There are a few ways of increasing the rpm of the induction motor. Increasing supply voltage is the easiest way. However, it also increases losses. Though, increase supply voltage can be used in limited cases when rpm has to be increased to a certain level only.

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Increase rotor diameter is another method of increasing rpm. Similarly, it increases mechanical power output by increasing induction motor speed. But again, an increase in diameter can be used only in limited cases.

Increase rotor circuit resistance is the third method of increasing rpm. Increasing rotor circuit resistance results in an increase in torque at a lower speed. But it also limits mechanical power output to some extend by preventing the motor from overspeeding at higher speeds.

The fourth way of increasing the rpm of the induction motor is to use a variable-frequency supply. It is a more practical solution as compared to the first three methods because it has the capability of increasing rpm over the entire speed range. Variable frequency supply also allows faster acceleration and deceleration rates without affecting mechanical power output.

Now let us compare different methods of increasing induction motor rpm with simulation using MATLAB/SIMULINK software package.

Suppose a five-phase induction motor is to be controlled so as to have a maximum speed of 3500 rpm. A three-phase supply voltage at 1500 V, 50 Hz, and a rotor resistance of .8 ohms are considered for this simulation. The variable frequency supply used in control is assumed to be a constant power type with a range from zero to 100%.

In the first simulation, an increase of rotor resistance from 0.8 ohms to 1.6 ohms is proposed so as to have the desired speed of 3500 rpm with a fixed supply voltage of 1500 V and 50 Hz. In this case, overshooting and hunting of speed is observed around 3000 rpm region due to limited bandwidth of the supply.

In the second simulation, a variable power supply is used with fixed resistance of the rotor circuit (Rotor Resistance = 1.6 ohms). The speed can be continuously increased up to 4400 rpm without overshooting and hunting around the 3000 rpm region due to limited bandwidth. However, maximum mechanical power output can be obtained only at 3500 rpm.