Light Load Control For Well Pump

Abstract

A control for turning on and off a motor driving a well pump which includes high pressure and low pressure detectors. The detectors are coupled to the gate circuits of respective silicon controlled detectors, or SCR's, which have their load circuits connected in a latching loop circuit which energizes a relay to operate the motor. To disable the relay when the pump is sucking air, an auxiliary SCR controls an auxiliary relay having normally closed contacts connected in the loop circuit. The auxiliary SCR is turned on by a source of positive voltage but a current transformer wired in series with the pump motor sets up a negative control voltage which is of sufficient level under normal current conditions to overpower the positive voltage to maintain the auxiliary SCR turned off. A capacitor connected to the gate insures against premature actuation of the gate by positive inrush voltage.

Parent Case Text

This is a division of application Ser. No. 359,102, filed May 10, 1973, now U.S. Pat. No. 3,801,889, issued Apr. 2, 1974.

Description

It is an object of the present invention to provide a control for a well pump which is reliable in turning the pump motor on at a predetermined low pressure and off at a predetermined high pressure and which positively disables the motor when the pump begins to suck air. It is another object to provide a control system for a well pump which makes use of the solid state SCR devices but in which such devices are completely deenergized during the long periods of standby when the pressure in the tank is up to the rated level. It is a general object to provide a control circuit for a well pump which is capable of operating without care of maintenance for long periods of time.

Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which:

FIG. 1 is a diagram showing a typical well pump system to which the invention is applied.

FIG. 2 is a schematic diagram illustrating a preferred embodiment of the invention.

While the invention has been described in connection with a preferred embodiment, it will be understood that I do not intend to be limited thereto but intend on the contrary to cover the various alternative and equivalent forms of the invention included within the spirit and scope of the appended claims.

Turning now to the drawings, a control package indicated at 10 serves to control a motor M driving a pump P which furnishes water under pressure to a tank T. Communicating with the tank are detectors H and L which are set to respond at respective high and low pressures, or water levels, and which are connected to terminals 11, 12, sharing a common terminal 13. The motor is powered through terminals 14, 15 and power is received from lines L1, L2 via terminals 16, 17. The water W, kept under head of air, leaves the tank through an outlet 18. It will be understood that detectors H and L include switches which show an open circuit when "satisfied," a drop in pressure closing the circuit to "call for water."

Turning now to FIG. 2, a first SCR 20 is provided having an anode 21, a cathode connection 22 and gate 23. The gate is connected to the low pressure detector L via a series resistor 24. The resistor 25 is the usual gate-cathode resistor and the diode 26 serves as a clamp to limit the negative voltage which can be applied to the gate.

Under the control of the high pressure detector H is an SCR device 30 having anode 31, cathode connection 32 and gate 33, the latter being connected to the high pressure detector via a series resistor 34 and with the gate being provided with a shunting resistor 35 and a clamping diode 36.

For the purpose of furnishing load current, a transformer 40 is provided having a primary winding 41 and a secondary winding 42. Connected in series, that is, in a loop, with the load circuits of the SCR's and with the transformer secondary winding is a motor relay having an input circuit, or winding, 50 and normally open contacts 51, 52 which are connected in series with the a.-c. supply lines. A protective diode 53 connected across the relay protects the SCR's against the inductive kick which occurs when the relay is deenergized.

In order to make the SCR 20 operate in the latching mode, such SCR, and the relay winding 50 which is in series with it, are shunted by a capacitor 54. Thus during positive pulses of current, the capacitor is charged to a voltage which corresponds to the sum of the voltage drops through the relay winding and anode-cathode circuit. The capacitor has a sufficiently high capacitance so as to maintain a positive voltage between the anode 21 and cathode 22, thereby to maintain the SCR 20 in the conductive state, bridging the negative half cycles supplied from the upper end of the transformer winding 42.

For the purpose of protecting the pump against the hazardous condition of "sucking air," means are provided for disabling the pump motor when the motor is turned on but when substantially less than rated current is flowing through the motor. This is accomplished by a current detecting circuit which employs an auxiliary SCR 60 having an anode 61, cathode 62 and a gate 63, the latter having a shunt resistor 64. The auxiliary SCR 60 is connected to operate an auxiliary relay having input and output circuits in the form of coil 65 and a normally closed contact 66. The relay is of the mechanically latching type having a latch 67 which serves to "hold in" the relay armature following energization with a reset button 68 provided for manual release. Connected in parallel with the relay coil is a protective diode 69.

For controlling the auxiliary SCR 60, that is, for keeping the same non-conductive as long as normal current is drawn by the motor M, a current transformer 70 is provided having a primary winding 71 which is wired in series with the motor and a secondary winding 72. Voltage from the secondary winding is rectified by a diode 73 and the level of the rectified voltage is set by a potentiometer 74 having a wiper 75, which wiper is connected via a series resistor 76 to the gate 63 of the auxiliary SCR. The diode 73 is so polarized that rated current, drawn by the motor, is effective to produce a negative voltage at the gate 63 of the auxiliary SCR which is sufficiently high as to keep the SCR turned off. However, for turning on the SCR when lower values of motor current are drawn, indicating the condition of sucking air, a positive source of voltage is provided which is made up of a diode 80 and series resistor 81. The diode 80, and the auxiliary SCR are powered through input terminals 82, 83 which are connected to the "switched" side of the contacts 51, 52 so that the current detecting circuit is completely dead and unresponsive during the times that the motor M is turned off.

In order to prevent the auxiliary SCR 60 from responding to an inrush of positive voltage when the contacts 51, 52 are initially closed, and before negative control voltage is developed by the current transformer and its associated diode, a high value capacitor, indicated at 90, is connected to the gate terminal of the SCR and specifically between the gate and cathode terminals. The capacitor increases the effective time constant of the gate circuit since no positive voltage can be developed at the lower end of the resistor 81 until the capacitor 90 has had opportunity to charge, and the rate of positive charging is limited by the value of the resistor 81. Once the initial inrush condition has been taken care of, the voltage across the capacitor will stabilize at a voltage and polarity which depends upon the relative magnitude of the voltages being supplied via the "negative" diode 73 and "positive" diode 80, the negative voltage normally predominating.

While the operation of the circuit will be apparent from the foregoing description, it will be helpful to review a typical operating sequence. Let it be assumed that the system occupies the initial condition shown in FIG. 1 with water at a low level within the tank and with a low pressure in the space above it, causing both of the detectors to be turned on, or closed, "calling for water." It is apparent that closure of the contacts L and H (FIG. 2) provides a path for positive control voltages to pass to the gates 23, 33 at the same time that positive pulses are applied to the anodes so that both SCR's conduct, and serve with one another, to energize the winding 50 of the motor relay, closing contacts 51, 52 and turning the motor on. The resultant operation of the pump then causes water to flow into the base of the tank to raise the water level and the pressure of the contained air.

When the pressure of the air becomes sufficient to satisfy the detector L, the latter becomes open circuited, thus deenergizing the gate terminal 23 of the SCR 20. However, because of the latching effect of the capacitor 54 the SCR 20 will continue to remain conductive and pressure will further increase until the high pressure detector H is finally satisfied opening its circuit (FIG. 2) and deenergizing the gate 33 of the SCR 30. Since the latter operates in the non-latching mode, such SCR will become non-conductive and no further current will flow through the loop circuit formed by the SCR's, causing the motor relay 50 to drop out, opening contacts 51, 52 and denergizing the motor. This causes the control circuitry to go into the quiescent state.

However, as water is drawn through the connection 18 from the tank the pressure will be gradually reduced until the detector H will, calling for water, close its circuit. No current will, however, flow through the loop circuit since the SCR 20 remains deenergized. However, when the water pressure falls an additional "differential" amount, the low pressure detector L, too, calls for water by closing its circuit thereby energizing the gate 23 so that the condition for conduction is satisfied in both of the SCR's thereby energizing the motor relay 50, closing contacts 51, 52 and turning on the motor.

The initial positive inrush through the diode 80 to the gate 63 of the auxiliary SCR is defeated by the capacitor 90. At rated motor current, the voltage generated in the current transformer and rectified by the diode 73 insures that a net negative voltage will exist at the gate 63 thereby maintaining the auxiliary SCR 60 turned off, and the auxiliary relay deenergized so that the loop circuit, including contact 66, remains intact.

However, let it be assumed that water cannot be replenished at a sufficient rate within the well to keep up with the rate of withdrawal by the pump, in which case the pump will "suck air." Such sucking of air effectively unloads the pump, and hence the motor, so that the current through the motor falls to only a fraction of rated value. This condition is reflected by a drop in voltage at the output of the current transformer and hence a drop in the negative voltage produced by the diode 73, so that the positive voltage from the diode 80 prevails, causing the auxiliary SCR to become conductive. This opens the contact 66 in the loop circuit, dropping out the motor relay 50 and opening the contacts 51, 52 to turn off the motor. In carrying out the invention the auxiliary, undercurrent relay is of the latching type so that the contact 66 remains open circuited, thus disabling the motor until the reset push button 68 is depressed, whereupon normal operation and control of water pressure will resume.

It has been found that in a normal installation, in which the leads of the detectors run closely parallel to one another, it is possible for the closing transient which occurs when the high pressure detector calls for water to induce a signal in the low pressure control line 11 which is sufficient to jar the SCR 20 prematurely into conduction. In order to avoid this possibility the time constant of the gate circuit of the transistor 20 is increased by addition of an auxiliary capacitor 95.

The term "detector" used herein is used in a generic sense to include both pressure-responsive devices which provide an indirect measure of water level or means to detect level directly. Also while the invention has been described as applied to a well pump it will be apparent that it is applicable to any pump which serves to replenish a reservoir.

Claims

I claim as my invention:

1. In a control for the circuit for a motor driving a well pump having an associated tank and intended for operation from the regular a.-c. supply line, a low-current lockout comprising, a current transformer having its primary winding wired in series with the pump motor and having a secondary winding, an latching type relay having an input coil and having a normally closed output contact, connecting means between the output contact and the motor so that the output contact controls the flow of current to the motor, a current detector circuit having an SCR including a gate and load circuit, the load circuit being coupled to the input of the latching type relay, the current detector circuit having power input terminals connected in shunt with the motor, the current detector circuit further having a source of positive voltage supplied by the power input terminals and capable of turning the SCR on and having a source of negative voltage supplied by the secondary winding of the current transformer, means for coupling the gate of the SCR to both sources, the source of negative voltage varying in accordance with the motor current and of such magnitude as to keep the SCR turned off as long as rated current is drawn by the motor but which permits the SCR to be turned on when less than rated current is drawn by the motor with resultant actuation of the latching type relay and interruption of the motor circuit for disablement of the motor under conditions when the pump is sucking air.

2. The combination as claimed in claim 1 in which means are provided for preventing inrush of positive voltage to the SCR when the motor is turned on so as to permit development of negative voltage corresponding to rated motor current to maintain the SCR turned off.

3. The combination as claimed in claim 1 in which a capacitor is connected from the gate to the cathode of the SCR so as to prevent the gate from going positive and hence turning on the SCR prior to the time that negative voltage is applied at the gate by reason of voltage induced in the secondary winding of the current transformer.