Motion Picture Herald (Oct-Dec 1931)

58 Better Theatres Section October 24, 1931 Chart showing flywheel effect on current pulsation. and this in turn draws a pulsating current from the power company's lines. Motor and compressor manufacturers have experimented for years on the correct application of flywheels to limit this current pulsation to values which the power companies feel will not affect their service to other consumers. A standard current pulsation of 66% or less has been found suitable for most power company requirements, although certain territories require 40% or less for most installations. Since these values have been fixed, and particularly since the speeds are increasing, it is usually economical, both from the cost and space saving standpoint, to incorporate the flywheel effect required in the rotor of the motor. As the fl)rwheel effect required, assuming everything else the same, varies inversely as the square of the speed, low speed machines may occasionally be supplied with an auxiliary flywheel, or economies can be effected by placing the rotor on the outside, since the flywheel effect of any rotor varies as the square of the distance from the center of the wheel to the effective center of its weight. For most compressors the greater the flywheel effect, the lower will be the current pulsation. However, it is interesting to note that in the case of two-cylinder, horizontal, double-acting machines, this is true to a certain point, beyond which increased flywheel effect tends to increase the current pulsation. Still further increase of flywheel effect will again bring the current pulsation down, but as the cost for this extra-large effect is usually very heavy, it is always desirable to try to design the equipment for the lower values. A curve showing this condition, with a typical curve for the two-cylinder, single-acting machines, is shown in the accompanying chart. From the foregoing, it is evident that the slow-speed synchronous motor does not lend itself to starting against full load. This is a characteristic of its design and it should not be expected to perform the impossible. When an attempt is made to start the motor against an excessive load, the rotor may not turn. Should the motor be left on the line in this condition, it is very liable to become damaged. It is evident by again referring to the table that the locked rotor inrush is about 300%, and this excessive current is more than the windings can continually stand. When the rotor is free to turn, the inrush falls as the speed increases, and thus for normal operation, the inrush is usually figured at 75% of the locked rotor value, which would be approximately 225% of full load current. The writer has been on jobs where the operator was trying to start the unit by throwing the motor continually on the line, although the rotor remained at a standstill. Upon investigation, it was found that the compressor was not unloaded, and the rotor couldn't be turned over with a crowbar. As soon as the condition was remedied, the unit operated satisfactorily. OFTEN THE LOAD On a compressor is such that the motor can start to turn over and come up to probably 50% or 75% of its speed, but cannot come up to a point close enough for synchronizing. If the motor is permitted to operate in this condition, which is as a squirrel-cage induction motor at reduced speed, it will be damaged in a short time. Fortunately the cost of pushbutton starters for synchronous motors is not much greater than that of hand-operated starters, and instead of the operator being in full control and able to keep the motor on the line and in synchronizing, this can be taken care of by the starter. By obtaining starters with the proper features, it is possible to protect almost completely the motor against poor handling. This brings us to the subject of the synchronous motor control. The function of the starting control is not only to throw the motor on the line and then to switch the direct current to the field circuit when the proper speed for synchronizing is obtained, but also it should protect the motor against excessive loads, single phasing, voltage failure, etc. Protection against loads should not only be on running loads, but on starting loads as well, for any continuous excessive load will dam age the motor. Any control will usually have overload protection, but when these overloads are set to stop the motor when the load reaches from 110% to 125% of normal, they cannot be used in the same circuit for starting protection and are either cut out of the starting circuit altogether, or on the better equipments, are used to protect the motor during starting by some means which will proportion the starting current to a value on which the overloads will operate. If this latter feature is installed on the control, and the operator holds the starting button trying to start against a dead load too excessive, or one which cannot be brought up to speed, the main switch will not stay in. It was previously explained that the discharge resistance was used also in starting so the control will then be arranged to insert this resistance in a closed circuit with the field during starting, and remove it from the circuit during running conditions. This is accomplished by the field switch with an auxiliary contact, but before discussing this, it might be well to understand the method of synchronizing. The tabulation indicates a pull in torque of 40%, which is at about 95% speed. Therefore, the motor must be synchronized at this speed. There are various methods of doing this, but whatever method is used, it must be relied on to act shortly after the motor reaches this speed and never before. As the motor speeds up, the current — induced in the field — falls, and this can then be employed as a means of synchronizing. Also, under normal conditions, the motor will come to speed in a definite period of time, which may also be used, provided precaution is taken that it will not synchronize after the elapsed time if conditions are not correct. Depending on the method used, the relays will serve to actuate an automatic contactor called the field switch, which places the direct current on the rotating field. This switch has an auxiliary contact that cuts in and out the field discharge resistance. The exciter motorgenerator set is either started by a separate pushbutton before the main motor is started, or it can be arranged to start on the same pushbutton as the motor. If a belted generator is used, excitation will be available when the motor comes up to speed. THE WRITER believes the operator can better care for a plant if he understands the functions of the equipment, as here explained. In addition it may be of value to mention a few other factors. The exciter brushes should be inspected and cleaned at stated intervals. Often the top brush sparks excessively, and the operator cannot seem to determine the cause. The reason is usually because it is carrying twice the load it was designed for, due to the lower brush sticking in the holder and not touching the commutator. Exciters are very often mounted on the floor. The lower brush is hard to get at and is never cleaned. Often oil dripping from an overflowing bearing binds the carbon dust in this holder, when all other brushes are apparently clean.