Title:
CONTROL MEANS FOR HIGH SPEED HOIST
United States Patent 3772579


Abstract:
A control means for a high speed electric hoist or crane having an induction motor permits motor operation in the "raise" or "lower" direction at controlled speeds up to full load hoisting speed (i.e., the approximately synchronous speed of the motor). The control means also permits the motor to operate automatically in the "lower" direction so as to differentiate between a "light" and a "heavy" load and to allow a light load to free fall up to 275 percent of motor synchronous speed but forcing a heavy load to be lowered normally at a restrained speed of about 100 percent of motor synchronous speed.



Inventors:
Stone, David W. (Franklin, WI)
Hansen, Manfred A. (Renton, WA)
Application Number:
05/287706
Publication Date:
11/13/1973
Filing Date:
09/11/1972
Assignee:
HARNISCHFEGER CORP,US
Primary Class:
International Classes:
B66B1/06; (IPC1-7): H02P1/40
Field of Search:
318/201,203,204,209
View Patent Images:
US Patent References:
3248625Electric motor control system1966-04-26Wycoff
3039036Control apparatus1962-06-12Wendelburg et al.
2963634Electric hoist control1960-12-06Cortelli et al.
2846631Eddy-current brake for hoists and the like1958-08-05Vogt
2823341Electric motor-brake hoist control system1958-02-11Smith et al.
2766415Magnetic amplifier motor control system1956-10-09Schurr



Primary Examiner:
Rubinson, Gene Z.
Claims:
We claim

1. A hoist comprising an induction type electric motor and control means for operating said motor in raise and lower directions, said control means comprising, a controller, electrically operable brake means for said motor, brake control means for controlling said brake means, first means responsive to regenerative conditions in said motor to effect operation of said brake control means, and second means responsive to torque conditions in said motor to effect operation of said brake control means, said first and second means, when said controller is operating said motor in the lower direction, being responsive to the regenerative and torque characteristics exhibited by said motor when subjected to various loads in the lower direction to operate said brake control means to effect motor operation at a first speed which is approximately the synchronous speed of the motor when a load is sufficiently light to be non-regenerative; to effect motor operation at a second speed which is approximately the synchronous speed of the motor when a load is sufficiently heavy to be regenerative and is producing more than a predetermined amount of torque; and to effect motor operation at a third speed which is in excess of the synchronous speed of the motor when a load is sufficiently heavy to be regenerative and is producing less than a predetermined amount of torque.

2. A hoist according to claim 1 wherein said first speed is slightly below a synchronous speed, wherein said second speed is slightly above motor synchronous speed, and wherein said third speed is substantially greater than motor synchronous speed.

3. A hoist comprising a wound rotor induction type electric motor and control means for operating said motor in raise and lower directions, said control means comprising:

4. A hoist according to claim 3 wherein said first means sense a non-regenerative load and operates said magnetic amplifier means to effect motor operation at a rate of speed slightly below synchronous speed; wherein said first and second means sense a regenerative load producing more than said predetermined amount of torque and operate said magnetic amplifier means to effect motor operation at a rate of speed slightly above synchronous speed; and wherein said first and second means sense a regenerative load producing less than said predetermined amount of torque and operate said magnetic amplifier means to effect motor operation at a rate of speed in excess of twice synchronous speed.

5. A hoist according to claim 4 wherein said predetermined amount of torque is approximately 15 percent of full load torque.

6. A hoist according to claim 5 wherein said rate of speed in excess of twice synchronous speed is approximately 270 percent of synchronous speed.

7. A hoist comprising a wound rotor induction type electric motor and an eddy current type load brake for said motor and control means for said brake and said motor comprising:

8. A hoist comprising a wound rotor induction type electric motor and an eddy current type load brake for said motor and control means for said brake and said motor comprising:

Description:
BACKGROUND OF THE INVENTION

1. Field of Use

This invention relates to control means for hoists or cranes which effect controlled "raise" and "lower" operations and also effect automatic high speed "lower" operation at two different speeds under light and heavy loads.

2. Description of the Prior Art

High speed AC electric hoists used, for example to transport concrete buckets to pouring levels on high rise buildings must operate at the highest rate of speed possible consistent with safety requirements. Heretofore, such hoists had control means to effect controlled "raise" and "lower" operations up to a certain speed (full load hoisting speed) and automatic high speed "lower" operation at some single speed well in excess of that speed. Such control means, when operating in automatic high speed "lower", did not distinguish speedwise between light or heavy loads. Thus, the braking means brought into play to maintain constant high speed lowering tended to be potentially less effective when heavy loads were involved and increased the danger of a runaway load.

SUMMARY OF THE PRESENT INVENTION

The present invention contemplates a high speed hoist comprising a hoist drum on a shaft driven by an AC electric motor. The hoist control means include an electrically operable mechanical brake for the hoist drum (i.e., motor shaft), a magnetic or eddy current brake coupled to the shaft, and an electric controller for operating the motor, the mechanical brake and the magnetic brake.

The hoist control means permits motor operation in the "raise" or "lower" direction at controlled speeds up to full load hoisting speed (i.e., the approximately synchronous speed of the motor). The control means also permits the motor to operate automatically in the "lower" direction so as to allow an empty bucket to free fall up to 275 percent of full load hoisting speed. However, any heavier load that cannot be safely handled at this high speed will be restrained to about 100 percent of full hoisting speed.

The electric controller of the control means comprises certain components or modules which come into play during "raise" and "lower" operation of the motor and during lightly-loaded or heavily loaded "lower" conditions, namely, a load detector module including a load detector relay, a regeneration module, a magnetic brake guard control module including a time delayed relay, a frequency detection module, a magnetic amplifier module, a controlled rectifier module, and an antihunt module. The time delayed relay and guard control module together, in effect, constitute a time delay relay.

The hoist control means operates as follows. Before electric power is applied, the motor shaft is held stationary by the mechanical brake. After electric power is applied, the magnetic brake is energized to an "off position" level to exert a nominal retarding torque to the motor shaft if it should move to prevent its runaway should the mechanical brake fail in operation. When electric power is applied to operate the motor in low-speed "raise" or "lower" directions, the mechanical brake releases and excitation of the magnetic brake means increases from the "off position" level to a "maximum" level. The stall motor torque is normally 10 percent for lowering and 140 percent for hoisting. When the motor is operated at full-speed "raise" or "lower", permissive interlocking functions are carried out by the controller as follows. The load detector module senses when the motor delivers 15 percent or more of full load torque and the load detector relay picks up (this relay also drops out at slightly lesser torque, i.e., 12 to 13 percent). The regeneration module senses when the motor exceeds synchronous speed and provides a control signal. The magnetic brake guard control module causes its time delayed relay (normally picked up) to drop outafter a short time delay in response to receiving both the control signal from the regeneration module and drop out of the load detector relay.

The net functional result of the operation of these modules is as follows. In the slow "raise" or slow "lower" conditions, the motor accelerates, stabilizes at a slow speed and continues running until stopped by the operator or by limit switches. In the full speed "lower" condition, stabilization of motor speed will occur at a running point which will be in one of three general conditions, namely: sub-synchronous (driving a light load down); restrained fall at 100 percent of hoisting speed with a heavy load causing the motor to regenerate more than 15 percent of full load torque; or free fall to 275 percent of full load hoisting speed and then controlled with a light load causing the motor to regenerate less than 15 percent of full load torque. If full speed "raise" is selected, the motor will be accellerated only if the time delay relay and load detector relay are picked up. The motor will continue until there is operator or limit switch intervention.

All three of these lowering conditions are stable and will continue until the motor is slowed or stopped by the operator or by limit switches which effect progressive slow down and stopping of the motor and simulate the effect of the operator's return of a master control switch toward neutral position.

In condition (1), the load detector relay may drop out, but nothing else.

In condition (2), the load detector relay drops out. The regeneration module provides a control signal at synchronous speed, but before the time delayed relay drops out, the load detector relay picks up again. Therefore, the time delayed relay remains picked up.

In condition (3), the load detector relay drops out. The regeneration module turns on at synchronous speed and, since the load detector relay does not pick up again, the time delayed relay eventually drops out. This causes all motor accelerator contacts to open and, with the secondary winding of the motor open, the motor exerts no torque and the secondary voltage and frequency is applied to the frequency detector module. The light load is then in free fall, restrained only by friction, windage and mass. When it accelerates to approximately 270 percent of full load hoist speed, the frequency detector module provides an output signal which starts to increase the magnetic brake excitation and the motor speed stabilizes at about 275 percent of full load hoisting speed.

DRAWINGS

FIG. 1 (sheets 1, 2 and 3) is a schematic diagram of a hoist and control means therefor in accordance with the invention;

FIG. 2 is a graph wherein motor speed is plotted against motor torque under various operating conditions, as described in legends on the graph;

FIG. 3 is a side view of the master control switch shown schematically in FIG. 1;

FIG. 4 is a diagrammatic view of the frequency detector module shown in FIG. 1 and includes details of a master reference rectifier;

FIG. 5 is a cross-section view of the magnetic or eddy current load brake shown in FIG. 1;

FIG. 6 is a graph showing the speed-torque characteristics of the magnetic load brake shown in FIGS. 1 and 3;

FIG. 7 is a diagrammatic view of the magnetic amplifier module shown in FIG. 1;

FIG. 8 is a diagrammatic view of the load detector module shown in FIG. 1;

FIG. 9 is a diagrammatic view of the regeneration module shown in FIG. 1; and

FIG. 10 is a diagrammatic view of the guard control module shown in FIG. 1;

DESCRIPTION OF A PREFERRED EMBODIMENT

General Arrangement and Operation

FIG. 1 (sheet 1) shows a hoist comprising a hoist drum 10 on a shaft 11 with a cable 12 to which a load 13 is attached. Shaft 11 is driven by a wound rotor induction motor 14 having primary and secondary windings. A variable resistance network 15 including accelerator contacts is connected to the motor secondary windings. A spring applied electrically releasable mechanical holding brake 16 is provided for shaft 11. An eddy current or magnetic load brake 17 is also provided for motor shaft 11. The primary of motor 14 is connectible for hoist and lower operation to an electrical power source comprising ac power lines L1, L2 and L3 by reversing contractor 18.

FIG. 1 (sheets 2 and 3) also shows a controlller for operating contactor 18 and motor 14, mechanical brake 16, eddy current brake 17, and resistance network 15. As FIG. 1 (sheet 2) shows, the controller comprises a control switch 19 having first and second point positions in both "lower" and "raise" (a total of five contact circuits) and push-button switches, relays and limit switches hereinafter described. As FIG. 1 (sheet 3) shows, the controller further comprises a regeneration module 20, a load detector module 21, and its load dector relay 22, a guard control module 23 and its time delayed relay 24, a frequency detector module 25, a magnetic amplifier module 26, a controlled rectifier module 27 and an anti-hunt module 28. All modules are hereinafter described in detail.

Before power is applied, shaft 11 is held stationary by mechanical brake 16. After power is applied, eddy current brake 17 is energized to an "off position" level to exert a nominal retarding torque on shaft 11 to prevent runaway if mechanical brake 16 fails. When master control switch 19 is moved to a first position to operate motor 14 for operation in either the raise or lower direction, mechanical brake 16 releases and eddy current brake 17 energization is increased from "off position" level to a "maximum" level. Stall motor torque exerted on shaft 11 is 10 percent for lowering ad 140 percent for hoisting. When master control switch 19 is moved beyond first point to second point position to operate motor 14 at full raise or lower speed, permissive interlocking functions are carried out by the controller as follows, with stall motor torque first going to 140 percent as during hoisting.

Load detector module 221 senses when motor 14 delivers 15 percent or more of full load torque and causes detector relay 22 to pick up (close) its contacts 22A and 22B. Module 21 also causes relay 22 to drop out (open) its contacts at a slightly lesser torque of about 12 or 13 percent. Regeneration module 20 senses when motor 14 reaches or exceeds synchronous speed and provides a control signal to guard control module 23. The guard control module 23 causes its time delayed relay 24 to drop out (open) its contacts 24A and 24B after an appropriate time interval if regeneration control module 20 is providing a signal and load detector module relay 22 is energized.

The result of the relays 22 and 24 both being energized when motor 14 is being operated in the raise direction is to bias the guard module 23 off at the same time as 3A, 4A and 5A in the controller sequentially operate their contacts in resistance network 15 to remove resistance and accelerate motor 14. Motor 14 stabilizes at a speed determined by the load and the permanent slip resistor and continues running until decreased or stopped by the hoist operator or slowed and stopped by the appropriate limit switches RL1 and RL2.

When the motor 14 operates in the lower direction the accelleration is accomplished in the same way as in second point raise. Since the final permanent resistance is the same, the reflected load will cause the motor to stabilize. The speed will be either barely sub-synchronous (for effectively no load); just over motor synchronous speed with a load sufficiently heavy to cause the motor to regenerate more than 15 percent of full load torque; or just over motor synchronous speed with a load causing the motor to regenerate less than 15 percent of full load torque. If it is the latter case, the controller will allow the load to accelerate the motor shaft to 275 percent of motor synchronous speed, at which point the eddy current brake will then regulate motor speed. Motor speed is stable at each of the three conditions and continues until slowed or stopped by the hoist operator or by limit switches LL1 and LL2.

It is to be understood that guard module 23 is always biased on and time delayed relay 24 is picked up (energized) at all times except when the interlock contacts L or R of lower relay L or raise relay R are closed and contact 22A of relay 22 is open and regeneration module 20 has an output at its terminals 5 and 6 and sufficient time has passed. The time delay is brought about by integration of the difference between the internal bias in guard module 23 and the current at terminals 1 and 2 of guard module 23 from regeneration module 20.

As movement of master control switch 19 continues, contactor 2A is energized and this causes rapid subsequent successive energization of the other contactors 3A, 4A and 5A. Contactor 2A is energized at first point raise and immediately in full (second point) lower. When contactor 2A is energized, relay 22 picks up and closes its contacts 22A and 22B. Therefore, at full raise or lower, relay 22 and time delay relay 24 are picked up. THis, in turn picks up relay CP, which in turn removes the excitation of eddy current brake 17.

This system uses a wound-rotor induction motor whose second point raise and lower speeds depend only upon the motor constant, the line voltage and fixed permanent secondary resistors and the reflected shaft load.

In order to drop out or unexcite the eddy current brake 17 when going from first point raise or lower to second point, both the time delayed relay 24 and the current (or load) sensing relay 22 must be energized and one of their N.O. contacts on each relay closed. This is for safety and required to prove the circuits are working.

In order to go to high speed, lower both time delayed relay 24 and the current sensing relay 22 must be deenergized and another of their N.O. contacts on each relay must be open.

The first point speeds depend upon the motor constants, secondary resistors, line voltage, eddy current brake 17 excitation and reflected shaft load. Full load first point raise is approximately 3 percent synchronous speed and empty bucket, first point lower is approximately 1 percent synchronous speed.

Assuming that the above described sequences are carried out to effect a lowering operation of the hoist, one of the three conditions specified hereinbefore occurs, namely:

1. The load, if any, is driven down undercontrol at some desired subsynchronous speed, or

2. The load descends at a restrained speed (slowly, over 102 percent of synchronous speed) with a heavy load causing motor 10 to regenerate more than 15 percent of full load torque, or

3. Free fall at 275 percent of synchronous speed with a light load 16 caused by motor 10 regenerating less than 15 percent of full load torque.

As previously noted, all three of these lowering conditions are stable and will continue until slowed or stopped by the operator or by the limit switches LL1, LL2, RL1 or RL2 in the control circuit. The limit switches are arranged to effect progressive slow down and stopping of the motor and simulate the effect of the operator's return of the master control switch toward neutral position.

In condition (1), the load detector relay 22 may drop out, but nothing else.

In condition (2), the load detector relay 22 drops out and the regeneration module 26 provides a control signal at synchronous speed. But before the time delayed relay 24 drops out, the load detector relay 222 picks up again. Therefore, time delayed relay 24 remains picked up.

In condition (3), load detector relay 22 drops and regeneration module 20 turns on at synchronous speed. Since load detect or relay 22 does not pick up again, time delayed relay 24 eventually drops out. This drops relay 1A, which drops 2A, causes all contacts of resistance network 15 to open and, with the secondary winding of motor 14 open, the motor exerts no torque and the secondary voltage and frequency is applied to frequency detector module 25. The light load 13 is then in free fall, restrained only by friction, windage and mass. When it accelerates to approximately 270 percent of full load hoist speed, frequency detector module 25 provides an output signal which starts to increase the excitation of eddy current brake 17 and motor speed stabilizes at about 275 percent of full load hoisting speed.

All of the operating conditions hereinbefore described are depicted in the graph in FIG. 2 wherein motor speed is polotted against motor torque for motor operation in the raise and lower directions.

A more detailed description of the construction and operation of controller components is as follows.

Master Control Switch

Master control switch 19, shown schematically in FIG. 1 (sheet 2) and in FIG. 3 operates contacts N, 40, 41, 42 and 43 to operate reversing contactor 18 and resistance network 15 to establish motor direction of speed. Section 47 of frequency detector module 25 (FIG. 4) comprises a full wave rectifier 48 to convert the ac signal to dc and a signal adjustment rhostat or potentiometer 49. As FIG. 1 (sheet 3) shows, the output terminals 3 and 4 of section 47 of module 25 are connected by conductors 51 and 52 to provide a variable dc speed reference signal to terminals 9 and 10 of magnetic amplifier module 26.

Master control switch 19 will not function until "start" button 60 is depressed with the induction master control switch in neutral with contact N closed. When "start" button 60 is depressed, UV relay coil 61 is energized and UV relay coil 61 is de-energized by emergency stop pushbutton 62, stop pushbutton 63, manual reset overspeed switch 64, or motor overload contacts 65 and 66.

The desired side of reversing contactor 18 will come in to energize motor 14 when master control switch 19 is moved to first point raise or lower position, provided the corresponding travel limit switches LL1 and RL1 are not open. With motor 14 energized for rotation in the desired direction, mechanical brake 16 releases and excitation of eddy current brake 17 increases from the "off position" level to the "maximum"level. Stall motor torque is 10 percent for lower and 140 percent for raise.

Eddy CurrentBrake

As FIG. 5 shows, eddy current brake 17 comprises an iron rotor 70 mounted on bearings 71 on shaft 11 inside a stationary field assembly having an iron stator 72 and coils 73. When direct current flows through coils 73, alternate magnetic poles are produced in stator 72 (i.e., north next to south, etc.) which induce alternate field in rotor 70 when the latter moves. This interaction induced by eddy currents in rotor 70 produces retarding torque at various speeds, as shown in the speed-torque graph in FIG. 6.

Excitation of the elctric brake coils 73 in eddy current brake 17 is provided by controlled rectifier module 27 which, in turn, is operated by magnetic amplifier module 26. Magnetic amplifier module 26, in turn is responsive to its own internal (off) bias and to the off biases from transformer T9 (rectified in rectifier section 47 of frequency detector module 25; the speed feedback signal from the secondary windings of motor 14 (as sensed and provided by frequency detector module 25); and an anti-signal from anti-hunt module 28.

Controlled Rectifier Module

As FIG. 1 (sheets 1 and 3) shows, controlled rectifier module 27 has power input terminals 80 and 81 which are supplied with power from correspondingly numbered terminals of an anode transformer 82 connected to lines L1 and L2 of the main power supply. Module 27 comprises two SCR's 83 and 84 (silicon controller rectifiers) which, when gated, supply ac current to the electric brake coils 73 of eddy current brake 17. The SCR's 83 and 84 block current in the forward direction until a positive gate signal is applied by magnetic amplifier module 26 to cause them to fire. The SCR's continue to conduct even if the gate signal is removed until anode current from transformer 82 falls to zero and reverses. Module 27 includes a surge suppression resistor 85 and capacitor 86 across the secondary of anode transformer 82 to dissipate harsh transients.

Magnetic Amplifier Module

As FIG. 7 shows, magnetic amplifier module 26 comprises a power supply transformer 90, a self-saturating magnetic amplifier havingiron cores 91 on which ac windings 92 are provided, three control windings 93, 94 and 95, and a bias winding 95a and small rectifiers 96 in series with each ac winding 92. As FIG. 1 (sheets 1 and 3) shows, the input terminals 1 and 2 of power supply transformer 90 of magnetic amplifier module 26 are connected by conductors 97 and 98 to correspondingly numbered terminals of anode transformer 82. The output terminals 4, 5 and 6 of magnetic amplifier module 27 are connected, respectively, to the gate of SCR 83, to one side of eddy current brake coils 73 and the SCR's 83 and 84, and to the gate of SCR 84. The other side of coil 73 is connected to terminal 99 of anode transformer 82 by a similarly numbered conductor. The terminals 7 and 8 of control winding 93 (speed feedback signal) are connected by conductors 100 and 101 to terminals A1 and A2 of frequency detector module 225. The terminals 9 and 10 of control winding 94 (speed reference signal) are connected by conductors 52 and 51 to terminals 4 and 3 of frequency detector module 25. The terminals 11 and 12 of control winding 95 (anti-hunt signal) are connected by conductors 103 and 104 to terminals 6 and 5 of anti-hunt module 28. The anti-hunt module 28 provides transient signals which compensate for system response delays which could result in hunting and comprises a resistor 105, an adjustable potentiometer 106, and a capacitor 107.

The magnetic amplifier in module 26 is self-saturating and, if not excited by the control windings 93, 94 and 95 or bias winding 95a, tends to turn the SCR's 83 and 84 full on to fully energize eddy current brake coil 73. Control current in one or more of the control windings 93, 94 and 95 or bias current in bias winding 95a resets the amplifier and it can be made to saturate over the entire ac half cycle, partial cycle or not at all. When the magnetic amplifier functions as an error detector, it continuously compares the off and bias signals with the speed feedback signal (and the anti-hunt signal) and providing SCR control output signals at terminals 4, 5 and 6 of module 26 which are the resultant thereof.

When the odd numbered terminal of a control winding is positive with respect to its even numbered terminal, the current in the control winding tends to turn the output pulse on. The reverse is true when the odd numbered terminal is negative with respect to its even numbered terminal. All of the control signals are of constant polarity except the anti-hunt signal. The values may vary from zero to a minimum, but never reverse in polarity. The anti-hunt signal is not constant, but allows current to flow in its control winding only under transient conditions.

Frequency Detector Module

When master control switch 19 is in first point LOWER position, motor 14 is energized but the rotor is restrained because magnetic amplifier module 26 is at maximum "on" and load brake 17 is excited. If motor 14 starts to turn, all signals remain constant. The frequency detector module 25 input is shorted by contacts 1A.

The frequency detector module 25 shown in FIGS. 1 and 4 has its input terminals 7 and 12 connected across the secondary winding of motor 14 by conductors 110 and 111, respectively, and provides, at its output terminals A1 and A2, a dc speed feedback signal which is proportional to the rotor frequency of motor 14, which in turn is proportional to motor slip and below motor synchronous speed. It may be assumed that motor 14 at rest produces a frequency of 60 Hertz and at synchronous speed produces a zero frequency. There is no input from the motor secondary into frequency detector module 25 whenever contactor 1A is energized. If motor 14 is driven faster than synchronous speed by an overhauling load, rotor frequency again increases. The magnitude of voltage induced in the rotor (secondary) windings varies directly with frequency. As FIG. 4 shows, frequency detector module 25 comprises an input transformer 112 and a saturating transformer 113 saturated by rotor voltage and having a square hysterisis loop core which tends to produce constant energy pulses for each half cycle fo input frequency. These pulses, when rectified and filtered by the rectifiers 114 and capacitors 115, produce a dc voltage signal (speed feedback signal) directly proportional to motor frequency at terminals A1 and A2.

Load Detector Module

Referring to FIGS. 1 (sheet 3) and 8, the load detector module 21 has power supply terminals 11 and 12 which are connected by conductors 118 and 119 to anode transformer 82. Module 12 also has signal input terminals 1, 2 and 3 which are connected by conductors 120, 121 and 122, respectively, to transformers 123, 124 and 125, respectively, which are connected to a portion of the permanent resistors for motor 14 to sense current conditions therein. Load detector module 21 which includes a signal detection section 126 and an amplifier section 127 senses when motor 14 is delivering over a maximum of 15 percent of full load torque and provides an output signal at its signal output terminals 6 and 7 to energize relay coil 22 connected thereto. Relay coil 22 operates relay contacts 22A and 22B. Load detector module 21 is designed to effect deenergization of relay coil 22 if motor torque drops to between about 12 and 13 percent of full load torque. The load detector module 21 has an "On" bias in it that varies with line voltage so that in the first approximation, the circuit trips on power and not current, i.e., shaft torque.

Regeneration Control Module

Referring to FIGS. 1 (sheet 3) and 9, the function of regeneration control module 20 is to provide an output signal at its terminals 5 and 6 for guard module 23 when motor 14 reaches and exceeds synchronous speed. Module 20 receives a voltage input signal at its terminals 1 and 2 from a voltage transformer 130 coupled to the main supply lines L1 and L2. Module 20 also receives a current input signal at its terminals 3 and 4 from a current transformer 131 coupled to main supply line L2. Power for module 20 is provided at its power supply terminals 7 and 8 from a transformer 132 understood to be connected to lines L1 and L2. The regeneration module 20 comprises a rectifier section 133 to combine and rectify the voltage and current signals and a level detector-switching section 134 to detect when the combined and rectified signal falls below a level and switches on a power level for delivery to signal output terminals 5 and 6.

Guard Control Module

Referring to FIGS. 1 (sheet 3) and 10, the guard control module 23 is designed to maintain relay 24 energized at all times, except under specified conditions. Module 23 comprises an input amplifier 140 having input terminals 1 and 2 for a first control winding 141. Terminal 2 is connected to terminal 5 of the regeneration module 20 and to terminal 3 of a second control winding 142, and the latter's terminal 4 is connected through a resistor 143 to the terminal 7 of a third control winding 144. The latter's terminal 8 is connected by a conductor 145 to terminal 10 of regeneration module 20. When the signal is present in the control winding 141 of guard control module 23 from terminals 1 and 2, the input amplifier 140 tends to turn off a second stage amplifier 146 of guard module 23 after a time delay. However, the presence of an input signal at input terminals 2 and 8 to control windings 142 and 144 of guard module 23 (from terminals 5 and 10 of the regeneration module 20) tends to turn on module 23. During subsynchronous operation only, the ON signal exists and the output terminals A3 and A4 are energized to energize, pick up or close time delay relay 24.

With a signal at terminals 1 and 2, the OFF signal cancels the ON signal and after a predetermined time delay output terminals A3 and A4 are deenergized, dropping out relay 24. Power to the module is supplied through terminals A1 and A2 from terminals 118 and 119 of anode transformer 82.