Single Phase Induction Motor-2 Study notes For Electrical Engineering

Making Single-Phase Induction Motor Self-Starting

• Since we have already know that the single-phase induction motor is not self-starting and it is undesirable to resort to the mechanical spinning of the shaft or pulling a belt to start it.
• To make a single-phase induction motor self-starting, we should somehow produce a revolving stator magnetic field. This may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding.
• When the motor attains sufficient speed, the starting means (i.e., additional winding) may be removed depending upon the type of the motor.

So as a matter of fact, single-phase induction motors are classified and named according to the method employed to make them self-starting.

• Split-phase type
• Capacitor start type
• Capacitor start capacitor run type
• Shaded-pole type

(i) Split-Phase Type

• The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M.
• The starting winding is located 90° electrical from the main winding and operates only during the brief period when the motor starts up.
• The two windings are so resigned that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance to be as inductance (the current delay with voltage) to make shifting current as shown in Figure.
• Consequently, the currents flowing in the two windings have a reasonable phase difference (25° to 30°) as shown in the pharos diagram this shifting in current its necessary for starting torque.

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Operation

• When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is.
• Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them.
• Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor.
• When the motor reaches about 80% of synchronous speed, the centrifugal switch opens the circuit of the starting winding.
• The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.

The Capacitor Start Motor

The capacitor-start motor is identical to a split-phase motor except that the starting winding has as many turns as the main winding. The picture of capacitor start induction motor is shown in figure below,

Moreover, a capacitor C (3-20 µF) is connected in series with the starting winding as shown in Figure.The value of the capacitor is so chosen that I_S leads I_m by about 80° which is considerably greater than 25° found in the split-phase motor.

Operation

• When the two stator windings are energized from a single-phase supply, the main winding carries current I_m while the starting winding carries current I_S.
• Due to capacitance, the currents Im and Is have a reasonable phase angle an (80°) between them.
• When starting torque is much more than that of a split-phase motor Again, the starting winding is opened by the centrifugal switch when the motor attains about 80% of synchronous speed.
• The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed.
• Capacitor-start motors are used where high starting torque is required and where the starting period may be long e.g., to drive:

(i) compressors (ii) large fans (iii) pumps (iv) high inertia loads The power rating of such motors lies between 120 W and 7-5 kW.

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Capacitor Start-Capacitor Run Motor

• This motor is identical to a capacitor-start motor except that starting winding is not opened after starting so that both the windings remain connected to the supply when running as well as at starting.
• Two designs are generally used

(i) In first it shows a picture of capacitor start capacitor run induction motor. This design eliminates the need for a centrifugal switch and at the same time improves the power factor and efficiency of the motor.

(ii) In the other design, two capacitors C1 and C2 are used in the starting winding.The value of the capacitor is so chosen that Is leads Im by about 80°.

• The smaller capacitor C1 required for optimum running conditions is permanently connected in series with the starting winding. The much larger capacitor C2 is connected in parallel with C1 for optimum starting and remains in the circuit during starting.
• The starting capacitor C2 is disconnected when the motor approaches about 80% of synchronous speed. The motor then runs as a two-phase induction motor.

Operation

• When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is.
• Due to capacitance C1 the currents Im and Is have a reasonable phase angle an (80°) between them.
• When The starting capacitor C2 is disconnected when the motor approaches about 80% of synchronous speed. The motor then runs as a two-phase induction motor.

Characteristics

• The starting winding and the capacitor can be designed for perfect 2-phase operation at any load. The motor then produces a constant torque and not a pulsating torque as in other single-phase motors.
• Because of constant torque, the motor is vibration free and can be used in: (a) hospitals (b) studios and (c) other places where silence is important.

Shaded-Pole Motor

A typical shaded-pole motor with a cage rotor is a single-phase induction motor, with main winding in the stator. A small portion of each pole is covered with a short-circuited, single-turn copper coil called the shading coil. The sinusoidally varying flux created by ac (single-phase) excitation of the main winding induces emf in the shading coil. As a result, induced currents flow in the shading coil producing their own flux in the shaded portion of the pole.

the main winding flux be φ_m = φ_max sinωt

where R is the reluctance of the path of φ_sc.

As per the above equations, the shading coil current (I_sc) and flux (φ_sc) phasors lag behind the induced emf (E_sc) by angle θsc; while the flux phasor leads the induced emf (E_sc) by 90^o .

• The reversal of the direction of rotation, where desired, can be achieved by providing two shading coils, one on each end of every pole, and by open-circuiting one set of shading coils and by short-circuiting the other set.
• The above is true due to the fact that the shaded-pole motor is single-winding (no auxiliary winding) self-starting one, makes it less costly and results in rugged construction.
• The motor has low efficiency and is usually available in a range of 1/300 to 1/20 kW. It is used for domestic fans, record players and tape recorders, humidifiers, slide projectors, small business machines, etc.
• The shaded-pole principle is used in starting electric clocks and other single-phase synchronous timing motors.

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