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

Crawling in Induction Motor (or Effect of Harmonics on the Performance of 3-Phase Induction Motor)The flux in the air-gap of an induction motor set up by the 3-phase stator windings carrying sinusoidal currents is of non-sinusoidal wave shape. According to the Fourier series analysis, any non-sinusoidal flux is equivalent to the combination of a number of sinusoidal fluxes of fundamental and higher order harmonics.Since the wave shapes of the air-gap flux have half-wave symmetry, hence all the even harmonics (i.e., 2 nd, 4th, 6th, … etc.) are absent in the Fourier series. Thus, a non-sinusoidal flux wave can be resolved ... Read More

A 3-phase induction motor has two main parts − Stator Rotor The rotor and stator are separated by a small air gap ranges from 0.5 mm to 4 mm depending on the power rating of the motor. Stator of Three Phase Induction Motor The stator is the stationary part of the motor. It consists of a steel frame which encloses a hollow cylindrical core. The core of the three phase induction motor is made up of thin laminations of silicon steel to reduce the eddy current and hysteresis losses. A number of equally spaced ... Read More

Complete Equivalent Circuit of Induction MotorIn order to obtain the complete per-phase equivalent circuit of an induction motor, it is required to refer the rotor part to the stator side frequency and voltage level.The transformation of the rotor circuit of the induction motor can be done by the means of effective turns ratio of the induction motor.The figure shows the complete equivalent circuit of the induction motor.Let suffixes "s" and "r" be used for stator and rotor quantities respectively.Then, $$\mathrm{𝑎_{𝑒𝑓𝑓}= Effective\:turns\: ratio \:of \:induction \:motor}$$$$\mathrm{𝑅′_{𝑟} = Resistance \:of \:the \:rotor\: winding\: per\: phase\: referred \:to \:stator}$$$$\mathrm{𝑋′_{𝑟0} = Standstill \:rotor \:reactance ... Read More

Circle Diagram of Induction MotorThe circle diagram is a graphical representation of the performances of an induction motor. It is very useful to study the performance of an induction motor under all operating conditions. The construction of the circle diagram of an induction motor is based on the equivalent circuit of the motor which is shown in Figure-1.By applying KCL in the equivalent circuit, we can write, $$\mathrm{𝐼_1 = 𝐼_0 + 𝐼′_2 … (1)}$$If the phase voltage V1 is taken along the vertical axis of OY as shown in Figure-2. Then, the no-load current (I0) lags behind the voltage V1 ... Read More

The figure shows the circuit diagram for the blocked rotor test of an induction motor. The blocked rotor test enables us to determine the efficiency and the circuit parameters of the equivalent circuit of a 3-phase induction motor.In the blocked rotor test, the shaft of the motor is locked so that it cannot rotate and the rotor winding is short circuited. In a slip-ring induction motor, the rotor winding is short-circuited through the slip-rings while in a squirrel cage induction motor, the rotor bars are permanently short-circuited with the help of end rings.In the blocked rotor test, a reduced voltage ... Read More