ESO 210

Introduction to Electrical Engineering Lectures-37

Polyphase (3-phase) Induction Motor

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Determination of Induction Machine Parameters

Three tests are needed to determine the parameters in an induction machine model. Detailed testing is quite involved and is specified in IEEE Standard 112.

1. DC Resistance Test 2. No-Load Test (Corresponds to Open circuit test in Transformer) 3. Block Rotor Test (Corresponds to Short circuit test in Transformer)

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V0

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Problem: A 4ooV, 6pole, 3-phase, 50 Hz star connected induction motor running at rated voltage takes 7.5 Amp with a power input of 700Watts. With the rotor blocked and 150 V applied to the stator the input current is 35Amp and The power input is 4000Watts the stator and the rotor copper losses are equal under this condition. The standstill leakage reactance of the stator and the rotor as seen from the stator are estimated to be in the ratio of 1: 0.5 Compute the parameters of equivalent circuit of Induction Motor.

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X= 𝑍2 − 𝑅2

Given Stator and rotor copper losses are equal here So 𝑟1 + 𝑟′2 = 1.09 𝑂ℎ𝑚

Given Stator leakage reactance is twice of the rotor leakage reactance

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Starting methods for induction motors Squirrel-cage rotor:

(i) Direct-on-line starting (DOL) With this method, starting current is high and may cause interference with supplies to other consumers.

(ii) Auto transformer starting With this method, an auto transformer is used to reduce the stator voltage, E1, and thus the starting current. However, the starting torque is seriously reduced so the voltage is reduced only sufficiently to give the required reduction of the starting current. A typical arrangement is shown in Figure below. A double-throw switch connects the auto transformer in circuit for starting, and when the motor is up to speed the switch is moved to the run position which connects the supply directly to the motor.

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(iii) Star-delta starting With this method, for starting, the connections to the stator phase winding are star-connected, so that the voltage across each phase winding is 1/√3 (i.e. 0.577) of the line voltage. For running, the windings are switched to delta-connection. A typical arrangement is shown in Figure below. This method of starting is less expensive than by auto transformer.

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Wound rotor When starting on load is necessary, a wound rotor induction motor must be used. This is because maximum torque at starting can be obtained by adding external resistance to the rotor circuit via slip rings. A face-plate type starter is used, and as the resistance is gradually reduced, the machine characteristics at each stage will be similar to Q, S, R and P. At each resistance step, the motor operation will transfer from one characteristic to the next so that the overall starting characteristic will be as shown by the bold line in Figure below. For very large induction motors, very gradual and smooth starting is achieved by a liquid type resistance.

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Advantages of squirrel-cage induction motors

The advantages of squirrel-cage motors compared with the wound rotor type are that they: (i) are cheaper and more robust

(ii) have slightly higher efficiency and power factor

(iii) are explosion-proof, since the risk of sparking is eliminated by the

(iv) absence of slip rings and brushes.

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Advantages of wound rotor induction motor

The advantages of the wound rotor motor compared with the cage type are that they: (i) have a much higher starting torque

(ii) have a much lower starting current (iii) have a means of varying speed by use of external rotor resistance.

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The advantages of squirrel-cage and wound rotor induction motors are combined in the double cage induction motor. This type of induction motor is specially constructed with the rotor having two cages, one inside the other. The outer cage has high resistance conductors so that maximum torque is achieved at or near starting. The inner cage has normal low resistance copper conductors but high reactance since it is embedded deep in the iron core. The torque-speed characteristic of the inner cage is that of a normal induction motor, as shown in Figure below. At starting, the outer cage produces the torque, but when running the inner cage produces the torque. The combined characteristic of inner and outer cages is shown in Figure below. The double cage induction motor is highly efficient when running.

Double cage induction motor

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Single-phase Induction Motor

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The winding used normally in the stator of the single-phase induction motor (IM) is a distributed one. The rotor is of squirrel cage type, which is a cheap one, as the rating of this type of motor is low, unlike that for a three-phase IM. As the stator winding is fed from a single-phase supply, the flux in the air gap is alternating only, not a synchronously rotating one produced by a poly-phase (may be two- or three-) winding in the stator of IM. This type of alternating field cannot produce a torque ( (T0)st=0.0), if the rotor is stationery ( ωr=0.0 ). So, a single-phase IM is not self-starting, unlike a three-phase one. However, as shown later, if the rotor is initially given some torque in either direction (ωr≠0.0), then immediately a torque is produced in the motor. The motor then accelerates to its final speed, which is lower than its synchronous speed. This is now explained using double field revolving theory.

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A typical shaded-pole motor with a cage rotor is shown in Fig. above. This is a single- phase induction motor, with main winding in the stator. A small portion of each pole is covered with a short-circuited, single-turn copper coil called the shading coil. The sinusoidally varying flux created by ac (single-phase) excitation of the main winding induces emf in the shading coil. As a result, induced currents flow in the shading coil producing their own flux in the shaded portion of the pole.

The reversal of the direction of rotation, where desired, can be achieved by providing two shading coils, one on each end of every pole, and by open-circuiting one set of shading coils and by short-circuiting the other set.

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Force of attraction between stator poles and rotor poles - resulting in production of torque in clockwise direction

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Methods of starting synchronous motor

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A phasor diagram shown in previous slide, illustrates the method of determining the counter EMF which is obtained from the phasor equation;

The phase angle δ between the terminal voltage VT and the excitation voltage Ef in is usually termed the torque angle. The torque angle is also called the load angle or power angle.

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Home Assignmen-3

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Last date April 21