US3743442A

US3743442A - Air compressing apparatus for railway passenger cars

Info

Publication number: US3743442A
Application number: US00150053A
Authority: US
Inventors: W Wilson
Original assignee: General Signal Corp
Current assignee: SPX Corp
Priority date: 1971-06-04
Filing date: 1971-06-04
Publication date: 1973-07-03
Anticipated expiration: 1990-07-03
Also published as: CA947845A

Abstract

The disclosure concerns air compressing apparatus for railway passenger cars which incorporates a continuously running, wet rotor, screw type air compressor driven by a DC shunt motor supplied with electrical power from the third rail. The drive motor is protected against damage from overloads and power interruptions, but the apparatus keeps the motor in operation as long as possible during the momentary interruptions in power attributable to rail gaps. The mechanism is adapted to utilize commercially available components, and, where possible, the controls for the compressor and its drive motor preferably are operated at the low voltage level of the car''s battery.

Description

tlnited States Patent [1 1 Wilson [451 July 3, 1973 [5 AIR COMPRESSING APPARATUS FOR 2,570,222 10/1951 Ell 417/45 RAILWAY PASSENGER CARS 2,793,803 5/1957 Yerge r 417/32 2,991,002 7/1961 Quertlerm 417/295 [75] Inventor: William Wilson, Ad m NY. 2,997,227 8/1961 Ternent 417/295 Assignee: General Signal corpuration New 3,448,916 6/1969 Fraser 417/295 York NY Primary Examiner-Wi1liam L. Freeh [22] Filed: June 4, 1971 Assistant Examiner-John T. Winburn Atl0rneyGeorge Vande Sande, Harold S. Wynn and [21] Appl. No.. 150,053 Jeffrey S. Mednick [52] U.S. C1. 417/12, 318/440, 318/459, [57] ABSTRACT 417/26 417/32 417/45 417/290 417/295 The disclosure concerns air compressing apparatus for [51] litt- Cl. F041) 49/00 railway passenger cars which incorporates a comimh [58] Field of Search 318/440, 442, 459, ously running, wet rotor Screw type air compressor 318/474; 417/12 290 driven by a DC shunt motor supplied with electrical power from the third rail. The drive motor is protected [56] References Cned against damage from overloads and power interrup- UNITED' STATES PATENTS tions, but the apparatus keeps the motor in operation 1112,47; 4/1906 Leonard 318/459 as g as p s l ing t m m ntary interruptions 1,208,226 12/1916 Storer 318/440 in power attributable to rail gaps. The mechanism is 2,067,837 7/1934 Frese 318/440 adapted to utilize commercially available components, 2,135,125 11/1938 Frese 313/440 and, where possible, the controls for the compressor lg; ffij and its drive motor preferably are operated at the low I y e 2 2,418,560 4,1947 gkggaw n gi lglfi voltage level of the car 5 battery. 2,483,515 10/1949 Alexander 318/459 9 Claims, 5 Drawing Figures se n- 39' L37 INVENTOR WILUAM G. WILSON w f 7 m ATTORNEYS PAFENFE am 3 R} I INVENTOR WILLIAM 6. WILSON BY Q/ZZZM ATTORNEYS WILLIAM G-WILSON ATTORNEYS AIR COMPRESSING APPARATUS FOR RAILWAY PASSENGER CARS BACKGROUND AND SUMMARY OF THE INVENTION At the present time, the air compressors carried by passenger cars used in railway service are ofthe reciprocating piston type and are driven intermittently as needed to maintain a predetermined pressure level in the air reservoirs. This kind of air supply inherently presents wear and maintenence problems both because of the frequency at which the equipment must cycle in service, and because of the high temperature to which the air is heated during compression.

The object of this invention is to provide an economical, improved air compressing apparatus adapted to be driven by the DC power taken from the third rail of the transit system. The invention employs an oil flooded or wet rotor screw compressor which is known for its ability to run continuously and to subject the air being compressed to only a small increase in temperature (e.g. 100F). This compressor does not require the high starting torque characteristic of a piston compressor; therefore, since it will run continuously sometimes loaded and sometimes unloaded it is driven by a shunt motor which possesses a relatively flat speed versus current curve and consequently drives the compressor at a substantially uniform speed. The motor is protected by a starting resistance which is automatically by passed in one step after the motor reaches a safe speed and is reinserted in series with the armature whenever the motor is stopped, and by a low voltage detector which disconnects the motor from the power lines when there is a power failure or the voltage is insufficient for proper operation. The detector, however, maintains the motor in operation as long as possible during the periodic, momentary interruptions in power which are commonly encountered in transit service as a result of gaps in the third rail. The preferred apparatus also provides protection against motor overload and various controls for the compressor. The system can use components which are commerically available, and, where possible, it is preferred that the control elements be arranged in a low voltage circuit supplied by the cars battery. This expedient tends to mimimize costs.

BRIEF DESCRIPTION OF THE DRAWINGS Several embodiments of the invention are described herein with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of the preferred air compressing apparatus for each car.

FIGS. 2 and 3 are schematic diagrams of alternative embodiments. 1

FIGS. 4 and 5 are schematic diagrams of two alternative subcircuits'for the FIG. 3 embodiments.

DESCRIPTION OF THE FIG. 1 EMBODIMENT As shown in FIG. 1, the preferred apparatus includes a wet rotor screw compressor 11 which is driven by a DC shunt motor 12 having an armature 13 and a field winding 14 and connected through leads l5 and 16 with the 600 volt power circuit of the car. Compressor 11 is equipped with an inlet duct 17 having an intake filter 17a, a discharge duct 18 which leads to the car's air storage reservoir 19, and an oil feed pipe 21. It will be understood that the complete compressor package includes other components, such as the air-oil separator, an oil cooler and oil feed pump, which are not illustrated.

The pneumatic portion of the compressing apparatus includes three control elements. The first is an unloader comprising a normally open valve 22 which is located in duct 17 and is closed by an air motor 22a, a solenoid valve 22b which selectively connects motor 22a with the atmosphere or with reservoir 1? depending upon whether or not it is energized, and a switch 23 which responds to the pressure in the reservoir. When reservoir pressure reaches the desired level and actuator 23a opens switch 23, valve 22b shifts to pressurizing position and causes motor 22a to close valve 22 and thereby unload compressor 1 l. The second control element is a normally closed solenoid valve 24 which is located in oil pipe 21 and which closes and prevents overflooding of compressor 11 whenever the latter is stopped. The final control element is a switch 25 which is actuaeted in response to compressor discharge temperature and which, as will appear later, serves to shut down the compressor when the temperature exceeds the desired maximum.

Motor 12 is connected with high

voltage power lines

15 and 16 through the starting contact 26a of a main relay 26. The coil 27 of this relay is connected directly with the negative side of the cars battery through low voltage lead 28 and is connected with the positive side of the battery through a circuit comprising a common portion and a pair of parallel branches. The common portion includes low voltage lead 29, the normally open contact 310 of a relay 31, conductor 32, high temperature switch 25, conductor 33, an overload switch 34 which is operated by a current responsive actuator 34a, and conductor 35. The first parallel branch of the coil circuit for relay 26 initiates energization of the coil and comprises the normally open contact 36a of a relay 36, whereas the second of said branches serves a holding function and comprises the normally open contact 26b of the main relay.

Relay 31 is a standard voltage sensitive relay whose coil 37 is rated for service at a voltage below that required by motor 12 and is connected across power leads 15 and 16 through a relatively large voltagedropping resistor 38. The relay inherently provides a differential between its kick in" and drop out voltages (i.e., the voltages at which it closes and opens, respectively, contact 3la), but the ratio of these voltages normally does not match the ratio of the voltages across

lines

15 and 16 at which it is desired to start and stop motor 12. Therefore, the circuit of coil 37 also includes a resistor 39 which is selectively by-passed through the normally closed contact 31b of the relay. In a typical case, relay 31 kicks in at 125 volts and drops out" at volts, and

resistors

38 and 39 are e.m.f. of at least 395 volts. Resistor 38 preferably is of the variable type so that variations in the kick in and drop out voltages of relay 31 attributable to manufacturing tolerances can be compensated.

It is appropriate to remark here that, while the preferred apparatus employs a voltage senstive relay 31, the low voltage detector function could also be performed by a current sensitive relay.

Relay 36 is a time delay relay of the off-delay type. Its coil 41 is connected directly with the negative low voltage lead 28, but is joined to the positive lead 29 via conductor 42, a normally closed contact 260 of main relay 26, conductor 35, switch 34, conductor 33, switch 25, conductor 32, and the contact 31a of relay 31. In addition to the contact 36a, through which main relay 26 is energized, relay 36 includes a normally closed contact 36b which controls a by-

pass path

43, 44 extending around the starting resistor 45 located in the armature circuit of motor 12. Relay 36 energizes and opens contact 36b as soon as relay 31 kicks in; therefore, by-

pass path

43, 44 is open when motor 12 is started, and resistance 45 is effective to protect annature 13 from current surges. However, as soon as relay 36 closes contact 36a, main relay 26 is energized to thereby open contact 260 and interrupt the energization circuit for the coil 41 of relay 36. After a short period, contact 36b will close to effectively remove resistance 45 from the armature circuit, and thereby minimize energy losses. The length of this period is determined by the delay characteristic of relay 36 and is selected to insure that motor 12 will have accelerated to a speed at which its back e.m.f. is sufficient to limit armature current to a safe value.

Feed valve 24 should be closed whenever compressor 11 is shut down, because overflooding can cause turbulence and splashing which might result in escape of oil through inlet duct 17, and might make sluggish subsequent start-up of the compressor. Therefore, in the illustrated embodiment, the solenoid of valve 24 is connected with the low voltage leads 28 and 29 through a circuit including high temperature switch 25, overload switch 34, and contact 31a of relay 31. This arrangement insures that valve 24 will open and allow oil to be delivered to compressor 11 when starting of motor 12 is initiated, and that the valve will close whenever main relay 26 is deenergized to stop the motor.

Assuming the low voltage leads 28 and 29 are receiving power from the cars battery, compressor 11 is put in operation by applying power to the high voltage leads and 16. When the voltage across the last mentioned leads exceeds the setting of relay 31, this relay I will kick in, thereby closing contact 31a and completing the energization circuits for feed valve 24 and time delay relay 36. At the same time, contact 31b is open to increase the resistance in the coil circuit of relay 31 and thereby insure that this relay will not drop out until line voltage decreases to a level substantially below the kick in" voltage. Energization of relay 36 has two effects: first, it opens contact 36b to interrupt by-

pass path

43, 44 and effectively insert starting resistor 45 into the circuit of motor armature 13, and second, it closes contact 36a and thereby completes the energization circuit for coil 27 of main relay 26. Actuation of main relay 26 effects closure of contact 26a, so now motor 12 commences to run. Simultaneously, contact 260 opens to interrupt the coil circuit of relay 36, andcontact 26b closes to complete the holding circuit for the coil 27 of the main relay. At the end of a predetermined delay interval, relay 36 will drop out and close contact 36b and short out starting resistor 45. Contact 36a will now be opened, but, since the holding circuit for coil 27 is complete through contact 26b, this action has no immediate effect.

If reservoir 19 is not fully charged when motor 12 is started, switch 23 will be closed; therefore, valve 22b will shift to vent psition when relay 31 kicks in". As a result, valve 22 will open to load compressor 11, and the latter will commence to deliver compressed air to the reservoir, as soon as the motor begins to run. When the pressure in reservoir 19 reaches the desired level, actuator 230 will open switch 23 to thereby effect closure of valve 22 and unloading of compressor 11, but motor 12 will continue to run. Thus,'during service, the compressor will run continuously and merely be loaded and unloaded by valve 22 as necessary to maintain a predetermined pressure in reservoir 19.

The low voltage detector relay 31 will remain in its shifted state, and keep motor 12 on the line, as long as the voltage across

leads

15 and 16 remains above its low voltage setting (i.e., 395 volts in the illustrated embodiment). Therefore, the power supplied to the lines by the propulsion motors, acting as generators, will be utilized as long as possible to drive compressor 11 during momentary interruptions in track power. However, if the line voltage drops to a very low level, relay 31 will drop out, thereby opening contact 31a and deenergizing

solenoid valves

22 and 24 and main relay 26. This action unloads and stops compressor 11 and interrupts the supply of oil throughpipe 21. Contact 31b closes when relay 31 shifts, so re-starting of motor 12 now cannot occur until line voltage is restored to the high 450 volt level. When power is restored to that level, the motor will be started again in the manner described earlier, and the armature 13 will be protect against a current surge by the inclusion of starting resistor 45.

If, during operation, compressor discharge temperature should reach an excessive level or current flow through armature 13 should become too high,

switch

25 or 34 will open and interrupt the energization circuits for main relay 26 and valve 24. As a result, the relay will drop out, thereby stopping motor 12, and oil feed valve 24 will close. Since opening of either of the

switches

25 and 34 also interrupts the energization circuit of time delay relay 36, the compressor will remain idle as long as the high temperature or high current condition persists. However, when this condition abates, and the opened

switch

25 or 34 again closes, valve 24 will open, and relay 36 will immediately kick in to reinsert starting resistor 45 in the armature circuit and reenergize main relay 26. Thus, motor 12 will be re-started automatically in the normal manner.

DESCRIPTION OF THE FIG. 2 EMBODIMENT In the FIG. 1 embodiment, contact 31a of relay- 31 applies power to the low voltage control circuits, and effects sarting of motor 12, as soon as power is supplied to the high voltage leads 15 and 16. This is considered the most convenient arrangement of the apparatus, but, in some cases, it might be desirable to incorporate a starting switch which can be operated independently of mainline voltage. Such a switch could be added to line 29 in series with contact 31a. Another possibility is shown in FIG. 2.

The FIG. 2 embodiment employs the same components as the first embodiment, but its control circuits contact 31a of its low voltage relay 31 controls only the energization and holding circuits for main relay 26. Thus, while this contact closes as soon as power is applied to main lines and 61, main relay 26 and time delay relay 36 will remain deenergized, and

valves

22 and 24 will remain closed, until switch 46 is closed. Except for this difference, the second embodiment operates in the same way and performs the same functions as the first embodiment.

DESCRIPTION OF THE EMBODIMENTS OF FIGS. 3-5

While each of the first two embodiments employs an off-delay relay 36 in the low voltage circuit to control the by-

pass path

43, 44 around starting resistor 45, it should be understood that other switching devices may be used. Three alternatives are depicted in FIGS. 3-5.

In FIG. 3, the by-

pass path

43, 44 is controlled by the contact 47a of a relay 47 which is connected across the armature 13 of motor 12 through variable resistor 48, and which senses the counter e.m.f. developed by the motor. Contact 47a is normally open, so starting resistance 45 is in the armature circuit under starting conditions. However, when the motor reachs a speed at which the current-limiting effect of the resistor is no longer needed, the counter e.m.f. developed by the motor will be sufficient to kick in" relay 47 and cause it to closed contact 470 and effectively remove resistance 45 from the armature circuit. In a similar manner, the FIG. 4 embodiment accomplishes cut-out of starting resistor 45 by means of the normally open contact 49a of an on-delay 49 whose coil is connected across the

main power lines

15 and 16 in a circuit containing variable resistor 51.

In the FIG. 5 embodiment, on the other hand, the cut-out function is performed by a static switch in the form of a silicon controlled rectifier 52. The rectifier is connected across resistor 45 and is fired by the difference between the voltage across an auxiliary resistor 53 located in the armature circuit, and the constant voltage at gate 52a developed by zener diode 54. At the desired operating speed of the motor, rectifier 52 will become conductive and short out starting resistor 45.

Although these three alternatives effect some simplification in the control circuits, each is considered inferior to the off-delay relay approach used in FIGS. 1 and 2. The FIG. 3 version is troublesome to apply because the counter e.m.f. of the motors used in the apparatus is relatively unpredictable, and consequently the coil circuit of the relay must be turned empirically. The delay characterisitic of the relay 49 used in FIG. 4 is'determined by a fluid dashpot, rather than by decay of magnetic flux lines as in the case of an off-delay relay; therefore, relay 49 is not as reliable as relay 36, and its performance is affected by temperature changes. Finally, the solid state switch of FIG. 5 is considered inferior because it is considerably more expensive than the standard time delay relay.

I claim:

1. Air compressing apparatus for a railway car comprising a. a wet rotor screw type air compressor;

b. a shunt motor connected to drive the compressor and supplied with DC power from a pair of high voltage leads;

c. a starting resistor connected in a series circuit with the armature of the motor;

d. a by-pass circuit connected in parallel with the starting resistor; e. a main relay having a starting contact in said series circuit which closes when the coil of the relay is energized; a low voltage DC circuit including a pair of power leads connected with the coil of the main relay through a contact of a detector means which is closed when the voltage supplied to the motor is above a first level and is open when said voltage is below a second, materially lower level;

g. delay means effective upon closure of the starting contact for maintaining the by-pass circuit open for a predetermined length of time and for thereafter completing saidcircuit;

h. the delay means comprises a time delay relay of the off-delay type including a contact in the by-pass circuit which is open when the relay is energized, and a coil connected with the low voltage power leads through an energizing circuit containing a start switch and a normally closed, second contact of the main relay; and

i. the coil of the main relay is connected with the low voltage power leads through an energizing circuit having a first branch containing said contact of the detector means and a normally open, second contact of the time delay relay, and a parallel, second branch containing said contact of the detector means and a normally open, third contact of the main relay.

2. Apparatus as defined in claim 1 in which the start switch is said contact of the detector means.

3. Apparatus as defined in claim 1 in which a. said start switch is separate from said contact of the detector means; and

b. each of said branches also contains the start switch.

4. Apparatus as defined in claim 1 including a. a pair of normally closed safety switches, both of which are included in the energizing circuit for the coil of the time delay relay as well as in each branch of the energizing circuit for the coil of the main relay;

b. means responsive to compressor discharge temperature for opening one safety switch; and

0. means responsive to current drawn by the motor for opening the other safety switch.

5. Apparatus as defined in claim 4 including a. an unloader for the compressor having a control solenoid which, when energized, causes the unloader to load the compressor;

b. a reservoir connected to receive and store air discharge by the compressor;

c. a normally closed third safety switch which is opened by means which responds to the pressure in the reservoir; and

d. an energizing circuit connecting the unloader solenoid with the power leads of the low voltage circuit and containing the third safety switch and the start switch, but otherwise being independent of the other energizing circuits.

6. Apparatus as defined in claim 5 including a. a feed pipe for delivering oil to the compressor and containing a normally closed, solenoid operated valve; and

' b. an energizing circuit connecting the solenoid of said valve with the power leads of the low voltage circuit through said pair of safety switches and said start switch. 7. Air compressing apparatus for a railway car comprising a. a wet rotor screw type air compressor;

d. a by-pass circuit connected in parallel with the starting resistor;

e. a main relay having a starting contact in said series circuit which closes when the coil of the relay is energized;

f. a low voltage DC circuit including a pair of power leads connected with the coil of the main relay through a contact of a detector means which is closed when the voltage supplied to the motor is above a first level and is open when said voltage is below a second, materially lower level;

. delay means effective upon closure of the starting contact for maintaining the by-pass circuit open for a predetermined length of time and for thereafter completing said circuit;

h. a pair of normally closed safety switches in the connection between the coil of the main relay and the power leads of the low voltage circuit, one switch being opened by means which responds to compressor discharge temperature and the other being opened by means which responds to the current drawn by the motor;

i. a feed pipe for delivering oil to the compressor and containing a normally closed, solenoid operated valve;

j. an energizing circuit connecting the solenoid of said valve with the power leads of the low voltage circuit through said pair of safety switches:

k. an unloader for the compressor having a control solenoid which, when energized, causes the unloader to load the compressor;

1. a reservoir connected to receive and store air discharge by the compressor;

m. a normally closed third safety switch which is opened by means which responds to the pressure in the reservoir; and

n. an energizing circuit connecting the unloader solenoid with the power leads of the low voltage circuit and which contains the third safety switch but is independent of the other safety switches.

8. Air compressing apparatus for a railway car comprising a. a wet rotor screw type air compressor,

1). a shunt motor connected to drive the compressor and supplied with DC power from a pair of high voltage leads,

0. a starting resistor connected in a series circuit with the armature of the rotor,

d. a by-pass circuit connected in parallel with the starting resistor,

e. a main relay having a starting contact in said series circuit which closes when the coil of the relay is energized,

f. detector means having a detector relay maintained in a first position by energization of its winding from the high voltage leads through a control resistance,

g. the detector means including a circuit for shunting the control resistance when the detector relay is in a second position,

h. a low voltage DC circuit including a pair of power leads connected with the coil of the main relay through a contact of the detector means which is closed when the voltage supplied to the motor is above a first level and is open when said voltage is below a second, materially lower level, and

i. delay means effective upon closure of the starting contact for maintaining the by-pass circuit open for a predetermined length of time for thereafter completing said circuit.

9. Apparatus as defined in claim 8 in which the control resistance includes both fixed and variable value portions, and the shunting circuit extends around only the fixed value portion.

Claims

1. Air compressing apparatus for a railway car comprising a. a wet rotor screw type air compressor; b. a shunt motor connected to drive the compressor and supplied with DC power from a pair of high voltage leads; c. a starting resistor connected in a series circuit with the armature of the motor; d. a by-pass circuit connected in parallel with the starting resistor; e. a main relay having a starting contact in said series circuit which closes when the coil of the relay is energized; f. a low voltage DC circuit including a pair of power leads connected with the coil of the main relay through a contact of a detector means which is closed when the voltage supplied to the motor is above a first level and is open when said voltage is below a second, materially lower level; g. delay means effective upon closure of the starting contact for maintaining the by-pass circuit open for a predetermined length of time and for thereafter completing said circuit; h. the delay means comprises a time delay relay of the off-delay type including a contact in the by-pass circuit which is open when the relay is energized, and a coil connected with the low voltage power leads through an energizing circuit containing a start switch and a normally Closed, second contact of the main relay; and i. the coil of the main relay is connected with the low voltage power leads through an energizing circuit having a first branch containing said contact of the detector means and a normally open, second contact of the time delay relay, and a parallel, second branch containing said contact of the detector means and a normally open, third contact of the main relay.

3. Apparatus as defined in claim 1 in which a. said start switch is separate from said contact of the detector means; and b. each of said branches also contains the start switch.

4. Apparatus as defined in claim 1 including a. a pair of normally closed safety switches, both of which are included in the energizing circuit for the coil of the time delay relay as well as in each branch of the energizing circuit for the coil of the main relay; b. means responsive to compressor discharge temperature for opening one safety switch; and c. means responsive to current drawn by the motor for opening the other safety switch.

5. Apparatus as defined in claim 4 including a. an unloader for the compressor having a control solenoid which, when energized, causes the unloader to load the compressor; b. a reservoir connected to receive and store air discharged by the compressor; c. a normally closed third safety switch which is opened by means which responds to the pressure in the reservoir; and d. an energizing circuit connecting the unloader solenoid with the power leads of the low voltage circuit and containing the third safety switch and the start switch, but otherwise being independent of the other energizing circuits.

6. Apparatus as defined in claim 5 including a. a feed pipe for delivering oil to the compressor and containing a normally closed, solenoid operated valve; and b. an energizing circuit connecting the solenoid of said valve with the power leads of the low voltage circuit through said pair of safety switches and said start switch.

7. Air compressing apparatus for a railway car comprising a. a wet rotor screw type air compressor; b. a shunt motor connected to drive the compressor and supplied with DC power from a pair of high voltage leads; c. a starting resistor connected in a series circuit with the armature of the motor; d. a by-pass circuit connected in parallel with the starting resistor; e. a main relay having a starting contact in said series circuit which closes when the coil of the relay is energized; f. a low voltage DC circuit including a pair of power leads connected with the coil of the main relay through a contact of a detector means which is closed when the voltage supplied to the motor is above a first level and is open when said voltage is below a second, materially lower level; g. delay means effective upon closure of the starting contact for maintaining the by-pass circuit open for a predetermined length of time and for thereafter completing said circuit; h. a pair of normally closed safety switches in the connection between the coil of the main relay and the power leads of the low voltage circuit, one switch being opened by means which responds to compressor discharge temperature and the other being opened by means which responds to the current drawn by the motor; i. a feed pipe for delivering oil to the compressor and containing a normally closed, solenoid operated valve; j. an energizing circuit connecting the solenoid of said valve with the power leads of the low voltage circuit through said pair of safety switches: k. an unloader for the compressor having a control solenoid which, when energized, causes the unloader to load the compressor; l. a reservoir connected to receive and store air discharged by the compressor; m. a normally closed third safety switch which is opened by means which responds to the prEssure in the reservoir; and n. an energizing circuit connecting the unloader solenoid with the power leads of the low voltage circuit and which contains the third safety switch but is independent of the other safety switches.

8. Air compressing apparatus for a railway car comprising a. a wet rotor screw type air compressor, b. a shunt motor connected to drive the compressor and supplied with DC power from a pair of high voltage leads, c. a starting resistor connected in a series circuit with the armature of the rotor, d. a by-pass circuit connected in parallel with the starting resistor, e. a main relay having a starting contact in said series circuit which closes when the coil of the relay is energized, f. detector means having a detector relay maintained in a first position by energization of its winding from the high voltage leads through a control resistance, g. the detector means including a circuit for shunting the control resistance when the detector relay is in a second position, h. a low voltage DC circuit including a pair of power leads connected with the coil of the main relay through a contact of the detector means which is closed when the voltage supplied to the motor is above a first level and is open when said voltage is below a second, materially lower level, and i. delay means effective upon closure of the starting contact for maintaining the by-pass circuit open for a predetermined length of time for thereafter completing said circuit.