Single Phase Induction Motors Study Notes for SSC JE & Other AE JE Exams

Principle of Operation of Single Phase Induction Motor

A single-phase induction motor is not self starting but requires some starting means.The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor. However, if the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors.

Double-Field Revolving Theory

When a 1–  supply is applied across stator containing 1-winding, a pulsating or alternating magnetic field is produced. It links with the rotor and produces induced emf rotor is essentially closed, it develops current and torque. But the rotor does not start. If any external force is applied. The rotor continues to rotate in the direction of force and reaches relative speed This behaviour is analysed according to double field revolving theory. According to this theory "a pulsating magnetic field contains two magnetic fields running at synchronous speed at equal magnitude ( _m/2) and in opposite directions".

Figure 1: Double revolving field theory representation

If N cos ϕ; no. of turns which are placed at a space angle θ and i(t)= I_mcosωt

By applying double field theory:

The 1- ϕ MMF can be divided two oppositely rotating MMF waves which rotate at synchronous speeds in opposite directions.

cos(θ + ωt) - backward rotating ; cos(θ – ωt) backward rotating  = F_f + F_b,

Both of these magnetic fields rotate with synchronous speed. Because of two fluxes, here two slips are present

Therefore, backward slip: 2-s.

Slip can be defined with the direction of rotation of rotor.

(2 – s) > > s and s_b > > s_f

Slip changes with direction of rotation of rotor. If rotor rotates in anticlockwise direction then s_f = 2 – s, and s_b = s.

Equivalent Circuit of a Single-Phase Induction Motor

Let R_l = Resistance of the main stator winding

X_l = Leakage reactance of the main stator winding

X_m = Magnetizing reactance

R₂ = Standstill rotor resistance referred to the main stator winding

X₂ = standstill rotor leakage reactance referred to the main stator winding

             Figure 2: Equivalent circuit representation  of 1-phase IM

The current in the stator winding is:

Speed Torque Characteristic of Single Phase Induction Motor

Figure 3: Torque Vs slip characteristics of 1-Phase IM

Here we can assume that the rotor is started by spinning the rotor or by using auxiliary circuit, in say clockwise direction. The flux rotating in the clockwise direction is the forward rotating flux (φ_f) and that in the other direction is the backward rotating flux (φ_b).

• The rotor rotates opposite to the rotation of the backward flux. If its slip with respect to forward field is s, what is the slip with respect to the backward field therefore, the slip w.r.t. the backward flux will be

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

Split-Phase Induction Motor

• 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.

Figure 4: Circuit diagram of split-phase induction motor

Figure 5: Characteristics of split-phase induction motor

Operation of  split-phase induction motor

• 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.

Capacitor-Start Induction 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.

Figure 6: Circuit diagram of Capacitor-Start Induction motor

Figure 7: Characteristics of Capacitor-Start Induction motor

Operation of Capacitor-Start Induction Motor

• 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.

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

1. 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.
2. 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 I_m 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 of Capacitor-Start Capacitor-run Induction Motor

• 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.

Figure 8: Circuit diagram of Capacitor-Start Capacitor-Run Induction motor

Characteristics of Capacitor-Start Capacitor-run Induction Motor

• 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.

Figure 9: Characteristics of Capacitor-Start Capacitor-Run Induction motor

Shaded-Pole Motor

The shaded-pole motor is very popular for ratings below 0.05 H.P. (~40 W) because of its
extremely simple construction. It has salient poles on the stator excited by single-phase supply
and a squirrel cage rotor as shown in Figure.
A portion of each pole is surrounded by a short-circuited turn of copper strip called shading
coil.

Figure 10: Shaded-Pole Motor

• 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.

You can avail of BYJU’S Exam Prep Online classroom program for all AE & JE Exams:

BYJU’S Exam Prep Online Classroom Program for AE & JE Exams (12+ Structured LIVE Courses)

You can avail of BYJU’S Exam Prep Test series specially designed for all AE & JE Exams:

BYJU’S Exam Prep Test Series AE & JE Get Unlimited Access to all (160+ Mock Tests)

Thanks

Team BYJU’S Exam Prep

Download BYJU’S Exam Prep APP, for the best Exam Preparation, Free Mock tests, Live Classes.