auxiliary rotor is almost same as its synchronous speed i.e N = NS2
   
   Now rearrange the above equation and find out the value of N, we get,
   
   
   This cascaded set of two motors will now run at new speed having number of poles (P1 + P2). In the above method the torque produced by the main and auxiliary motor will act in same direction, resulting in number of poles (P1 + P2). Such type of cascading is called cumulative cascading. There is one more type of cascading in which the torque produced by the main motor is in opposite direction to that of auxiliary motor. Such type of cascading is called differential cascading; resulting in speed corresponds to number of poles (P1 – P2).
   In this method of speed control of three phase induction motor, four different speeds can be obtained
   1. When only main induction motor work, having speed corresponds to NS1= 120 F / P1.
   2. When only auxiliary induction motor work, having speed corresponds to NS2 = 120 F / P2.
   3. When cumulative cascading is done, then the complete set runs at a speed of N = 120F / (P1 + P2).
   4. When differential cascading is done, then the complete set runs at a speed of N = 120F / (P1 – P2).
3. Injecting slip frequency emf into rotor side - when the speed control of three phase induction motor is done by adding resistance in rotor circuit, some part of power called, the slip power is lost as I2R losses. Therefore the efficiency of three phase induction motor is reduced by this method of speed control. This slip power loss can be recovered and supplied back in order to improve the overall efficiency of three phase induction motor and this scheme of recovering the power is called slip power recovery scheme and this is done by connecting an external source of emf of slip frequency to the rotor circuit. The injected emf can either oppose the rotor induced emf or aids the rotor induced emf. If it oppose the rotor induced emf, the total rotor resistance increases and hence speed decreases and if the injected emf aids the main rotor emf the totalr esistance decreases and hence speed increases. Therefore by injecting induced emf in rotor circuit the speed can be easily controlled. The main advantage of this type of speed control of three phase induction motor is that wide range of speed control is possible whether its above normal or below normal speed.

WORLD OF ELECTRIC MOTOR (Types Classification and History of Motor)

The motor or an electrical motor is a device that has brought about one of the biggest advancements in the fields of engineering and technology ever since the invention of electricity. A motor is nothing but an electro-mechanical device that converts electrical energy to mechanical energy. Its because of motors, life is what it is today in the 21st century. Without motor we had still been living in Sir Thomas Edison’s Era where the only purpose of electricity would have been to glow bulbs. There are different types of motor have been developed for different specific purposes.

In simple words we can say a device that produces rotational force is a motor. The very basic principal of functioning of an electrical motor lies on the fact that force is experienced in the direction perpendicular to magnetic field and the current, when field and electric current are made to interact with each other. Ever since the invention of motors, a lot of advancements has taken place in this field of engineering and it has become a subject of extreme importance for modern engineers. This particular webpage takes into consideration, the above mentioned fact and provides a detailed description on all major electrical motors and motoring parts being used in the present era.

Classification or Types of Motor

The primary classification of motor or types of motor can be tabulated as shown below,

Click to Enlarge

In the year 1821 British scientist Michael Faraday explained the conversion of electrical energy into mechanical energy by placing a current carrying conductor in a magnetic field which resulted in the rotation of the conductor due to torque produced by the mutual action of electrical current and field. Based on his principal the most primitive of machines a D.C.(direct current) machine was designed by another British scientist William Sturgeon in the year 1832. But his model was overly expensive and wasn’t used for any practical purpose. Later in the year 1886 the first electrical motor was invented by scientist Frank Julian Sprague. That was capable of rotating at a constant speed under a varied range of load, and thus derived motoring action.

MAIN CLASSIFICATION 

1) DC Motor

2) Synchronous Motor

3) 3 Phase Induction Motor

4) 1 Phase Induction Motor

5) Special Types of Motor

Among the four basic classification of motors mentioned above the DC motor as the name suggests, is the only one that is driven by direct current. It’s the most primitive version of the electric motor where rotating torque is produced due to flow of electric current through the conductor inside a magnetic field.

Rest all are A.C. electrical motors, and are driven by alternating current, for e.g. the synchronous motor, which always runs at synchronous speed. Here the rotor is an electro – magnet which is magnetically locked with stator rotating magnetic field and rotates with it. The speed of these machines are varied by varying the frequency (f) and number of poles (P), as Ns = 120 f/P.

In another type of AC motor where rotating magnetic field cuts the rotor conductors, hence circulating current induced in these short circuited rotor conductors. Due to interaction of the magnetic field and these circulating currents the rotor starts rotates and continues its rotation. This is induction motor which is also known as asynchronous motor runs at a speed lesser than synchronous speed, and the rotating torque, and speed is governed by varying the slip which gives the difference between synchronous speed Ns , and rotor speed speed Nr,

It runs governing the principal of EMF induction due to varying flux density, hence the name induction machine comes. Single phase induction motor like a 3 phase, runs by the principal of emf induction due to flux, but the only difference is, it runs on single phase supply and its starting methods are governed by two well established theories, namely the Double Revolving field theory and the Cross field theory.

Apart from the four basic types of motor mentioned above, there are several types Of special electrical motors like Linear Induction motor(LIM),Stepper motor, Servo motor etc with special features that has been developed according to the needs of the industry or for a particular particular gadget like the use of hysteresis motor in hand watches because of its compactness.