CHAPTER LX

REGULATING DEVICES

~Regulation of Alternators.~--Practically all the methods employed for regulating the voltage of direct current dynamos and circuits, are applicable to alternators and alternating current circuits. For example: in order that they shall automatically maintain a constant or rising voltage with increase of load, alternators are provided with composite winding similar to the compound winding of direct current dynamos, but since the alternating current cannot be used directly for exciting the field magnets, an accessory apparatus is required to rectify it or change it into direct current before it is used for that purpose.

It is a fact, however, that composite wound alternators do not regulate properly for inductive as well as non-inductive loads.

In order to overcome this defect compensated field alternators have been designed which automatically adjust the voltage for all variations of load and lag. These machines have already been described.

~Alternating Current Feeder Regulation.~--With slight modification, the various methods of feeder regulation employed with direct current, may be applied to alternating current distribution circuits. For instance, if a non-inductive resistance be introduced in any electric circuit, the consequent drop in voltage will be equal to the current multiplied by the resistance. Therefore, feeder regulation by means of rheostats is practically the same in the case of alternating current as in that of direct current. In the case of the former, however, the effect of self-induction may also be utilized to produce a drop in voltage. In practice, this is accomplished by the use of self-induction coils which are commonly known as reactance coils.

~Application of Induction Type Regulators.~--In supplying lighting systems, where the load and consequently the pressure drop in the line increases or decreases, it becomes necessary to raise or lower the voltage of an alternating current, in order to regulate the voltage delivered at the distant ends of the system. This is usually accomplished by means of _alternating current regulators or induction regulators_. A device of this kind is essentially a transformer, the primary of which is excited by being connected directly across the circuit, while the secondary is in series with the circuit as shown in fig. 2,414. By this method the circuit receives the voltage generated in the secondary.

~Ques. Name two types of pressure regulator.~

Ans. The induction regulator, and the variable ratio transformer regulator.

~Ques. Of what does an induction regulator consist?~

Ans. It consists of a primary winding or exciting coil, a secondary winding which carries the entire load current.

The primary is wound for the full transmission voltage, and is connected across the line, while the secondary is connected in series with the line.

~Ques. What is its principle of operation?~

Ans. When the primary coil is turned to various positions the magnetic flux sent through the secondary coil varies in value, thereby causing corresponding variation in the secondary voltage, the character of which depends upon the value and direction of the flux.

~Ques. What is the effect of turning the secondary coil to a position at right angles with the primary coil?~

Ans. The primary will not induce any voltage in the secondary, and accordingly it has no effect on the feeder voltage.

~Ques. What is this position called?~

Ans. The neutral position.

~Ques. What are the effects of revolving the primary coil from the neutral position first in one direction then in the other?~

Ans. Turning the primary in one direction increases the voltage induced in the secondary, thus increasing the feeder voltage like the action of a booster on a direct current circuit while turning the primary in the opposite direction from the neutral position, correspondingly decreases the feeder voltage.

~Ques. It was stated that for neutral position the primary had no effect on the secondary; does the secondary have any effect on the feeder voltage?~

Ans. The secondary tends to create a magnetic field of its own self-induction, and has the effect of a choke coil.

~Ques. How is this tendency overcome?~

Ans. The primary is provided with a short circuited winding, placed at right angles to the exciting winding. In the neutral position of the regulator, this short circuited winding acts like the short circuited secondary of a series transformer, thus preventing a choking effect in the secondary of the regulator.

~Ques. What would be the effect if the short circuited winding were not employed?~

Ans. The voltage required to face the full load current through the secondary would increase as the primary is turned away from either the position of maximum or minimum regulation, reaching its highest value at the neutral position.

The short circuited winding so cuts down this voltage of self-induction that the voltage necessary to force the full load current through the secondary when the regulator is in the neutral position is very little more than that necessary to overcome the ohmic resistance of the secondary.

~Ques. What effect is noticeable in the operation of a single phase induction regulator?~

Ans. It has a tendency to vibrate similar to that of a single phase magnet or transformer.

~Ques. Why?~

Ans. It is due to the action of the magnetizing field varying in strength from zero to maximum value with each alteration of the exciting current, thus causing a pulsating force to act across the air gap, which tends to cause vibration when the moving part is not in perfect alignment.

~Ques. Explain the effect produced by bad alignment?~

Ans. If the bearings of the primary be not in perfect alignment with the bore of the secondary, thereby making the air gap on one side smaller than that on the other, the crowding over of the flux to the smaller air gap will cause an intermittent pull in that direction, which will develop vibration unless the primary bearings are tight and the shaft sufficiently stiff to withstand the pull.

~Ques. Upon what does the regulator capacity for any given service depend?~

Ans. It depends upon the range of regulation required and the total load on the feeder.

~Ques. How is the capacity stated?~

Ans. In percentage of the full load of the feeder.

For instance, on a 100 kilowatt circuit, a 10 kw. regulator will give 10 per cent. regulation, and a 5 kw. regulator, 5 per cent. regulation.

~Polyphase Induction Regulators.~--The polyphase induction regulator is similar to the single phase regulator except that both the primary and secondary elements are wound with as many sets of coil as there are phases in the circuit.

In construction these windings are distributed throughout the complete circumference of the stationary and moving elements and closely resemble the windings of an induction motor.

Polyphase regulators have but little tendency to vibrate because the field across the air gap is the resultant of two or more single phase fields and is of a constant value at all times. This field rotates at a rate depending upon the number of poles and the frequency of the circuit. This produces a mechanical pull of constant value which rotates with the magnetic field varying its position from instant to instant.

It is evident that this pull is of an entirely different character from that produced by the single phase field and that there is no tendency to set up the vibration that the mechanical pull of the single phase regulator tends to establish.

There is, however, considerable torque developed, and the device for revolving the moving element must be liberally designed so as to withstand the excess torque caused by temporary overloads or short circuits.

~Ques. In what respects do polyphase induction regulators differ in principle from single phase regulators?~

Ans. The induced voltage in the secondary has a constant value, and the regulation is effected by varying the phase relation between the line voltage and the regulator voltage.

~Ques. How is the primary wound?~

Ans. It is wound with as many separate windings as there are phases in the circuit, and these primary or shunt windings are connected to the corresponding phases of the feeder.

~Ques. What kind of magnetizing flux is produced by the primary windings?~

Ans. A practically constant flux which varies in direction.

~Ques. How is the secondary wound?~

Ans. There is a separate winding for each phase.

~Ques. Why is the voltage induced in the secondary constant?~

Ans. Because of the constant magnetizing flux.

~Ques. How is the line voltage varied by a polyphase regulator?~

Ans. When the regulator is in the position of maximum boost, the line AB, fig. 2,425 represents the normal busbar voltage, BC the regulator voltage, and AC the resultant feeder voltage. When the regulator voltage is displaced 180 degrees from this position, the regulator is in the position to deliver minimum voltage to the feeder, the regulator voltage being then represented by BD, and the resultant feeder voltage by AD. When the regulator voltage is displaced angularly in the direction BF, so that the resultant feeder voltage AF becomes equal to the normal busbar voltage AB, the regulator is in the neutral position. Intermediate resultant voltages for compensating the voltage variations in the feeders may be obtained by rotating the moving element or primary in either direction from the neutral position. For example, by rotating the primary through the angle FBE, the resultant voltage may be made equal to AE or AJ, thereby increasing the feeder voltage by an amount BJ; or by rotating it in the opposite direction through the angle FBG, the feeder voltage may be reduced by an amount BH.

~Ques. How are induction regulators operated?~

Ans. By hand or automatically.

~Ques. How is automatic operation secured?~

Ans. By means of a small motor, controlled by voltage regulating relays.

~Ques. How is the control apparatus arranged?~

Ans. Two relays are employed with each regulator, a primary relay connected to the feeder circuit and operating under changes of voltage therein, and a secondary relay connected between the primary relay and the motor, and operated by the contacts of the former, for starting, stopping and reversing the motor in accordance with changes in the feeder voltage, thereby causing the regulator to maintain that voltage at its predetermined normal value.

~Ques. Why are two relays used?~

Ans. For the reason that a primary relay, of sufficient accuracy and freedom from errors due to temperature and frequency variations, could not be made sufficiently powerful to carry the relatively large current required for operating the motor.

~Ques. What names are given to the relays?~

Ans. Primary and secondary.

~Ques. What difficulties were encountered in the operation of relays? ~

Ans. Vibration or chattering at the contacts of both relays and tendency of the movable contact arm of the primary relay to hug closer to one of the stationary contact points than to the other, thereby operating too often.

~Ques. What causes vibration or chattering at the contacts?~

Ans. This is due to the voltage frequently approximating the value required for closing a contact, thereby causing the contact points to barely touch and make several poor contacts in succession.

~Ques. What objectionable action is produced by vibration at the contacts?~

Ans. Arcing, burning and pitting of the contacts, even when alloys of the rarer metals are used, such as those of the platinum group, having extreme hardness and high melting points.

~Ques. What effect is produced by poor contact of the primary relay?~

Ans. It causes chattering in the secondary relay; which burns out and wears away its contact points, increasing the heating of the motor, creating objectionable noise and entailing wear and tear on the whole outfit.

~Ques. Why does the movable contact arm of the primary relay tend to remain nearer one of the stationary contact points than the other?~

Ans. This is due to the tendency of the relay to open the contact whenever the voltage equals that at which the contact closes.

~Ques. What provision is made in the primary relay to prevent vibration or chattering?~

Ans. Two auxiliary windings are provided: one in series with each of the stationary contact points and so arranged as to assist in making the contact by increasing the pressure on the contact points at the instant of closure.

The best effect of the compounding action of the auxiliary coils is obtainable when arranged for ¾ per cent. of the torque of the main coil.

A non-inductive resistance placed in parallel with each coil of the secondary relay, takes current approximately in phase with the current in the main coil of the primary relay, and of proper strength to make the number of ampere turns in the auxiliary coil three-fourths per cent. of the number in the main coil. The resistances have the additional effect of absorbing the "discharge" from the main coils of the secondary relay when the contacts are broken, thereby obviating sparking at the primary contact points.

~Variable Ratio Transformer Voltage Regulators.~--The principle of operation of this class of regulator is virtually the same as that of the induction type regulator; that is to say, both consist of regulating transformers, but ~in the variable ratio method~ _the primary or series coil is divided into a number of sections which may be successively cut in or out of the circuit to be regulated_, instead of varying the flux through the entire coil, as in the induction type. There are two distinct mechanical forms of variable ratio regulator:

1. Drum type; 2. Dial type.

~Drum Type Regulators.~--This form of variable ratio transformer consists essentially of a drum and finger type switch, similar to a railway controller.

There are many contacts, giving a large number of points of regulation, obtained by the use of changing switches and floating coils.

The floating coil is a part of the secondary winding of the regulating transformer which is insulated from the main portion of the winding, and is sub-divided by taps into a number of equal sections.

The sub-divisions of the main secondary winding are much larger, each one being equivalent to the whole of the floating coil.

~Ques. Describe the operation of a drum regulator.~

Ans. The floating coil and main windings are first connected in series with each other and with the line to be regulated. The floating coil is then cut out of the circuit step by step. When entirely cut out it is transferred to the next lower tap on the main winding, after which it is again cut out step by step and then transferred again. By continuing this process a large number of steps are provided with but comparatively few actual taps on the transformer.

~Ques. How many floating coils are used and why?~

Ans. Two floating coils are included in each regulator so that one can be transferred while the other is supplying the current to the line.

~Dial Type Regulators.~--This form of variable ratio transformer regulator consists of a regulating transformer and a dial type switch as shown in the accompanying illustrations. The regulating transformer is similar to a standard transformer except that the secondary winding is provided with a number of taps leading to the contact of the dial switch as shown in the diagram fig. 2,437.

~Ques. What modification is made to adapt dial regulators for heavy current?~

Ans. A dial with a series transformer, and a shunt or auto-transformer are employed as shown in fig. 2,436.

~Ques. Why is such modification desirable?~

Ans. Because, the additional cost of a series transformer is small in comparison with the cost of building a dial with a large current carrying capacity, and the cost of bringing out a number of heavy leads from a small transformer.

~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.

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.

~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.

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.

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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

~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.

[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.

~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.

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.

~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.

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.

~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.