E3106/08/17
ELECTRICAL MACHINERY & CONTROL
UNIT 8
ALTERNATING CURRENT (AC) MACHINERY (PART IV)
OBJECTIVES
GENERAL OBJECTIVE
To interpret the concept of single-phase motors
Specific Objectives
By the end of this unit, you would be able to:
- explain the operation principle of single-phase motors
- explain the development of rotating magnetic field in single-phase
- identify auxiliary winding and main winding in split-phase motor
- summarize the types and characteristics for motors
INPUT
- OPERATION PRINCIPLE OF SINGLE-PHASE
Single-phase motors are the most familiar of all electric motors because they are used in home appliances and portable machine tools. There are many kinds of single-phase motors on the market, each designed to meet a specific application.
Figure 8.1: Current induced in the rotor bars due
to rotation
The principle of operation of a single-phase
induction motor is quite complex and it can
be explained by the cross-field theory. As
soon as the rotor begins turning, a speed
electric magnetic field ( emf ), E is induced
in the rotor conductors as they cut the stator
flux Fs ( see Figure 8.1). This voltage
increases as the rotor speed increases. It causes currents to flow in the rotor bars facing the stator poles. These currents produce an ac flux Fr which acts at right angles to the stator flux Fs.
Equally important is the fact that Fr does not reach its maximum value at the same time as Fs does. In effect, Fr lags almost 900 behind s due to the inductance of the rotor.
The combined action of Fs and Fr produces a revolving magnetic field, similar to that in a three phase motor. The value of Fr increases with the increasing speed, becoming almost equal to Fs at synchronous speed. This explains in part why the torque increases as the motor speeds up.
- THE DEVELOPMENT OF ROTATING MAGNETIC FIELD IN
SINGLE-PHASE
The major problem associated to the design of single-phase induction motors is that, unlike three-phase power source, a single-phase source does not produce a rotating magnetic field. Therefore the problems which we find are that the single-phase motor is not self-starting and we cannot be sure in which direction the rotation will take place.
Let us consider an explanation of the action of a single-phase motor. Because there is only single-phase winding, the field can only act in one direction, varying in magnitude with that of the excitation current. However, we can also represent it by the combined effects of two rotating fields of equal magnitude, but rotating in opposite directions. These are illustrated in Table 8.1.
Table 8.1: Representation of a one-phase field
Representation of single-phase field |
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Explanation |
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Representation of single-phase field |
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Explanation |
c) In this position, there are no vertical components hence the total field is zero
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d)
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e) Finally, we have almost returned to the situation associated with ( a ) except that the field acts in the opposite direction |
Therefore we can consider the single-phase winding is effective to set up two rotating magnetic fields, equal in magnitude but opposite in direction. If the rotor is moving, it must be moving in the direction of one of these fields and rotating in the opposite direction to that of the other field.
Test your UNDERSTANDING before you continue to the next input
ACTIVITY 8 A
- What is the problem that we will face with the single-phase motor since is not self starting?
- Based on question (8.1), how can the problems can be solved?
FEEDBACK TO ACTIVITY 8 A
- The major problem we face with the single-phase motor is it does not produces rotating magnetic field and we cannot be sure in which rotation will take place.
- The problem can be solved by combining the two rotating fields of equal magnitude, but rotating in opposite direction.
INPUT
- AUXILIARY WINDING AND MAIN WINDING IN SPLIT-PHASE MOTOR
A split-phase motor is a single-phase induction motor with two stator windings, a main stator winding (M) and an auxiliary starting winding (A). Figure 8.2 shows two windings are set 900 apart along the stator of the motor and the auxiliary windings is designed to be switched out of the circuit at some set speed by a centrifugal switch. The auxiliary winding is designed to have a higher resistance or reactance ratio than the main winding, so that the current in the auxiliary winding leads the current in the main winding. The higher R/X ratio is usually accomplished by using smaller wire for the auxiliary winding. Smaller wire is permissible in the auxiliary winding because it is used only for starting and therefore does not have to take full current continuously.
Figure 8.2: A split-phase induction motor
(Source: Electrical Machinery Fundamentals (3rd edition) Stephen J. Chapman)
To understand the function of the auxiliary winding, we will see Figure 8.3. Since the current in the auxiliary winding leads the current in the main winding, the magnetic field BA peaks before the main magnetic field BM. Since BA peaks first and then BM there is a net counterclockwise rotation in the magnetic field. In other words, the auxiliary winding makes one of the opposite rotating stator magnetic fields larger than the other one and provides a net starting torque for the motor.
Figure 8.3: Relationship of main auxiliary magnetic fields
(Source: Electrical Machinery Fundamentals (3rd edition) Stephen J. Chapman)
In a split-phase induction motor, the current in the auxiliary windings always peaks before the current in the main winding, and therefore the magnetic field from the auxiliary winding always peaks before the magnetic field from the main winding. The direction of rotation of the motor is determined by whether the space angle of the magnetic field from the auxiliary winding is 900 ahead or 900 behind the angle of the main winding. Since that angle can be changed from 900 ahead to 900 behind just by switching the connections on the auxiliary winding, the direction of rotation of the motor can be reversed by switching the connection of the auxiliary winding while leaving the main winding’s connection unchanged.
Test your UNDERSTANDING before you continue to the next input
ACTIVITY 8 B
- What are the differences between main winding and auxiliary winding?
- How can the direction of rotation of a split-phase induction motor be reversed?
- Complete the diagram below
(iv) …………….
(iii) …………………...
(ii) …………………...
VAC
FEEDBACK TO ACTIVITY 8 B
- The differences between auxiliary winding and main winding are auxiliary winding is designed to be switched out of the circuit at some set speed by a centrifugal switch. The auxiliary winding is designed to have a higher resistance / reactance ratio than the main winding, so that the current in the auxiliary winding leads the current in the main winding.
- The direction of rotation of a split-phase induction motor can be reversed by switching the connections of the auxiliary winding while leaving the main winding’s connections unchanged.
8.5
(iii) centrifugal switch
(ii) auxiliary winding
(i) main winding
VAC
INPUT
8.4 TYPES AND CHARACTERISTICS FOR SINGLE-PHASE INDUCTION MOTOR
- The equivalent circuit
- Current angles at starting in this motor
Electrical Machinery Fundamentals (3rd edition) Stephen J. Chapman
There are three major types for single phase induction motor:- capacitor-start motors
- shaded-pole motors
- universal motors
- Capacitor-start motors
For some applications, the starting torque
supplied by a split-phase motor is
insufficient to start the load on a motor’s
shaft. In those cases, a capacitor-start motors
may be used, refer Figure 8.4.
In a capacitor-start motor, a capacitor is
placed in the series with the auxiliary winding Figure 8.4: Capacitor start motorof the motor. By proper selection of capacitor size, the magnetomotive force of the starting current in the auxiliary winding can be adjusted to be equal to the magnetomotive force of the current in the main winding, and the phase angle of the current in the auxiliary winding can be made to lead the current in the main winding by 900. Since the two windings are physically separated by 900, a 900 phase difference in current will yield a single uniform rotating stator magnetic, and the motor will behave just as though it were starting from a three-phase power source. In this case, the starting torque of the motor can be more than 300 percent of its rated value.
Capacitor-start motors are more expensive than split-phase motors, and they are used in applications where a high starting torque is absolutely required. Typical applications for such motors are compressors, pumps, air conditioners and other pieces of equipment that must start under a load.
8.4.2 Shaded-pole motors
A shaded-pole induction motor is an induction motor with only a main winding. Instead of having an auxiliary winding, it has salient poles, and one portion of each pole is surrounded by a short-circuited coil called a shading coil ( Figure 8.5).
Figure 8.5: A basic shaded-pole
induction motor
A time varying flux is induced in the poles by the
main winding. When the pole flux varies, it induces a
voltage and a current in the shading coil which
opposes the original change in flux. This opposition
retards the flux changes under the shaded portions of the coils and therefore produces a slight imbalance between the two opposite rotating stator magnetics fields. The net rotation is in the direction from the unshaded to the shaded portion of the pole face.
Shaded poles produce less starting torque than any other type of induction motor starting system. They are much less efficient and have a much higher slip than other types of single-phase induction motors. Such poles are used only in very small motors (1/20 hp and less) with very low starting requirements. Where it is possible to use them, shaded-pole motors are the cheapest design available. Because shaded-pole motors rely on a shading coil for their starting torque, there is no easy way to reverse the direction of rotation of such a motor. To achieve reversal, it is necessary to install two shading coils on each pole face and to selectively short one or the other of them.
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(a) Cutaway view of shaded-pole induction motor |
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(Source: Electrical Machinery Fundamentals (3rd edition) Stephen J. Chapman)
Information Gallery
- UNIVERSAL MOTOR
In order for a series DC motor to function effectively on AC, its field poles and stator frame must be completely laminated. If they were not completely laminated, their core losses would be enormous. When the poles and stator are laminated, this motor is often called a universal motor, since it can run either an AC or a DC source.
When the motor is running from AC source, the commutation will be much poorer than it would be with a DC source. The extra sparkling at the brushes is caused by transformer action inducing voltages in the coils undergoing commutation. Typical applications for this motor are vacuum, cleaners, drills , similar portable tools and kitchen appliances.
Test your UNDERSTANDING before you continue to the next input
ACTIVITY 8 C
- How is the current phase shift accomplished in the auxiliary winding of a capacitor-start induction motor?
- How is starting torque produced in a shaded-pole motor?
- Where the universal motors are can be used?
FEEDBACK TO ACTIVITY 8 C
- The current phase shift accomplished in the auxiliary winding of a capacitor-start induction motor can be adjusted to be equal to the magnetomotive force of the current in the main winding and the phase angle of the current in the auxiliary winding can be made to lead the current in the main winding by 900.
- Starting torque produced in a shaded-pole motor is less than any other type of induction motor starting system. Shaded-pole motors have a very small effective phase shift and therefore a small starting torque. They are much less efficient and have a much higher slip than other types of single-phase induction motors. Such poles are used only in very small motors with very low starting torque requirements.
- The universal motors can be used in vacuum, cleaners, drills , similar portable tools and kitchen appliances.
SELF-ASSESMENT
If you face any problem, discuss it with your lecturer
You are approaching success. TRY all the questions in this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page.
Question 8-1
A. When a basic single-phase motor is switched on, it is possible that it might not start, or, if it does start, we cannot be certain of the direction of rotor rotation. Explain the reason for this uncertainty and hence describe a development that will ensure that the rotor starts in a predetermined direction.
Question 8-2
A. State the main difference between a split-phase motor and a capacitor-start motor.
Question 8-3
A. Explain briefly how a shaded-pole motor operates.
FEEDBACK TO SELF-ASSESMENT
Question 8-1
A. Refer the note 8.1
Question 8-2
A.
a) A split-phase motor is a single-phase induction with two stator windings, a main stator winding and an auxiliary starting winding. The split-phase construction provides only a small phase difference between the main and auxiliary windings its starting torque modest.
- In a capacitor-start motor, a capacitor is placed in series with the auxiliary winding of the motor. Capacitor-start motors have auxiliary windings with an approximately 900 phase shift, so they have large starting torques.
Question 8-3
A. Refer the note 8.4.2