CHAPTER XXXV

OPERATION OF MOTORS

In operating motors of any considerable size, whether connected to the public supply mains of a central generating station for combined lighting and power service, or to power service mains only, there are certain precautions to be observed in starting, stopping, and regulating the motor, in order that the efficiency of the supply, and indirectly the working of other motors and lamps connected to the mains in the immediate neighborhood, may not be affected by abnormal variations of pressure. These precautions should be observed also to prevent any danger of the motor itself being subjected to detrimental mechanical shocks and excessive temperatures in the working parts.

Before Starting a Motor.--The general instructions relating to inspection and adjustment, lubrication, etc., which have already been given, should be carefully followed preparatory to starting[E].

[E] NOTE.--In starting a motor, first see that the bearings contain sufficient oil and that the brushes bear evenly on the commutator. If a circuit breaker be used, close it; then close the main switch. Rotate slowly the handle of the starting rheostat as far as it will go. Care should be taken, in starting the motor, that the handle of the rheostat be not rotated too fast. To stop a motor, open the circuit breaker or switch, which will cut in the resistance of the starting box. Never attempt to stop a motor by forcibly pulling open the starting box, _Disregard of these instructions may cause burning out of the field coils._

Starting a Motor.--In starting a motor, resistance must be put in series with the armature because, since there is no reverse electromotive force to counteract the applied voltage when the motor is at rest, the switching of the latter direct to the motor would result in an abnormal rush of current. This, in addition to being uneconomical and productive of a drop of voltage in the mains, would injure all except the smallest motors. Hence motors above two horse power usually require a rheostat.

Ques. Describe a rheostat or "starting box."

Ans. It consists essentially of a suitable resistance to be inserted at starting to reduce the initial rush of current, and which can be cut out in sections by successive movements of a lever as the speed increases.

Ques. Describe what occurs in starting a motor.

Ans. When the lever of the starting box is moved to the first contact some of the resistance is cut out of the circuit and current flows through the motor. This produces a torque and starts the armature rotating. The movement of the armature induces a reverse voltage, which, as the speed increases, gradually reduces the applied current. With this reduction of current, the torque is reduced and the speed not accelerated as quickly as at first. When the applied current has been reduced to a certain value by the increasing reverse current, the handle of the starting box is moved to the next contact, and so on till all the resistance in the starting box has been cut out, the motor then attaining its normal speed.

Ques. What is the difference between a starting box and a speed regulator?

Ans. Motor starting rheostats or "starting boxes," are designed to start a motor and bring it gradually from rest to full speed. They are _not_ intended to regulate the speed and must not be used for such purpose.

_Failure to observe this caution will result in burning out the resistance which, in a motor starter, is sufficient to carry the current for a limited time only_, whereas in the case of speed regulators sufficient resistance is provided to carry the full load current continuously.

Ques. For what kinds of service are speed regulators used?

Ans. In cases when the speed must be varied, as in traction motors, organ blowers, machine tool drive, etc.

Ques. How long does it take to start a motor?

Ans. Usually from five to ten seconds.

Ques. How is the starting lever operated?

Ans. It is moved progressively from contact to contact, pausing long enough on each contact for the motor to accelerate its speed before passing to the next.

Ques. What are the conditions at starting in a series motor?

Ans. There is a rush of current, the magnitude of which depends on the amount of resistance cut out at each movement of the starting lever.

Ques. How are small series motors started on battery circuits?

Ans. By simply closing a switch to complete the circuit, the resistance of the battery being sufficient to prevent a great rush of current while starting.

Ques. How is a shunt motor started?

Ans. In starting a shunt motor, no trouble is likely to occur in connecting the field coils to the circuit. Since the resistance of the armature is very low, it is necessary on constant voltage circuits to use a starting rheostat in series with the armature.

The necessary connections are shown in fig. 756. The switch is first closed thus sending current through the field coils, before any passes through the armature. The rheostat lever P is then moved to the first contact to allow a moderate amount of current to pass through the armature. The resistance of the rheostat is gradually cut out by further movement of the lever P, thus bringing the motor up to speed.

Ques. How does the reverse voltage affect the starting of a motor?

Ans. When a motor is standing still, there is no reverse voltage, and the current taken at first is governed principally by the resistance of the circuit. If the motor be series wound, there is a momentary reverse voltage, due to self-induction while the field is building up. If the motor be shunt wound, self-induction delays the current through the field coils, but that through the armature is not impeded by such cause. When the armature begins to revolve, reverse voltage is developed which increases with the speed. The resistance of the starting box may be gradually cut out as the armature comes to speed. Thus the reverse voltage gradually replaces ohmic drop in limiting the current as the motor comes to speed.

Failure to Start.--This fault, which is liable to occur in a motor of any description, is similar to failure to excite in a dynamo, and is liable to be produced by any of the causes mentioned in connection with the latter fault, excluding insufficient speed, and insufficient residual magnetism.

When a motor fails to start, it should first be ascertained if a supply of electrical energy be available in the mains. This may readily be discovered by means of a voltmeter, or if low tension service, by means of the fingers bridging across the main terminals. If the supply of energy be present, the contact arm of the starter should be moved into such position that all resistance is inserted into circuit with the motor. This is important, as the motor may start suddenly while trying to ascertain the cause of the stoppage.

Having closed the switch, if the motor fail to start, it will be advisable to remove the load if possible, as the failure may arise from an overload of the machine. This being effected and the motor not starting, the terminals of the latter should be tested by the means already described for voltage. If no voltage be generated, a broken circuit or a defective contact may be looked for in the main fuse, switch, or starting box. The resistance coils of the latter, through the heat developed, frequently break in positions out of sight. If a defective contact of this nature cannot readily be seen, the contact arm should be moved slowly over the contacts, as it is possible the broken coil may be cut out of circuit by this means.

If a difference of pressure exist between the motor terminals, the field magnets will, if shunt or compound wound and in good order, be excited, which may be ascertained by means of a bar of iron. If no magnetism be present, it will of course, indicate a broken or bad connection, either between the terminals of the field coils, or one or more of the coils themselves. If the bar pull strongly, the position of the brushes upon the commutator in regard to the neutral points should be ascertained, and the rocker adjusted, if necessary, to bring them into their correct positions. If this fail to start the motor, the connecting leads from the motor terminals to the brushes and the brushes themselves should be carefully examined for broken or bad connections, and defective contact of the brushes with the commutator. In the latter case, it may arise from a dirty state of the commutator, or from the brushes not being fed properly. If due to these causes, pressing the brushes down upon the commutator with the fingers will probably start the motor. If the failure to start arise from none of these causes, it is probably due to the field coils acting in opposition, or to a short circuited armature. This latter remark applies more especially to motors provided with drum armatures.

Precautions with Shunt Motors.--With motors of this type, because of the large amount of self-induction in the shunt windings, it is important to note: 1, that in switching on the field magnet, the current may take an appreciable time to grow to its normal value, and 2, that in switching off, especially with quick break switches, high voltages are induced in the windings, which may break down the insulation.

Ques. What provision is made so that the magnetizing current will have time to reach its normal value?

Ans. The field connections are generally separated from the actual starter, and taken to the main switch, so that wherever the main switch is closed, the current flows through the field coils, before the starting lever is moved.

Ques. How are the connections arranged to avoid excessive voltage in the windings due to self-induction?

Ans. Generally the armature and field magnet circuits are placed in a closed circuit that is never opened.

In other cases, in order that the rise of voltage may not injure the insulation when the shunt is opened, a special form of main switch is sometimes used which, before breaking from the supply, puts a non-inductive resistance across the shunt of the motor. This is known as a _flashing resistance_.

Ques. How can shunt motors be controlled from a distant point?

Ans. The starter and switch are placed at the desired point and the two main wires and the field wires run from that point to the motor.

This requires additional wire which increases the cost and line loss.

Regulation of Motor Speed.--Motors are generally run on constant voltage circuits. Under these conditions, the speed of series motors varies with the load and at light loads becomes excessive. Shunt motors run at nearly constant speeds.

For many purposes, particularly for traction, and for driving tools, it is desirable to have speed regulation, so that motors running on constant voltage circuits may be made to run at different speeds.

The following two methods are generally used for regulating the speed of motors operated on constant voltage circuits:

1. By inserting resistance in the armature circuit of a shunt wound motor;

2. By varying the field strength of series motors by switching sections of the field coils in or out of circuit.

Ques. Describe the first method.

Ans. This method is illustrated in fig. 756. When the main switch is closed, the field becomes excited, then by moving the lever P of the starting rheostat the various contacts (1, 2, 3, 4, 5), more or less of the rheostat resistance is cut out of the armature circuit, thus varying the speed correspondingly.

This is the same as the method of starting a motor, that is, _by variation of resistance in armature circuit_, but it should be noted that when this method is used for speed regulation, _a speed regulating rheostat should be used instead of the ordinary starting box_, because the latter, not being designed for the purpose, _will overheat and probably burn out_.

Ques. Describe the second method.

Ans. This method of regulating the speed of a series motor is shown in fig. 757. The current through the armature will flow through all the field windings when the position of the switch lever S, is on contact 4, and the strength of the field will be the maximum. By moving the arm to contact 3, 2, etc., sections of the field winding are cut out, thus reducing the strength of field and varying the speed.

Ques. How does the speed vary with respect to variation of field strength?

Ans. Decreasing the field strength of a motor increases its speed, while increasing the field strength decreases the speed.

Under the conditions of maximum field strength, as with switch S on point 1, the torque will be greatest for any given current strength and the reverse voltage also greatest at any given speed. The current through the armature of the motor, to perform any given work, will thus be a minimum, as well as the speed at which the motor has to run, in order to develop sufficient reverse voltage to permit this current to flow. Regulation of speed by varying the field strength is limited in range of action, since the field saturation point is soon reached, moreover, with too low a field strength, armature reaction produces excessive field distortion, sparking, etc.

Ques. How is the speed of shunt and compound motors varied with respect to the normal speed in the two methods?

Ans. The first method (variable resistance in armature circuit) reduces the speed _below_ the normal or rated speed of the machine, while the second method increases the speed _above_ the normal.

In the first method the amount of speed reduction depends partly upon the amount of resistance introduced into the armature circuit, and partly upon the load.

In the second method the amount of speed increase depends entirely upon the amount of resistance placed in the shunt winding circuit.

Eighty-five per cent. is about the maximum speed reduction obtainable by armature resistance but so great a reduction is seldom satisfactory since comparatively slight increases in the load will cause the motor to stall.

Shunt field regulation may be obtained up to any point for which the motor is suited, the only limitation in this case being the maximum speed at which the motor may be safely operated.

It should be remembered, however, that speed increase by shunt field weakening increases the current in proportion to the increase in speed, and care should be taken not to overload the armature.

NOTE.--A compound motor may be made to run at constant speed, if the current in the series winding of the field be arranged to act in opposition to that of the shunt winding. In such case, an increase of load will weaken the fields and allow more current to flow through the armature without decreasing the speed of the armature, as would be necessary in a shunt motor. Such motors, however, are not very often used, since an overload would weaken the fields too much and cause trouble. If the current in the series field act in the same direction as that in the shunt fields, the motor will slow up some when a heavy load comes on, but will take care of the load without much trouble.

NOTE.--Motors have much the same faults as dynamos, but they make themselves manifest in a different way. An open field circuit will prevent the motor starting, and will cause the melting of fuses or burning out of the armature. A short circuit in the fields, if it cut out only a part of the winding, will cause the motor to run faster and very likely spark badly. If the brushes be not set exactly opposite each other, there will also be bad sparking. If they be not at the neutral point, the motor will spark badly. Brushes should always be set at the point of least sparking. If it become necessary to open the field circuit, it should be done slowly, letting the arc gradually die out. A quick break of a circuit in connection with any dynamo, or motor is not advisable, as it is very likely to break down the insulation of the machine. The ordinary starting box for motors is wound with comparatively fine wire and will get very hot if left in circuit long. The movement of the arm from the first to the last point should not occupy more than thirty seconds and if the armature do not begin to move at the first point, the arm should be thrown back and the trouble located.

Ques. How is a wide range of speed regulation secured?

Ans. By a combination of the two methods.

Regulation by Armature Resistance.--Speed regulators for this method of regulation, are designed to carry the normal current on any contact without overheating and when all the resistance is in the circuit, they will reduce the speed of the motor about 50 per cent. provided the motor be taking the normal current. When operating without resistance in the armature circuit, shunt wound and compound wound motors will regulate to approximately constant speed regardless of load. This characteristic of inherent regulation is lost, however, when armature resistance is employed to reduce the speed of the motor, fluctuations in load resulting in fluctuations in speed, which become more noticeable as the amount of resistance inserted in the armature circuit is increased. Accordingly, it becomes necessary to move the lever of the speed regulator forward or backward to again obtain the speed at which the machine was operating before the load changed.

When the speed of a motor driving a constant torque machine is reduced by inserting resistance in the armature circuit there is no corresponding reduction in current consumed. The motor runs more slowly simply because a part of the energy impelling it is shunted into the resistance and there dissipated in the form of heat. Hence, whether the motor be operating at full speed or half speed, the amount of current consumed is the same; the only difference being that in the one case all the energy taken from the line is expended in driving the motor while in the other case only one half is utilized for power, the other half being dissipated in the resistance. Speed regulation by armature resistance only is therefore open to two objections: 1, the difficulty of maintaining constant speed under varying load conditions, and 2, the necessity of wasting energy to secure speed reduction. These objections are, in part, offset by the fact that speed reduction by armature resistance may be applied to any motor of standard design and requires nothing more than the simplest and least expensive speed regulating rheostat.

In cases where the motor will be operated nearly always at full speed, the difference in first cost of the installation may justify the use of the armature resistance method of control. As a rule, speed regulation by shunt field resistance is preferable.

Regulation by Shunt Field Resistance.--Since regulation by this method is for speeds above normal, a starter must be used to bring the motor up to its rated speed. Usually the starter is combined with the regulator, as shown in fig. 761, the device being called a _compound starter_.