Page 115 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

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Ze = √3x7.5 = 4.1Ω

Xc = √Ze -Re = √4.1 -2.5 = 3.4Ω
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Full load copper loss = 3xI x Re =3*7.52*2.5 = 421.875 watts
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Speed torque characteristics of induction motor
For a constant torque, the slip is directly proportional to the rotor resistance. If the resistance in the rotor circuit is
increased, the slip is increased and the speed of the rotor is decreased. This method is applicable only to wound
rotor induction motor.
From the figure 1 it is clear how does the speed vary with the change of rotor resistance.
At the normal operating condition, the slip increases with increasing torque hence they are obeying a linear
characteristic. For a fixed load curve, the speed is downward from n to n . From this figure we can also obtain the
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maximum torque at the starting at the resistance r ’’. So, this method has an advantage of achieving maximum
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torque at the starting period.
This type of speed control is used when the intermittent speed control is required. It has some drawbacks
1 The rheostat which is used to vary the resistance per phase causes unbalancing in rotor.
2 The resistances generate huge losses and generate heat in the system.
3 In case of a large machine the size of the rheostat will be large and in such case it is not easily portable.
4 It requires more maintenance, hence the cost associated with it is more.

5 This method cannot be used for industrial automation purpose since we have to change manually the value of
the resistance.
Crawling of induction motor

Squirrel-cage motors sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their
synchronous speed Ns. This phenomenon is known as crawling of an induction motor. It is produced by the
7th harmonics in the windings.By proper design to reduce harmonics, the crawling can be reduced. When a
sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage
and impedance and follows the envelope of the voltage waveform. These loads are referred to as linear loads
Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous
motors. Some loads cause the current to vary disproportionately with the voltage during each half cycle. These
loads are classified as nonlinear loads, and the current and voltage have waveforms that are no sinusoidal,
containing distortions, whereby the 50-Hz waveform has numerous additional waveforms superimposed upon it,
creating multiple frequencies within the normal 50-Hz sine wave. The multiple frequencies are harmonics of the
fundamental frequency. Application of non-sinusoidal voltages to motors results in harmonic current circulation in
the windings of motors. Irms = √[(I ) + (I ) + (I ) + …], where the subscripts 1, 2, 3, etc. represent the different
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harmonic currents. The harmonics are grouped into positive (+), negative (-) and zero (0) sequence components.
Positive sequence harmonics (harmonic numbers 1, 4, 7, 10, 13, etc.) produce magnetic fields and currents
rotating in the same direction as the fundamental frequency harmonic. Negative sequence harmonics (harmonic
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CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75