Found 1006 Articles for Electronics & Electrical

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

For a double cage induction motor, the suffixes "s" and "r" are used to denote stator and rotor respectively.Let, 𝑅𝑠 = Stator resistance per phase𝑋𝑠 = Stator reactance per phase𝑅′𝑟𝑜 = Rotor resistance per phase of outer cage referred to stator𝑋′𝑟𝑜 = Standstill leakage reactance per phase of outer cage referred to stator𝑅′𝑟𝑖 = Rotor resistance per phase of inner cage referred to stator𝑋′𝑟𝑖 = Standstill leakage reactance per phase of inner cage referred to stator𝑠 = Fractional slipIf it is considered that the main flux completely links both the cages, then the impedances of the two cages can be ... Read More

The operation of the induction motor is based on the principle of induction of voltages and currents in its rotor circuit from the stator circuit. The performance characteristics for steady-state conditions of the induction motor can be evaluated using the equivalent circuit of it. As the 3-phase induction motor represents a balanced load, therefore, the equivalent circuit of it is drawn only for one phase.Stator Circuit Model of Induction MotorThe stator circuit model or the stator equivalent circuit of the induction motor is shown in the figure.The stator circuit model of the induction motor consists of a stator phase winding ... Read More

Wiring Diagram and Working Principle of DOL StarterThe connection diagram of the Direct On Line (DOL) starter is shown in the figure. In the DOL method of starting a squirrel cage induction motor, the motor is connected to the full supply voltage through a starter.The direct-on line starter consists of a coil operated contactor C which is controlled by start (normally open) push button and stop (normally closed) push button.When the start push button (S1) is pressed, the contactor coil C is energies from two line conductors R and Y. The three main contacts (M) and the auxiliary contact (A) ... Read More

There are two types of 3-phase induction motors viz. squirrel-cage induction motor and slip-ring or wound-rotor induction motor.Squirrel cage induction motors are widely used in industrial applications because they are very cheap, robust, efficient and reliable.On the other hand, slip-ring or wound-rotor induction motors are rarely used in industries since these motors have several disadvantages such as high copper loss and frequent maintenance.Following are the key differences between slip ring and squirrel cage induction motors −The construction of a slip-ring induction motor is complicated because it consists of slip-rings and brushes while the squirrel cage induction motor has simple construction.The ... Read More

Let us first take a look at the construction diagrams of single-cage and double-cage induction motors −Following are the key differences between a single-cage induction motor and a double-cage induction motor of the same rating −A double-cage induction motor has high starting torque and low starting current as compared to a single-cage induction motor. Therefore, the double-cage motors are more suitable for direct on-line starting.A double-cage induction motor has high rotor resistance. Consequently, a large amount of heat is generated in the rotor circuit of a double-cage motor at the time of starting as compared to a single-cage motor.Due to ... Read More