Questions. . . ! Single phase induction motor

Q1: Where do we require single-phase induction motors?

Ans: Single-phase induction motors are required where

(i) 3 phase supply is not available 

(ii) efficiency is of lesser importance

(iii) Rating is less than one H.P. 

(iv) Equipment is portable

Q2: Why is the power factor of a single-phase induction motor low?

Ans: It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature.

Q3: What is the function of centrifugal starting switch in a single-phase induction motor?

Ans: The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.

Q4: What happens when the centrifugal starting switch fails to open?

Ans: If the starting switch fails to open when needed, then the starting winding will overheat and burn out and motor will not start next time.

Q5: What happens when the centrifugal switch fails to close when needed?

Ans: If the centrifugal starting switch fails to close, the motor will overheat the main winding without any failure of the main winding.

Q6: Why are resistance split-phase inductions motors most popular?

Ans: These motors are most popular due to their low cost. They are used where moderate starting torque is required and where the starting periods are not frequent. They drive fans, pumps, washing machines, small machine tools etc. They have power ranting between 60 watts and 250 watts.

Q7: What is the draw back of the resistance split-phase induction motor?

Ans: The starting winding has a relatively small number of turns of fine wire and its resistance is higher than that of the main winding. Therefore the current density is high and the winding heats up quickly. If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in-thermal relay.

Q8: Why is the starting torque of a resistance split-phase induction motor not high?

Ans: The starting torque is given as, Ts = K Im Is Sin Ф

Where

            K = constant whose magnitude depends upon the design of the motor

(i) The angle between Is and Im is small (approximately 25 degree) in a resistance split-phase induction motor, so the starting torque is small.

(ii) Since currents Is and Im are not equal in magnitude, the rotating magnetic field is not uniform and the starting torque produced is small.

Q9: Why is the starting torque of a capacitor start induction motor high?

Ans: The capacitor C in the starting winding is so chosen that Is leads Im by 75 degree. Since the starting torque is directly proportional to Sin Ф, and it is quite high in capacitor-start induction motor.

Q10: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?

Ans: Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q11: What is the principle of operation of shaded-pole induction motor?

Ans: A shaded-pole motor is basically a small single-phase squirrel cage motor in which the starting winding is composed of short-circuited copper ring (called shading coil) surrounding one-third of each pole. The effect of the shading coil is to cause a flux to sweep across the pole faces, from unshaded to shaded portion of the pole, producing a weak rotating magnetic field. As a result, the rotor is set in motion due to induction principle.

Q12: Which type of torque is developed in single phase motors?

Ans: Pulsating torque is produced.

Q13: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?

Ans: Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q14: What is a fractional H.P. motor?

Ans: A small motor having H.P. less than unit is called fractional H.P. motor.

Q15: Which type of rotor is used in single phase motors?

Ans: Squirrel cage type

Q16: How the starting winding produce rotation in a single phase resistance start induction motor?

Ans: The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.    

    

Q17: How the starting winding is made resistive?

Ans: It consists of only few turns of smaller diameter.

Q18: How the speed of rotation of a split phase induction motor is reversed?

Ans: The terminal connections of the starting windings are reversed with respect to main running windings.

Q19: What will happen if the centrifugal switch fails to open the starting winding?

Ans: Excessive heat will be produced due to high resistance of the starting winding due to which stator temperature will rise and eventually both windings will burn.

Q20: How speed control is made in single phase motors?

Ans: It is usually controlled by applying a variable voltage from tapped transformers, variacs, potentio meters, and tapped reactors.

Q21: Is there any relation between the capacitances of two capacitors used in two value capacitor motor?

Ans: Starting capacitor has about 10 – 15 times high capacity than the value of running capacitor.

Q22:  What is size of shaded-pole motor?

Ans: These are usually built in small fractional H.P, not exceed 1/4 H.P.

Q23: Why shaded-pole single phase induction motor does not need any special starting technique like capacitors and auxiliary winding etc.

Ans: Because it is inherently self started motor. The construction of the poles is such that they give a sweep to the magnetic flux and motor starts rotating.

Q24: How can a universal motor be reversed?

Ans: By reversing either the field leads or armature leads but not both.

Q25: What are applications of Stepper motors?

Ans: (i) Paper feed motors in typewriters and printers 
(ii) Positioning of print heads 
(iii) Pens in XY-plotters 
(iv) Recording heads in computer disc drives etc.

Q26: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?

Ans: Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q27: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?

Ans: Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q28: What is a fractional H.P. motor?

Ans: A small motor having H.P. less than unit is called fractional H.P. motor.

Q29: Which type of rotor is used in single phase motors?

Ans: Squirrel cage type

Q30: How the starting winding produce rotation in a single phase resistance start induction motor?

Ans: The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.  

      

Q31: How the starting winding is made resistive?

Ans: It consists of only few turns of smaller diameter.

Q32: How the speed of rotation of a split phase induction motor is reversed?

Ans: The terminal connections of the starting windings are reversed with respect to main running windings.

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Split – Phase Induction Motor

• Split phase induction motor has two windings its stator, a main winding and an auxiliary winding or starting winding. The starting winding is located 90^o electrical from the main winding and operates for a short time when the motor starts up.
• The two windings are so designed that the starting winding has a high resistance and relatively small reactance while the main winding has relatively low resistance and large reactance. This can be done by having main winding of large diameter and number of turns while starting winding of a fine wire of a small number of turns. The currents taken by these two windings are out of phase (25^o to 30^o). Schematic diagram and phasor diagram of split phase induction motor are shown in figure
• Operation: When the stator windings are energised from a single phase supply, the main winding carries current Im and the starting winding carries current Is. These two currents are out of phase (25^o to 30^o) and produce a weak rotating field which starts the motor. The starting torque is given as,

                                    Ts = K Im Is Sin Ф

Where

            K = constant whose magnitude depends upon the design of the motor

When the motor reaches about 75% of synchronous speed, then the centrifugal switch or relay 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 is below the synchronous speed and depends upon the load on the motor.

Characteristics of Split-Phase I / M:

1. The starting torque is 1.5 to 2 times the full load torque and the starting current is 6 to 8 times the full-load current.
2. The power ratings of such motors are from 60 Watts to 250 Watts.
3. They have efficiency of 50 – 70 % and overload of 50%.
4. As they have moderate starting torque and low starting current, so they are commonly used to drive fans, blowers, centrifugal pumps, domestic refrigerators, washing machines, oil burners, small machine tools, etc.
5. These motors are essentially constant-speed motors. The speed variation is 2 to 5 % from no-load to full-load.
6. The speed range of such motors is from 2875 to 700 r.p.m.
7. Such a motor can be operated with a power factor of 0.55 – 0.65.
8. The percent slip for such motors is about 4 – 6 percent.

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A.C. Series Motor OR Universal Motor

§  A universal motor is built like a series d.c. motor with the difference that both its stator and armature are laminated. A universal motor is defined as a motor which can be operated on both a.c. supply and d.c. supply at approximately the same speed and output.

§  A d.c. series motor (both armature and field windings are in series) can be operated on a single phase a.c. supply. It is then called an a.c. series motor. Following changes are made in d.c. series motor to operate satisfactory on a.c. supply.

1.    The entire magnetic circuit is laminated in order to reduce the eddy current loss. Hence an a.c. series motor requires a more expensive construction than a d.c. series motor.

2.    The series field winding uses a few turns to reduce the reactance of the field winding. This reduces the voltage drop across the field winding.

3.    A high field flux is obtained by using a low – reluctance magnetic circuit.

4.    When the motor is operated on a.c. supply, there will be possible sparking between the brushes and the commutator. This can be eliminated by using high-resistance leads to connect the coils to the commutator segments.

• Operation: When the motor is connected to a.c. supply, the same alternating current flows in the field and armature windings. The field winding produces an alternating flux that reacts with the current flowing in the armature to produce a torque. The flux produced is only alternating not the rotating.

Characteristics of Universal Motor:

1. It is self starting motor, starting torque is twice or thrice, therefore no starting device is required.
2. The speed increases to a high value with a decrease in load. In small motors, the losses are enough at no-load to limit the speed to a definite value. However, in a d.c. series motor, the speed at no-load may rise to a dangerously high value.
3. The motor torque is high for large armature currents, thus giving a high starting torque.
4. At full load, the power factor is about 90%.
5. They operate at high speeds (1500 – 15000 r.p.m)

Application:

These motors are used to drive

(1) high speed vacuum cleaners   

(2) Sewing machines 

(3) Electric shavers 

(4) Drills 

(5) Machine tools etc.

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Single Phase Motors

"Single phase motors are also used to convert Electrical energy to mechanical energy but the difference is that they are operated on single phase supply".

Types of single phase motors:

Single phase motors are generally built in the fractional –horsepower range and are classified into following four basic types:

1. Single-Phase Induction Motors

      (i) Split-phase type

     (ii) Capacitor type 

     (iii) Shaded-pole type

2. AC Series Motor or Universal Motor
3. Repulsion Motors

      (i) Repulsion-start induction-run motor 

      (ii) Repulsion-induction motor

4. Synchronous Motors

            (i) Reluctance motor 
           (ii) Hysteresis motor

Single phase Induction motors:

Split – Phase Induction Motor

• Split phase induction motor has two windings its stator, a main winding and an auxiliary winding or starting winding. The starting winding is located 90^o electrical from the main winding and operates for a short time when the motor starts up.

• When the motor reaches about 75% of synchronous speed, then the centrifugal switch or relay 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 is below the synchronous speed and depends upon the load on the motor.

Capacitor-Start Motor

• The capacitor motor is similar to a split-phase induction motor but with two differences: (i) the starting winding of capacitor start motor have as many turns as the main winding and (ii) a capacitor is connected in series with the starting winding.
• Capacitors are used to improve
• The starting-performance
• Running performance
• Both starting and running performance of a split-phase induction motor.

Capacitor-Start Capacitor-Run Motor

• This motor is similar to capacitor-start motor except that starting winding is not opened after starting so that both the windings of the stator remain connected to the supply when running as well as at starting.

Shaded-Pole Motor

• A shaded-pole motor has salient pole stator similar to the stator of dc machine. The pole is laminated to reduce the core losses. The pole is physically divided into two sections as shown in figure F6. A short-circuited turn of copper ring called shading coil is placed on the smaller section of the pole. This section covers around one-third of the pole arc and is called the shaded portion of the pole. The remaining two-third section of the pole arc is called unshaded portion. The main single phase winding is wound on the entire pole section.  The rotor used is similar to the rotor of any other single phase induction motor.

Single phase Synchronous  Motor:

Synchronous Reluctance Motor:

This motor has variable air-gap which decides the reluctance of the magnetic path. More the air gap, more is the reluctance and vice versa. The reluctance is greatest when the magnetic field of the stator aligns with the cut out portion of the rotor whereas reluctance is minimum when it aligns with the center position of the rotor teeth. Therefore the reluctance of the magnetic circuit is a function of the air-gap.  The salient poles created on the rotor must be equal to the poles on the stator. The rotor salient poles offer low reluctance to the stator flux and therefore, become strongly magnetized.

A.C. Series Motor OR Universal Motor:

§  A universal motor is built like a series d.c. motor with the difference that both its stator and armature are laminated. A universal motor is defined as a motor which can be operated on both a.c. supply and d.c. supply at approximately the same speed and output.

§  A d.c. series motor (both armature and field windings are in series) can be operated on a single phase a.c. supply. It is then called an a.c. series motor. Following changes are made in d.c. series motor to operate satisfactory on a.c. supply.

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Construction of alternator

Construction of Alternator

An alternator consists of:

-> stator & rotor

Stator:

§  Stator is the stationary part of the alternator and contains 3-phase armature windings. Stator core is built up of silicon steel laminations to reduce eddy current losses. The laminations are provided with slots on its inner periphery and are packed tightly together by cast iron frame.

§  Open slots are used allowing easy installation of stator coils and easy removal in case of repair. Coils are insulated before inserting in the slots and are further protected by fiber.

§  The three phase windings are placed in these slots and serves as the armature windings of the alternator. The armature windings are always connected in star and the neutral is connected to ground.

Rotor:

§  The rotor is rotating part of the alternator. It carries a field winding which is supplied with dc current through two slip rings by a separate dc source. This dc source (exciter) is generally a small dc generator mounted on the shaft of the alternator.

§  There are two types of rotors:

 -> Salient pole type rotor 

 -> non-salient pole type rotor.

Salient pole Alternator:

§  Salient means sticking out or projected out. A salient pole is a magnetic pole that is projected out of the rotor surface.

§  The salient pole alternators are slow-speed machines, speed varying from 150 to 600 rpm. These alternators are driven by hydraulic turbines. They are also called water-wheel generators or hydro-generators. Pelton wheel, Francis turbine and Kaplan turbine are types of hydraulic turbines used with these alternators:

§  Salient type rotor has large diameter, small length and low speed. Diameter is usually between 3-15 m.

§  Salient type rotor has non-uniform air-gap and two or four poles.

§  Low and medium speed alternators (120 – 400 rpm) driven by diesel engines or water turbines have salient pole type rotors due to following reasons:

o   The salient field poles would cause an excessive windage loss if driven at high speed and would tend to produce noise.

o   Salient-pole construction can not be made strong enough to withstand the mechanical stress at higher speeds.

Non-Salient pole Alternator:

§  Non-salient pole is non-projecting surface type.

§  Non-salient type rotor has small diameter and large length.

§  Non-salient type rotor is used for high speed and has uniform air-gap.

§  Non-salient pole rotors have four or more poles.

§  High speed alternators (1500 – 3000 rpm) are driven by steam turbines and use non-salient type rotors due to following reasons:

o   Gives noiseless operation at high speeds.

o   Flux is uniformly distributed along the periphery, so proper sine wave is obtained which gives better emf.

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Alternator

An alternator: It is a machine which converts mechanical energy into electrical energy and delivers ac current. It is also called ac generator or synchronous generator.

Principle of operation:

§  The alternator works on the principle of Faraday’s laws of electromagnetic induction. “Whenever a conductor links with a magnetic field, either the conductor is moving or the field is moving then an emf is induced in the conductor”.

§  The rotor winding is energized from the dc exciter and alternate N and S poles are developed on the rotor. When the rotor is rotated in anticlockwise direction by a prime-mover, the stator or armature conductors are cut by the magnetic flux of rotor poles and hence emf is induced in the armature (stator) conductors due to electromagnetic induction.

§  The frequency of the induced emf in the armature conductors is given by,

f = NP/120

            Where N = speed of rotor in rpm,     P = number of rotor poles

Why is it called synchronous generator: An alternator is also known as synchronous generator because it rotates with synchronous speed and gives a perfect constant relationship between the speed, frequency and the number of poles.

Difference between ac generator and dc generator:

           

S #	DC Generator	AC Generator
1	Field is static while armature is rotating	Either field or armature can be made rotating
2	Commutator required to convert  ac into dc	No commutator required
3	Complicated design in the presence of commutator	Simple construction having           no commutator
4	Not used commercially	Used commercially

Advantages of Stationary Armature: In an alternator, the field winding is placed on the rotor and is connected to dc supply through two slip rings. The three phase armature winding is placed on the stator. This arrangement has the following advantages:

(1)  It is easier to insulate stationary high voltage windings of the armature.

(2)  The stationary three phase armature can be directly connected to the load without using slip rings and brushes.

(3)   As the exciting current is small, so two small slip rings and brushes are required for dc supply to field winding on the rotor.

(4)  Higher speed of rotating dc field is obtained due to simple and robust construction of the rotor. This will increase output of the alternator.

(5)  The stationary armature can be cooled more efficiently because the stator can be made large enough and with many air passages or cooling ducts for forced air circulation.

(6)  The weight and inertia of rotor is reduced.

(7)  The strength of armature teeth is increased.

(8)  Armature reactance is reduced.

Disadvantages of Revolving Armature:

(1)  It has small KVA capacity and low-voltage rating.

(2)  Insulation of revolving high voltage armature will be difficult.

(3)  Windings can be cooled properly.

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