How three phase induction motor works?speed control method

 Asynchronous motors behave electrically like transformers. The secondary winding is the rotor and the mechanical power output of the motor acts on the primary side like a – variable – load resistance. If no mechanical power output is produced at rest (on initiation of start-up), this load resistance is zero, i.e. the transformer is in effect secondarily shorted.

Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans

An electric motor converts electrical energy into a mechanical energy which is given to different types of loads. A.c. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device.

A 3 phase induction motor has its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections.

The stator of an induction motor consists of a number of overlapping windings offset by an electrical angle of 120°. When the primary winding or stator is connected to a three phase A.C supply, it create a rotating magnetic field which rotates at a synchronous speed.

Synchronous Speed equation

                                        

The direction of rotation of the motor depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. Thus interchanging the connection of any two primary terminals to the supply will reverse the direction of rotation.

The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. Motors are commonly configured to have 2, 4, 6 or 8 poles. The synchronous speed, a term given to the speed at which the field produced by primary currents will rotate, is determined by the following expression.

Types for induction motor: 

1) Slip-ring motors

Principal diagram of a slip-ring motor with external rotor resistances

With slip-ring motors, the rotor winding is connected to slip rings and terminated with external resistances. The resistance of the external resistors influences the current flowing through the rotor and the speed-torque characteristic.

Principal diagram of a slip-ring motor with external rotor resistances Slip-ring motors represent the conventional method of controlling starting torque (and the current consumption) by selection of the rotor resistances. The highest attainable starting torque corresponds to the breakdown torque of the motor. This is independent of the magnitude of the rotor resistance. The primary current consumption of slip-ring motors is proportional to the rotor current. Thanks to these characteristics, slip-ring motors can achieve a high starting torque with relatively low current consumption.
The external resistors are usually changed in steps during motor startup. The rotor windings are shorted in normal continuous duty. By designing the rotor resistances for continuous duty, it is even possible to continuously influence the speed, albeit at the cost of high heat dissipation.

2)  Squirrel-cage induction motors

In the case of asynchronous machines with squirrel-cage induction rotors, the rotor consists of a grooved cylindrical laminated rotor package with rods of highly conductive metal (preferably aluminum), that is joined on the face side by rings to form a closed cage. The cage – at least in the case of small motors – is usually cast into the rotor.
To reduce the starting current and influence the starting torque characteristic, the coil rods are specially designed so that they create a high rotor resistance at rest and at low speeds by current displacement. They are usually placed crosswise at an angle to the axis of rotation to avoid variations in torque and to ensure smooth running characteristics.

Speed control methods of three phase Induction Motor :

figure: induction motor speed control

Control From Stator Side:

it includes following methods

 

fig:speed equation of induction motor

fig:Rotor resistance starter

 If the rotor resistance is increased,  the torque produced decreases. But when  the load on the motor is same,  motor has to supply same   torque as load demands.  So motor reacts by increasing its slip to compensate decrease in  T torque  due to resistance  R and maintains the load torque R constant So due to additional rotor resistance motor slip increases ie the speed of the motor decreases Thus by increasing the rotor resistance R.  speeds below normal value can be achieved Another advantage of this method is that the starting torque of the motor increases Fi proportional to rotor resistance  

But this method has following disadvantages

1. The large speed changes are not possible This is because for large speed change. required large in rotar which causes large rotor losses, which reduce the efficiency of motordi

1)  this method can not be used for the squirrel cage induction motors 
2)  The speed above the normal values can not be obtained
3)  Large power looses occur hence sufficient cooling arrangements required
4)  Due to large power losses,  efficiency is low Thus the method is rarely used


tags: dol starter,motor starter,star delta starter,single phase motor,three phase motor,3 phase induction motor, single phase to three phase,three phase induction motor,4 phase motor,dol starter,motor starter,star delta starter,single phase motor,three phase motor

read also..

What is VFD ( veriable frequency drive) and how it works? 

Why single-phase induction motor is not self-starting and how can make it self starting