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30-12-2010, 12:26 PM


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3-Phase Induction Motors Introduction
• 3-phase induction motors
are simple, low cost,
and easy to maintain.
• They run at essentially
constant speed from
zero-to-full load.
• Therefore, they are the
motors most frequently
encountered in industry.

Induction Motor Components
A 3-phase induction motor has two main parts:
• A stator – consisting of a steel frame that supports a hollow,
cylindrical core of stacked laminations. Slots on the internal.

• A rotor – also composed of punched laminations, with rotor slots for
the rotor winding.

Induction Motor Components
Squirrel cage rotor consists
of copper bars, slightly
longer than the rotor,
which are pushed into the
The ends are welded to
copper end rings, so that
all the bars are short
In small motors, the bars
and end-rings are diecast
in aluminium to form
an integral block.

Induction Motor Components
A wound rotor has a 3-phase winding, similar to the stator winding.
The rotor winding terminals are connected to three slip rings which turn with the rotor. The slip rings/brushes allow external resistors to be connected in series with the winding.
The external resistors are mainly used during start-up –
under normal running conditions the windings shortcircuited
Induction Motor: Operating principle

The following sequence of events takes place:

1. A voltage E = BLv is induced in each conductor while it is being cut by the Flux (Faraday’s Law)
2. The induced voltage produces currents which circulate in a loop around the conductors (through the bars).
3. Since the current-carrying conductors
lie in a magnetic field, they experience a mechanical force (Lorentz force).
4. The force always acts in a direction to
drag the conductor along with the
magnetic field.
Induction Motor – Rotating Field: Direction of rotation
The phase current waveforms
follow each other in the
sequence A-B-C.
This produces a clockwise rotating
magnetic field.
If we interchange any two of the
lines connected to the stator,
the new phase sequence will
be A-C-B.
This will produce a counter clockwise rotating field,
reversing the motor direction.

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26-07-2012, 03:58 PM

Electrical Machines

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Induction Machines


The induction machine was invented by NIKOLA TESLA in 1888. Right from its incep-
tion its ease of manufacture and its robustness have made it a very strong candidate for
electromechanical energy conversion. It is available from fractional horsepower ratings to
megawatt levels. It finds very wide usage in all various application areas. The induction
machine is an AC electromechanical energy conversion device. The machine interfaces with
the external world through two connections (ports) one mechanical and one electrical. The
mechanical port is in the form of a rotating shaft and the electrical port is in the form of
terminals where AC supply is connected. There are machines available to operate from three
phase or single phase electrical input. In this module we will be discussing the three phase
induction machine. Single phase machines are restricted to small power levels.
seminar flower
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17-09-2012, 04:54 PM

Electrical Machines

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Testing of Transformers

The structure of the circuit equivalent of a practical transformer is developed earlier.
The performance parameters of interest can be obtained by solving that circuit for any load
conditions. The equivalent circuit parameters are available to the designer of the transformers
from the various expressions that he uses for designing the transformers. But for a user
these are not available most of the times. Also when a transformer is rewound with different
primary and secondary windings the equivalent circuit also changes. In order to get the
equivalent circuit parameters test methods are heavily depended upon. From the analysis of
the equivalent circuit one can determine the electrical parameters. But if the temperature
rise of the transformer is required, then test method is the most dependable one. There are
several tests that can be done on the transformer; however a few common ones are discussed

Winding resistance test

This is nothing but the resistance measurement of the windings by applying a small
d.c voltage to the winding and measuring the current through the same. The ratio gives
the winding resistance, more commonly feasible with high voltage windings. For low voltage
windings a resistance-bridge method can be used. From the d.c resistance one can get the
a.c. resistance by applying skin effect corrections.

Polarity Test

This is needed for identifying the primary and secondary phasor polarities. It is
a must for poly phase connections. Both a.c. and d.c methods can be used for detecting
the polarities of the induced emfs. The dot method discussed earlier is used to indicate the
polarities. The transformer is connected to a low voltage a.c. source with the connections
made as shown in the fig. 18(a). A supply voltage Vs is applied to the primary and the
readings of the voltmeters V1, V2 and V3 are noted. V1 : V2 gives the turns ratio. If V3 reads
V1−V2 then assumed dot locations are correct (for the connection shown). The beginning and
end of the primary and secondary may then be marked by A1 −A2 and a1 −a2 respectively.
If the voltage rises from A1 to A2 in the primary, at any instant it does so from a1 to a2 in
the secondary. If more secondary terminals are present due to taps taken from the windings
they can be labeled as a3, a4, a5, a6. It is the voltage rising from smaller number towards
larger ones in each winding.

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