In a typical AC electric motor, a rotating magnetic field is set up by the current flowing through windings(coil winding machine) in the stator. This current also causes an "induced" current to flow through the bars in the rotor (hence the term "induction" motor). The resultant force causes the rotor to rotate as it continually "chases" the rotating magnetic field and, since the rotor is firmly fixed to the shaft, the shaft also rotates.
The basic construction of AC induction motors three phase motor has changed very little over the years which are widely used in pump motor,compressor,range hood motor etc
THE STATOR WINDING
The windings in a motor are there to provide a path for the AC current to flow which in turn produces the magnetic field which will cause the rotor to rotate. The windings are insulated copper wire and inserted into slots in the stator laminations(Some plant insert windings manually and some plant do it by machinery). These slots have insulation between the windings and the steel laminations(please refer to slot cell inserting machine). This is known as the "stator pack". The windings are designed to provide the output and speed required. The stator pack is, in turn, inserted into the motor casing known as the "stator frame". The ends of the winding are brought out through the motor casing to terminals in a terminal box mounted on the frame. This is where the mains leads are connected.
THE ROTOR
This consists of laminations, shaft, bearings and a "winding". The type of "winding" will depend on the type of motor required.
If the rotor has a winding similar to that of the stator it is known as a "wound rotor motor". The wound rotor motor also is provided with copper or brass rings on the shaft and brushes. These transfer the current generated in the rotor to external resistance banks used to bring the motor up to speed or control the speed. When the motor is up to the full load speed the slip rings are shorted together to enable the motor to run continuously at the full load speed.
If the "winding" consists of solid bars that are joined either end by a shorting ring, it is known as a "squirrel cage rotor" motor. This is because the cage of the rotor resembles the cage that squirrels use to play with when in captivity. The bars are generally aluminium but can be copper or any such material. The squirrel cage rotor motor is the most common type in use today as it requires simple control gear and, in most cases, can be used instead of a wound rotor motor.
The bearings are used to support the shaft and to enable it to rotate.
POLARITY AND SPEED
You will remember the earlier reference to the rotating magnetic field and how the rotor "chases" it. In theory, if there were just the one magnetic 'pole', the rotor would rotate at a rate equal to twice the frequency of the supply, that is to say, for a 50 hertz ( or 50 cycles per second) supply the rotational speed would be 100 revolutions per second, or 6000 rpm.
In practice it is not possible to create one magnetic pole without at the same time creating an equal and opposite pole, so the highest achievable speed for an ac induction motor using a 50 HZ supply is 3000 rpm.
It is possible to arrange the stator windings in such formations as to provide any number of PAIRS of poles and so we can offer 2,4,6,8,10,12 pole motors etc. Motors over 12 pole are available if required but are not in common use.
Poles and Synchronous Speed
The number of poles are determined by the number of magnet poles. They should be an even number.
Remember that as the number of poles increase, so the speed decreases.
We call the hypothetical speed "synchronous" speed because it is the speed that would be obtained if the rotor rotated in "synchrony" with the magnetic field. In any ac induction motor the synchronous speed is never achievable, since friction losses in the bearings, air resistance within the motor and additional drag imposed by the load combine to cause the rotor to lag slightly behind the rotational speed of the magnetic field. This lagging effect is known as the "slip".
The "synchronous" speed of a motor can be determined by the formula:synchronous speed = 120 x f Poles where : Speed is expressed in rpm f equals frequency in HZ poles is an even number. (ie 2,4,6 etc)
If the frequency varies, the speed varies in a direct ratio.
Pearl
Emily