We know that induction motor cannot run at synchronous speed, its speed is always less than that of synchronous speed. The difference between the synchronous speed Ns and actual rotor speed Nr is called slip.
Fractional Slip (s) = ( Ns – Nr ) / Ns
The rotor speed can be written as Nr = (1-s) Ns
It is also known that Ns = 120f /P where f and P is the supply frequency and number of poles respectively. Therefore, Nr = 120f /P (1-s) ……………… (i)
From the above equation (i) it is observed that there are three factors which controls the speed of induction motor. These factors are: Supply frequency f, Number of poles P, Slip s. Methods of speed control are distinguished according to the main action on the motor – from the stator side and from the rotor side.
Various methods of speed control from stator side are:
a) Variation of supply frequency
b) Variation of applied voltage
c) Changing the number of poles
From the rotor side the speed may be controlled in the following ways:
a) Changing resistance in the rotor circuit and
b) Introducing additional emf ( same frequency as fundamental emf ) into rotor circuit.
Now we will go through a small description of each of the speed control methods of an induction motor. The methods are:
Speed control by variation of supply frequency
This method provides wide range of speed control with gradual variation of speed throughout this range. The major difficulty with this method is how to get the variable frequency supply. The auxillary equipment required for this purpose results in a high cost, increased maintenance and lowering of overall efficiency. That is why this method is not generally used but there are certain applications where this method is very suitable. If an induction motor is to be operated at different frequencies (with constant values of p.f, efficiency, slip, overload capacity) then its supply voltage V can be varied with the frequency change according to the following equation: V/ / V = f/ / f √ T/ / T
For constant torque T/ = T we have,
V/ / V = f/ / f i.e voltage applied to the stator must vary in proportion
to the frequency. The synchronous speed of an induction motor is given by Ns = 120f /P. The synchronous speed and therefore the speed of the motor can be controlled by varying supply frequency. The emf induced in the stator of the induction motor is given by E1 = 4.44 k f φ T1. From the emf equation it can be understood that a change of frequency will result in a change of flux level unless the induced emf is changed in the same ratio. An imbalance will result in an excessive flux and saturation or reduced flux and reduced torque per ampere of current. Excessive flux will cause increase in iron losses. In order to avoid saturation and to minimize losses, motor is operated at rated air gap flux by varying terminal voltage with frequency so as to maintain constant (V/f) ratio. This control method is known as constant volts per hertz. The variable frequency supply is obtained by the following devices :-
- Inverter (converts fixed voltage dc to fixed/variable voltage ac of variable frequency) – voltage source inverter (VSI) and current source inverter (CSI).
- Cycloconverter (converts fixed voltage, frequency ac to variable voltage and frequency ac)
Speed control by variation of supply /stator voltage
By varying the supply voltage (V) i,e the voltage supplied to the stator we can vary the speed of an induction motor. This is a slip control method with constant frequency variable supply voltage. From the torque eq. we come to know that the torque developed by the induction motor is proportional to the square of the supply voltage. It is also known that slip at maximum torque is independent of supply voltage. Here speed control is obtained by varying supply voltage until the torque required by the load is developed at desired speed. This method of speed control is suitable where load torque decreases with speed eg: fan load.
We have already discussed earlier that torque developed by the induction motor is proportional to the square of the supply voltage and current proportional to voltage. So when we reduce the voltage to reduce speed of the motor for same current value, the torque definitely reduces. The stator voltage control is most suitable where intermittent drive operation is required. This method is most used for fan / pump loads.
- Speed control in single ph IM (domestic fan) is obtained by triac controller. Speed control is done by varying firing angle of the triac.
- Speed control of three ph IM is done by connecting three pairs of back to back connected thyristors. Speed control is obtained by varying the conduction period of thyristors.
- For speed control of small ac motors (single phase supply) we use two thyristors (back to back) in anti-parallel.
Speed control by changing number of poles
This method of speed control is applicable for squirrel cage motors but not for wound motors. It is suitable for cage motors because it automatically develops number of poles equal to the poles of stator windings. The number of stator poles can be changed by the following ways- a) Using multiple stator poles, b) method of consequent poles and c) pole amplitude modulation (PAM).
In multiple stator pole method, the stator is provided with two separate independent windings, each wound for different number of poles, placed in same stator slots. One winding is energized at a time. For example a motor has two windings for 6 and 4 poles which will give synchronous speeds of 1000 rpm and 1500 rpm with a supply frequency of 50 Hz. If full load slip is 5% then the operating speed will be 950 rpm and 1425 rpm respectively. This method is more costly and efficiency is less.
Rotor resistance control
As the name implies this method is applicable or only wound rotor induction motor and it is not applicable for squirrel cage motors. By connecting external resistances in the rotor circuit through slip rings, this control method is achieved. Speed control is provided from rated speed to lower speed. This method of speed control is very simple. It is possible to have a huge starting torque, low starting current and large pull-out torque at slip value. Disadvantages of this method are as :
- Reduction in speed is done by reducing in efficiency
- External rotor resistors are bulky and expensive.
This method is used in fan or pump drives and also in cranes, Ward-Leonard Ilgner drives where load is intermittent.
Other speed control methods:
- Kramer system of speed control - This system is based on extraction of power from the rotor circuit of a polyphase wound rotor induction motor via slip rings. This power at slip frequency is supplied to a synchronous converter and the dc current is used to supply the armature of a separately excited dc motor coupled to the induction motor.
- Leblanc system of speed control - This system consists of a frequency convertor mechanically coupled to the shaft of the slip ring induction motor. The speed and p.f of which is controlled by injecting voltage at slip frequency of the rotor circuit.
- Scherbius system of speed control – Here the slip energy is not converted into dc and fed to a dc motor, rather it is fed directly to a special 3-ph commutator motor called a Scherbius machine.
Hope you have got an overall idea about the speed control of an induction motor.