The 3-phase input power fed to the stator of a 3-phase induction motor is given by, $$\mathrm{𝑃_{𝑖𝑠} = \sqrt{3} 𝑉_𝐿𝐼_𝐿 cos \varphi_i = 3 𝑉_{𝑠𝑝ℎ} 𝐼_{𝑠𝑝ℎ} cos \varphi_i}$$Where, VL = Line voltageIL = Line currentVsph = Stator phase voltageIsph = Stator phase currentcos \varphii = input power factorStator LossesStator copper losses or I2R losses in the stator winding resistances, which are given as follows −$$\mathrm{𝑃_{𝑠𝑐𝑢} = 3 𝐼_{𝑠𝑝ℎ}^{2} 𝑅_{𝑠𝑝ℎ}}$$Hysteresis and eddy current losses in the stator core, which are known as stator-core losses and are given by −$$\mathrm{𝑃_{𝑠𝐶} = 𝑃_{𝑠ℎ} + 𝑃_{𝑠𝑒}}$$Therefore, the power output of the stator will be, ... Read More

The rotor speed (Nr) of an induction motor is given by, $$\mathrm{𝑁_𝑟 = (1 − 𝑠)𝑁_𝑠}$$And the synchronous speed is given by, $$\mathrm{𝑁_𝑠 =\frac{120𝑓}{𝑃}}$$Therefore, $$\mathrm{𝑁_𝑟 = (1 − 𝑠) (\frac{120𝑓}{𝑃}) … (1)}$$It is clear from eqn. (1) that the speed of the induction motor can be changed by varying the frequency (f), number of poles (P) or slip (s).Speed Control of Induction Motor by Pole Changing MethodBy changing the stator poles, the speed of the induction motor can be changed. The number of stator poles can be changed by, Multiple Stator WindingsMethod of consequent poles, andPole-Amplitude Modulation (PAM)The pole changing ... Read More

Theory and Circuit DiagramThe no-load test or open-circuit test of an induction motor enables us to determine the efficiency and the circuit parameters of the equivalent circuit of the 3-phase induction motor. The circuit arrangement for the no-load test of the induction motor is shown in the figure.In the no-load test, the motor is uncoupled from the load and the rated voltage at the rated frequency is applied to the stator of the motor to run it. To measure the input power, two-wattmeter method is used.A voltmeter and an ammeter being connected as shown in the circuit diagram (see the ... Read More

An induction machine can also work as a generator without any external supply system. It's called an isolated induction generator. To provide the necessary excitation in the machine, a delta connected capacitor bank is connected across the terminals of it (see the figure).The presence of residual magnetic flux is necessary to provide the initial excitation in the machine. If there is no residual magnetic flux, then, initially, the machine has to run as an induction motor to create residual magnetic flux.At no load, the induction motor is run slightly above the synchronous speed with the help of a prime mover. ... Read More

In a conventional 3-phase induction motor, a balanced 3-phase supply is fed to the stator of the motor from the supply mains. Whereas, in an inverted or rotor fed induction motor, the rotor has 3-phase windings and being fed with the 3-phase balanced supply from the AC mains. The rotor windings of the inverted induction motor must be in star configuration.Construction of Inverted Induction MotorThe figure shows the block diagram of a typical inverted or rotor fed induction motor.The stator of the inverted or rotor fed induction motor has three phase windings which are short-circuited. The rotor also has 3-phase ... Read More

The speed of an induction motor can also be changed with the help of pole-amplitude modulation (PAM) technique. The pole amplitude modulation technique is a flexible method of pole changing which is used in the applications where the speed ratios are other than 2:1. The induction motors designed for the speed control based on pole amplitude modulation technique are called as PAM motors.To understand the pole amplitude modulation technique, consider the MMF distribution in the air gap of a 3-phase induction motor written as follows$$\mathrm{𝐹_𝑅 = 𝐹_{𝑚𝑅} \:sin \:𝑝 \theta … (1)}$$$$\mathrm{𝐹_𝑌 = 𝐹_{𝑚𝑌} \:sin (𝑝 \theta −\frac{2\pi}{3}) … (2)}$$$$\mathrm{𝐹_𝐵 ... Read More

Sometimes, a 3-phase induction machine is used as a generator. Initially, the induction machine is started as a motor and it draws lagging reactive power from the supply mains. Then, the speed of the machine is increased above the synchronous speed by an external prime mover in the same direction as the rotating magnetic field of the stator so that the induction machine will now operate as an induction generator and will produce a generating torque.The direction of this generating torque is opposite to the rotation of the rotor. Hence, under these conditions, the slip in the machine is negative ... Read More

Synchronous SpeedIn an induction motor, the speed at which the rotating magnetic field (RMF) rotates is known as synchronous speed (NS).The value of the synchronous speed depends upon the number of stator poles (P) in the motor and the supply frequency (f). Therefore, for a given motor of P-poles, the synchronous speed is, $$\mathrm{Synchronous \:speed, 𝑁_𝑆 =\frac{120𝑓}{𝑃}RPM}$$Slip in Induction MotorAn induction motor cannot run at synchronous speed. If it runs at synchronous speed, there would be no cutting of the flux by the rotor conductors and there would be no induced EMF, no current and no torque. Therefore, the rotor ... Read More

The primary disadvantage of a conventional squirrel cage induction motor is the low starting torque and high starting current because of low rotor resistance.The starting torque of the motor can be increased by using rotor bar material of high resistivity. The high rotor resistance gives a higher starting torque and lower starting current at a higher power factor. Although the higher rotor resistance decreases the full-load speed, it increases the rotor I2R losses, which results in lower efficiency of the motor. Therefore, a low rotor resistance is required for normal operation of the motor so that the slip is low ... Read More

Consider three identical coils located on axes physically at 120° in space and each coil is supplied from one phase of a balanced 3-phase supply. Thus, each coil will produce an alternating flux along its own axis. Let the instantaneous values of the fluxes are given by, $$\mathrm{\varphi_{1} = \varphi_{m} sin \omega t … (1)}$$$$\mathrm{\varphi_{2} = \varphi_{m} sin(\omega t − 120°) … (2)}$$$$\mathrm{\varphi_{3} = \varphi_{m} sin(\omega t + 120°) … (3)}$$The waveforms of the fluxes produced by the three coils are shown in Figure-1. The resultant flux (ϕr) at any instant is equal to the phasor sum of the fluxes ... Read More