Jenneson6e Vol2 PPT Ch06

PowerPoint slides t/a Jenneson and Harper, Electrical Principles for the Electrical Trades (Machines) 6e, Vol 2Slides prepared by Andrew O’Connell

Copyright © 2012 McGraw-Hill Australia Pty Ltd

6–1

Chapter 6Single-phase motors



6–2

Purpose

• This chapter describes the construction and performance of single-phase motors.

• It provides a comparison of single-phase and three-phase motors.

• It describes abnormal operating conditions for AC induction motors.



6–3

Single-phase induction motors

• Single-phase motors do not have the same inherent rotating magnetic field as three-phase motors.

• Single-phase motors require some method of creating a rotating magnetic field. This can be achieved by having two windings that are out of phase with each other.



6–4

Single-phase induction motors (continued)

• Out of phase windings can be obtained by having windings of different resistance and reactance values, or by the addition of a capacitor in series with one of the windings.

• Once the motor has reached sufficient speed, one of the windings can be disconnected and the motor will continue to rotate.



6–5

• The single-phase motor:– has a characteristic vibration at twice the

supply frequency– is nosier than a three-phase motor– has a rather high no-load current at low

power factor– has a power factor that improves with the

addition of load– requires special starting techniques– has several versions based on the method of

starting.

Single-phase induction motors (continued)



6–6

Split-phase induction motor

• The split-phase motor has two dissimilar windings.

• The run winding is connected to the supply whenever the power is applied.

• The start winding is only connected to the supply during the start sequence.



6–7

Split-phase induction motor (continued)



6–8

• The run winding has a low resistance and a high reactance.

• The start winding has a high resistance and a low reactance.

• The two windings have different phase angles and are spaced around the stator. This produces the rotating magnetic field that is necessary for starting the motor.




6–9

• The speed of the rotating magnetic field can be determined by:

n = 120f/p

Where:n = speed of the rotating magnetic field in RPM

f = supply frequency in Hz

p = number of poles




6–10

• To reverse the direction of rotation, the connections for only one of the windings must be reversed.

• The rotating flux is not uniform and an elliptical field pattern is produced. This produces considerable humming noise and vibration during starting.




6–11




6–12

• The rotating stator field cuts the rotor conductors and induces a current into the short circuited rotor conductors.

• The magnetic field that is produced by the current flowing in the rotor conductors interacts with the stator field and motor action occurs.




6–13

• The starting torque depends on the relative strengths of the start and run fluxes and the phase displacement between them.

• When the rotor reaches approximately 75% of the rated speed, the start winding is disconnected from the supply. This is usually achieved by a centrifugal switch connected in series with the start winding.




6–14

• Once the start winding is disconnected the stator field becomes a stationary pulsating field.

• The currents in the rotor lag the stator field by almost 90o due to the reactance of the rotor conductors.

• Since the two magnetic fields are at right angles the resultant field is known as the 'cross field'.




6–15

• The two fields effectively combine to form a rotating magnetic field.

• The rotor then follows this field at a slightly slower speed due to losses in the rotor.




6–16

• The typical torque/speed curve for a split-phase motor displays the change from the starting condition to the running condition.




6–17

• Split-phase motors have only moderate starting torque and are typically used for:– washing machines– blowers– buffing machines– grinders– machine tools.




6–18

Capacitor-start motor

• The split-phase induction motor is limited to a maximum of about 30oE between the starting and running winding currents.

• To increase this angle and produce improved starting characteristics, a capacitor is connected in series with the starting winding.



6–19

Capacitor-start motor (continued)

• A correctly chosen capacitor will improve the starting torque, but care must be taken to avoid resonance.

• The improved torque derives from the greater phase displacement between the two winding currents.



6–20

• Other aspects of the capacitor-start motor, such as switching of the start winding and reversal of rotation, are similar to the split-phase motor.




6–21

• Capacitor-start motors are used in general purpose heavy duty applications requiring high locked rotor starting torque, such as:– refrigerators– air conditioners.




6–22

Capacitor-start, capacitor-run motor

• The capacitor-start, capacitor-run motor consists of two permanently connected windings, the run and auxiliary windings.

• The auxiliary winding is connected in series with a capacitor. During starting, a second capacitor is connected in parallel with the run capacitor.



6–23

Capacitor-start, capacitor-run motor (continued)

• The starting capacitor provides the necessary phase displacement between the winding currents for maximum torque at start.

• At approximately 75% of the rated speed, the centrifugal switch disconnects the starting capacitor.



6–24

• During operation the run capacitor ensures the correct phase displacement between the two currents in the windings, so providing a constant strength rotating magnetic field.

• The run capacitor:– increases the breakdown torque– improves full load efficiency and power

factor– reduces operational noise– increases locked rotor torque.




6–25

• Reversal of the capacitor-start, capacitor-run motor is achieved by reversing the connections to one winding only.

• Capacitor-start, capacitor-run motors are used for heavy duty loads requiring quiet operation and substantial starting torque, such as wall-mounted air conditioners.




6–26

Permanently split capacitor motor• The permanently split capacitor motor has

two identical windings.

• A capacitor is connected in series with either the main or the auxiliary winding at all times. The connection is controlled by a switch.

• The direction of rotation is determined by which winding is connected in series with the capacitor at that time. Hence the switch controls the direction of rotation.



6–27

Permanently split capacitor motor (continued)



6–28

• The permanently split capacitor motor is suitable for light applications with low starting torque that may need to be reversed, such as:– ceiling fans– blowers– air flow regulators.

• The speed of these motors can be varied fairly easily with series inductances.

Permanently split capacitor motor (continued)



6–29

Shaded-pole motor

• The shaded-pole motor has a cage rotor with salient poles in the stator. On one side of each pole a slot is cut and a copper shading ring is embedded in the slot.



6–30

Shaded-pole motor (continued)

• The supply current produces an alternating flux which induces a current in the shading ring.

• The induced flux opposes changes in the main flux resulting in a delay in the magnetic field at the shaded end of the magnetic pole.

• The magnetic axis shifts across the pole face from the unshaded part to the shaded part of the pole.



6–31

• The shifting flux is similar to a rotating magnetic field and it produces a small torque, causing the rotor to rotate in the direction of the flux, towards the shaded section of the pole.




6–32

• The shaded-pole motor:– has a low torque– runs with a high slip speed– is a simple construction– is low cost– is reliable and low maintenance– has low efficiency– is restricted to low power ratings.




6–33




6–34

• To reverse the direction of rotation requires either changing the shading ring from one side to the other or swapping the placement of the stator in the frame.




6–35

• The shaded-pole motor is suitable for:– fans and blowers– advertising signs– damper controllers– hair dryers.




6–36

Series motor

• The series motor is often called a universal motor since it can be used on either an AC or a DC supply.

• The series motor has a highly variable speed characteristic. Under some circumstances governors must be used to restrict speeds to safe values.



6–37

Series motor (continued)

• The field coils are concentrated type windings fitted closely around salient poles.

• The armature has laminations, a commutator and windings.

• The armature windings are connected in series with the field coils via carbon brushes.



6–38




6–39


• As the supply alternates, the armature connections change polarity so the stator and armature fluxes remain in phase.



6–40


• This constant phase relationship causes a steady rotation in the one direction.

• To reverse the direction of rotation requires reversal of the connections to either the field or the armature but not both.



6–41


• At heavy loads the motor speed is low. At light loads the motor speed is high.



6–42


• The series universal motor runs at a relatively high speed and has good starting and running torque characteristics considering its small size.

• It is popular for use in:– saws and drills– sewing machines– business machines– food mixers– small washing machines and vacuum

cleaners.



6–43




6–44

Comparison of single-phase and three-phase motors

• Advantages of single-phase motors include:– only two windings.– reduced construction cost due to

automatic machine winding– only one active and one neutral conductor

required in most cases.



6–45

Comparison of single-phase and three-phase motors (continued)

• Advantages of three-phase motors include:– smaller physical size for the same output– more efficient use of the iron core– higher efficiency– smaller line currents for the same output– suitable for higher powerline frequencies– less mechanical vibration– inherently self-starting due to rotating magnetic field– no starting mechanism required– reduced complications for difficult installations (e.g.

submersible pumps).– easily reversible by interchanging two supply

connections– simpler control of starting currents without loss of torque.



6–46

• Disadvantages of single-phase motors include:– higher line currents for the same power– energy supplier limitations on the use of

larger motors– motor reversal requires internal changes

in most types of single-phase motors.




6–47

• Disadvantages of three-phase motors include:– three identical windings required– three active conductors required– more labour intensive during construction.




6–48

Abnormal operating conditions for AC induction motors

• The torque produced by a motor is proportional to the voltage squared.

• A reduction in the supply voltage will cause a greater percentage reduction in the torque. This may cause the motor to overheat or stall.

• An increase in the supply voltage will cause an increase in torque. If this increased torque is utilised, the additional load may cause a temperature rise in the motor windings.



6–49

Abnormal operating conditions for AC induction motors (continued)

• Therefore, voltage fluctuations will have a negative impact on the performance of the motor.

• Motors are usually given a full-time rating for a specified temperature rise. The motor may need to be switched off after a duty period and be allowed to cool down.



6–50

• Motors that operate at higher operating temperatures have a greatly reduced lifespan.

• Common causes of overheating are inadequate or restricted ventilation and overloading.

• The higher temperature causes accelerated deterioration of lubricants and winding insulation.

• Overheating can be avoided by decreasing the load and increasing the cooling system’s efficiency.




6–51

• Variation of the supply frequency affects motor speed, power factor, efficiency and torque.

• Supply system frequency variations usually occur when there are comparatively few large loads connected to a small supply.

• A decrease in the supply frequency causes a decrease in speed, a decrease of the power factor and a decrease in efficiency.




6–52

• Short duration overloads (e.g. 150% for 15 seconds) produce a temperature rise that can normally be dealt with by the motor’s cooling system.

• If overloading persists, the increased temperature of the motor can lead to a shortened motor life.




6–53

• Other effects of overloading include:– a slight decrease in speed– decreased efficiency– decreased power factor– increased possibility of the motor stalling.

• A stalled motor draws starting current and produces large amounts of heat until the protection system operates.




6–54

• Some types of motors are given restricted duty cycles. They can be overloaded for a short time period, but they must then be allowed to cool down to room temperature.




6–55

• When a motor is started, high current flows and generates heat in excess of the normal amount.

• If a motor is subjected to frequent starting, the heat is not sufficiently dissipated and the motor windings suffer.

• Drawing starting values of current (for starting, reversing or braking) stresses the windings. Unless the windings are designed for this type of operation, the windings may eventually become short-circuited.




6–56

• Other abnormal conditions include exposure to:– corrosive fumes– explosive vapour– dust– steam– salt air– high humidity– extremes of temperature.




6–57

• Damage due to these factors is often due to the selection of an incorrect enclosure.

• The motor must be rated for the conditions under which it will operate.




6–58

• Probably the most common abnormal operating condition for single-phase motors is encountered with centrifugal switch failure.

• If the centrifugal switch does not open, the start windings will overheat and can be permanently damaged.

• If the centrifugal switch does not close, the motor will not start and the starting current will cause excessive heating of the run winding and it can be permanently damaged.




6–59

• If the centrifugal switch fails in either way, the heat in one winding can be transferred to the other winding, and without the intervention of the motor protection device the motor can be damaged.




6–60

Quick quiz

1. Which type of single-phase motor has copper rings embedded in one side of its salient poles?

2. Which type of single-phase motor can also operate from a DC supply?

3. Which type of single-phase motor has two identical windings and a reversing switch?

4. Which type of single-phase motor has high torque and is suitable for use in air conditioners?

5. Which type of single-phase motor has two dissimilar windings and is used for washing machines?



6–61

Quick quiz—answers

1. The shaded-pole motor

2. The series universal motor

3. The permanently split capacitor motor

4. The capacitor-start, capacitor-run motor

5. The split-phase motor



6–62

Summary

• A single-phase supply produces a stationary, pulsating magnetic field when it is connected to a motor. While this field will cause a rotor to continue rotating, it will not produce a starting torque.

• Different types of single-phase motors use different methods for producing a rotating magnetic field.



6–63

Summary (continued)

• The split-phase motor has two dissimilar windings which are out of phase.

• The phase difference and the physical displacement of the two windings around the stator produces a rotating magnetic field.

• The induced current in the rotor produces a magnetic field and the interaction of these two fields causes motor action.



6–64

• Once the motor reaches 75% of the full load speed, the start winding is disconnected from the supply by a centrifugal switch.

• The cross field of the rotor interacts with the stator field and a rotating magnetic field results. Thus the motor continues to spin even if the start winding is disconnected.

Summary (continued)



6–65

Summary (continued)

• The addition of a capacitor in series with the start winding causes a greater phase difference between the start and run windings and therefore a greater starting torque. This is known as the capacitor-start motor.



6–66

• The capacitor-start, capacitor-run motor has two windings permanently connected to the supply.

• One of these windings is connected in series with a capacitor. During starting an additional capacitor is connected to increase the starting torque.

Summary (continued)



6–67

• The capacitor-start, capacitor-run motor has: – improved locked rotor and breakdown

torque– improved full load efficiency– improved full load power factor– reduced operational noise and vibration.

• The capacitor-start, capacitor-run motor is suitable for heavy loads where high starting torque and quietness are required.

Summary (continued)



6–68

• The permanently split capacitor motor has two identical windings and a reversing switch that controls the connection of a capacitor in series with one of the windings.

• It is suitable for low torque applications that require ease of motor reversal such as ceiling fans.

Summary (continued)



6–69

• The shaded-pole motor has a shading ring embedded in one side of each of its salient poles.

• The magnetic axis shifts across the pole face thus producing a rotating magnetic field and hence a starting torque.

• This type of motor is simple and robust, with a low starting torque.

Summary (continued)



6–70

• The series universal motor consists of a wound armature that is connected in series with the field winding through carbon brushes.

• This motor has good starting and running torque and it can operate from either an AC or a DC supply.

Summary (continued)



6–71

• Compared to three-phase motors,single-phase motors:– are noisier– are physically larger for the same power output– draw higher current for the same power output– are not self-starting– are more difficult to reverse– may have energy supplier limitations– are easier to manufacture– only require a single-phase supply.

Summary (continued)



6–72

• Abnormal operating conditions for single-phase motors include:– voltage fluctuation– higher operating temperatures– frequency variation– overloading– frequent starting– inappropriate external conditions– malfunction of the starting switch.

Summary (continued)

Jenneson6e Vol2 PPT Ch06

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