N = Ns(1-s)
and Ns = 120f/P
This shows the speed of three phase induction motor depends upon synchronous speed (Ns) and slip(s). The synchronous speed of induction motor depends upon supply frequency and the number of stator poles.
So by changing the supply frequency, the number of stator poles and slip we can change the speed of 3 phase induction motor. Therefore, speed control of 3 phase induction motor can be achieved by following methods:
Stator frequency — 3 phase motor speed control methods
The synchronous speed can be changed by changing the stator supply frequency (f). But only change in supply frequency, keeping supply voltage constant has an adverse effect on the air gap flux. Because air gap flux is proportional to the ratio of supply voltage and supply frequency.
Hence the ratio of supply voltage and supply frequency is kept constant by changing the stator voltage and frequency simultaneously. This is necessary to keep the air gap flux constant. So this method is also known as constant (V/f) control.
In this 3 phase motor speed control method, the AC input of constant voltage and constant frequency is given to a rectifier which converts AC into DC. The output of the rectifier is filtered by a capacitor bank and given to an inverter circuit. The inverter converts the DC voltage into a 3 phase variable voltage variable frequency supply.
This supply is applied to the stator winding of the motor. Thus we get the constant V/f speed control of induction motor.
In this method, air gap flux remains constant. Therefore smooth speed control can be obtained. By this method, it is possible to get maximum torque at all speeds.
Because of large capital outlay, this system is justified only for drives wherein the rugged, maintenance-free characteristics of the induction motor are essential. Otherwise, a DC motor with converter control is the logical and cheap alternative.
Stator voltage — 3 phase motor speed control methods
In this 3 phase motor speed control method, only stator voltage is varied to vary the speed of the motor, the stator frequency is kept constant. We can vary the motor speed by varying the stator voltage. With the increase in stator voltage, the motor speed will increase.
In this speed control method of 3 phase induction motor when the stator voltage is reduced, the air gap flux and the motor torque both will reduce. The speed control is obtained at the cost of reduction of motor torque.
In this method, very low starting torque is produced. Therefore, this type of control is not suitable for constant torque loads. This type of control is preferred in the applications such as fans, centrifugal pumps and blowers where low starting torque is required.
This method is only applicable for small motors and for fan type loads where the load torque increases with the speed. The motor tends to get overheated with other loads. It is a commonly used method for ceiling fans driven by single-phase induction motors which have large standstill impedance limiting the current drawn by the motor.
Stator pole — 3 phase motor speed control methods
By means of a suitable switch, the stator connection may be changed in such a manner that the number of stator poles is changed. This changes the actual speed of motor since the actual speed of the motor is approximately inversely proportional to the number of poles.
By suitable connections, one winding can give two different speeds. If more than two speeds are required, two separate windings are placed in the same slot. Now each winding can give two speeds and two windings can give four different speeds. In a slpring motor, it is necessary to change the rotor poles accordingly. Therefore it is difficult to apply this method to a slipring motor.
Rotor resistance control
As the name indicates, this 3 phase motor speed control method is only applicable to the slip-ring induction motor.
As we know that sub-synchronous speed (speeds below synchronous speed) control of the slip-ring induction is possible by introducing a variable resistance in the rotor circuit of the motor. This method is based upon the general principle that the induction motor slip increases (i.e. speed decreases) as more power loss occurs on the rotor circuit.
This method provides a wide range of speed and good starting torque. The maximum torque in this case remains constant. Good regulation of speed can be achieved for both constant-torque and fan-type loads.
The conventional method of rotor resistance control demands simultaneous and precise variation of all the three balanced resistors in each phase. Often this is difficult to achieve.
To overcome this problem, a high-frequency thyristor chopper which enables external resistance to be varied simultaneously and sleeplessly is employed.
Figure shows a circuit in which the rotor slip power is rectified in a three-phase bridge rectifier and fed through a filtering choke to an external resistance.
The thyristor (shown by the switch symbol in Figure) in the chopper connected across the resistor is switched on and off at a high frequency. The ratio of on-time to off-time determines the effective value of rotor circuit resistance and thus controls the motor speed by changing its speed-torque characteristics.
The greatest drawback of this method is its poor efficiency due to power wasted in the external rotor resistance. Therefore, this method is adopted for a narrow speed range and usually for a short time duration.
Rather than wasting the slip energy in external rotor resistance, it can be feedback to supply to increase the efficiency of this speed control scheme. This is achieved by using a converter and inverter in the rotor circuit as shown in Figure.
By feeding the electric power into the rotor circuit (negative rotor power loss), super synchronous speed operation (i.e. speeds above synchronous speed) becomes possible.
Speed Control of Induction Motor by Cascade Method
In this method, two machines are mechanically coupled. Mostly both motors are of the slipring type. In this case, supply is connected to the stator of one of the induction motors and induced EMF of the rotor is fed to the stator of another motor.
If P1 and P2 are the numbers of poles of the two machines and f is the supply frequency, then the set can give following different speeds:
- When machine 1 works alone,
the synchronous speed = 120f/P1
- When machine 2 works alone,
the synchronous speed = 120f/P2
- When machine 1 and 2 is running so that the torque of the two motors is in the same direction,
the synchronous speed of the set = 120f/(P1 + P2).
- When machine 1 and 2 is running so that the torque of the two motors is in opposite direction,
the synchronous speed of the set = 120f/(P1 – P2).
Thanks for reading about 3 phase motor speed control methods.
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