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TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES

Cubao, Quezon City

ELECTROMECHANICAL ENERGY CONVERSION

Assignment No. 4

Chapter 5

“DIRECT CURRENT

MOTOR CHARACTERISTICS”

Submitted by: Ezekiel M. Brizuela

EC41FA1

Engr. Jurieve Bagay

Instructor

January 22, 2012

Date

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1. What is meant by a load on a generator? A load on a motor?

A load on a generator constitutes the electrical devices that convert electrical energy

into other forms of energy. The load on a motor constitutes the force that tends to

oppose rotation which is called a countertorque.

2. List several practical types of loads applied to motors.

The practical types of loads that applied to motors are: fan blades, pumps, grinder,

boring mills, crushers, excavators, elevators, turntables, churns, drills, food mixers

and host of other commonly used machines.

3. When the load changes, what tends to change in a generator? In a motor?

The voltage regulator tends to change when the load changes; in shunt generators, a

load increase is always accompanied by a drop in terminal voltage, while in

compound generators, the voltage may fall, rise or even remain constant as the load

changes. In case of the motor, the speed of rotation tends to change as the load

varies; an increase in load causes the speed of a shunt motor to drop slightly that of a

compound motor to drop considerably, and that of a series motor to drop greatly.

4. What methods are usually employed to adjust the voltage of a generator? The speed

of a motor?

The voltage of a generator can always be adjusted by doing these methods: (1)

changing the speed and (2) changing the strength of the magnetic field. The speed

of rotation of a dc motor can be employed by varying either or both of two things: (1)

the strength of the magnetic field and (2) the voltage impressed across the armature

terminals.

5. Generally speaking, what factor is kept constant when a generator is in operation?

When a motor is in operation?

For all practical purpose, it is usually true that the impressed emf across the motor

terminals is substantially constant, except in the case of special motors or

applications in which the power supply constitutes a separate source; in such cases,

the voltage of the generator is varied to change the speed of the motor.

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6. Are generators started under load? Explain.

Generators are always started without electrical loads; the procedure is to bring

them up to speed, adjust the voltage and then close the main switch that permits the

machine to deliver current.

7. Are motors started under load? Explain.

Motors may or may not have a mechanical load when they are started; as a matter

of practical significance, it is quite customary for a motor to start a load that is often

equal to or greater than the rated name-plate value.

8. Is it possible to operate a dc generator as a motor and vice versa? Explain.

In fact, they may often be operated either as generators or motors with complete

satisfaction if certain conditions are fulfilled. In some cases, when a dynamic or

regenerative braking is employed, they operate as motors most of the time and as

generators during the braking periods.

9. Name the three general types of dc motor.

The tree general types of dc motors are: (1) series motor, (2) shunt motor and (3)

compound motor.

10. Indicate, in general way, how the speeds of the three types of dc motor are affected

by an increase in load.

In general, if a change from no mechanical load to full mechanical load causes the

speed to drop approximately 8 percent or less, the moor is said to be constant-

speed type – shunt motor falls into this classification. Motor in which the speed

changes by greater value than indicated here are regarded as falling into the

variable-speed of classification; series and compound motor falls into this.

11. What is meant by a constant-speed motor? What type of motor exhibits constant-

speed characteristics?

A motor might be regarded as falling into the constant-speed class, when the

variation is as much as 10 or 12 percent. The point is that the terms “constant

speed” and the “variable-speed” are relative, like “tall” and “short” and must be

applied advisedly in each particular instance.

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12. What is meant by a variable-speed motor? What type of motor exhibits constant-

speed characteristics?

There is absolutely no question about the speed classification; series motors or

compound motors with strong series fields, are definitely variable speed machines.

13. What is meant by an adjustable-speed motor?

Whenever the speed of the motor can be controlled by an operator who makes a

manual adjustment, it is said to be of the adjustable-speed motor. The difference

between the variable and adjustable speed type are; the loading conditions, the

speed changes only because the operator or automatic control adjustment has

made an adjustment or some sort.

14. Under what condition, would a motor be called a constant-speed-adjustable-speed

motor? A variable-speed-adjustable-speed motor?

To have a constant-speed-adjustable-speed motor; a shunt motor with a field

rheostat-control would fall into such a classification. A variable-speed-adjustable-

speed motor might be a series motor with a line rheostat; such an arrangement is

used on a hoist.

15. Why is the generated emf in a dc motor called a counter emf?

The generated voltages are indicated by crosses and dots below the circles and are

in direction opposite to the flow of the current. Since the generated voltage opposes

the flow of current, it is called counter electromotive force.

16. Can the counter emf ever be equal to the impressed voltage in a motor? Gevie

reasons for your answer.

This counter emf clearly can never be equal to, and must always be less than, the

voltage impressed across the armature terminals, because the direction in which the

current flows determines first the direction of rotation and thus the direction of the

counter emf.

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17. How does the counter emf control the value of the armature current?

It only means that the armature current is controlled and limited by the counter emf

because the direction in which the current flows determines first the direction of

rotation and thus the direction of the counter emf.

18. Upon what two factors does the counter emf depend in a given motor?

The two factors that the counter emf depends in a given motor are: (1) the flux per

pole and (2) the speed of rotation in revolution per minute.

19. When a load upon a shunt motor is increased, what electrical factor affects speed?

If the load of the motor is increased, the armature current will rise.

20. When a load upon a compound or series motor is increased, what two factors affect

speed?

The two factors that affect speed are: (1) the increased load, which requires an

increased armature current; (2) an increase in the flux because of the series field of

the compound motor.

21. Approximately, what percentage of the impressed voltage is the counter emf in a dc

motor?

As the matter of practical importance, it should be stated that the counter emf

developed in the armature of a motor is usually between 80 and 95 percent of

voltage impressed across the terminal voltage.

22. Explain why the power developed by a dc motor is determined by the value of the

counter emf.

The power in watts developed by the armature is counter emf multiplied by the

armature current because it is equal to the power in watts supplies to the aremature

of the armature voltage multiplied by armature current deducted the whole thing to

the copper loss in the squared of the armature current multiplied by armature

resistance.

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23. What limits the armature current in a dc motor at the instant of starting?

At the instant a dc motor is started, the counter emf is zero because the armature is

not revolving. As the armature accelerates to full speed, the value of counter emf

rises to a value that causes the proper value of armature current to slow.

24. How is it possible to keep the armature current down to a reasonable value when a

dc motor is started?

The counter emf limits the current in the low-resistance armature winding, it should

be slear that the instant of starting, when the counter emf is zero, the armature

current would be extremely high unless some resistance were added to offset the

lack of counter emf.

25. Why is it not particularly serious to start a small motor directly from the line without

the use of external resistors?

For the small motors, it is usually the fractional-horsepower sizes up to ¾

horsepower, no starting resistor is necessary. The reasons are the resistance and

the inductance of the armature winding generally sufficiently high to limit the initial

rush. Second the inertia of a small armature is generally so low that it comes up to

speed very quickly.

26. What is the primary function of a starter for a dc motor?

The primary function is to limit the current in the armature circuit during the starting

or accelerating period.

27. What are the two general types of the manual starter for dc motors?

The two general types of the manual starter are: (1) three-point type and (2) four-

point type.

28. How are starters rated?

Starters rated always on the basis of horsepower and voltage of the motors with

which they are to be used.

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29. When starting a dc motor with a manually operated starter, why is it not permissible

to hold the handle on an intermediate stud for a considerable length of time? Give

reasons for your answer.

The entire process should take from 5 to 10 seconds only. It will result power failure

and the field circuit will be opened accidentally, the starter arm will fall back to its off

position. If power fails and the starter arm is not restored to the off position the motor

might be damaged should the power come on again and if the shunt field circuit

were opened accidentally and the starter arm did not return to the off position, the

motor speed might become dangerously high.

30. Explain exactly how a dc motor should be properly started with a manual starter.

To start the motor, one hand is held on the handle of the open main switch while the

starter arm is moved to the first stud with the other hand; then the main switch is

closed. If all the wiring is correct and the armature is free to turn, the motor will start.

31. How many electrical circuits are there in a three-point starter? A four-point starter?

There are two (2) electrical circuits in a three-point starter and three (3) electrical

circuits are in four-point starter.

32. What is the disadvantage of the three-point starter? How is this disadvantage

overcome in the four-point starter?

Three-point starter are not completely satisfactory when used with motors whose

speed must be controlled by inserting resistance in the shunt-field circuit. It is not

desirable feature of this type that makes it unsuitable for use with speed-controlled

motors.

33. What is the function of the holding coil?

Holding coil is a separate relay coil that is energized by contacts which close when a

relay pulls in; to hold the relay in its energized position after the orginal operating

circuit is opened.

34. What is a controller? What functions, other than starting, can it perform?

Whenever a starter is equipped with some means for varying the speed of the motor

to which it is connected, it is called a controller. Controller may also be designed to

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permit reversing the direction of rotation and may include protective features such as

overload relays, undervoltage relays and open-field devices.

35. What two important advantages are possessed by a manual controller for a shunt or

compound motor?

The two important advantages that a manual controller possessed for a shunt or

compound motor are; (1) as resistance is cut in, the speed increases; and (2) at a

comparatively high speed, the field must be weakened considerably.

36. Explain the operation of a four-point controller for a shunt or compound motor. Refer

to the wiring diagram of Fig. 101 in doing this.

When the motor is started, the two arms move forward simultaneously, the long one

pushing the short one. Resistance is first inserted and then cut out the armature

circuit in the usual way as the armature accelerates. The field excited without any

rheostat resistance in the field circuit because the current passes directly.

37. In an automatic starter distinguish between: a relay and a contactor; normally open

and normally closed contacts.

A relay is an electrically operated switch. Many relays use an electromagnet to

operate a switching mechanism mechanically, but other operating principles are also

used. Relays are used where it is necessary to control a circuit by a low-power

signal with complete electrical isolation between control and controlled circuits, or

where several circuits must be controlled by one signal. A contactor is an electrically

controlled switch used for switching a power circuit, similar to a relay except with

higher current ratings. A contactor is controlled by a circuit which has a much lower

power level than the switched circuit.

38. What is a timing relay? Explain its operation.

A timing relay is a type of relay that delays changing position when the coil is

energized or de-energized. When the start button is pressed, the control relay picks

up; this closes the sealing contacts and the contacts that feed the four contactors.

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39. In what aspects do the counter emf, time-limit, and current-limit automatic types of

starter differ from one another in operation? What advantages are possessed by

each?

The counter emf type of starter, is a number of relays that are connected across the

armature where the counter emf increases as the motor accelerates and the former

are adjusted to pick up at predetermined values of voltage. The tile-limit starter is a

group of relays that ate timed to operate at preset intervals of time by means of

devices that function mechanically, pneumatically or electrically. The current-limit

starter is the relays that are designed in which they are sensitive to current changes

in the armature circuit.

40. Describe the operation of the counter-emf automatic starter of Fig. 102.

The counter-emf method is the shunt motor started by pressing the start button. This

energizes the main contactor which instantly closes the auxiliary contacts and the

main contacts.

41. Describe the operation of the time-limit automatic starter of Fig. 103.

A time-limit acceleration starter is connected to a compound motor. In the design

there are a group of three contactors which each has one pair of instantaneously

closing contacts across a block of armature resistance and another pair of timed

contacts that close with a time delay after the coil is energized.

42. Describe the operation of the current-limit automatic starter of Fig. 104.

The current-limit acceleration starter functions in still another way, depending for the

motor’s increase in speed upon the current taken by the armature circuit; this

permits the motor to start more slowly when the load is heavy and more rapidly

under light-load condition.

43. In the push-button automatic starter, is the “START” button normally open or closed?

Is the “STOP” button normally open or closed? Are the overload relay contacts

normally open or closed?

When the start button is pressed, the contactor is energized and this causes

contacts to the other contactor to close. The motor now starts as current passes

through the resistors and series relays which open the normally closed contacts

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instantly and before the contacts close. The motor now runs with all of the armature-

circuit resistance cut out. The motor is stopped in the same way as was previously

described.

44. Explain why the armature of a dc motor automatically draws more current from the

source when the load is increased.

When a generator delivers electrical power to a load, its terminal voltage tends to

change. The operator has practical control over this tendency on the part of the

generator to change its terminal voltage. Voltage control of a generator is generally

exercised through the medium of flux adjustment or control.

45. What is meant by the normal speed of a motor?

The speed at which a motor operates when it is driving its rated load is so-called

rated horsepower or simply the normal speed of a motor.

46. What general statements can be made with regard to the change in speed with load

for shunt motors? Compound motors? Series motors?

If the mechanical load is completely removed from shunt motor, so that it is merely

overcoming its own bearing, brush, and wind friction, it will operate at a speed only

slightly higher than the normal speed; this will generally be between 2 and 8 percent

higher than the normal speed. Doing the same thing to a compound motor will result

in a rise speed of about 10 to 25 percent.

47. What is meant by torque? In what units is it usually expressed?

Torque is a force that produces rotation. It normally expressed in pound-feet (lb-ft)

in English system or Newton-meter (N-m) in SI system.

48. In general, upon what two factors does the torque of a motor depend?

The two factors that the torque of a motor depends is: (1) the flux created by the

main poles and (2) the current flowing in the armature winding. The torque is

independent of the speed of the motor.

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49. How much power must a motor develop? Be explicit.

The power developed in watts, is the power to drive the mechanical load and the

power necessary to overcome the motor’s own rotational losses.

50. Why is the torque of a shunt motor directly proportional to the armature current?

The torque of a shunt motor is directly proportional to the armature current because

the current through the shunt field is constant and is fixed only by the shunt-field

resistance and the terminal voltage. This means that the shunt-field is independent

of the load and is substantially constant because the flux depends on the field

current.

51. Explain how the torque varies with increased load upon a compound motor; a series

motor.

The torque developed by the series motor depends upon the armature current and

the flux that the current produces in passing through the series field. At light loads,

when the magnetic circuit iron is not saturated having the equation of the parabola.

The torque in the compound motor combines the torque-load characteristic pf the

shunt and series motors. As load on the motor increases, the armature, or load

current passing through the series field creates flux that adds to the constant shunt-

field flux.

52. Under what operating conditions is it desirable to use a shunt motor? A series

motor? A compound motor?

By arranging for some of the field magnetomotive force to be provided by a series

winding and some to be provided by a shunt winding, it is possible to obtain motors

with a wide variety of inherent torque -speed characteristics. In practice most

compound motors have the bulk of the field MMF provided by a shunt field winding,

so that they behave more or less like a shunt connected motor. The series winding

MMF is relatively small, and is used to allow the torque -speed curve to be trimmed

to meet a particular load requirement. When the series field is connected so that its

MMF reinforces the shunt field MMF, the motor is said to be 'cumulatively

compounded'.

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53. Are shunt and compound types of motor stable at no load?

Yes, the shunt and compound motor is more stable at no load because the speed

regulates at higher value of rpm.

54. What precautions must be taken in operating a series motor, the load on which

varies over wide limit?

In operating a series motor, great care must be taken not to permit the load to be

reduced to such an extent that the speed becomes excessive.

55. Two similar shunt motors are changed to compound machines. If one of them is

wound with twice as many series-field turns per pole as the other, which will have:

(a) the greater speed change with load? (b) the greater starting torque? (c) the

greater overload torque?

The speed of a shunt motor is substantially constant and has a very definite no load

value. The speed of a compound motor varies considerably and also has very

definite no-load value. The series motor operates over an extremely wide speed

range and tends to “run away” at light loads – it should never be used with a belt

drive or when the load is such that the torque might drop to approximately 15

percent of the full load.

56. Define speed regulation.

Speed regulation is the percentage of speed change; generally this is only

calculated on devices which are attempting to maintain a constant speed. For

example, on some industry motors, they need to maintain a constant RPM. When

the motor has weight bearing on it, it is called a load, and the speed may change

slightly.

57. What approximate values of speed regulation can be assigned to shunt and

compound motors?

Shunt motor are generally regarded as constant-speed motors because their

percent speed regulation is very small. They are not constant-speed motors in the

strictest case, but their speed varies little between full load and no load. Compound

motors are properly considered to be variable-speed motors because their percent

speed regulation is comparatively high.

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58. What is it improper to speak of the speed regulation of a series motor?

A shunt motor has good speed regulation while a series motor has poor speed

regulation. For some applications such as cranes or hoists, the series motor has an

advantage since it results in the more deliberate movement of heavier loads. Also,

the slowing down of the series motor is better for heavy starting loads. However, for

many applications the shunt motor is preferred.

59. Distinguish between the terms speed regulation and voltage regulation.

Speed regulation is the change in speed with the change in load torque, other

conditions being constant. A motor has good regulation if the change between the

no load speed and full load speed is small. A voltage regulator is an electrical

regulator designed to automatically maintain a constant voltage level. A voltage

regulator may be a simple "feed-forward" design or may include negative feedback

control loops.

60. Why are shunt motors generally referred to as constant-speed motors?

The shunt motor's speed can be controlled. The ability of the motor to maintain a set

rpm at high speed when the load changes is due to the characteristic of the shunt

field and armature. Since the armature begins to produce back EMF as soon as it

starts to rotate, it will use the back EMF to maintain its rpm at high speed. If the load

increases slightly and causes the armature shaft to slow down, less back EMF will

be produced. This will allow the difference between the back EMF and applied

voltage to become larger, which will cause more current to flow. The extra current

provides the motor with the extra torque required to regain its rpm when this load is

increased slightly. The shunt motor's speed can be varied in two different ways.

These include varying the amount of current supplied to the shunt field and

controlling the amount of current supplied to the armature. Controlling the current to

the shunt field allows the rpm to be changed 10-20% when the motor is at full rpm.

61. Why are compound and series motors generally referred to as variable-speed

motors?

Compound and series motors generally referred to as variable-speed motors

because their percent speed regulation is comparatively high. They may be

considerable variation between the regulations of motors of different design, but this

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is generally because of the number of series field ampere turns as compared with

the shunt field.

62. Give several practical applications for shunt (constant-speed) motors.

Wood planers, circular saws, grinder, polishers and line shafts have been found that

constant-speed shunt motors perform mostly satisfactory.

63. Give several practical applications for compound motors.

Compressors, pumps and power blowers are several application of the compound

motor.

64. Give several practical applications for series motors.

Some common application of series motors are streetcars, turntables, cranes,

bucket, and mine hoist and the operation of large valves.

65. Why are differential-compound motors unstable at heavy loads? Explain carefully.

Under heavy load the speed of the differential compound motor is unstable; and if

the overload current is very heavy, the direction of rotation may be reversed.

Thereafter, the motor will run as a series motor with the danger of over speed on no

load that is the inherent characteristic of all series motor.

66. Under what conditions is it permissible and desirable to use differential-compound

motors?

Differential-compound motor have few applications; they may be used in special

cases in which it is desirable to have a better constant-speed characteristic than has

the shunt type of motor.

67. What precaution must be taken when starting a differential-compound motor?

Differential compound must be started with caution, preferably with the series field,

short-circuited, because a large starting series-field current may be sufficient to

reverse the normal magnetic polarities and cause the motor to start up in the wrong

direction.

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68. Distinguish between speed control and speed regulation.

Speed controller is to take a signal representing the demanded speed, and to

drive a motor at that speed. The controller may or may not actually measure the

speed of the motor. Speed regulation is the ability of a motor to maintain its

speed when a load is applied. A motor's speed regulation is fixed based on its

design.

69. How does the speed vary when the shunt-field rheostat is adjusted?

In shunt-field rheostat, the shunt field current has been rising, when the motor

eventually comes to stop because the reduced torque, the counter emf is zero; a

heavy surge of current now passes through the armature and series field.

70. How does the speed vary when the armature rheostat is adjusted?

The motor thus speeds up quickly and in doing so, generates a considerable counter

emf, takes a progressively smaller current, and develops diminishing value of torque.

71. How does the speed vary when the armature voltage is adjusted?

When the generator is low, high field resistance, the motor speed is low; when the

generator is high, low field resistance, the motor speed is high. This variable-voltage

control system has many important applications when extremely wide speed ranges.

72. Using the fundamental equation of the speed of a dc motor [Equation 18] , justify the

answers to questions 69 to 71.

The speed decreases as resistance is inserted in the armature circuit. This is

equivalent to increasing the second term in the numerator because Ia(Ra+R); the

greater the value of the inserted resistance R, the lower becomes the speed.

73. Describe the Ward Leonard system of control for a shunt motor. Refer to Fig. 109 in

doing this.

The War Leonard system is made up of a driving motor which runs at almost

constant speed and powers a dc generator as shown in the diagram. The generator

output is fed to a dc motor. By varying the generator field current, its output voltage

will change. The speed of the controlled motor thus can be varied smoothly from

zero to full speed. A Ward Leonard drive is a high-power amplifier in the multi-

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kilowatt range, built from rotating electrical machinery. A Ward Leonard drive unit

consists of a motor and generator with shafts coupled together. The motor, which

turns at a constant speed, may be AC or DC powered. The generator is a DC

generator, with field windings and armature windings. The input to the amplifier is

applied to the field windings, and the output comes from the armature windings.

74. Compare the power losses in the rheostats of field- and armature-resistance

methods of control.

The insertion of a resistance in the field or armature circuit of an adjustable-speed

motor always involves a power loss. This power loss in watts is generally a small

percent of the total power input to the motor if the field-resistance method of control

is employed. On the other hand, if the armature-resistance method of control is

employed, the power loss may be quite large because the current in the armature

circuit is nearly equal to the line current.

75. What advantages are possessed by Ward Leonard system of control? What are its

disadvantages?

The advantages of Ward Leonard system are: four quadrant control which means

full speed control on rotation sides, braking and regenerating power.

The disadvantage disadvantages: high cost you need 3 machines, low efficiency. It

must be said that it provides excellent stepless speed control for a motor which must

have a very wide range of speed.

76. List several practical applications of the Ward Leonard control system.

It is frequently application on electric excavators, on freight-handling ships, and in

blooming and paper mills and for the operation of passenger elevators in tall

building.

77. Describe the operation of the Ward Leonard system of control of Fig. 111, in which

two exciters are used.

The speed of motor is controlled by varying the voltage fed from the generator,

which varies the output voltage of the generator. The varied output voltage will

change the voltage of the motor, since they are connected directly through the

armature. Consequently changing the generator voltage will control the speed of the

motor. The picture of the right shows the Ward Leonard control system, with the

generated voltage feeding the generator and emf feeding the motor.

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78. What advantages are possessed by the modified Ward Leonard control system of

Fig.111?

The system is greatly simplified scheme of connections dispenses with the need for

separate excitation and thereby reduces the cost of the installation somewhat.

79. Explain why the voltages of the main and intermediate exciters of Fig. 111, is the

modified Ward Leonard system of control, must never be equal. What would happen

if they were.

The amplifier output is usually connected to a second motor, which moves the load,

such as an elevator. With this arrangement, small changes in current applied to the

input, and thus the generator field, result in large changes in the output, allowing

smooth speed control. Armature voltage control only controls the motor speed from

zero to motor base speed. If higher motor speeds are needed the motor field current

can be lowered, however by doing this the available torque at the motor armature

will be reduced. Another advantage for this method is that the speed of the motor

can be controlled in both directions of rotation.

80. Describe the operation of the simplified Ward Leonard system of control of Fig. 112,

in which two series machines are used. What important magnetic design features

must the machine possess for good operation?

The controlling generator is driven by a prime mover; usually a constant speed ac

motor and speed control of the controlled motor is affected by shunting the series

field on the generator with a variable resistance. The terminal voltage of a series

generator depends upon the series-field current or excitation.

81. How does the effect of armature reaction in motors differ from its effect in operators?

The armature current in a motor is opposite to that of a generator for the same

direction of rotation, it follows that the shift of the magnetic axis is opposite to the

shift produced in the generator. The result is that the magnetic neutral in a motor

always tends to shift in a direction opposite to that on the armature rotation.

82. If no interpoles are used, how must the brushes be shifted in generators? In motors?

When the interpoles are not used, brushes must be shifted backward beyond the

resultant magnetic neutral, so that armature reactance may be effectively

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neutralized. It is only the way that the currents in the coils may be made to reverse

effectively and smoothly without the objectionable commutator sparking.

83. What are the polarities of the interpoles with respect to the main poles in

generators? In motors?

The interpoles for dc motors are always made somewhat stronger than would be

required to neutralize the armature reaction flux in the interpolar zones. He reason

for this is exactly the same way as was given in the discussion of generators; that is,

the interpoles must help the commutated coils generate sufficient voltage to

overcome the reactance voltage due to the inductance and thus anticipate the new

current directions.

84. Why is it possible to use half as many interpoles as main poles in some small

motors? What advantage would this have?

In small machine, it is often found possible to use half as many interpoles as main

poles, thus reducing the cost of manufacture. The reason for this practice is that the

span of every coil is 180 electrical degrees. Therefore, if an interpole is made doubly

strong, its effect on one coil side is equivalent to the action of two interpoles, each

acting on one coil side.

85. What two fundamental methods may be used to reverse a dc motor? Which is

preferable in compound machines?

The two general methods for reversing the direction of rotation of a dc motor (1)

changing the direction of current flow through the armature and (2) changing the

direction of current flow through the circuit on circuits. In compound motor, it is

necessary to reverse the current flow through the armature winding only.

86. When a DPDT switch is used in the field circuit of a shunt motor for reversing

purposes, what precaution must be taken when the motor is started? Is this

precaution necessary if the DPDT switch is placed in the armature circuit?

When the switch is closed to the left, the current will be down through both field and

armature. When the switch is closed to the right, the current will be up through the

one of the elements and down through the other. The wiring is much simpler when

the first two method is used because the reversal of the current through a single

element.

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EMB

87. Describe the operation of the automatic reversing starter of Fig. 118.

It is provided with two acceleration contactors and resistors, designated by 1A, 2A

and R1, R2. Arrangement is made for armature reversing through forward contacts F

and reversing contacts R. The push button station is equipped with for and rev

buttons, each of which, when pressed, closes one set of contacts and

simultaneously opens another set.

88. Carefully explain why the control relay CR in Fig. 118 has a normally closed contact

in the F contactor circuit.

When the “for” button is pressed, the F contactor is energized and the R circuit is

opened at f as a safety measure; this seals the “for” button at F1, closes the F

contacts and the current passes through the armature circuit from a to b.

89. In fig. 118, explain why each of the push buttons, “for” and “rev”, has one normally

open and one normally closed set of contacts.

The motor is permitted to come to rest; then, the “rev” button is pressed. This

energizes the control relay which opens normally and momentarily opens the

contacts as a further safety measure. The “for” button is pressed, the F contactor is

energized and the R circuit is opened at f as a safety measure; this seals the “for”

button at F1, closes the F contacts and the current passes through the armature

circuit from a to b.

90. What would happen in Fig. 118 if the “for” and “rev” buttons were pressed

simultaneously?

It normally contacts then close to energize the contactor, and the operation of the

latter seals the reversing circuit, closes contacts and causes the main contacts to

close. The motor now picks up speed in the opposite direction since current passes

through the armature circuit fro b to a. further actions of the starter proceed as

explained for forward action.