CHAPTER XVII.

MANAGEMENT OF GENERATORS AND MOTORS.

_Operation of Generators._--The generator should be located in a clean dry place. If it is belt driven, the belt should be run horizontal if possible and so that the slack side will be on top. This increases the arc of contact with the pulleys and allows the belt to run with less tightening. The frame should be provided with a slide for the purpose of adjusting and tightening. The proportion between the largest and the smallest pulleys used close together should not be greater than about 6 to 1.

To start a dynamo it is best first to disconnect it from the switchboard. Start it running and adjust the voltage by the field rheostat. When the voltage has arisen to its proper value and everything is running smoothly, the main switch may be closed. If there is much of a load, it will probably be found that the voltage has fallen off a little and it will be necessary to re-adjust it. Next, look carefully after the brushes and set them at the points where there will be the least sparking. A good modern generator should not spark at all. All of the bearings must be carefully looked after and watched for heating. If they are not properly oiled or in good condition, they may heat considerably. The armature should run with considerable end play as this helps to distribute the oil over the bearings and even up the wear on the commutator surface.

Small generators sometimes lose their residual magnetism and it is then impossible to start them generating. In such a case the fields may be connected to a live-lighting circuit; or a small exciting current may be obtained from a battery. One should know which is the positive pole of the field and apply the battery or line current accordingly. A test for polarity may be made by placing the ends of wires of opposite polarity in a vessel with water and bringing them within an inch or so of each other. Under these conditions bubbles will be given off at the negative pole. The polarity of the generator will vary with the polarity of the field, the direction of rotation, and the connection of the brushes. By reversing any one of these we may reverse the polarity of the current delivered. Shunt dynamos cannot well be operated in parallel; where it is desired to operate several dynamos together, compound-wound machines are used.

_Operation of Motors._--The speed of a direct-current motor is always such that the counter e.m.f. of the motor becomes nearly equal to the impressed e.m.f. of the line. In order to speed up a motor it is necessary to weaken the fields; and conversely, to slow it down we must strengthen the fields. The above methods are necessary if the motor is to run at a nearly fixed speed with a variable load. The speed can also be controlled by a variable resistance placed in the armature circuit. This method, however, does not result in a steady speed with a variable load. It has very little effect if the load is light, and very much if it is heavy.

In order to start a motor it is necessary to have some resistance in the armature circuit. In very small motors the armatures are usually wound with sufficient resistance so that no external resistance is required. The larger motors are, however, equipped with starting boxes which limit the current through the armature until it has attained sufficient speed so that its counter e.m.f. will keep the current in check.

These starting boxes are usually wound with fine wire and cannot stand the starting current very long. The handle must be moved over steadily and slowly and not allowed to remain on an intermediate position unless it is known that the box is meant to be used as a speed controller as well as a starting box. The direction of rotation of a direct-current motor can be reversed by reversing either the field or the armature current. If both are reversed, it will continue to run in the same direction.

_Alternating-Current Motors._--Synchronous alternating-current motors are not used in theater work but the rotary converter is frequently used and may be considered as such. This machine must run at a certain speed which depends upon the frequency of the current supplied and the number of poles on the machine.

Rotary converters are of different types and may be started either from the direct-current side or from the alternating-current side. Some of them are provided with connections so that the alternating current may be applied on the direct-current side of the armature. For starting and operating these converters, the instruction of the maker should be consulted.

For motor-generators where alternating current is used, the induction motor is generally employed. It may be either single-phase, two-phase, or three-phase. All of these are essentially constant-speed motors. Simple repulsion motors and single-phase induction motors that start as repulsion motors are reversed by shifting the brushes.

Alternating-current series motors are reversed in the same manner as direct-current motors by reversing either field or armature. If both are reversed, the motor will continue to run in the same direction.

A three-phase induction motor will reverse its direction of rotation if any two of the line wires connected to the primary winding be reversed. If all three wires are changed in order, the direction of rotation will remain unchanged. Synchronous motors when started as induction motors are also governed in this way.

These motors of the larger size are generally started through an auto-transformer. For the smaller ones, it is customary to provide a throw-over switch, one side only of the switch being fused. The motor at starting takes very strong currents which would blow the running fuse. To reverse a two-phase induction motor the two wires of one phase must be reversed.

GENERAL HINTS.

Be sure that all belts are sufficiently tight.

See that all bearings are well oiled.

Let all shafts have sufficient end play.

Use no oil cans of iron around dynamos or motors.

Keep files and other iron or steel away.

See that all connections are good and tight.

Allow no metal dust or gritty substances to accumulate at the insulation of exposed parts.

See that the brushes fit properly and do not cut or scratch.

Use no emery paper on commutators.

Lubricate the commutators very sparingly and wipe off as much of the lubricant as you can.

Keep everything about the machines clean and allow no oil drippings to accumulate.

Place the starting box for a motor so you can see the motor start from the box.

Always place a switch which will disconnect all of the wires close to the motor.

If possible arrange motors so they may start without load.

Allow no motor or generator to be placed in the operating room of a moving-picture theater.