STARTING METHODS AND TYPES OF SINGLE-PHASE INDUCTION MOTOR
We have seen that some means should be used to start the single-phase induction motor. Mechanically methods are impractical and, therefore, the motor is started temporarily converting it into two-phase motor. Single phase induction motors are usually classified according to the auxiliary means used to start the motor .they are classified as follows: 1. split-phase motor 2. capacitor-start motor 3. permanent-split capacitor(PSC)motor(or single value capacitor motor) 4. shaded-pole motor All these starting methods depend on two alternating fields displaced in space and
phase.
The resultant of two fields is a rotating field. This rotating field reacts with the cage rotor to provide the starting torque.one field is produced by the main winding and the other by the auxiliary winding .the auxiliary winding is also called starting winding.
(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 Im and Ia in the main winding and auxiliary winding lag behind the supply voltage V, Ia leading the current Im Fig. 9.5(b). This means the current through auxiliary winding reaches maximum value first and the mmf or flux due to Ia lies along the axis of the auxiliary winding and after some time (t = ??the current Im reaches ? ?? maximum value and the mmf or flux due to Im 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 torque is
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developed and the motor becomes a self-starting motor. After the motor starts, the auxiliary winding is disconnected usually by means of centrifugal switch that operates at about 75 per cent of synchronous speed. Finally the motor runs because of the main winding. Since this being single phase some level of humming noise is always associated with the motor during running. A typical torque speed characteristic is shown in Fig. 9.5 (c). It is to be noted that the direction of rotation of the motor can be reversed by reversing the connection to either the main winding or the auxiliary windings.
(b) Capacitor starts induction motor. Capacitors are used to improve the starting and Running performance of the single phase inductions motors. 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 Ia the current in The auxiliary coil leads the current Im in the main coil by 90 electrical degrees in time phase so That the starting torque is maximum for certain values of Ia and Im. This becomes a balanced 2-Phase motor if the magnitude of Ia and Im 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 capacitors 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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Since the auxiliary winding and capacitor are to be used intermittently, these can be designed for minimum cost. However, it is found that the best compromise among the factors of starting torque, starting current and costs results with a phase angle somewhat less than 90° between Im and Ia. A typical torque-speed characteristic is shown in Fig. 9.6 (c) high starting torque being an outstanding feature. (c) Capacitor start capacitor run motor. If two capacitors are used with the auxiliary winding as shown in Fig. 9.8 (a), one for starting and other during the start and run, theoretically optimum starting and running performance can both be achieved. Fig.9.8. (=) Capacitor start capacitor run motor (>) Torque-speed characteristic. The small value capacitor required for optimum running conditions is permanently connected in series with the auxiliary winding and the much larger value required for starting is obtained by a capacitor connected in parallel with the running capacitor. The starting capacitor is disconnected after the motor starts.
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Since this capacitor must carry current for a short starting period, the capacitor is a special compact ac electrolytic type made for motor starting duty. However, the capacitor permanently connected has a typical rating of 40 ??; since it is connected ? permanently, the capacitor is an ac paper, foil and oil type. The cost of the motor is related to the performance; the permanent capacitor motor is the lowest cost, the capacitor start motor next and the capacitor start capacitor run has the highest cost.
(d) Shaded pole induction motor. Fig. 9.9 (a) shows schematic diagram of shaded pole induction motor. The stator has salient poles with one portion of each pole surrounded by a short-circuited turn of copper called a shading coil. Induced currents in the shading coil (acts as an inductor) cause the flux in the shaded portion of the pole to lag the flux in the other portion. Hence the flux under the unshaded pole leads the flux under the shaded pole which results in a rotating field moving in the direction from unshaded to the shaded portion of the pole and a low starting torque is produced which rotates the rotor in the direction from unshaded to the shaded pole. A typical torque speed characteristic is shown in Fig. 9.9 (b). The efficiency is low. These motors are the least expensive type of fraction rotation built up to about 1/2hp since the of the motor is in the direction from unshaded towards the shaded part of the pole, a shaded pole motor can be reversed only by providing two sets of shading coils which may be opened and closed or it may be reversed permanently by inverting the corel horse power motor.
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doc_784288615.doc
We have seen that some means should be used to start the single-phase induction motor. Mechanically methods are impractical and, therefore, the motor is started temporarily converting it into two-phase motor. Single phase induction motors are usually classified according to the auxiliary means used to start the motor .they are classified as follows: 1. split-phase motor 2. capacitor-start motor 3. permanent-split capacitor(PSC)motor(or single value capacitor motor) 4. shaded-pole motor All these starting methods depend on two alternating fields displaced in space and
phase.
The resultant of two fields is a rotating field. This rotating field reacts with the cage rotor to provide the starting torque.one field is produced by the main winding and the other by the auxiliary winding .the auxiliary winding is also called starting winding.
(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 Im and Ia in the main winding and auxiliary winding lag behind the supply voltage V, Ia leading the current Im Fig. 9.5(b). This means the current through auxiliary winding reaches maximum value first and the mmf or flux due to Ia lies along the axis of the auxiliary winding and after some time (t = ??the current Im reaches ? ?? maximum value and the mmf or flux due to Im 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 torque is
1
developed and the motor becomes a self-starting motor. After the motor starts, the auxiliary winding is disconnected usually by means of centrifugal switch that operates at about 75 per cent of synchronous speed. Finally the motor runs because of the main winding. Since this being single phase some level of humming noise is always associated with the motor during running. A typical torque speed characteristic is shown in Fig. 9.5 (c). It is to be noted that the direction of rotation of the motor can be reversed by reversing the connection to either the main winding or the auxiliary windings.
(b) Capacitor starts induction motor. Capacitors are used to improve the starting and Running performance of the single phase inductions motors. 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 Ia the current in The auxiliary coil leads the current Im in the main coil by 90 electrical degrees in time phase so That the starting torque is maximum for certain values of Ia and Im. This becomes a balanced 2-Phase motor if the magnitude of Ia and Im 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 capacitors 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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Since the auxiliary winding and capacitor are to be used intermittently, these can be designed for minimum cost. However, it is found that the best compromise among the factors of starting torque, starting current and costs results with a phase angle somewhat less than 90° between Im and Ia. A typical torque-speed characteristic is shown in Fig. 9.6 (c) high starting torque being an outstanding feature. (c) Capacitor start capacitor run motor. If two capacitors are used with the auxiliary winding as shown in Fig. 9.8 (a), one for starting and other during the start and run, theoretically optimum starting and running performance can both be achieved. Fig.9.8. (=) Capacitor start capacitor run motor (>) Torque-speed characteristic. The small value capacitor required for optimum running conditions is permanently connected in series with the auxiliary winding and the much larger value required for starting is obtained by a capacitor connected in parallel with the running capacitor. The starting capacitor is disconnected after the motor starts.
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Since this capacitor must carry current for a short starting period, the capacitor is a special compact ac electrolytic type made for motor starting duty. However, the capacitor permanently connected has a typical rating of 40 ??; since it is connected ? permanently, the capacitor is an ac paper, foil and oil type. The cost of the motor is related to the performance; the permanent capacitor motor is the lowest cost, the capacitor start motor next and the capacitor start capacitor run has the highest cost.
(d) Shaded pole induction motor. Fig. 9.9 (a) shows schematic diagram of shaded pole induction motor. The stator has salient poles with one portion of each pole surrounded by a short-circuited turn of copper called a shading coil. Induced currents in the shading coil (acts as an inductor) cause the flux in the shaded portion of the pole to lag the flux in the other portion. Hence the flux under the unshaded pole leads the flux under the shaded pole which results in a rotating field moving in the direction from unshaded to the shaded portion of the pole and a low starting torque is produced which rotates the rotor in the direction from unshaded to the shaded pole. A typical torque speed characteristic is shown in Fig. 9.9 (b). The efficiency is low. These motors are the least expensive type of fraction rotation built up to about 1/2hp since the of the motor is in the direction from unshaded towards the shaded part of the pole, a shaded pole motor can be reversed only by providing two sets of shading coils which may be opened and closed or it may be reversed permanently by inverting the corel horse power motor.
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doc_784288615.doc