Chapter 41. Induction Motors (Single Phase) – Electrical Technology, Vol2: Machines and Measurements, 1/e

41

Induction Motors (Single Phase)

OBJECTIVES

In this chapter you will learn about:

  • Phase-splitting circuits
  • Pole–speed relationship
  • Single-phase induction motor
  • The shaded-pole motor
  • The universal motor
  • The synchronous motor
  • Getting the motor started
  • Centrifugal switch
  • The capacitor start split-phase motor
  • Resistance start split-phase motor
  • Quadrature windings: running and starting
  • Torque-speed characteristic
  • Two-value capacitor motors
  • Permanent-split capacitor motors
  • Reversing single-phase induction motors
  • Dual-voltage operation

Single-phase induction motors

41.1  INTRODUCTION

Single-phase induction motors are used extensively in industrial, commercial, and domestic applications. They are used in clocks, fans, blowers, pumps, washing machines, and machine tools, as well as range in size from a fraction of a horse power (HP) to about 15 HP.

Large single-phase induction motors are split-phase machines that have two separate windings physically displaced by ninety electrical degrees and phase-splitting circuits are those that cause the current and associated flux of one winding to lag or lead the current and associated flux of the other winding (Figure 41.1). The net effect is the production of a rotating magnetic field that sweeps a squirrel-cage rotor developing induction motor action. Smaller single-phase induction motors use a much simpler device called a shading coil to provide the phase-splitting effect.

Figure 41.1 Single-Phase Induction Motor (a) The Squirrel-cage Rotor (b) Rotor and Starting Switch of a Split-phase Motor

Phase-splitting circuits are also used to operate three-phase induction motors from a single-phase source. This enables larger motors to be operated in isolated areas where three-phase sources are not available.

Note: Generally, the term small motor means a motor of less than 1 HP, i.e. fractional HP motor.

41.2  CLASSES OF INDUCTION MOTORS

There are four basic classes of single-phase motors that are used roughly in equal quantities, which are as follows:

  1. Single-phase induction motors are used for personal and small business tasks; furnace oil burner pumps, hot water circulators, or hot air circulators are some of their typical uses. Refrigerator compressors and power tools, such as lathes and bench-mounted circular saws are also powered with induction motors. Their power outputs usually range from about 1/6 HP (0.125 kW) up to ¾ HP (0.560 kW), although higher ranges are also used. Four-pole motors are usually used, with lesser quantities of two and six poles. The pole–speed relationship is similar to that of two three-phase motors, because the operation of the motor is essentially the same.

     

    S = 120 f / p r.p.m.     or     ω = 4π f / p rad/sec         (41.1)

     

  2. Shaded-pole motors, as shown in Figure 41.2, are used in smaller sizes for quiet, low-cost applications. Typical uses are in small fans and blowers with less power ratings than those described in (1). There is, of course, an overlap in use. The shaded-pole motor is simple and reliable but has a low starting torque and efficiency relative to that of the induction motor.
  3. The universal motor closely resembles a d.c. series motor and, as its name implies, will operate on any a.c. frequency or on direct current. They are widely used because they can develop very high speed while loaded and very high power for their size. Any service that requires a speed beyond that possible with a two-pole induction motor, where S = 3600 r.p.m., is a normal use for a universal motor.
  4. The last basic type of single-phase motor is the synchronous motor. The single-phase synchronous motor, as shown in Figure 41.3, is one of the various forms of magnetized but unwound motors. Consequently, they do not have the ability to control power factor. The exact speed relation of a synchronous motor is used in electric clocks and various cycle-timing devices. These motors are built in the smallest of sizes, where the power output is only a few Watts.

Figure 41.2 Shaded-pole Motor (the Copper Strap is Necessary for Starting the Motor)

Figure 41.3 Synchronous Motor

The relative quantities of these four basic types of single-phase motors are almost evenly distributed on a one-for-one basis in modern homes. The single-phase induction motor usually has a very high power rating compared with the other types mentioned here; they collectively develop as much power as the aggregate of the other three put together.

The split-phase motors employ two separate windings having different reactance resistance ratios. The current reaches its maximum with high-reactance winding at a later time and the rotor experiences a shift in magnetic field, providing the necessary starting torque. When the motor is nearly up to speed, the high-resistance winding is disconnected by a centrifugal switch.

The capacitor motor shown in Figure 41.4 employs a capacitor in series with an auxiliary winding to provide the necessary phase shift. For improved performance, two capacitors are used. The larger capacitor provides good starting torque and it is then switched out by a centrifugal switch. The smaller one remains in the circuit to provide better operating efficiency and power factor.

Figure 41.4 Single-phase Induction Motors (a) Shaded Pole (b) Split Phase (c) Capcitor

41.3  GETTING THE ROTOR STARTED

The problem in single-phase induction motor design is to get the rotor started. There are several ingenious methods of doing this. In the shaded-pole motor, Figure 41.4(a), a heavy copper coil is wound around one half of each salient stator pole. Induced circuits in the shorted turn delay the build-up of magnetic flux in that region of the pole. The magnetic flux vector appears to shift as a function of time and the rotor experiences the effect of a partially rotating field.

Figure 41.5 Operation of a Centrifugal Switch

The running winding consists of insulated copper wire. It is placed at the bottom of the stator slots. The wire size in the starter winding is smaller than that of the running winding. These coils are placed on top of the running winding coils in the stator slots closest to the rotor.

Both the starting and running windings are connected in parallel to the single-phase line when the motor is started. After the motor accelerates to speed (approximately two-thirds to three quarter of the rated speed), the starting winding is disconnected automatically from the line by means of a centrifugal switch.

The rotor for the split-phase motor is similar to that of the squirrel-cage induction motor, that is, the rotor consists of a cylindrical core, assembled from steel laminations. Copper bars are mounted near the surface of the rotor. The bars are brazed or welded to two copper endings. In some motors, the rotor is a one-piece cast aluminium unit. The rotor fans are a part of the squirrel-cage rotor assembly. These rotor fans maintain air circulation through the motor to prevent large increase in the temperature of the rotor windings.

The centrifugal switch is mounted inside the motor as shown in Figure 41.5, and disconnects the starting winding after the rotor reaches a predetermined speed. The switch consists of a stationary part and rotating part. The stationary part is mounted on one of the end shields and has two contacts that act like a single-pole single-throw switch. The rotating part of the centrifugal switch is mounted on the rotor.

A simple diagram illustrating the operation of the centrifugal switch is given in Figure 41.5. When the rotor is at a standstill, the pressure of the spring on the fibre ring of the rotating part keeps the contacts closed. When the rotor reaches approximately three-quarters of the rated speed, the centrifugal action of the rotor causes the rotor to release its pressure on the fibre ring and the contacts open. As a result, the starting winding is disconnected from the line.

41.4  CONSTRUCTION

The split-phase induction motor consists of a stator, a rotor, a centrifugal switch located inside the motor (Figure 41.6), two end shields housing the bearings that support the rotor shaft and a cast steel frame into which the stator core is pressed. The two end shields are bolted to the cast steel frame. The bearings housed in the end shields keep the rotor centred within the stator so that it rotates with a minimum of friction and without striking or rubbing the stator core.

The stator for a split-phase motor consists of two windings held in place in the slots of a laminated steel core. The two windings consist of insulated coils distributed and connected to make up two windings spaced 90° apart. One winding is the running winding and the second one is the starting winding.

Figure 41.6 Cut-away View of a Split-Phase Motor

41.5  PHASE SPLITTING

Initiating a rotating magnetic field from a single-phase source, without resorting to mechanical means, requires the use of two stator windings and a phase splitting circuit. The physical layout of the windings for an elementary two-pole split-phase motor is shown in Figure 41.7(a), and the corresponding equivalent circuit diagram of the motor is shown in Figure 41.7(b). The main winding supplies the direct-axis flux (ϕd) and an auxiliary winding displaced at ninety electrical degrees from the main winding supplies the quadrature flux (ϕq). The auxiliary winding is also called the starting winding.

The phase splitter is connected in series with the auxiliary winding, causing the current in the auxiliary winding to be out of phase with the current in the main winding. As the magnetic field due to a current is in phase with the current that produces it, the quadrature field and the main field will be out of phase, resulting in a rotating flux and induction motor action.

Figure 41.7 (a) Elementary Two-pole Single-phase Motor with Phase Splitter (b) Equivalent Circuit Diagram

Phase splitting may be accomplished through the use of capacitance or resistance. If accomplished through the use of capacitance, the motor is called a capacitance-start split-phase motor; if it is accomplished through the use of resistance, it is called a resistance-start split-phase motor. Regardless of the means used to start the rotor turning (be it phase-splitting or mechanical action), once it starts turning, self-excitation will maintain the quadrature field, and the auxiliary winding with its phase splitter may be disconnected.

41.6  LOCKED-ROTOR TORQUE

The locked-rotor torque of a split-phase motor is proportional to the magnitudes of the locked-rotor current in each winding times the sum of the angle of phase displacement between the two currents. Expressed mathematically,

 

Tlr = ksp · Imw · Iaw sin α        (41.2)

 

α =│θi,mwθi,aw│        (41.3)

 

where, ksp = machine constant, split-phase motor

Iaw = current in auxiliary winding (A)

 Imw = current in main winding (A)

θi,aw = phase angle of current in auxiliary winding

θi,mw = phase angle of current in main winding

α = phase displacement between Iaw and Imw.

 

Example 41.1

The main and auxiliary windings of a hypothetical 120 V, 60 Hz, split-phase motor have the following locked-rotor parameters:

 

Rmw = 2.00 Ω   Xmw = 3.50 Ω


Raw = 9.15 Ω   Xaw = 8.40 Ω

 

The motor is connected to a 120 V, 60 Hz system. Determine the (1) locked-rotor current in each winding; (2) phase displacement angle between the two currents; (3) locked-rotor torque in terms of the machine constant; (4) external resistance required with the auxiliary winding to obtain a 30° phase displacement between the two currents; (5) locked-rotor torque for the conditions in (3); and (6) per cent increase in torque due to the addition of external resistance.

 

Solution:

The circuit for the original conditions is shown in Figure 41.8(a)

  1.  
  2.  α = │θi,mwθi,aw│ = │−60.2551 − (−42.5530)│ = 17.7021°
    = 17.7°
  3. Tlr = kspImwIaw sin α
    = k × 29.7688 × 9.6610 × sin 17.7021
    = 87.45ksp

     

  4. The circuit for the new condition, with a resistor in series with the auxiliary winding, is shown in Figure 41.8(b). A phasor diagram showing the respective currents for the old condition and the desired location of the new, auxiliary winding current is shown in Figure 41.8(c). The required phase angle for is as follows:

     

    θi,aw = –60.2551° + 30° = –30.2551°

    Applying Ohm’s law to the auxiliary branch in Figure 41.8(b), we obtain

    From the impedance diagram for the new auxiliary circuit branch in Figure 41.8(d), we obtain

  5.  
    Tlr = kspImwIaw sin α

    = ksp × 29.7668 × 7.1979 × sin 30°
    = 107.1 ksp
  6.  

Figure 41.8 For Example 41.1 (a) Original Circuit (b) Modified Circuit (c) Phase Diagram for Determining the Required Phase Angle of Auxiliary Current for New Conditions (d) Impedance Diagram for the New Auxiliary-Circuit Branch

Note: The added resistance in the auxiliary winding circuit decreased the auxiliary winding current, but increased the locked-rotor torque.

 

Example 41.2

Carry out a graphical analysis for Example 41.1

 

Solution:

Because only the auxiliary winding has series-connected elements to provide phase splitting, the current in the main winding may be assumed to be constant, permitting Equation 41.2 to be written as

 

Tlr α Iaw sin α        (41.4)

 

Graphs of Іaw, 1 and (Іαw sin α) as a total resistance of the auxiliary winding circuit in Example 41.4, as Rx is increased from 0 Ω to 20 Ω, are shown in Figure 41.9. Note the following:

  1. The current in the auxiliary winding decreases with increasing resistance.
  2. Angle α increases with increasing resistance.
  3. The locked-rotor torque Iaw sin α1 reaches a peak value with an auxiliary circuit resistance of approximately 14.2 Ω and decreases with increase in resistance.

Note: For every split-phase motor there is an optimum value of auxiliary circuit resistance that will maximize the locked rotor torque. The phase displacement for this optimum value of resistance is generally between 25° and 30°.

41.7  RESISTANCE-START SPLIT-PHASE MOTORS

The circuit diagram for a general-purpose resistance-start split-phase motor is shown in Figure 41.10(a). The auxiliary winding is wound with a smaller diameter than the main winding, causing the auxiliary winding to have a higher ratio of resistance to reactance than the main winding. The switch with auxiliary circuit is a magnetic relay, a solid-state switch, or a centrifugally operated switch. The centrifugally operated switch, as shown in Figure 41.10(a), is closed when the motor is at rest and open when the rotor is at 75–80 per cent synchronous speed. A solid-state switch, called a triac, is shown with broken lines in Figure 41.10(a); the switch closes when starting and is set to open at approximately 75 per cent synchronous speed. A magnetic relay (not shown) is closed by a high motor-staring current, and springs open when the acceleration of the motor reduces the current to approximately 80 per cent of the locked-rotor current. A representative phasor diagram for the motor (when starting) is shown in Figure 41.10(b).

Figure 41.9 Graphs of Auxiliary Winding Current, Phase-displacement Angle α, and Locked-rotor Torque Represented by Iaw Sin α, for the Split-phase Motor in Example 41.1

A typical torque-speed characteristic for a resistance-start split-phase motor is shown Figure 41.10(c).

This motor is adaptable to loads such as centrifugal pumps, oil burners, blowers and other loads of similar characteristics that require moderate torques and constant speed. This motor offers no means for speed control from a fixed frequency source other than that obtained by recounting for different pole arrangements.

Figure 41.10 Resistance-start Split-phase Motor (a) Circuit Diagram (b) Phase Diagram (c) Torque-speed Characteristic

41.8  CAPACITOR-START SPLIT-PHASE MOTORS

A capacitor-start split-phase motor develops a much larger (Іaw, sin α), and hence a much larger locked-rotor torque, than does the resistance-start split-phase motor. The value of capacitance that produces the greatest locked-rotor torque in a capacitor-start split-phase motor causes a phase-displacement angle α of between 75° and 88° compared with the 25° and 33° phase-displacement angle of the resistance-start split-phase motor. The circuit diagram and phase relationships for the capacitor-start split-phase motor are shown in Figure [41.11(a, b)], respectively.

A typical torque-speed characteristic for the motor is shown in Figure 41.11(c). The starting curve shows the motor characteristics, with both the auxiliary and main windings energized. The running curve shows the characteristic behaviour after the auxiliary winding is disconnected. A comparison of the characteristic with that of the resistance start split-phase motor in Figure 41.10(c) shows that the running characteristics of both machines are essentially the same. The significant difference between the two machines is the starting torque; about 130 per cent rated for the resistance-starts split-phase motor and 300 per cent rated for the capacitor-start split-phase motor. The high starting torque and good speed regulation of the capacitor-start motor make it well suited for applications in stokers, compressors, reciprocating pumps and other loads of similar characteristics. This motor offers no means for speed control from a fixed frequency source other than that obtainable by reconnecting for a different number of poles.

Figure 41.11 Capacitor Motors (a) Circuit for Capacitor-start Motor (b) Phase Diagram Corresponding to (a) (c) Torque-speed Characteristic for Motor in (a) (d) Permanent-split Capacitor Motor (e) Two-value Capacitor Motor

Neither the resistance-start split-phase motor nor the capacitor-start split-phase motor can attain synchronous speed. The rotating flux depends on current in the rotor to produce the quadrature field. As the rotor approaches synchronous speed, the speed-voltage is induced in the rotor, the associated current in the rotor and the quadrature flux approaches zero. Hence, the accelerating torque will become zero at slightly below synchronous speeds. However, permanent-split capacitor motors and two-value capacitor motors are in effect two-phase motors, and at no load could attain synchronous speed.

41.8.1  Permanent-split Capacitor Motors

A permanent-split capacitor motor utilizes a permanently connected auxiliary circuit containing a capacitor. There is no switch in the auxiliary circuit, and its operation is smoother and quieter than a capacitor-start or resistance-start motor of the same power rating. The values of capacitance for this type of motor are smaller than the one used in the capacitor-start motor and is a compromise between the best starting and best running performances. The primary field of application for a permanent-split capacitor motor is for shaft-mounted fans used in heaters and for ventilating fans. Its speed may be varied by a tapped or slide-wire autotransformer in the main line, as shown in Figure 41.11(d); by using an external resistor or reactor in series with the main winding or in series with both windings or by adjusting the number of turns in the main winding through the use of taps and a selector switch or by solid-state control.

41.8.2  Two-value Capacitor Motor

A two-value capacitor motor, shown in Figure 41.11(e), provides a greater amount of capacitance for starting than for running. This provides a greater locked-rotor torque than is obtainable with the permanent-split capacitor motor, and a reduced capacitance when running results in improved power factor, improved efficiency and higher breakdown torque.

41.9  REVERSING SINGLE-PHASE INDUCTION MOTORS

The reversing of the direction of rotation of the motor is accomplished by stopping the machine, interchanging the leads to the auxiliary circuit and then restarting. This reverses the quadrature-axis flux, causing flux rotation to be in the opposite direction. This is illustrated in Figure 41.12.

Figure 41.12 Reversing the Direction of Rotation of a Split-Phase Induction Motor

41.10  DUAL-VOLTAGE OPERATION

Single-phase motors often have dual-voltage ratings. To obtain these ratings, the running winding consists of two sections. Each section of the winding is rated. With reference to Figure 41.13, the motor is rated at 115/230 V. One section of the running winding is marked T1 and T2 and the other section is marked T3 and T4. If the motor is to be operated on 230 V, the two 115 V windings are connected in series across the 230 V line. If the motor is to be operated on 115 V, then two windings are connected in parallel across the 115 V line. This is illustrated in Figure 41.13.

The starting winding, however, consists of only one 115 V winding. The leads of the starting winding are generally marked T5 and T6. If the motor is to be operated on 115 V, both sections of running winding are connected in parallel with the starting winding, as shown in Figure 41.13(a).

For 230 V operation, the connection jumpers are changed with the terminal box so that the two 115 V sections of the running winding are connected in series across the 230 V line. This is shown in Figure 41.13(b). The 115 V starting winding is connected in parallel with one section of the running winding. If the voltage drop across this section of the running winding is 115 V, then the voltage across the starting winding is also 115 V.

Some dual-voltage split-phase motors, as shown in Figure 41.14, have a starting winding with two sections and a running winding with two sections. The running winding sections are marked T1 and T2 for one section and T3 and T4 for the other section. One section of the starting winding is marked T5 and T6 and the other section of the winding is marked T7 and T8.

Figure 41.13 shows the winding arrangement for a dual-voltage motor with one starting winding and two running windings. The correct connections for 115 V operation and for 230 V operation are given in the table shown in Figure 41.14.

Figure 41.13 Dual-voltage Operation of Single-phase Motors: (a) Motor Connected for 115 V (b) Motor Connected for 230 V

Figure 41.14 Winding Arrangement with Two Starting and Two Running Windings

Example 41.3

Using the given data for the split-phase motor windings in Example 41.1, determine (1) the capacitor required in series with the auxiliary winding to obtain a 90° phase displacement between the current in the main winding and the current with auxiliary winding at locked rotor; (2) locked-rotor torque in terms of the machine constant.

 

Solution:

  1. The winding impedances in Example 41.1 are

    The circuit for original conditions is shown in Figure 41.15(a)

    The circuit diagram for the new condition (with a capacitor in series with the auxiliary winding) is shown in Figure 41.15(b), and a phasor diagram showing the respective currents for the original condition and the desired location of the new auxiliary-winding current is shown in Figure 41.15(c). The required phase angle for is

    Applying Ohm’s law to the auxiliary branch in Figure 41.15(b), we get

    Figure 41.15 For Example 37.3 (a) Original Circuit (b) Modified Circuit (c) Phasor Diagram for Determining Required Phase Angle of Auxiliary Current for New Conditions, (d) Impedance Diagram for New Auxiliary-circuit Branch

    Thus

    From the impedance diagram shown in Figure 41.15(d) for the new auxiliary circuit branch, we get

    Xc  = 8.40 − 9.15 × tan (−29.74°) = 13.628 Ω
  2.  
    Tlr = kspImwIaw, sin α = ksp × 29.7688 × 11.387 × sin 90°
    Tlr = 338.9 ksp

Note: The per cent increase in locked-rotor torque obtained by capacitor start in Example 41.3 with respect to the locked-rotor torque obtained by resistor start in Example 41.1 is

Example 41.4

Carry out a graphical analysis of Example 41.3.

Graphs of Iaw, α and (Iaw sin α), plotted against Xc for Example 41.3, are shown in Figure 41.16. The current in the auxiliary winding increases and then decreases with increasing capacitive reactance (resonance phenomena); angle α increases with increasing capacitive reactance and the locked-rotor torque, represented by Iaw sin α, increases to some peak value and then decreases with increasing capacitive reactance. Note that for the given winding parameters, the optimum value of capacitive reactance that resulted in a phase displacement angle of approximately 75° produced the greatest locked-rotor torque.

A comparison with the Iaw sin α curve for the capacitor motor in Figure 41.16(c) with that of the split-phase motor in Figure 41.9(c) shows that with the same windings, phase shifting with capacitance can produces significantly greater locked-rotor torques than can phase shifting with resistance.

Figure 41.16 For Example 41.4 Graphs of Auxiliary Winding Current, Phase Displacement Angle α, Locked-Rotor Torque Represented by Iaw sin α, for the Capacitor Start Motor in Example 41.3

41.11  SHADED-POLE MOTORS

The starting methods employed so far are generally based on the principle of producing a rotating magnetic field to initiate rotor rotation. Split-phase motors employ stators with uniform air gaps with respect to their rotor and stator windings, which are uniformly distributed around the periphery of the stator.

The shaded-pole motor, illustrated in Figure 41.17(a), utilizes a short-circuited coil or copper ring, called a shading coil, to provide the starting torque. The shading coil is wound around a part of the pole face and acts as the short-circuited secondary of a transformer.

Figure 41.17(a) shows the general construction of a salient two-pole shaded-pole motor. The special pole pieces are made up of laminations, and a short-circuited shading coil is wound around the smallest segment of the pole piece. The shading coil, separated from the main a.c. field winding, serves to provide a phase-splitting of the main field flux by delaying the change of flux with the smaller segment.

As shown in Figure 41.17(b), when the flux with field poles tends to increase, a short-circuit current is induced in the shading coils, which by Lenz’s law opposes the force and flux producing it. Thus, as the flux increases in each field pole, there is a concentration of flux in the main segment of each pole, while the shaded segment opposes the main field flux.

At point C, as shown in Figure 41.17(e), the rate of change of flux and current is zero, and no voltage is induced in the shaded coil. Consequently, the flux is uniformly distributed across the poles. When the flux decreases, the current reverses in the shaded coil to maintain the flux in the same direction. The result is that the flux now crowds in the shaded segment of the pole.

An examination of Figs. 41.17(b, c, d) will reveal that at intervals b, c, and d, the net effect of the flux distribution in the pole has been to produce a sweeping motion of flux across the pole face representing a clockwise rotation. The flux with the shaded-pole segment is always lagging the flux in the main segment in time as well as in physical space (although a true 90° relation does not exist between them). The result is that a rotating magnetic field is produced, sufficient to cause unbalance in rotor torques (double-revolving field theory) such that the clockwise torque exceeds the counter-clockwise torque (or vice versa) and the rotor always turns in the direction of the rotating field.

Figure 41.17 Shaded-pole Motor Construction, Operation and Characteristics

The electrical characteristics of shaded-pole motor are shown in the torque-slip curve of Figure 41.17(f). The staring torques are very small and nominally about 25 per cent of the full-load torque. Rated torque, depending on the horse power occurs nominally at about 10–25 per cent slip. Maximum breakdown torque is slightly higher than rated and occurs at slips between 30 and 40 per cent. Efficiencies vary from 5 to 35 per cent.

Split-phase induction motors are manufactured in both fractional and integral HP motor sizes. The shaded-pole motor is usually a small fractional HP motor not exceeding 1/10 HP, but motors up to ¼ HP have been produced. The great advantage of this motor lies in its utter simplicity – a single-phase rotor winding, a cast squirrel-cage rotor and special pole pieces. No centrifugal switches, capacitors, special starting windings or commutators are used.

41.11.1  Reversing Shaded-pole Motors

To reverse the direction of rotor rotation, it would be necessary to unbolt the pole structure and reverse it physically. To eliminate such a slow and complicated process, newer techniques have been devised for producing reversible shaded-pole motors.

The first of these techniques is to connect the shading coils in series on corresponding shading segments and short-circuit them through a switch. As shown in Figure 41.18(a), the shading coils on trailing salient pole tips on one side are short-circuited for CW rotation and those on trailing pole tips on the opposite side of the pole are short-circuited for CCW rotation. At no time, however, are both sets of trailing-pole tips short-circuited.

The second method is generally used with non-salient pole stators. Two separate distributed windings, 90° in space with respect to the short-circuited shaded poles, are shown as windings A and B in Figure 41.18(b). When winding A, not shown as distributed, is energized, the flux pattern is in a CW order: winding A, shaded pole A′, winding A (in the location of B) and shaded pole B′. When winding B is energized, the flux pattern is CCW. Winding B, shaded coil A′, winding B distributed at A and shaded coil B′.

The third method, as shown in Figure 41.18(c), also employs a single continuous distributed winding with appropriate taps at the 90° points. When the taps of one set are energized by the double-pole double-throw, switch, the rotor rotates clockwise. When the taps of a second set, displaced by 90° with respect to the shading coils, are energized the motor rotates in the counter-clockwise direction.

Note: While the shaded-pole motor using the methods described in Figure 41.18 is a reversible motor, it is not a reversing motor. Once started in a specified direction of rotation, it must be brought to a standstill before the motor direction is reversed.

Figure 41.18 Methods of Reversing Shaded-pole Motors

SUMMARY
  1. The stator of single-phase induction motors has a starting winding and a running winding.
  2. The rotor of a single-phase induction motor is similar in construction to the rotor of a two-phase squirrel-cage motor.
  3. The centrifugal switch disconnects the starting winding after the rotor reaches a predetermined speed, usually two-third or three-quarter of the rated speed.
  4. The motor is called a split-phase motor because it starts like a two-phase motor from a single-phase line.
  5. The motor must have both the starting and running winding energized at the instant the motor circuit is closed to create the necessary starting torque.
  6. If the mechanical load is too great when a split-phase motor is started, or if the terminal voltage is too low, then the motor may fail to reach the speed required to operate the centrifugal switch.
  7. To reverse the direction of rotation of the motor, simply interchange the leads of the starting winding.
  8. The speed regulation of a split-phase induction motor is very good.
  9. The starting torque of the split-phase motor is comparatively poor.
  10. In a capacitor-start induction-run motor, the capacitor provides a higher starting torque than is obtainable with a standard split-phase motor.
  11. The capacitor limits the starting surge current to a lower value than is developed by the standard split-phase motor.
  12. The capacitor is used to improve the starting torque and does not improve the power factor of the motor.
  13. The capacitor-start induction-run motor is used in those applications where there are relatively few starts in a short period of time.
  14. In a capacitor-start capacitor-run motor, the starting winding and the capacitor are connected in the circuit at all times.
  15. The capacitor-start capacitor-run motor has a very good starting torque.
  16. The problem in single-phase induction motors is to get the rotor started.
  17. The auxiliary winding has a higher ratio of resistance to reactance.
  18. For dual-voltage operation, the running winding consists of two sections.
  19. The shaded-pole motor utilizes a short-circulated coil or copper ring called a shading coil.
  20. The shading coil acts as the short-circuited secondary of a transformer.
  21. While a shaded-pole motor is a reversible motor, it is not a reversing motor.
MULTIPLE CHOICE QUESTIONS (MCQ)
  1. The starting torque in a single-phase induction motor is
    1. High
    2. Very high
    3. Low
    4. Very low
    5. Zero
  2. The efficiency of a single-phase induction motor as compared to that of a three-phase induction motor of the same power rating is
    1. Higher
    2. Much higher
    3. Lower
    4. Much lower
  3. The no-load current of a single-phase induction motor is of the order of
    1. 30 per cent
    2. 45 per cent
    3. 60 per cent
    4. 75 per cent
  4. The centrifugal switch closes when the rotor reaches approximately
    1. One-fourth of the rated speed
    2. Two-third of the rated speed
    3. Three-fourth of the rated speed
    4. Rated speed
  5. Initiating a rotating magnetic field from a single-phase source requires
    1. Two stator windings
    2. A phase-splitting circuit
    3. Two stator windings and a phase splitting circuit
    4. None of the above
  6. Shaded-pole motor is a
    1. Reversible motor
    2. Reversing motor
    3. None of the above
ANSWERS (MCQ)
  1. (e)
  2. (c)
  3. (b)
  4. (c)
  5. (c)
  6. (a)
CONVENTIONAL QUESTIONS (CQ)
  1. What prevents a single-phase induction motor from being self-starting unless it has special starting circuit provisions?
  2. Describe the basis of the double-revolving field theory.
  3. How does the creation of a second artificial phase enable a single-phase-motor to develop starting torque?
  4. How is the required phase shift accomplished in a resistance split-phase motor?
  5. What is the function of the centrifugal switch in a single-phase motor?
  6. What happens when the centrifugal switch fails to open?
  7. How is the required phase shift accomplished in a capacitor-start induction motor?
  8. What circuit change enables a resistance split-phase or capacitor-start induction motor to be reversed?
  9. What advantage does a capacitor-start capacitor-run motor have over a capacitor-start motor?
  10. How does an auto transformer enable a capacitor to perform as two different values of capacitor in a capacitor-start capacitor-run motor?
  11. What future limits the utility of a permanent-split capacitor motor?
  12. Why is a single-phase induction motor less efficient than a comparable power three-phase induction motor?