N. Hawkins

“ 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 ”

“ One of the commonest errors is to select a rheostat of insufficient capacity. If the current required to operate the motor at full speed with no resistance in circuit be greater than the rated capacity of the rheostat, overheating of the resistance will result. An increase in temperature even to a point where the hand cannot be held on the enclosing case need cause no apprehension, but should the resistance become red hot it indicates that the apparatus is being worked far beyond its capacity, and the load on the motor should be reduced or a regulator of greater capacity substituted. ”

“ NOTE.--The action of an equalizing bar in equalizing the load on compound dynamos run in parallel may be explained as follows: The compound winding of a dynamo raises the pressure in proportion to the current flowing through it, and if, in a system of parallel operated compound dynamos without the equalizing connection, the current given by one machine were slightly greater than the currents from the others, the pressure of that machine would increase. With this increase in pressure above the other machines, a still greater current would flow, and so raise the pressure further. ”

“ When the machine heats, it is a common mistake to suppose that any part found to be hot is the seat of the trouble. Hot bearings may cause the armature or commutator to heat, or vice versa.
All parts of the machine should be tested to ascertain which is the hottest, since heat generated in one part is rapidly diffused. This is best done by starting with the machine cold; any serious trouble from heating is usually perceptible after a run of a few minutes at full speed with the field magnets excited. ”

“ What will be the result if the connections of some of the field coils of a dynamo be reversed?
Ans. If one-half the number of coils oppose the other half, the field magnetism will be neutralized and the machine will not build up at all; but if one of the coils be opposed to the others, the machine might build up, but the generated voltage will be low, and there will be considerable sparking at some of the brushes. ”

“ 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. ”

“ Of course when the solder begins to melt at the commutator connections and shellac begins to "fry out" of the armature and an odor of burnt cotton begins to pervade the air, the safe limit has been far exceeded, and in most cases, as a matter of fact serious damage is the result. To be more definite, no part of the dynamo or motor should be allowed to rise in temperature more than 80 degrees F. above the temperature of the surrounding air, excepting in the case of commutators where no solder has been used to connect the leads. These can be allowed to rise to a still higher temperature. ”

“ 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 [664] the mains, would injure all except the smallest motors. Hence motors above two horse power usually require a rheostat. ”

“ During the operation of starting the motor, the field resistance is short circuited by an auxiliary contact (the slotted metal strip shown near center of rheostat) but as soon as the starting lever touches the no voltage release magnet or, in other words, when the motor has been accelerated to normal speed, this short circuit is removed, and the field resistance becomes effective for speed regulation. The motor is accelerated from rest to normal speed by moving both levers from left to right, while increases in speed above normal are obtained by moving the upper lever from right to left. ”

“ When all energy has been delivered, the circuit switch is opened so that no more current can be obtained until one or more coins have been deposited. The indicating mechanism shows only the number of coins which stand to the credit of the customer, but, by consulting the meter dial, one can determine what fractional part of the prepayment next to be cancelled remains to the credit of the customer. A coin or washer larger than the coin for which the device is designed cannot be introduced into the receiving slot and a smaller one will not operate the device. ”

“ 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. ”

“ With a given current, the total magnetizing force which deflects the needle is the same, but with a short coil, it is produced by a large current circulating around a few turns, instead of a small current circulating around thousands of turns as in the long coil. The short coil being of low resistance is used to measure the current, and the long coil with high resistance, is suitable for measuring the pressure. Hence, a short coil instrument with its scale directly graduated in amperes is an ammeter, and the long coil type with graduation in volts is a voltmeter. ”

“ In the direct reading type, the scale is so cut that when the galvanometer is balanced, the pointer of the instrument indicates the value of the resistance between the X posts. The scale is calibrated for any desired range. These ohmmeters being slide wire bridges, the greatest accuracy is at the center of the scale, hence one should be selected that will bring the part of the scale likely to be the most used at or near the center. A convenient type is that in which the scale is cut in per cent., 100 per cent. being at the center of the scale. ”

“ The field coils of series machines are sometimes provided with short circuiting switches or resistances arranged to shunt the current across the field coils. If too much of the current be shunted across, the switch should be opened, or if there be a regulator, it should be so adjusted that it will pass enough current through the field windings to excite the machine. ”

“ If a dynamo fail to excite, the operator should first see that the brushes are in the proper position and making good contact, and that the external circuit is open if the machine be shunt wound, and closed if series wound.
In starting a dynamo it should be remembered that shunt and compound machines require an appreciable time to build up, hence, it is best not to be too hasty in hunting for faults. ”

“ In the tangent galvanometer this principle is employed in the measurement of currents. A magnetic needle is pivoted in a field of known strength. The current to be measured is passed round a coil (or coils) which generates a field at right angles to the original field. The needle then lies along the direction of the resultant field, and by finding the tangent of its angle of deflection, and knowing the field strength produced by unit current in the coil, the current strength can be found. ”

“ 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. ”

“ The positive and equalizer switches of each machine differ in pressure only by the slight drop in the series coil, and in some large stations these two switches are placed side by side on a pedestal near the machine. In such cases, the equalizer and positive bus bars are often placed under the floor near the machines, so that all leads may be as short as possible. If all the dynamos be of equal capacity, all the leads to bus bars should be of the same length, and it is sometimes necessary to loop some of them. ”

“ The function of the core is to secure a uniform field. The moving coil is wound upon a form of aluminum, which serves the purpose of damping by the generation of eddy currents. The winding of the coil is of fine copper wire, to which current is conveyed by means of the controlling springs and which, in the case of a voltmeter, is connected in series with a resistance, and in the case of an ammeter, across the terminals of a shunt. ”

“ An ammeter or ampere meter is simply a commercial form of galvanometer so constructed that the deflection of the needle indicates directly the strength of current in amperes. A good ammeter should have a very low resistance so that very little of the energy of the current will be absorbed; the needle should be dead beat, and sufficiently sensitive to respond to minute variations of current. ”

“ At the control stations, the operator can distinguish the accelerating button from the retarding button by the sense of touch and obviously he can in the same manner ascertain the position of the lever. The position of the lever whether at start, stop or safety, can be readily observed at a distance. When the lever of either control station is placed at stop, the current is disconnected from the motor and a powerful dynamic brake brings the press to rest without delay and without shock or harmful strain. ”

“ Under such circumstances, the pressure at the bus bars is liable to great variations, and there is also liability of blowing the fuses of the overloaded dynamos, thus precipitating a general breakdown. To cause an equal division of the load among all the dynamos, under such circumstances, it is needful to insert a small resistance in the armature circuits of such dynamos as possess the straightest characteristics, or of such dynamos as take more than their share of an increase of load. ”

“ The single plug used with each decade is never out of use, being either in the zero position or set on some value, and is therefore not easily lost by being laid aside. The use of only one plug in a decade makes it easy to ascertain that the plug is making good contact as only one block in a row is plugged at a time, the other blocks are not kept under a strain by having plugs forced tightly between them. ”

“ Since the function of the particular galvanometer is simply to show a deflection when a current is passing, and to mark zero when there is none, a coil of thick wire with a pocket compass in the center will do all that is required, but care must be taken to remove it sufficiently far away from the disturbing effects of the armature magnetism. ”

“ The Potentiometer.--For the rapid and accurate measurement of voltage, current, and resistance, the potentiometer can be recommended. Those in charge of electric light and power companies, and also those who purchase large amounts of electrical energy are realizing, more and more, the necessity of having satisfactory primary standards with which to check their volt-, ampere-, and watt-meters. ”

“ Short Circuits.--In a series or compound dynamo a short circuit or heavy load will overload the machine and cause the fuses to blow. A shunt machine will not excite under these circumstances, for the reason that practically the whole of the current generated in the armature passes direct to the external circuit, and the difference of potential between the shunt terminals is practically nil. ”

“ As this, however, is liable to burn the terminals, a better plan is to fix a short piece of scrap wire in one terminal, and then with another piece of insulated wire make momentary contacts with the other terminal and the short piece of wire. If the machine excite, it will be at once evident by the arc which occurs between the two pieces of wire. As the voltage of a series machine when induced to build in this manner generally rises very rapidly, great care should be taken that the contact is at first only momentary, merely a rubbing or scraping touch of the wires. ”

“ When dynamos feeding current to motors are to be shut down, the switches on the motors should first be opened. Otherwise some of the motor fuses will blow. As the voltage goes down the motors will draw more current to do the work. If a plant be shut down with the motor switches "in" it will generally be found impossible to start a shunt dynamo, the low resistance in the mains not allowing enough current to flow around the shunt fields to energize them. ”