Motor Inductions

8/7/2019 Motor Inductions

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A HISTORICAL TOUCH

Faraday discovered the electromagnetic induction law around 1831 and Maxwell formulated the

laws of electricity (or Maxwells equations) around 1860. The knowledge was ripe for the

invention of the induction machine which has two fathers: Galileo Ferraris (1885) and Nicola

Tesla (1886). Their induction machines are shown in Figure 1.1 and Figure 1.2.

Both motors have been supplied from a two-phase a.c. power source and thus contained two

phase concentrated coil windings 1-1 and 2-2 on the ferromagnetic stator core.In Ferraris patent the rotor was made of a copper cylinder, while in the Teslas patent the rotor

was made of a ferromagnetic cylinder provided with a short-circuited winding.That is, amultiphase a.c. stator winding produces a traveling field which induces voltages that produce

currents in the short-circuited (or closed) windings of the rotor. The interaction between thestator produced field and the rotor induced currents produces torque and thus operates the

induction motor.As the torque at zero rotor speed is nonzero, the induction motor is self-starting.The three-phase a.c. power grid capable of delivering energy at a distance to induction motors

and other consumers has been put forward by Dolivo- Dobrovolsky around 1880.In 1889, Dolivo-Dobrovolsky invented the induction motor with the wound rotor and

subsequently the cage rotor in a topology very similar to that used today. He also invented thedouble-cage rotor. Thus, around 1900 the induction motor was ready for wide industrial use.

No wonder that before 1910, in Europe, locomotives provided with induction motor propulsion,were capable of delivering 200 km/h.However, at least for transportation, the d.c. motor took

over all markets until around 1985 when the IGBT PWM inverter was provided for efficientfrequency changers. This promoted the induction motor spectacular comeback in variable speed

drives with applications in all industries.

Mainly due to power electronics and digital control, the induction motor may add to its oldnickname of the workhorse of industry the label of the racehorse of high-tech.


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INDUCTION MACHINES IN APPLICATIONSInduction motors are, in general, supplied from single-phase or three-phase a.c. power grids.

Single-phase supply motors, which have two phase stator windings to provide selfstarting, areused mainly for home applications (fans, washing machines, etc.): 2.2 to 3 kW. A typical

contemporary single-phase induction motor with dual (start and run) capacitor in the auxiliaryphase is shown in Figure 1.4. Three-phase induction motors are sometimes built with aluminum

frames for general purpose applications below 55 kW (Figure 1.5).


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Ideal multiphase mmfsa.) two-phase machine b.) three-phase machine

Lap a.). and wave b.) single-turn (bar) coils


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Single-layer a.) and double-layer b.) coils (windings)

Single-layer three-phase winding for 2p1 = 4 poles and q = 1 slots/pole/phase:

a.)slot/phase allocation;b.), c.), d.) ideal mmf distribution for the three phases when their currents are maximum;

e.) star series connection of coils/phase; f.) parallel connection of coils/phase


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2/4 pole winding (Ns = 24)a.) emf star, b.) slot/phase allocation, c.) coils of phase A, d.), e.) mmf for 2p2 = 4 and 2p1 = 2


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.


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Equivalent Circuit of 3-Phase Induction Motor at Any Slip


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But the rotor is revolving at a speed of N relative to the stator core. Therefore, the

speed of rotor field relative to stator core

Equivalent Circuit of the RotorWe shall now see how mechanical load of the motor is replaced by the equivalentelectrical load. shows the equivalent circuit per phase of the rotor at slip s. The

rotor phase current is given by;


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Transformer Equivalent Circuit of Induction Motor


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Power Relations


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Starting of 3-Phase Induction Motors


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This method also aims at connecting the induction motor to a reduced supply at

starting and then connecting it to the full voltage as the motor picks up sufficient

speed. Fig shows the circuit arrangement for autotransformer starting.

The tapping on the autotransformer is so set that when it is in the circuit, 65% to80% of line voltage is applied to the motor.

At the instant of starting, the change-over switch is thrown to start position.

This puts the autotransformer in the circuit and thus reduced voltage is applied

to the circuit. Consequently, starting current is limited to safe value. When the

motor attains about 80% of normal speed, the changeover switch is thrown torun

position. This takes out the autotransformer from the circuit and puts the

motor to full line voltage. Autotransformer starting has several advantages viz

low power loss, low starting current and less radiated heat. For large machines

(over 25 H.P.), this method of starting is often used. This method can be usedfor both star and delta connected motors.


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outer winding has relatively open slots and a poorer flux path around itsbars, it has a low inductance. Thus the resistance of the outer squirrel-cage

winding is high and its inductance is low.

(ii) The inner winding consists of bars of greater cross-section short-circuited

by end rings. Therefore, the resistance of this winding is low. Since the

bars of the inner winding are thoroughly buried in iron, it has a high


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inductance . Thus the resistance of the inner squirrelcage

winding is low and its inductance is high.

Dual cage rotor induction motors for large starting torque (source: ABB)


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Figure 1.9 Dual stator winding induction generator for wind turbines (source: ABB)


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Motor Inductions