1. Why is starter necessary for a three-phase induction motor?
Need for starter
Why is starter necessary for a three-phase induction motor?
At starting ,the speed of motor is zero so that the back e.m.f. In the armature is zero.
Armature resistance is so low, if it is connected to power supply directly ; huge current will pass thru armature.
The huge current may damage the machine, major heat, very high speed in case.

2. DOL Starter C/B Start M 1 1 K1.4 K1.2 K1.1 K1.3 2 TOL Stop 3 TOL 43~ 5

3. Motor Starting Characteristics on DOL Starter:
Available starting current : %.
Peak starting current : to 8 Full Load Current.
Peak starting torque : %
Advantages of DOL Starter:
Most Economical and Cheapest Starter
Simple to establish, operate and maintain
Simple Control Circuitry
Easy to understand and trouble‐shoot.
It provides 100% torque at the time of starting.
Only one set of cable is required from starter to motor.
Motor is connected in delta at motor terminals.

4. Disadvantages of DOL Starter:
It does not reduce the starting current of the motor.
High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).
Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.
Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors
High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.

5. STAR DELTA STARTER
Stator winding is connected on star at starting
Phase voltage is reduced by 1/√3
Due to reduced voltage
starting current is Limited
Stator winding is connected in delta
when motor gathers sufficient speed
( running Condition )

6. Primary Resistance starter or Stator Resistance Starter
Running
Start
Reduced voltage to stator,
Three resistance is added in series to the stator winding
Initial R MAX
Due to large voltage drop in R HIGH starting current is reduced

7. Auto transformer Starter

9. 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.
If Speed is achieved by some means, the performance of induction motor in terms of Power Factor, Efficiency gets affected.
Synchronous speed 
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,

10. The Speed of Induction Motor is can be controlled by TWO methods
Control from Stator Side
Control from Rotor Side
The speed control of three phase induction motor from stator side are further classified as :
Supply frequency control or V / f control
Changing the number of stator poles.
Controlling supply voltage.
Adding rheostat in the stator circuit
The speed control 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 an emf (Voltage) in the rotor side.

11. By changing the SUPPLY frequency or Applied frequency
Thus, synchronous speed changes with change in supply frequency, and thus running speed also changes.
This method is not widely used.
This method is used where, only the induction motor is supplied by a generator (so that frequency can be easily change by changing the speed of prime mover).
Induction motor is equal to transformer
E or V = 4.44 f T K Ф
Variable V,F DC
3Ф IM
Rectifier
Inverter AC

12. By changing the applied voltage
This method is the easiest and cheapest, still rarely used because-
1) A large change in supply voltage is required for relatively small change in speed.
2) Large change in supply voltage will result in large change in flux density, hence disturbing the magnetic conditions of the motor
Methods of controlling
Using auto transformer
Primary resistance connected in series in stator winding
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

13. Changing the number of stator poles
This method is generally used for squirrel cage induction motors as squirrel cage rotor adapts itself for any number of stator poles.
Change in stator poles is achieved by two or more independent stator windings wound for different number of poles in same slots.
For example, a stator is wound with two 3phase windings, one for 4 poles and other for 6 poles.
for supply frequency of 50 Hz
synchronous speed when 4 pole winding is connected, Ns = 120*50/4 = 1500 RPM
ii) synchronous speed when 6 pole winding is connected, Ns = 120*50/6 = 1000 RPM

14. The speed control 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 an emf (Voltage) in the rotor side.

15. Injecting Slip-Frequency E.M.F. into Rotor Circuit
In this method, a voltage is injected in the rotor circuit.
The frequency of rotor circuit is a slip frequency and hence the voltage to be injected must be at a slip frequency.
It is possible that the injected voltage may oppose the rotor induced e.m.f. or may assist the rotor induced e.m.f.
If it is in the phase opposition, effective rotor resistance increases.
If it is in phase of rotor induced e.m.f., effective rotor resistance decreases.
Thus by controlling the magnitude of the injected e.m.f., rotor resistance and effectively speed can be controlled.
Practically two methods are available which use this principle. These methods are,
1. Kramer system Scherbius system

16. Kramer system

17. Scherbius system