US2245924A - Electrical system - Google Patents

Description

June 17, 1941.

J. J. KENNEDY ELECTRICAL SYSTEM Filed June 12, 1940 INVENTOR John J Kennedy /ATTORNEY Patented June 17, 1941 ELECTRICAL SYSTEM John J. Kennedy, New Haven, Conn, assignor to The Safety Car Heating and Lighting Company, Inc., a, corporation of Delaware Application June 12, 1940, Serial No. 349,931

(Cl. l'lL-l23) 3 Claims.

This invention relates to the conversion of electrical power from one form to another, and more in particular to a system for providing an alternating current source for fluorescent lights on railway cars where direct current power is available.

An object of this invention is to provide means for converting power received from a direct current source to power in the form of alternating current. A further object is to provide control means which is automatically operable in response to changes in the supply voltage and the load to maintain the output voltage and frequency within satisfactory limits. A further object is to provide apparatus of the above character which will produce alternating current of substantially constant voltage and frequency throughout a wide range of conditions of use. A further object is to provide apparatus of the above character which is simple and sturdy in construction and which. is inexpensive to manufacture and efficient in operation. A further object is to provide durable apparatus which is light in Weight and which is dependable in operation. Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly consists in the features of construction, combinations of elements, and arrangements of parts as will be exemplified in the structure to be hereinafter described and the scope of the application of which will be indicated in the following claims.

The single figure of the drawing is a circuit diagram of one embodiment of the invention with certain details omitted for clarity.

On railway cars, such as coaches, pullman cars, and the like, each car is provided with its own individual electrical system; this system generally includes a set of storage batteries, lighting and other equipment and anaxle-driven generator to supply power for the equipment and for charging the batteries. Additional equipment is provided under some circumstances for supplying power to the system when the car is stationary in the yard, and in some systems means is provided for driving air-conditioning equipment directly from the axle when the car is traveling at a proper speed. In all systems, the equipment should be sturdy and dependable in action. Furthermore, it is difiicult to insure that the equipment will receive proper attention during use. and it is therefore desirable that the equipment operate with a minimum; of adjustment and repair. In addition to these considerations, it must be borne in mind that the equipment must be light in weight and efficient in operation.

It has been found that fluorescent lights are very well suited for railway lighting, and it is therefore desirable to provide for their use by furnishing a reliable and eflicient source of alternating current on railway cars. It is not practical to provide only an axle-driven alternator as this would not operate when the car was stationary or moving at a slow speed. Further, the frequency would vary over too wide a range to be practical. It is proposed to provide an alternator driven by a direct current motor deriving its power from the battery-generator system of the car. However, this is difiicult because of many problems such as those referred to above. Further, it should be noted that during operation, the voltage of the car battery-generator system varies depending upon the condition of charge of the battery, and upon whether or not power is being supplied by the generator which is mechanically connected to the car axle. With the usual type of car system, the normal voltage variation may be from 28 to 45 volts. It is an object of this invention to provide a source of alternating current which will have the proper characteristics under the above conditions.

In the present embodiment of my invention, apparatus is provided which derives its power from the battery-generator system of the car and which produces alternating current of substantially constant frequency and voltage. The present invention is capable of receiving power over the wide range of voltage of the batterygenerator system, and efficiently delivers alterhating current of substantially constant voltage and frequency. During operation there are changes in the alternating current load, and the present apparatus is capable of maintaining reasonably constant voltage and frequency changes in the load.

Referring particularly to the left-hand side of the drawing, a generator 2 is provided with a controller 4 which maintains the proper current in the generator field One side of the generator is connected through a line 1 and a switch it to a line 8 which is connected to a set of batteries i2, and the other side of the generator is connected through a line It to the other side of the batteries. Lines 8 and It are the main power lines of the battery-generator system which carries a load It consuming direct current. The voltage between lines 3 and I l depends upon the state of charge of the set of batteries [2 and upon the rate at Which the batteries are being charged or discharged. Connected across lines 8 and I4 is a direct current motor [8 which has a main shunt field winding 11, a starting winding 19 and an armature 20. Armature 20 is connected through a shaft 22 to drive the armature 24 of an alternator indicated at 2'0 and having a main field winding 25 and an auxiliary field winding 21. The alternator output brushes are connected to a pair of lines 28 and 30 which supply alternating current to a load 32 which, in this embodiment, is a bank of fluorescent lights and their attendant auxiliaries.

The main field winding 25 of the alternator has one side connected directly to line 8 and the other side connected through a resistor 36 to line Hi. The alternator auxiliary field winding 21 is connected in series with the motor armature 20 and the starting winding I9. In this way, both windings l9 and 21 carry the motor armature current, all as will be more fully discussed below.

The main motor field winding IT has one side connected directly to line 8 and the other side connected through a resistance unit 42 to line 14. Resistance unit 42 is provided with two taps indicated at M and 4%. Tap 45 is connected through a lead 48 to one terminal of a solenoid-operated switch 56 having an armature 52 connected through a lead a to line I4. Switch 50 is provided with a solenoid 56 which is connected directly between lines 8 and [4 so that when the battery-generator voltage between these lines reaches a predetermined value, armature 52 is raised closing switch 58 and connecting tap 66 directly to line M. This shorts out the lower portion of resistance unit 42, decreasing the resistance of the circuit of the main motor field winding ll. Tap 44 is connected by a lead 58 to a similar solenoid-operated switch 69, having an armature 62 connected through a lead 64 to line M. Switch 69 has a solenoid 66 which is' connected across lines 8 and I4 and is so adjusted that it will lift armature 62 and thus close switch 68 at a voltageabove the voltage necessary to close switch 59. Thus, with the voltage between lines 8 and M at such a value that switches 58 and 60 are both open, as the voltage rises to a predetermined value, switch closes and shorts out the lower portion of resistance unit 42. Upon an additional predetermined rise in voltage, switch closes and shorts out an additional portion of resistance unit 62. At low voltage, the current of motor field llflows through the entire resistance unit 42 and the field current is held at a relatively low value.

The iron core of the motor is such that the iron is not saturated and thus any increase in field current is accompanied by a proportionate increase in magnetic flux. An increase in line voltage therefore causes a proportionate increase in field current, and as a result, the change in motor speed is comparatively small throughout a reasonable range of change of voltage. When the voltage does increase to an extent that the motor increases and approaches the maximum permissible speed, switch 5!] is closed, and the resistance of the field circuit is thereby decreased, so that the field current increases. This increase in field current causes a corresponding increase in magnetic fiux, which results in a reduction in the speed of the motor. The motor then operates within a second range of change of voltage with the motor speed maintained between the permissible limits.

If at any time there is a further increase in and M varies over a wide range.

the voltage to a point where the motor speed again approaches the maximum limit, switch 69 closes, thereby further decreasing the resistance of the field circuit and further increasing the field current. This further increase in field current decreases the speed of the motor again in the same manner as the closing of switch 50, and the motor then operates within the permissible range of speed over a third range of change of voltage. When the voltage decreases, the reverse of the above operation occurs with switch 6E3 first opening, and then switch 50 opening. By proper design, these increases and decreases in the speed may be carefully adjusted to be within permissible limits, and it is contemplated that the number of voltage-responsive switches, such as switches 50 and 60, will be such that the motor speed will be maintained within permissible limits throughout the range of the change of voltage.

During operation, the temperature of field winding [1 may vary to thereby change the resistance of the field winding. However, by mak ing the resistance unit E2 large in proportion to the resistance of the motor field winding and with a zero temperature-resistance coefficient, any change in the resistance of field winding I! is a small part of the total resistance of the circuit, and temperature changes do not materially affect the operation of the motor. The auxiliary motor winding [9 is effective during starting of the motor to provide sufficient flux for rapid starting of the motor. After the motor has come up to speed, the flux of the auxiliary field winding is not great and affects the operation of the motor to only a small degree- As indicated above, the magnetic field of alternator 26 i produced by the main alternator field winding 25 and the auxiliary alternator field winding 21. Main field winding 25 is connected across lines 8 and I4 in series with resistance unit 36 which has a zero temperature-resistance coefiicient and which reduces the effect of temperature Change in the field Winding, in the manner of resistance unit 42. The iron core of a1- ternator 26 is such that the flux produced by alternator field Winding 25 is substantially constant even though the voltage between lines 8 In this manner, the flux produced by main field winding 25 tends to produce a constant voltage across the output lines 28 and 30.

However, the load of the bank of lights and auxiliaries varies over a wide range so that the output current of alternator 26 varies, and an increase in load current tends to cause a voltage drop at the terminals of the alternator due to armature reaction and it is desirable to compensate for this voltage drop by increasing the excitation of the alternator. Accordingly, auxiliary field winding 21 carries the motor armature current, which current varies with the load upon alternator 26. Thus, as the alternator current is increased, there is a corresponding increase in the mechanical load upon motor l8 and a resulting increase in the motor armature current. This produces an increase in the flux of auxiliary field winding 21 which adds to the flux pro the alternator, which increase is sufficient to make up for the armature reaction. As a result, the voltage of the alternator remains constant.

As many possible embodiments may be made ofthe above invention and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth, or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. In an electrical system for supplying alternating current of substantially constant frequency and voltage to a load wherein power is derived from a direct current source the voltage of which may vary over a Wide range, the combination of, a direct current motor armature connected to carry current which varies with the load and with changes in the voltage source, a motor field Winding connected to receive current from said source and to produce magnetic flux linking said armature, a current control mechanism including a resistance unit in series with said motor field winding and means to short out a portion of said resistance unit dependent upon and responsive to a predetermined increase in the voltage of said source, an alternator armature mechanically connected to said motor armature and having its output terminals connected to deliver alternating current to the load, a main alternator field winding connected to receive current from said source and thereby produce a magnetic fiux for said alternator armature, and an auxiliary alternator field winding connected in series with said motor armature to cooperate with said main alternator field Winding in producing flux for said alternator armature.

2. In a railway car electrical system for supplying alternating current of substantially constant frequency and voltage wherein power is available in direct current form from a batterygenerator system, the combination of a direct current motor having a field winding and an armature; means connecting said field winding in series with a stepped resistance unit across the source of direct current; means responsive to the increase in voltage of the source of direct current to progressively remove portions of said resistance unit from the series circuit of said field winding; an alternator having an armature and a main field winding and an auxiliary field winding; means connecting said main field winding to supply the main magnetic fiux for said alternator; means mechanically connecting the armature of said motor to the armature of said alternator so that said alternator is driven by said motor and the load on said motor varies directly with the electrical load of said alternator; and means connecting said auxiliary field winding to carry the armature current of said motor so that said auxiliary field winding produces a magnetic flux for said alternator which is additive to the magnetic fiux produced by said main field winding and which additional magnetic flux varies with the load upon said alternator.

3. In apparatus of the class described, the combination of, an alternator having an armature and a main field winding, an auxiliary field winding for said alternator, a direct current motor mechanically connected to drive said alternator and having an armature and a main field winding, means connecting said auxiliary field winding in series with the armature of said motor so that said auxiliary field winding tends to produce a magnetic flux which varies with the armature current of said motor, means comprising a source of direct current to drive said motor, means connecting the main field Winding of said motor across said source including a stepped resistance unit, and voltage-responsive means including a plurality of switch units which are progressively closed in response to predetermined rises in the voltage of said source to progressively short out portions of said resistance unit as the voltage of said source increases, whereby the current carried by the field winding of said motor is increased as the voltage of said source increases, and the rate of change in current is greater than the rate of change in voltage.

JOHN J. KENNEDY.

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