Transformer & Induction M/C
:
10EE46
1.
Why starter are required for 3 phase induction motor ? (June/July2014)
The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is maximum (Q s = 1). Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current (4 to 10 times the full-load current) in the stator at low power factor and consequently the value of starting torque is low.
Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally. However, this large starting current will produce large linevoltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.
2.
Write a short note on : Star-delta starter for three phase IM (Dec 2012)
Star-Delta Starter
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Transformer & Induction M/C 10EE46
It is cheaper as compared to A. T. starter. This method of starting is used for motors designed to operate normally in delta. The six terminals from the three phases of the stator must be available : a, A : Terminals of phase A b, B : Terminals of phase B c, C : Terminals of phase C
Commercially, the terminals are marked A 1, A 2; B 1, B 2 and C 1, C 2 respectively. The motor is started with TPDT switch in position 1 and subsequently switched to position 2.
Position 1 : Starting-windings connected in Y
Position 2 : Running-windings get connected in D
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Transformer & Induction M/C 10EE46
Fig: Star-Delta Starter
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Transformer & Induction M/C 10EE46
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Transformer & Induction M/C 10EE46
3.
With neat sketches explain the construction, working and applications of i) Split phase and ii) Capacitor start single phase induction motors. (June/July 2013) ( Dec 2012)
(a) Split-phase induction motor. The stator of a split phase induction motor has two windings, the main winding and the auxiliary winding. These windings are displaced in space by 90 electrical degrees as shown in Fig. 9.5 (a). The auxiliary winding is made of thin wire (super enamel copper wire) so that it has a high R/X ratio as compared to the main winding which has thick super enamel copper wire. Since the two windings are connected across the supply the current I m and I a
in the main winding and auxiliary winding lag behind the supply voltage V, I a
leading the current I m
Fig. 9.5(b). This means the current through auxiliary winding reaches maximum value first and the mmf or flux due to I a
lies along the axis of the auxiliary winding and after some time
(t = θ/w) the current I m
reaches maximum value and the mmf or flux due to I m
lies along the main winding axis. Thus the motor becomes a 2-phase unbalanced motor. It is unbalanced since the two currents are not exactly 90 degrees apart. Because of these two fields a starting
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Transformer & Induction M/C 10EE46
Split phase induction motor (=) Connection
(>) Phasor diagram at starting (?) Typical torque-speed characteristic.
The capacitor start induction motor is also a split phase motor. The capacitor of suitable value is connected in series with the auxiliary coil through a switch such that I a
the current in the auxiliary coil leads the current I m
in the main coil by 90 electrical degrees in time phase so that the starting torque is maximum for certain values of I a
and I m
. This becomes a balanced 2-phase motor if the magnitude of I a
and I m
are equal and are displaced in time phase by 90° electrical degrees. Since the two windings are displaced in space by 90 electrical degrees as shown in Fig. 9.6 maximum torque is developed at start. However, the auxiliary winding and capacitor are disconnected after the motor has picked up 75 per cent of the synchronous speed. The motor will start without any humming noise. However, after the auxiliary winding is disconnected, there will be some humming noise.
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Transformer & Induction M/C 10EE46
4.
Explain why single phase induction motor is not self starting. Describe anyone method of starting of a single phase induction motor. (Dec/Jan 2015)
In this type of starter, (A. T.) it attains the reduced voltage by means of an auto transformer at the start. After a definite time interval (about 15 sec.), and after the motor accelerates, it is transferred from the reduced voltage to 133 the full voltage in the second step. A. T. are generally provided with voltage drops to give 40%, 60%, 75% and 100% line voltage. The starting current and starting torque depends on the tapping selected. In the third step, the change-over switch may be hand operated or automatic through time relay which connects the motor finally to the line by changing over from position A to B .
The capacitor start induction motor is also a split phase motor. The capacitor of suitable value is connected in series with the auxiliary coil through a switch such that I a
the current in the auxiliary coil leads the current I m
in the main coil by 90 electrical degrees in time phase so that the starting torque is maximum for certain values of I a
and I m
. This becomes a balanced 2-phase motor if the magnitude of I a
and I m
are equal and are displaced in time phase by 90° electrical degrees. Since the two windings are displaced in space by 90 electrical degrees as shown in Fig. 9.6 maximum torque is developed at start. However, the auxiliary winding and capacitor are disconnected after the motor has picked up 75 per cent of the synchronous speed. The motor will start without any humming noise. However, after the auxiliary winding is disconnected, there will be some humming noise.
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Transformer & Induction M/C 10EE46
5.Why starter is necessary to start all induction motor? Explain in detail Star-delta starter for three phase IM. (Dec/Jan 2014) (Dec/Jan 2015)
The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is maximum (Q s = 1). Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current (4 to 10 times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally. However, this large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.
Star-Delta Starter
It is cheaper as compared to A. T. starter. This method of starting is used for motors designed to operate normally in delta. The six terminals from the three phases of the stator must be available : a, A : Terminals of phase A b, B : Terminals of phase B c, C : Terminals of phase C
Commercially, the terminals are marked A 1, A 2; B 1, B 2 and C 1, C 2 respectively. The motor is started with TPDT switch in position 1 and subsequently switched to position 2.
Position 1 : Starting-windings connected in Y
Position 2 : Running-windings get connected in D
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Transformer & Induction M/C 10EE46
Fig: Star-Delta Starter
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