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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 constantspeed 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 constantspeed 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 constantspeed 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-adjustablespeed 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) fourpoint 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 armaturecircuit 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 shuntfield 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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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.
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