Hawkins Electrical Guide v. 07 (of 10) Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

CHAPTER LX

REGULATING DEVICES Fig. 2,436. —Diagram of connections for Westinghouse 11 point dial, series transformer and auto-transformer . The auto-transformer has a number of taps connected across the line, the series transformer is placed in series with one side of the line, and connected to a dial, as shown. Ques. How are dial regulators modified for high voltage? Ans. Standard dials may be used with series and shunt transformers similar to the method used for heavy current circuits. Ques. Describe the connections. Ans. The primary of the shunt transformer is connected across the line and the secondary has a number of taps which are connected to contacts on the dial. The primary of the series transformer is connected in series with the line and two leads from the secondary winding are connected to the dial. The connections are similar to those shown in fig. 2,437, except that shunt transformers are used instead of auto-transformers. Fig . 2,437.—Diagram of connections for Westinghouse dial type variable ratio voltage transformer. In construction the secondary winding of the transformer is divided into 10, 14, or 20 parts giving 11, 15, or 21 taps which are brought out from the secondary winding and connected to the various points of the dial. The diagram shows connections for an 11 point dial and regulating transformer. Since there is a difference of voltage between adjacent contacts, the contact arm must not touch the contact toward which it is moving until after it has left the contact upon which it was resting . Moreover, it is undesirable to open the circuit each time in moving from one contact to the next. These conflicting requirements are met by the use of arcing tips which are placed on the contact arm so that a very close adjustment can be obtained, and so arranged that the contacts are not short circuited but always have a gap of from one-sixteenth to one-eighth inch in the circuit during the time of changing from one contact to the next. The air gaps form a "preventive resistance." A quick moving mechanism is used to accelerate the movement from one contact to the next, a very quick movement being necessary to avoid undue arcing. The capacity of the regulator is 200 amperes at 2,200 volts, being arranged to give a maximum increase in voltage of 400 volts. The maximum pressure between contacts is 25 volts. Fig. 2,438. —Diagram of connections of General Electric pole type regulator. The operation of the regulator is obtained by means of a small single phase motor which is in continuous operation, and which by mechanical means may be connected to the regulator shaft. The control of the mechanism is obtained by means of a voltage relay. The operating motor, which is of the drawn shell type, is provided with a starting clutch and will consequently start up with full load. Under actual operating conditions it will, of course, be comparatively seldom that the motor will be called upon to start up. A non-inductive resistance , made up from standard units, is connected in series with the relay winding and several taps are provided, so that the relay can be adjusted for any voltage from 10 per cent. below normal. In order to readily dissipate the heat developed in the resistance, it has been mounted in a pocket on the back of the tank, openings being provided for natural air ventilation. The relay plunger is hinged to one end of a balance arm, which arm is provided with two trip pins to control the mechanism. An adjustable helical spring is attached to the other end of the arm to assist the magnetic pull of the coil in balancing the plunger and also for adjustment. The relay is not provided with series winding for line drop compensation, but it may be used with a standard line drop compensator, which then has to be installed outside of the regulator. The voltage relay must be connected to the feeder side of the regulator, the necessary low voltage to be obtained from a distributing transformer, or if this should not be available in the immediate vicinity, a 200 watt step down transformer will be satisfactory. The motor is designed to operate in parallel with the relay, the normal connections being as shown. The speed of the motor and the ratio of the gearing is such that it requires about 90 seconds to operate the regulator from limit to limit, but, as this regulator is not intended to take care of sudden voltage fluctuations, the comparatively long time of operation will not be objectionable. Figs. 2,439 to 2,443. —General Electric pole type regulator removed from tank. It consists essentially of a primary and secondary coil, operating motor, and voltage relay mechanism . The regulator and mechanism is suspended in a cast iron tank, the lower part, containing the regulator core and coils, being filled with oil. The leads for the regulator are brought out at the upper part of the tank. The outgoing leads are compressed into bushings and connected to the leads of the regulator by means of terminals, the arrangements being such that the regulator with mechanism can be removed from the tank without difficulty. Besides the cover, the tank is also provided with a hinged door on the front side so as to give access to the mechanism. The door is provided with a gasket and the construction is practically rain and dust proof. However as there is always danger of the door not being clamped down perfectly, thus making it possible for water to enter the tank, a pocket has been provided inside the tank and underneath the door to collect the water. Capacity up to 2.3 kw., to control 2,300 volts, 60 cycle, 10 ampere feeders, and for a voltage range of 10 per cent. above or below normal, the operating motor and relay being designed for 110 or 220 volts. No provision is made for line drop compensation, although this can be obtained by installing a current transformer and a line drop compensator externally to the regulator. It will be seen that the circuit comprising the dial, the secondary of the shunt, transformer and the secondary of the series transformer form a circuit which is not electrically connected to the main circuit. It can therefore be grounded without disturbing the main circuit as a safeguard to render it impossible for the pressure of the dial to be higher above the ground than the secondary voltage of the shunt transformer. Figs . 2,444 to 2,446.—Sectional views of General Electric pole type regulator winding and core. The secondary core has only two slots containing a single coil, while the rotor or primary core has four slots. Two of these slots are occupied by a single primary coil, and the two circular slots in quadrature thereto contain the compensating or short circuit winding. This winding also serves to hold the primary punchings together, and it consists of two copper rods riveted to the two cast brass flanges. The secondary coil is form wound, while the primary coil is wound directly on the core. The rotor flanges, both top and bottom, are provided with discs which are turned in alignment with the punchings, and these discs bear against the top and bottom flanges between which the secondary punchings are clamped. These secondary flanges are also turned in alignment with the secondary punchings, so that an even air gap between the primary and the secondary is assured. The secondary coil is wound with an opening in the upper horizontal part which affords passage for the operating shaft of the rotor. A bearing for this shaft is provided in the table which supports the mechanism and from which the regulator is suspended. Flexible leads are brought out from the rotor and twisted around the shaft as in standard regulator practice. The regulator being two pole, the rotor is turned through an angle of 180 deg. to obtain the full range of the regulator. Small Feeder Voltage Regulators. —In some generating stations the voltage is maintained constant at the busbars and the line drop compensated by automatically operated regulators connected in the main feeders. It is possible in this way to obtain constant voltage at all loads at the various distribution centers, that is, at those points on the feeders where the lines of the majority of consumers are connected as shown in fig. 2,447. Figs. 2,447 and 2,448.—Systems of distribution illustrating use of small feeder or pole type voltage regulators . It is evident, however, that, while the voltage at the center of distribution can be maintained constant, no account can be taken of the drop in the lines between this center and the consumers. This drop is generally negligible, except in some particularly long lines, as, for example, consumer B in fig. 2,447. In order to obtain perfect regulation at B, it would be necessary to install a separate regulator in that line, this regulator to be installed either at the center C or preferably at B. In a great many cases the power distribution is not as ideal as indicated in fig. 2,447, but rather as shown in fig. 2,448, that is, the consumers are connected all along the feeder. In this case there is no definite center of distribution, and the automatic regulator installed in the station can be adjusted to give only approximately constant voltage at an imaginary center of distribution C; that is, the voltage cannot be held constant at any definite point during changes of load distribution. Fig . 2,449.—General Electric pole type regulator in service; its construction is shown in fig. 2,450. The majority of the consumers may, however, obtain sufficiently good voltage while a few may have reason for criticism. To overcome this difficulty it is necessary either to increase the copper in the feeder or else to install small automatic regulators. There are also cases where a small amount of power is transmitted a long distance through a feeder direct from the station. The amount of copper required to reduce the line drop is usually too great to be considered and the cost of the ordinary automatic regulator is also comparatively high. In such cases small pole type regulators as shown in fig. 2,449 are desirable. Ques. Describe the operation of the regulator mechanism shown in fig. 2,450. Ans. Assuming the voltage to be normal, the balance arm of the relay will be held horizontal, the trips F will not engage with the triggers E, and no movement is therefore transmitted to the ratchet wheel C. If the voltage drops below normal, the left hand trip will descend until it finally gets in the way of the left hand trigger just before it reaches the limit of its counterclockwise travel. This trigger will therefore release the left pawl D, which will engage with the ratchet wheel and will consequently turn it clockwise until the rocker arm reaches its right hand limit. Before the rocker arm reaches the left hand limit, the released pawl must be locked by its trigger, so that if the voltage has reached its normal value, further movement of the ratchet wheel will not take place, whereas if the voltage be still too low, the trigger will again release the pawl by striking the trip of the relay. Fig . 2,450.—Mechanism of General Electric pole type regulator. The operating motor (described in fig. 2,438) is direct connected to a worm and gear, the shaft of which is provided with a bell crank. A rod A, connects the crank with the rocker arm B, which thus may be caused to oscillate over a ratchet wheel C. The rocker arm is provided with two pawls D, which can engage with the teeth of the ratchet wheel, so that this wheel can be rotated one way or the other. The ratchet wheel is mounted on the same shaft as a worm, which engages with the gear segment carried by the regulator shaft, so that the movement of the ratchet wheel is directly transmitted to the regulator. Besides the two large pawls D, the rocker arm also carries two smaller ones E, called the triggers. These triggers usually hold the pawls locked in such positions as not to engage with the ratchet wheel, but the pawls will be released when the triggers strike the trips F of the relay arm. A limiting device for the movement of the ratchet wheel and the regulator segment is provided, as shown. This device consists of two cams K, mounted on a common arm, which can turn on the shaft of the ratchet wheel. Normally these cams are not within reach of the pawls, but through a lever arrangement, controlled by the regulator segment, the arm holding the cams may be rotated so that, if the trigger has been raised, so as to release the pawl, the tip of the pawl will bear on the cam of the limiting device, and before the pawl can engage with the ratchet wheel it has already been locked by its trigger. A further movement of the ratchet wheel in that particular direction is therefore impossible, while it is free to be moved the other way. A positive stop for the gear segment is also provided. The motor is provided with oil ring bearings, and the gear for the motor worm runs in oil, the supporting casting forming a well therefor. Fig . 2,451.—Diagram of connections of General Electric direct current voltage regulator (form T) with two dynamos and one exciter. In cases where several shunt or compound wound direct current machines are operating in parallel, either on two wire or three wire systems, a good arrangement for voltage regulation and line drop compensation is obtained by using this regulator and a separate exciter. The compensating shunt as well as pressure wires can be used to maintain a constant pressure at the center of distribution. Ques. How is this automatic locking of the pawl obtained? Ans. By having a lip G on the under side of the pawl strike a finger H fastened to the bearings in front of the ratchet wheel. The pawl is thus raised just before it reaches the limit of its clockwise travel sufficient to be locked by its trigger. Fig. 2,452. — Condenser sections and method of assembling same with tripod. The tripod bolts are made of extra length to accommodate the addition of extra condenser sections if necessary. The illustration shows three sections in position. Ques. How does the mechanism operate when the voltage rises above normal? Ans. As described above, with the exception that the right hand trip causes a rotation of the regulator in the opposite direction. Ques. How is adjustment made for various voltages? Ans. Taps are provided on the resistance in series with the relay, and finer adjustment can be obtained by means of the helical spring on the right hand end of the balance arm. In order to adjust the sensitiveness of regulation, the bearing for the balance arm can be raised or lowered by means of a stud J, fig. 2,450, connecting this bearing with the bearing of the operating shaft, and the regulator can be made to maintain the voltage within 1 per cent. above or below normal. Fig. 2,453. —Westinghouse unit switch type pressure regulator, designed for handling heavy currents where a variable ratio transformer type of regulator is desired. The regulator consists of a number of electrically operated switches controlled from a master switch. These switches are arranged to perform practically the same cycle of operation as previously described for the drum type regulators. The transformer windings are divided into sections, and two floating coils are provided which are connected to various taps on the main auto-transformer. These floating coils have intermediate steps, and the successive operation of the switches connects the floating coils in proper sequence to the main auto-transformer, and transfers the line connection from one point of the floating coil to the next. In this way a 23 point regulator with sixteen switches, and a 71 point regulator with 21 switches may be supplied. The master switches are arranged with an automatic lock to prevent their being operated too rapidly. The magnet switches themselves are so interlocked that the proper sequence of operation is insured. The electrically operated switches may be of the open type, mounted on a slate or marble switchboard, when the whole control outfit is placed in a room which is comparatively free from dust or dirt of any kind, and where there is no danger of employees coming in contact with the switches. The other type of switch is entirely enclosed, the main contacts being oil immersed. The frames of these switches are grounded and the whole design is arranged to operate under ordinary dirty conditions. All of these switches, however, should receive the necessary inspection and attention. The contacts have a long life and are easily renewed. Regulators of this type are adapted for metallurgical purposes where the regulation is effected in the primary circuit and the secondary circuit is of very low voltage but large current capacity and is used for supplying power to the furnaces. These regulators have been built in capacities up to 800 amperes at 3,300 volts. Ques. What provision is made for convenient inspection? Ans. A snap switch is provided by means of which the power to the motor and relay can be disconnected. Figs. 2,454 and 2,455.—Front and rear views of General Electric automatic voltage regulator. The regulator has a direct current control magnet , an alternating current control magnet, and a relay . The direct current control magnet is connected to the exciter bus bars. This magnet has a fixed stop core in the bottom and a movable core in the top which is attached to a pivoted lever having at the opposite end a flexible contact pulled downward by four spiral springs. For clearness, however, only one spring is shown in the figure. Opposite the direct current control magnet is the alternating current control magnet which has a pressure winding connected by means of a pressure transformer to the alternator or bus bars. There is an adjustable compensating winding on the alternating current magnet connected through a current transformer to the principal lighting feeder. The object of this winding is to raise the voltage of the alternating current bus bars as the load increases. The alternating current control magnet has a movable core and a lever and contacts similar to those of the direct current control magnet, and the two combined produce what is known as the "floating main contacts." The relay consists of a U shaped magnet core having a differential winding and a pivoted armature controlling the contacts which open and close the shunt circuit across the exciter field rheostat . One of the differential windings of the relay is permanently connected across the exciter bus bars and tends to keep the contacts open; the other winding is connected to the exciter bus bars through the floating main contacts and when the latter are closed, neutralizes the effect of the first winding and allows the relay contacts to short circuit the exciter field rheostat. Condensers are connected across the relay contacts to prevent severe arcing and possible injury. Automatic Voltage Regulators for Alternators. —The accurate regulation of voltage on any alternating current system is of importance. The desired voltage may be maintained constant at the alternator terminals by rapidly opening and closing a shunt circuit across the exciter field rheostat. Fig. 2,456. —Diagram of connections of General Electric contact making ammeter for operating on alternating current circuits. The instrument is designed to indicate with the aid of a current transformer, certain values of current in an alternating current system . This value depends upon the setting of the regulating rheostat in parallel with the pressure coil of the ammeter. It is also possible with this instrument, together with the necessary control apparatus, to hold certain values of current. By using a different magnet coil this meter may be connected to a shunt instead of a current transformer and used on a direct current system. Ques. Describe in more detail this method of regulation. Ans. The rheostat is first turned in until the exciter voltage is greatly reduced and the regulator circuit is then closed. This short circuits the rheostat through contacts in the regulator and the voltage of the exciter and alternator immediately rise. At a predetermined point, the regulator contacts are automatically opened and the field current of the exciter must again pass through the rheostat. The resulting reduction in voltage is arrested at once by the closing of the regulator contacts which continue to vibrate in this manner and keep the generator voltage within the desired limits. The connections are shown in fig. 2,457. Fig. 2,457. —Diagram of General Electric automatic voltage regulator connections with alternator and exciter. In operation , the circuit shunting the exciter field rheostat through the relay contacts is opened by means of a single pole switch at the bottom of the regulator panel and the rheostat turned in until the alternating current voltage is reduced 65 per cent. below normal. This weakens both of the control magnets and the floating main contacts are closed. This closes the relay circuit and demagnetizes the relay magnet, releasing the relay armature, and the spring closes the relay contacts. The single pole switch is then closed and as the exciter field rheostat is short circuited, the exciter voltage will at once rise and bring up the voltage of the alternator. This will strengthen the alternating current and direct current control magnets, and at the voltage for which the counterweight has been previously adjusted, the main contacts will open. The relay magnet will then attract its armature and by opening the shunt circuit at the relay contacts will throw the full resistance into the exciter field circuit tending to lower the exciter and alternator voltage. The main contacts will then be again closed, the exciter field rheostat short circuited through the relay contacts and the cycle repeated. This operation is continued at a high rate of vibration due to the sensitiveness of the control magnets and maintains a steady exciter voltage. Line Drop Compensators. —In order that the actual voltage at a distant point on a distribution system may be read at the station some provision must be made to compensate for the line drop, that is to say, for the difference in voltage between the alternator and the center of distribution. In order to do this a device which is known as a "line drop compensator" is placed in the voltmeter circuit as shown in the diagram, fig. 2,458. Ques. What are the essential parts of a line drop compensator? Ans. The elements of a line drop compensator are a variable resistance, and a variable inductance. Ques. Describe the connections. Ans. The secondary of a pressure transformer is connected in series with the compensator inductance and resistance, and the secondary of a current transformer as shown in the diagram, fig. 2,458. Fig. 2,458 —Diagram showing essential parts and connections for a line drop compensator . The compensator corrects the voltmeter indication at the supply end of a feeder for the ohmic and inductive drop in pressure between that point and the point of consumption , so that the reading of the station voltmeter corresponds with the actual voltage at the point of consumption, independent of the power factor and current. It is especially useful for adjusting pressure regulators. Ques. How are the inductance and resistance wound? Ans. They are wound so that any proportion of the winding of either can be put in or out of the voltmeter circuit. Fig. 2,459. —General Electric line drop compensator. It has two dial switches with many taps to the resistance and reactance in the box so that it can be adjusted to compensate accurately for line losses with loads of varying power factor. Dial R changes resistance, and dial X, reactance. Fig. 2,460. —General Electric line drop compensator. This compensator contains besides resistance and inductance, a current transformer, the secondary of the transformer being connected in series with the resistance and inductance; the primary of the contained current transformer is connected to an external current transformer. The reactance and resistance are both so wound that any proportion of the winding can be cut in or out of the voltmeter circuit. Both elements have 12 points of adjustment of one volt each, giving a total combined drop at maximum setting of about 17 volts. Ques. How can the voltmeter indicate the pressure at the center of distribution? Ans. If the amount of inductance and resistance be properly adjusted, there will be produced a local circuit corresponding exactly in all its characteristics to the main circuit. Hence, any change in the main circuit produces a corresponding change in the local circuit, and causes the voltmeter to always indicate the pressure at the end of the line or center of distribution or at any point for which the adjustment is made. Fig. 2,461. —Westinghouse line drop compensator. For single phase circuits, one compensator and one series transformer, that is the instrument as listed with transformers, will give correct indications for a single phase circuit. The same voltage transformer serves for both voltmeter and compensator. For balanced two phase circuits one compensator and one transformer connected in one of the phases is sufficient. Two single phase compensators should be used for unbalanced two phase circuits. For three phase circuits the compensator should be connected by means of two series transformers. Ques. How should the adjustment be made? Ans. It is advisable to calculate the ohmic drop for full load and set the resistance arm at the point which will give the required compensation and then adjust the inductance arm until the voltmeter reading corresponds to the voltage at the point on the line selected for normal voltage. Fig. 2,462. —Diagram of automatic voltage regulator , using line drop compensator . For ordinary installations the compensating winding on the alternating current control magnet is connected to a current transformer in the main feeder. A dial switch is provided by which the strength of the alternating current control magnet can be varied and the regulator made to compensate for any desired line drop up to 15 per cent. according to the line requirements. Where the power factor of the load has a wide range of variation, a special line drop compensator, such as shown in fig. 2,459, adapted to the regulator would be desirable. The connections are readily understood by the diagram. The number of condenser sections which will prevent undue arcing at the relay contacts depends on the characteristics of the exciter. They may be roughly estimated by allowing one section for each 15 kw. capacity for exciters with laminated poles, and one for each 22 kw. capacity for exciters with solid steel poles. It is necessary though to have one condenser section for each pair of relay contacts, and at times it becomes necessary to apply a double section for each pair of contacts. In the lower part of the figure the line drop compensation and connections is reproduced in more detail on a larger scale. [2] [2] NOTE.—It is desirable, in any system of distribution, to read the active voltage at the point of distribution, by means of the voltmeters in the station. A compensator proper consists of a variable resistance and a variable inductance, and sometimes a current transformer. In wiring, the voltmeter, instead of being connected directly across the secondaries of a pressure transformer, has inserted in series with it, portions of the resistance and inductance of the compensator. These are so connected that the drop in pressure across them will be combined with that of the pressure transformer, so that the voltmeter reading indicates the pressure at the center of distribution or end of the line. Starting Compensators. —These are used for starting induction motors and consist of inductive windings (one for each phase) with a number of taps connecting with switch contacts as shown in fig. 2,463. A starting compensator is similar to a rheostat except that inductive windings are used in place of the resistance grids. Fig. 2,463. —Diagram of connections of General Electric two phase starting compensator with no voltage release and fuses. Ques. Describe the inductive windings. Ans. The compensator winding consists of an inductive coil in each phase with each coil placed on a separate leg of a laminated iron core. Each coil is provided with several taps so located that a number of sub-voltages may be obtained. Ques. Are starting compensators necessary for small motors? Why? Ans. No, because the full voltage starting current taken, although equal to several times the load current, is nevertheless so small, compared with the capacity of the station alternators or feeders, that it does not materially affect the regulation of the circuit. Fig. 2,464. —Diagram of connections of General Electric three phase starting compensator with low voltage release and fuses. Fig. 2,465. —Diagram of connections of General Electric two phase starting compensator with no voltage release and overload relays for 1,040 to 2,500 volt circuits. Motors larger than about 7 horse power cause an objectionably heavy rush of current if thrown directly on the line. Starting compensators obviate such sudden variations of line load and are accordingly recommended for motors above 7 horse power except in cases where voltage variations and excessive starting currents are not objectionable. Figs. 2,466 and 2,467.—General Electric three phase hand operated starting compensator . Fig. 2,466, compensator in case; fig. 2,467, compensator with case removed. The compensator consists of a core and windings, a cable clamp, and a switch, assembled in a substantial metal case with external operating handle and release lever . The windings consist of coils wound on separate legs of a laminated core, and tapped at several points, the connections terminating at the switch contacts. The shaft of the switch extends through the sides of the compensator case, and is operated by a lever at the right, being held in the running position by a lever at the left. It is provided with wiping contacts. The switch is immersed in oil, and is intended to be used as a line switch as well as for starting the motor. The lever has three positions: "off," "starting," and "running." In the off position , both compensator and motor windings are disconnected from the line . In the starting position , the switch connects the line to the ends and the motor to the taps of the compensator winding without overload relays or fuses in circuit. In the running position , the compensation winding is cut out and the motor is connected to the line through suitable fuses or overload relays mounted directly above the compensator. To prevent the attendant throwing the motor directly on the line, and thereby causing a rush of current which it is the object of the compensator to avoid, an automatic latch is provided and so arranged that the lever at off position can be thrown only into the starting position (backward); and can be thrown thence into the running position (forward) only by a quick throw of the lever, whereby any appreciable drop in speed and consequent increase in current in passing from the starting into the running position is avoided. Ques. What should be noted with respect to the compensator winding taps? Ans. The choice of a tap giving so low a voltage as to require over one minute for starting should be avoided so as to prevent the overheating to which starting compensators, in common with other motor starting devices, are liable if left in circuit unnecessarily long, or if the motor be started several times in rapid succession. Fig. 2,468. —Diagram of connections of General Electric three phase starting compensator with no voltage release and overload relays. It should also be noted that the starting current diminishes rapidly as full speed is approached. It is, therefore, important that the switch be kept in the starting position until the motor has finished accelerating to prevent any unnecessary rush of current when the switch is thrown to the running position. Fig. 2,469. —General Electric starting compensator with low voltage release and overload relays. On the switch shaft there are mounted two levers, held together with a strong spring which operates in either direction and prevents the switch being left on the starting position. On the running side it is held by the external low voltage release lever until released either by hand or by the action of a low voltage relay. The low voltage release consists of a cast iron frame open at the bottom and totally enclosing the coil. A laminated plunger is used to hold the tripping lever, the latter engaging with the lever mounted on the switch shaft. The compensator cannot be thrown into the running position without first going to the starting position and it cannot be left on the starting position. Ques. What is the usual arrangement of starting compensators for large motors? Ans. Starting compensators may be wound for any voltage or current for which it is practicable to build motors. For very large motors the switching device is generally separate from, the compensator itself and consists of triple and four pole switches for three phase and two phase motors respectively. One double throw switch or two interlocked single throw switches are required for the motor and a single throw switch for energizing the compensator, the running side of the motor circuit being provided with fuses or automatic circuit breakers, or the switches provided with low voltage and overload release attachments. Star Delta Switches. —These are starting switches, designed for use with small three phase squirrel cage motors having their windings so arranged that they may be connected in star for starting and in delta for running. Ques. Describe the operation of a star delta switch. Ans. In starting the motor, the drum lever is thrown in the starting direction which connects the field windings of the motor in star . When the motor has accelerated and has come partially up to speed the starting lever is quickly thrown to the running position in which position the field windings are connected in delta . The effect of connecting the field winding in star at starting is to reduce the voltage applied to each phase winding, while in the running position each phase of the field winding has full line voltage impressed upon it.

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