IV. Read and translate the text
When a 3-phase AC power supply is connected to the stator terminals of an induction motor, 3-phase alternating currents flow in the stator windings. These currents set up a changing magnetic field (flux pattern), which rotates around the inside of the stator. The speed of rotation is in synchronism with the electric power frequency and is called the synchronous speed.
In the simplest type of 3-phase induction motor, the rotating field is produced by 3 fixed stator windings, spaced 120 apart around the perimeter of the stator. When the three stator windings are connected to the 3-phases power supply, the flux completes one rotation for every cycle of the supply voltage. On a 50 Hz power supply, the stator flux rotates at a speed of 50 revolutions per second, or 50 x 60 = 3000 rev per minute.
Basic (simplified) principle of a 2 pole motor
A motor with only one set of stator electrical windings per phase, as described above, is called a 2 pole motor (2p) because the rotating magnetic field comprises 2 rotating poles, one North-pole and one South-pole. In some countries, motors with 2 rotating poles are also sometimes called a 1 pole-pair motor.
If there was a permanent magnet inside the rotor, it would follow in synchronism with the rotating magnetic field. The rotor magnetic field interacts with the rotating stator flux to produce a rotational force. A permanent magnet is only being mentioned because the principle of operation is easy to understand. The magnetic field in a normal induction motor is induced across the rotor air-gap as described below.
If the three windings of the stator were re-arranged to fit into half of the stator slots, there would be space for another 3 windings in the other half of the stator. The resulting rotating magnetic field would then have 4 poles (two North and two South), called a 4 pole motor. Since the rotating field only passes 3 stator windings for each power supply cycle, it will rotate at half the speed of the above example, 1500 rev/min.
Consequently, induction motors can be designed and manufactured with the number of stator windings to suit the base speed required for different applications:
- 2 pole motors, stator flux rotates at 3000 rev/min
- 4 pole motors, stator flux rotates at 1500 rev/min
- 6 pole motors, stator flux rotates at 1000 rev/min
- 8 pole motors, stator flux rotates at 750 rev/min
- etc
Flux distribution in a 4 pole machine at any one moment
The speed at which the stator flux rotates is called the synchronous speed and, as shown above, depends on the number of poles of the motor and the power supply frequency.
Where: no = Synchronous rotational speed in rev/min
f = Power supply frequency in Hz
p = Number of motor poles
To establish a current flow in the rotor, there must first be a voltage present across the rotor bars. This voltage is supplied by the magnetic field created by the stator current. The rotating stator magnetic flux, which rotates at synchronous speed, passes from the stator iron path, across the air-gap between the stator and rotor and penetrates the rotor iron path as shown in Figure 2.4. As the magnetic field rotates, the lines of flux cut across the rotor conductors. In accordance with Faraday’s Law, this induces a voltage in the rotor windings, which is dependent on the rate of change of flux.
Since the rotor bars are short circuited by the end-rings, current flows in these bars will set up its own magnetic field. This field interacts with the rotating stator flux to produce the rotational force. In accordance with Lenz’s Law, the direction of the force is that which tends to reduce the changes in flux field, which means that the rotor will accelerate to follow the direction of the rotating flux.
At starting, while the rotor is stationary, the magnetic flux cuts the rotor at synchronous speed and induces the highest rotor voltage and, consequently, the highest rotor current. Once the rotor starts to accelerate in the direction of the rotating field, the rate at which the magnetic flux cuts the rotor windings reduces and the induced rotor voltage decreases proportionately. The frequency of the rotor voltage and current also reduces.
When the speed of the rotor approaches synchronous speed at no load, both the magnitude and frequency of the rotor voltage becomes small. If the rotor reached synchronous speed, the rotor windings would be moving at the same speed as the rotating flux, and the induced voltage (and current) in the rotor would be zero. Without rotor current, there would be no rotor field and consequently no rotor torque. To produce torque, the rotor must rotate at a speed slower (or faster) than the synchronous speed.
Consequently, the rotor settles at a speed slightly less than the rotating flux, which provides enough torque to overcome bearing friction and windage. The actual speed of the rotor is called the slip speed and the difference in speed is called the slip. Consequently, induction motors are often referred to as asynchronous motors because the rotor speed is not quite in synchronism with the rotating stator flux. The amount of slip is determined by the load torque, which is the torque required to turn the rotor shaft.
For example, on a 4 pole motor, with the rotor running at 1490 r/min on no-load, the rotor frequency is 10/1500 of 50 Hz and the induced voltage is approximately 10/1500 of its value at starting. At no-load, the rotor torque associated with this voltage is required to overcome the frictional and windage losses of the motor.
As shaft load torque increases, the slip increases and more flux lines cut the rotor windings, which in turn increases rotor current, which increases the rotor magnetic field and consequently the rotor torque. Typically, the slip varies between about 1% of synchronous speed at no-load to about 6% of synchronous speed at full-load.
and actual rotational speed is
Where no = Synchronous rotational speed in rev/min
n = Actual rotational speed in rev/min
s = Slip in per-unit
The direction of the rotating stator flux depends on the phase sequence of the power supply connected to the stator windings. The phase sequence is the sequence in which the voltage in the 3-phases rises and reaches a peak. Usually the phase sequence is designated A-B-C, L1-L2-L3 or R-W-B (Red-White-Blue). In Europe this is often designated as U-V-W and many IEC style motors use this terminal designation. If two supply connections are changed, the phase sequence A-C-B would result in a reversal of the direction of the rotating stator flux and the direction of the rotor.