CHAPTER 13

SINGLE-PHASE MOTORS 13.1 Introduction There are two basic forms of construction for single-phase

motors. One is almost identical to that of the three-phase induction motor while the other is of a form similar to that

of the d.c. series motor. Both types are popular but for larger sizes the induction motor is the most highly regarded because of its simplicity, ruggedness and reliability. The series or "universal" motor is more popular in smaller sizes where its high speed and light weight give it many advantages. The three-phase induction motor discussed in the previous chapter had an inherent starting torque because of its rotating field, but the single-phase induction motor initially has no rotating field, and so no starting torque. Special techniques have to be adopted to ensure the starting of the single-phase induction motor and because of the various starting methods employed there are several versions of the single-

phase induction motor.

13.2 Producing starting torque single-phase motor In the three-phase motor the supply consist< identical currents being supplied to thre• windings in the motor. The resultant ma! rotated at constant speed and strength. Ide• identical currents at 90° E could be suppli windings in the two-phase motor. then ~ magnetic field would rotate at synchronorn motor can be wound with two windings but on it to a single-phase supply the two cum probably be in phase with each other and no re would be produced. The rotating magnel produced in the single-phase induction simulating the effects of a two-phase motor. problem is to ensure the motor has two cur appropriate phase angle to each other. 11 achieved by having windings of different indrn sometimes by adding a capacitor in series witt windings. Once the motor is rotating at a sui one of the windings can be disconnected and will continue to rotate. There are two chc peculiar to single-phase motors. The motor vibration of twice line frequency when runni tends to make the motor more noisy in opera ti three-phase motor. The second is that the mot' an amount of negative torque which is a func slip speed. This results in a rather high no-load low power factor. When a load is applied to th current changes only marginally, but the pc improves in a similar fashion to the three-phc

13.3 Single-phase induction m1

Fig. 13.1 A single-phase 180 W motor

244

13.3.1 Split-phase motor The standard split-phase induction moto separate windings (start and run) connected supply during the starting process. For norm however, only the run winding is used. The run winding consists of a numb1 connected in series to form a set number
" is zero and the start flux ii> s is 50% of the maximum value in a positive direction. The resultant

stator flux at position a is shown in Figure 13.5(b). At position b in Figure 13.4, ii>" is 50% of its

direction. That is, the direction of rotation of

reversed by changing the direction of cu through one winding. This is done by exchan1 end connections of any one winding.

As seen in Figure 13.5(b), the rotating s not of uniform value and an elliptical fie!< produced. This produces considerable vib: humming noise during starting.

The rotating stator field cuts the rote induces a voltage in them and, because they out, a current flows through the bars and pro( flux. The stator flux and the rotor flux interac a force on the rotor bars, causing the rotor tc direction in which the stator flux is rotating. 1

maximum value and sis 86.6% of its maximum value, and these combine to form the resultant stator flux b in

Figure 13.5(b). At position cin Figure 13.4, il>.is 86.6% of its maximum value and il>sis 100%, and the resultant stator flux c is shown in Figure 13.5(b). By taking each position from a to I in Figure 13.4 it can be seen that the stator flux rotates one full revolution for one full cycle. The stator flux rotates at a speed governed by the supply frequency and the number of poles in a winding.

In = l~Of I

i.e. where

f

=

frequency

p = number of poles

n =speed in r/min For a two-pole machine on a 50 Hz supply, n = 3000r/min For a four-pole machine,

n

= 1500 r/min.

Fig. 13.6 Types of switching mechanisms for s, motors. Also shown is a centrifugal a1 lower left. SIMPSON AP

SINGLE-PHASE MOTORS

247 A

,,0

/0 10 10 \0 \ 0

'

A

I

I

'

I 4

I )

I I \

t

,

~\

..

I

.-;

/e

r:\ ..

"')

e"'/

\

G> I

;;;

( I

\ I I

..

0\ 01

t

t

'-

., E• \

01

I'

I

I

0;

-'~

c

c

(a) First hall cycle

(b) Second half cycle Fig. 13.7 Pulsating stator field

force is called the starting torque and largely depends upon the relative strengths of the start and run fluxes, and the phase .displacement between the currents flowing through both windings. The start and run windings are connected in parallel across the supply voltage. When the rotor has reached sufficient speed to provide a strong cross flux, the start winding can be open-circuited. This is usually done by connecting a centrifugally operated switch in series with the start winding (Fig. l 3.2(b )). The centrifugal switch is usually set to open when the rotor speed reaches approximately 75% of the rated speed of the motor. When the motor is switched off, the rotor slows down and the centrifugal mechanism operates, closing the switch contacts again in readiness for the next starting operation.

A

Because the start winding is only connected during the starting procedure, it is designed for a very short duty cycle. If the centrifugal switch fails to operate, the start winding will quickly overheat and burn out. Running

When the rotor speed of the standard split-phase motor reaches approximately 75% of the synchronous speed, the centrifugal switch open-circuits the start winding and only the run winding is connected to the supply. For a two-pole motor when the stator current flows in one direction for one half-cycle, a magnetic field is produced in the direction C-A in Figure 13.7(a). During the next half-cycle when the stator current is reversed, the magnetic field also reverses and is in the direction A-C in Figure 13.7(b).

A

8

c '

(a) Stator flux

c (b) Rotor flux

Fig. 13.8 Magnetic fields in a rotating single-phase motor

248

ELECTRICAL PRINCIPLES FOR THE ELECTRICAL V rotor

This stator field, produced by the run windings, varies in strength and direction according to the supply, but it does not rotate. It is a stationary pulsating field. This is the reason why some form of starting (i.e. start winding) is required for split-phase motors.

When the stator winding is connected to the a.c. supply and the rotor is turning, the rotor bars cut the stator flux, causing an e.m.f. to be generated in them. In Figure l 3.8(a) on page 24 7 the rotor is revolving in a clockwise direction, and the stator field is acting in the direction C-A. By Fleming's right-hand rule (sect. 6.1.1) the generated e.m.f. in the rotor acts in the direction shown (out of the page) in all rotor bars above the axis D-B (indicated by the dots), and into the page in all rotor bars below the axis D-B. The induced rotor voltages are in phase with the stator flux and cause a rotor current to flow. Because of the low resistance and high inductance of the rotor bars, these currents lag the induced rotor voltage by nearly 90°. Consequently, the rotor currents produce a rotor flux lagging almost 90° behind the stator flux, and acting in the direction D-B, as shown in Figure l 3.8(b). Because the rotor flux is at right angles to the stator flux it is often referred to as the "cross field". The two fields effectively combine to form a rotating field, which tends to force the rotor bars in the direction in which the field rotates. For one full cycle of the a.c. supply the resultant field rotates 360°E. For the two-pole machine described, this constitutes one full revolution. For a four-pole machine, it will rotate a half revolution for each full cycle of the a.c. supply. Due to the internal losses within the rotor, however, the rotor itself will not rotate at synchronous speed, but at a slightly slower speed. Figure 13.10 shows a typical torque/ speed characteristic for a split-phase motor. The break in the curve is caused by the switch operating to disconnect the starting winding. This is necessary to limit the losses in the motor and to protect the starting winding. The torque curves between the running and starting sequences normally do not coincide, so the speed and torque values have to adjust when the switch operates. The values shown on the curve must be considered as representative only. They will vary from one make to another and even within the one make because of design changes.

a

c

d

Fig.13.9 Phase relation of s, VR, R

obtained. Any value capacitor will increase th1 values that enable the starting winding tc resonance must be avoided. For this reason it to ensure that too large a value of capacitor is than a small value. The phasors in Figm indicate the ideal phase displacement of 90° E of phase displacement between I. and ls prov uniform strength of stator flux during starting Figures l 3.5(b) and 13.12(b). Due to this m01 strength, the starting torque is higher than th< sized split-phase motor. Figure 13.13 speed/torque curve for a capacitor-start n drawn in the same proportions as that of Figt give a framework for comparison. It can be see is a large increase in starting torque due to the the capacitor, while the torque is the same c: phase motor after the switch has operated. I fashion to the split-phase motor the switch approximately 75% of full-load speed. The act windings for the two types of motor may different data. Reversal of rotation is achie same principles applying to the split-phase mol the motor can be reversed by changing over th of any one winding but not both.

Uses For general-purpose heavy-duty application; high locked rotor starting torque, such ; refrigerators and air compressors. s

Uses

400

Split-phase motors have only moderate starting torque so they are limited to such typical uses as washing machines, blowers, buffing machines, grinders and machine tools.

300

13.3.2 Capacitor-start motor Design limitations restrict the split-phase motor to a maximum of about 30° E betw~en the starting and running winding currents. To increase this angle and produce improved characteristics a capacitor is connected in series with the starting winding (see Fig. 13.1 l(a)). If the correct size capacitor is selected then the two currents are at 90°E to each other and improved starting torque is

b

200 Torque %

100

I

\ ___ ________ _ ....._Rated torque

Speed

Flg.13.10 Speed/torque curve fora split-pha~

249

SINGLE-PHASE MOTORS

Run winding

Start winding

90"E Capacitor

a.c. supply (a} Electrical connections /A

(b) Phasors

Flg.13.11 Capacitor-start, induction-run motor

k

/ h

"

I

\

I I

\ I Ia I

g

I

90°E

\

f

a

b

c

d

e

g

h

k

I

a d

(a) Waveforms

-

b

/

c

(b) Relative field strength

Fig.13.12 Rotating field in a capacitor-start motor

Switch speed

400

300

Rated speed

I I

200

I

I

Torque %

100

Rated torque -----------

I

Speed

Fig. 13.13 Speed/torque curve for a capacitor-start motor

13.3.3 Capacitor-start, capacitor-run motor This type of motor has both windings permanently connected across the supply; these are referred to as the main and auxiliary windings. During starting, additional capacitance is connected in series with the auxiliary winding to provide the necessary phase displacement between the winding currents for maximum torque. The starting capacitor is therefore connected in para1Iel with the running capacitor. When the rotor speed reaches about 75% of the rated speed, the centrifugal switch disconnects the starting capacitor from the circuit as shown in Figure 13.14. During operating conditions the running capacitor ensures the correct phase displacement between the two currents in the windings, so providing a constant strength rotating magnetic field. It should be noted that the starting capacitor can be rated for intermittent duty, but the running capacitor must be of a construction suitable for continuous rating

250

ELECTRICAL PRINCIPLES FOR THE ELECTRICAi Running capacitor

Auxiliary winding

Starting capacitor

Main winding

Fig. 13.14 Capacitor-start, capacitor-run motor

such as the paper-spaced oil-filled type. The twocapacitor motor provides substantially the same running torque as the capacitor-starting, induction-run type, but

there are beneficial effects. Adding the second capacitor: I. increases the breakdown torque; 2. improves full-load efficiency and power factor; 3. reduces operational noise and vibration; 4. increases locked rotor torque. The direction of rotation can be reversed by changing over the two leads of any one winding but not both. This changes the direction of rotation of the magnetic field in the stator.

Uses Heavy-duty loads where quietness is a consideration and substantial starting torque is necessary: wall mounted air-conditioning units where high head pressures are encountered in hot weather, for example.

13.3.4 Permanently split capacitor motor The permanently split motor also has both windings permanently connected across the supply, with a capacitor in series with one of them as shown in Figure 13. 15. For this type of niotor, both windings are identical in wire size and the nun1ber of turns, and are also referred to as the main and auxiliary windings. Because the

capacitor is in series with one winding, th~ that winding leads the current in the other, p· necessary phase displacement to produce a ro field. However, the phase displacement betw fluxes is relatively small, and so the startir. low. By interchanging the line connection fro1 Figure 13.15, the capacitor is then in series w instead of the auxiliary winding. The current winding leads that in the auxiliary winding a1 runs in the reverse direction. These motors are suitable for unit heate because their speed can be varied fairly easil~ inductances.

Uses Light applications with low starting torque, e blowers which may need to be reversed fre1 remote control of induction regulators and c regulating air flow in air-conditioning system

13.3.5 Shaded-pole motor The shaded-pole motor has a cage rotor with' in the stator. On one side of each pole, a slot shading ring is embedded into it, as show1 13.16. The shading rings are made of copper into a closed loop, providing a low resis through the ring. The supply current produces an alterna· each pole. This alternating flux cuts the sh inducing an e.m.f. in it. Because of the low resi~ the current flowing through the ring is rela Also, according to Lenz's Jaw, the induced cu shading ring will produce a flux that will tern the change of the main flux. When the supply current rises rapidly fro1 in Figure I 3. I 7(d) an induced voltage is establ shading ring. The current in the ring prod which opposes the build-up of the main flux. the main flux is concentrated in the unshadet the pole, as in Figure I 3. I 7(a).

A Main winding

L,

c \

)t---0

L,

\

\

Shading ring

\

\

\

\

\

', B

' Fig. 13.15 Permanently split motor circuit

Flg.13.16 Salient poles and shading

rin~

251

SINGLE-PHASE MOTORS

B

D

A Time

(a)

(d)

(c)

(b)

Flg.13.17 Centre line of flux moves towards the shading ring, giving the effect of a moving field

When the current changes from B to C, there is little change in value ofcurrent and very little voltage is induced in the shading ring. Consequently, practically no current nor flux is produced in the shading ring. The main flux is at this time nearly always at maximum value, and is uniformly distributed over the whole pole face, as seen in Figure 13.17(b). When the supply current drops rapidly from C to D, an induced voltage is established in the shading ring. The current in the shading ring produces a flux which opposes the collapse of the main flux. The concentration of flux therefore occurs in the shaded action of the pole, as shown in Figure 13.17(c). The magnetic axis shifts across the pole face, from the unshaded part to the shaded part of the pole. This shifting flux is similar to a rotating field, and produces a small torque, causing the rotor to rotate in the direction of the flux, towards the shaded section of the pole. The starting torque is very low as indicated in Figure I 3. 18 and the motor runs with a slip speed slightly higher than the single-phase motors described above. It is simple in construction, low in cost and reliable. There are no switches, slip-rings, brushes or capacitors that may require maintenance. The motor efficiency is down and this tends to restrict its use to low power ratings. Direction of rotation has to be reversed by altering the direction of the rotating magnetic field across the pole face. This is done by shifting the shading ring to the other

side of the pole face. Some poles are fitted with slots on both sides for this purpose but with others the only method is to remove the stator from its housing and replace it the other way around in the frame.

Uses Because its speed can be varied within a limited range by a series resistor or inductor it is suitable for fans and blowers, advertising signs, damper controllers, hair dryers and other uses where the starting torque requirements are minimal. 1

13.4 Commutator induction motors This type of single-phase motor is becoming scarce but is included here for interest. Called repulsion or repulsioninduction motors, all have a generally common build with minor variations only. The motor has a smooth bore stator with concentric windings forming the field poles. The rotor is somewhat similar to the armature of a d.c. machine, with either a radial or axial commutator (see Fig. I 3. I 9). The stator winding is connected to the supply while the rotor winding is connected to the bars of the

300

Rated

200

speed I

Torque %

I

100

Speed

n,,,.

Fig. 13.18 Speed/torque curve for a shaded/pole motor

(a) Radial

(b) Axial

Fig. 13.19 Repulsion motor armatures

252

ELECTRICAL PRINCIPLES FOR THE ELECTRICA

(±)@ ®®i:B

e

A

N

®

s_@ ®0~

®r:i

0 0 8

s

A

0 0 0 80000 No rotation

Fig. 13.20 Neutral brush position in a repulsion motor

commutator with permanently short-circuited brushes to provide current paths. Construction costs are high when compared to the normal cage-rotor induction motor but for amperes of starting current the starting torque is very high. There are three major forms of repulsion motor, all variations of the one type.

13.4.1 Repulsion motor This motor behaves in a similar fashion to the three-phase wound-rotor motor in that the speed is dependent to some extent on the applied load. Note that the windings on the repulsion-type rotor are not shorted out by a switch but by the brushes. In Figure 13.20 where the field poles are shown as salient poles for the sake of clarity, the rotor winding acts as the secondary of a transformer, and an e.m.f. is induced in each conductor. By Lenz's law the direction of these induced voltages is such that they tend to oppose the stator flux. If the brushes are along the axis A-A in Figure 13.20, the direction of current flow is as shown. The resultant rotor flux is also along the same axis A-A, but opposing the stator flux so no tangential force is produced, no torque is developed, and the motor cannot rotate. If the brush axis is rotated in a clockwise direction to position B-B, the direction of current flow in the rotor conductors is as indicated in Figure l3_2l(a). The rotor flux is along the axis B-B while the stator flux is still along axis A-A. The two fluxes interact, causing repulsion between them and a torque is developed causing the rotor to move in a clockwise direction and bringing successive armature conductors into an appropriate position to continue creating torque. If the brush axis is rotated anticlockwise to position C-C, as shown in Figure 13.21(b), the repulsion between the two fluxes causes the rotor to move in an anticlockwise direction. The normal method for changing the direction of rotation is by changing the brush position to either side of the marked centre position. Because of the high induced voltages in the rotor, the rotor current and flux is large. Consequently the repulsion motor develops a high starting torque. As the motor

accelerates so the induced voltage, rotor currj decrease. The motor torque then reduces dow running torque. Starting torque typically is i that of rated running torque.

13.4.2 Repulsion-start, induction-run m1 This motor has the same basic construction 1 addition of a centrifugally operated switch. commences rotation as a repulsion moto1 attaining about 75% of its rated speed the swil a mechanism that short-circuits all the segn commutator, effectively converting it into a motor. Reversal of rotation is effected by ~ brush position as for the previous type. Starti typically about 450% that of rated runn' although high-torque versions will give up to a A characteristic speed/torque curve for suet given in Figure 13.22. 13.4.3 Repulsion-induction motor This is the true RI motor, although theterm is applied to all forms of repulsion motor. construction to the repulsion motor, it has mechanism-instead it has a modified form cage under the rotor windings. Its operation the windings being more effective for startin high inductance cage becomes more effecti· motor has accelerated up to its rated speed. Th to give the more constant running speed of cage-induction motor. Reversal of rotation i the other two types in that direction is altered I the brush position. Starting torque is typicall~ of rated running torque.

Uses A very popular motor until three-phase s1 three-phase motors became readily availabl now restricted. Its attractiveness lay in its h torque with comparatively low starting cu1 power factor about 10% higher than the induction motor. Any high inertia load that ti

253

SINGLE-PHASE MOTORS

B

A

N

A

B (a) Clockwise rotation

~ @G:l (±)(fl EB s

A

N

c

©@a

(±) -

-_ 0

G:l

00

-

8

s

A

0 80000

c (b) Anticlockwise rotation

Fig. 13.21 Reversal of rotation by shifting the brush position in a repulsion motor

be accelerated up to speed may require such a motor. Many commercial grade refrigerators used them, as did older style industrial plants that used a central motor with many belts and pulleys.

600

500

13.5 Series motor The series motor is often called a universal motor because it can operate effectively on d.c. and a.c. up to power line frequencies. Like the normal d.c. series motor, it has a highly variable speed characteristic, with speeds up to 15 000 r/min in domestic appliances. Under some circumstances governors have to be used to restrict speeds to safe values. Field pole construction consists of a number of lamination stampings riveted together to form salient poles. The field coils are concentrated-type windings fitting closely around the salient poles. The armature construction is similar to that of a d.c. armature, with laminations, commutator and windings. The armature windings are connected in series with the

Switch speed

400

300 Torque %

Rated speed

200

I 100

Rated torque

I

Speed

Fig. 13.22 Speed/torque curve for a high-torque repulsion start induction-run motor

254

ELECTRICAL PRINCIPLES FOR THE ELECTRICAi A

s

N

N

s

B

B (a) First half cycle

(b) Second half cycle

Fig. 13.23 Torque production in a universal motor

two field coils by means of carbon brushes running on the commutator (see Fig. 13.23). There is a common current flowing through both windings, so the two magnetic fluxes produced are in phase with each other. Interaction of the fluxes produces

the speed is low; at light loads the speed is very the very small series motor in domestic use, t

losses (such as friction and windage) are large limit the speed to a safe value. The motor

torque to turn the armature. As the a.c. supply alternates

the fluxes change in unison so remaining in phase. When the line current flows from A to B in Figure l 3.23(a), north and south poles are produced as shown. Assuming the armature current is in the direction

indicated by the dots and crosses, the flux produced around the armature conductors interacts with the field flux producing an anticlockwise rotation.

When the line current flows from B to A on the alternate half-cycle, the polarities of the main fields are reversed as shown in Figure 13.23(b). The current through the armature also reverses, so reversing the

Speed

armature flux. The resultant torque is still in the anticlockwise direction so a steady rotation in one direction is maintained. Reversal of rotation is obtained

by changing the direction of current flow through the armature with respect to the field. That is, changing over the leads to the armature or the fields but not both (refer to Fig. 13.25). The speed/load characteristic for the series motor is shown in Figure 13.24. When the load is heavy,

Load

Fig. 13.24 Universal motor speed/load chara1

255~

SINGLE-PHASE MOTORS

'}...

Lt'.)

;,_

""

loc

,_,

-1-

,_ / ::;,

0-..

lo (a)

(b)

Fig. 13.25 Reversing direction of rotation in a universal motor

relatively high speed and has good starting and running torque characteristics considering its small size.

\.

~

reduced starting, running and breakdown torques, increased running noise and vibration, full load speed reduction and a higher operating temperature.

Uses Popular in portable appliances such as saws and drills, sewing machines, business machines, food mixers, small washing machines and vacuum cleaners.

13.6 Abnormal operating conditions In section 12. l l, abnormal operating conditions applicable only to three-phase motors were discussed. In this section more conditions are discussed, and apply to both single- and three-phase motors.

13.6.1 Voltage fluctuation This can be of two types: voltage rise and fall where the voltages remain symmetrical; and variation in individual phase voltages. This latter is especially detrimental to the performance of three-phase motors. Previously it has been shown that the torque produced is proportional to the square of the voltage. That is, ifthe voltage drops to 90% of its nominal value the torque reduces to 81 % of its rated value, Similarly, if the voltage rises to I 10%, then the torque increases to 121 % of its rated value. For example, a IO kW motor with a 10% voltage variation should now be rated at 8.1 kW or 12.1 kW. Under normal operating conditions a voltage variation of this magnitude should make only minor differences to the motor's characteristics. With a voltage increase for example, the increased torque reduces the slip only slightly so a motor rotatingat 1450 r /min on 50 Hz would increase its speed to approximately 1455 r /min. If advantage is taken of the increased torque, then the

operating temperature could be expected to increase. With voltage variations greater than I 0% the motor must be derated to prevent excessive temperature rise. Motors are normally given a full-time rating for a specified temperature rise, and under these conditions may have to be switched off after a duty period and be allowed to cool down. Starting and breakdown torque values are also affected. A voltage rise increases torque while a voltage reduction decreases torque. In the latter case care must be taken to see that the motor does not stall under load. For three-phase motors a more serious problem occurs when only one phase shifts in value. The phase current is affected to a greater proportion, which affects the rotating magnetic field in the motor. This results in

~

.\-

13.6.2 Higher operating temperatures Common causes of overheating in motors are inadequate or restricted ventilation and overloading, Apart from accelerated deterioration of lubricants, possibly the most serious effect is on the insulation. At increased temperatures there is a marked reduction in the life of insu1ation. For example, insulation designed to work at 90°C may have an expected life of 25 years. If the operating temperature is doubled to 180°C, the life expectancy is reduced to about 1.25 years. The cure is increased efficiency of the cooling system and a decrease of the load applied to the motor.

13.6.3 Frequency variation Motor speed can be regulated by the frequency of the supply and allowances are made when selecting a motor for this purpose, but a variation in supply frequency under

other circumstances can affect motor operation. The obvious effect is a change in speed, but there are also changes in power factor, efficiency and torque. A higher

frequency causes an increase in power factor, a slight increase in efficiency and a decrease in torque. The opposite occurs, with a decreased frequency. A variation in the frequency of supply usually occurs where there are comparatively few large loads connected to a smaller supply.

13.6.4 Overloading Manufacturers build into their motors the capability to handle short duration overloads as specified in AS 1359.41, In broad general terms the standard requires the motor to be able to withstand 1.5 times full load for a period of 15 seconds without appreciable change in speed or excessive heating. For a motor to operate under these conditions means that it must also have a breakdown torque in excess of the overload test figure, The heating effect in a machine winding is related to the square of the current and the time it is flowing, so any excess current must result in a temperature rise. With short time overloads the amount of heat generated is small and can expect to be dissipated by the normal cooling process. Long periods of overload, however, can lead to excessive increases in temperature, in turn leading to a shortened motor life. Other effects are a slight decrease in speed, decreased efficiency, decreased power factor, and an increased possibility of stalling because the working

\

'\__L

256

ELECTRICAL PRINCIPLES FOR THE ELECTRIC,!

torque is closer to the breakdown torque. When the motor stalls it draws starting current at full line voltage until its protection system operates. Large amounts of heat can be generated in short periods of time under these circumstances. It should be noted that special types of motors are given restricted duty cycles. In effect they are overloaded for short periods of time, after which they must be switched off and allowed to cool down to room temperature. If this type of motor is required to run on a continuous duty cycle, then it must be derated to a lower power value.

13.6.5 Frequent starting Motors in general are mechanically strong enough to handle normal loads with a fair safety factor. The number of times a motor is started, however, is not within the scope of the manufacturer unless specifically requested at time of purchase. When a motor is started there is a high current flow that decreases as the motor accelerates up to its operational speed. While this current is flowing, heat is being generated within the windings at a rate in excess of the usual heat dissipation rate. Under normal conditions this excess heat is removed by the cooling system while the motor is in operation. With repetitive starting, however, the heat generated does not have sufficient time to be removed and the temperature of the motor rises. The circumstances are similar for repeated reversing and plug braking. Starting current values, whether being used for starting, reversing or braking, repeatedly stress the windings. Unless the coils are firmly braced they rub against one another, eventually rubbing through the insulation and so causing short circuits within the windings of the motor. 13.6.6 Other factors Motors are designed to withstand normal operating conditions. Conditions other than these must be considered abnormal and need special consideration. Some of these are exposure to corrosive fumes, explosive vapours, dust, steam, salt air, high humidity, operation in ambient temperature of below approximately 10° C or above 40°C, or operation at altitudes in excess of !000 metres. Generally, all these factors can subject a motor to damage of some kind, but initially they are due to selection of the wrong type of stator housing at the time of purchase. Even motors selected for operation at elevated altitudes are subject to the same restriction and must be rated by the manufacturer at the time of purchase for the conditions under which they will work.

13.7 Single-phase synchronous motors Unlike other types of motors, the speed of synchronous motors is constant and is determined by the number of

poles and the frequency of the supply, whicr within very fine limits to the standard freqw Hz). This constant speed is the feature tha small single-phase synchronous motor suita applications as clocks, timers or recording d

13.7.1 Reluctance motors The stator winding of the reluctance motor the split-phase or capacitor-start motor. however, is assembled from laminations fr number of teeth are cut to form definite salie1 the windings are of the usual squirrel-cage t; The motor starts as an induction mol starting winding is open-circuited by the switch at approximately 75% synchror Because the load applied to this type < comparatively light, there is small slip. The poles tend to become permanently magne stator poles and become "locked" together poles are changing at the rate of twice frequency. The rotor is attracted by the stator the periods of the cycle when they are fully During the period when the stator flux is 10\ of the rotor carries it past the position of one and it is then attracted by the next stator pol build-up of the stator flux. Each rotor pc travels through the space of two stator poles supply frequency. The reluctance motor starts as an induc locks into synchronism and continues to run; synchronous speed. If the number of salient rotor is some multiple of the stator poles, th operate at a constant speed which is a subm1 synchronous speed. This is called a sub' reluctance motor. 13.7.2 Hysteresis motors With this type of motor, the rotor is canst specially hardened steel rings instead of the laminations. The effect of hysteresis is theret and opposes any change in magnetic polaritie. once they are established. The rotor poles "le stator poles of the opposite polarities. Normally, the synchronous motor is not; One method that is used to provide mover rotor is the shaded-pole principle. The mov< stator flux across the pole face pulls the rota it. Because the stator and rotor fluxes are 1 "locked" together, the rotor runs at a synchr determined by the number of stator poles an frequency. There are many variations to the singlt chronous motors mentioned, most of which basic principle of either the reluctance or hyst