DEVICE AND METHOD FOR SWITCHING IN AND OUT A START WINDING OF A SINGLE PHASE AC MOTOR

Abstract

A device for switching in and out a start winding of a single phase AC motor using an energizer winding to generate voltage which can be used to power a simple timing switch circuit. The AC voltage is rectified and converted to DC voltage. The DC voltage is then used to drive a normally closed solid state switch to an open state which in turn inactivates a triac connected to the start winding in the motor. The “on time” of the start winding is controlled by a RC circuit that ramps the voltage to the gate of a FET that drives current through the normally closed solid state switch.

Claims

1. A single phase AC motor containing a start winding circuit comprising a rotor, and a stator, wherein the stator is wound with a main winding, a starting winding, and an energizer winding, wherein the energizer winding is connected to an AC-DC convertor to generate DC voltage to power a timing switch circuit to activate and inactivate the starting winding of the motor.

2. A start winding cut-out circuit for a single phase AC electric motor including a stator, a main winding and a start winding, said main winding being adapted to be connected across AC power supply lines, said circuit comprising an electronic switch having a pair of power terminals and a gate which closes said switch in response to triggering voltage appearing thereon, said power terminals and said start winding being connected in series and connected across said main winding, timing means operating through a timing interval during current flow therethrough, said timing means being connected to said gate and forming a source of trigger current to said gate, and an auxiliary coil wound on said stator and adapted to have voltage induced therein, said auxiliary coil being connected to provide said voltage as a timing means to provide said trigger current for said time interval after initial energization of said motor.

3. The circuit as in claim 2, wherein said coil is wound on the axis of said start winding.

4. The circuit as in claim 2, wherein said coil is wound on the axis of said main winding.

5. The circuit as in claim 2, wherein said coil is wound between said start and main windings.

6. The circuit as in claim 2, wherein said coil and said trigger means are connected to one side of said start winding.

7. The circuit as in claim 2, wherein said coil is substantially seventy electrical degrees from the main winding.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic diagram of a control circuit in accordance with the invention.

[0018] FIG. 2 is a schematic diagram similar to FIG. 1 but illustrating an alternate form of the invention.

[0019] FIG. 3 is a schematic diagram similar to FIG. 1 but illustrating another alternate form of the invention.

[0020] FIG. 4 is a schematic diagram similar to FIG. 1 but illustrating yet another alternate form of the invention.

[0021] Components of these Figures are shown with component numbers in a description added after the Detailed Description.

DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

[0022] FIG. 1 schematically illustrates an AC electric motor of the split-phase induction type, including a main winding 10 and a start winding 11. The two windings 10 and 11 are wound on the stator (not shown) of an electric motor and are angularly displaced by 90 electrical degrees. As is well known to those skilled in the art, starting torque for the motor is attained by providing the start winding 11 with a different phase angle from the main winding 10, thereby producing a time or phase displacement between the two windings. One side of each of the windings 10 and 11 is connected to a power supply line 12, and the other side of the main winding 10 is connected to a power line 13. The two lines 12 and 13 are adapted to be connected to an AC power supply 14 such as a 60-cycle single phase AC power source.

[0023] While the main winding 10 is connected directly across the two power supply lines 12 and 13, the start winding 11 is connectable across the two supply lines by a control circuit which enables energization of the start winding 11 during a short timing interval or period at initial energization of the motor. This time interval may, for example, be one-half second in length. As will be described hereinafter, the start winding 11 may also be reenergized if the motor stalls. The control circuit comprises a selectively closable bi-directional electronic switch such as the type commonly known as a triac, indicated by the reference numeral 16. The triac 16 comprises a first main or power terminal 17, a second main or power terminal 18, and a gate 19. The triac 16 offers a low resistance path between the main terminals 17 and 18, when in each AC half cycle, a potential appears across the main terminals and a trigger signal appears on the gate 19. The triac 16 otherwise presents a very high resistance, or open circuit to current flow between the main terminals 17 and 18. The main terminal 17 is connected to the start winding 11 and the other main terminal 18 is connected to the power line 13. It will be apparent therefore that the start winding 11 and the triac 16 are connected in series and they are connected across the power lines 12 and 13 in parallel with the main winding 10, and when the triac 16 is closed or triggered, the start winding 11 will be energized.

[0024] A trigger circuit 15 is also provided in the control circuit to trigger the triac 16 into conduction. The trigger circuit 15 uses the exciter winding coil (EW) 31 which generates a voltage (to be explained below) which can be used to power a simple timing switch circuit.

[0025] The EW 31 is connected between the Schottky diodes 20, 21 and the power line 13. The EW 31 is mounted on the motor stator concentric with or on the axis of the start winding 11 so that voltage will be induced in the EW 31.

[0026] Considering the operation of the circuit illustrated in FIG. 1, when the power lines 12 and 13 are connected to an AC power supply, current flows through the main winding 10. Normally closed solid state switch (NCSS) 24 is defaulted so that the start winding 11 is connected when power is applied to the motor. This is important because without the start winding 11 being engaged, insufficient voltage is generated in the EW 31 to actively turn on the switch. Since the NCSS 24 is normally closed, the triac 16 is activated allowing current to flow through the start winding 11 within a half cycle of supply voltage. The storage capacitors 22, 23 for the DC voltage have to be of sufficient capacity to maintain the needed voltage throughout the 60 Hz cycle.

[0027] As shown in FIG. 1, the auxiliary coil 31 is mounted concentrically with or on the axis of the start winding 11, and consequently current flowing through the start winding 11 induces a voltage in the coil 31. The induced AC voltage is rectified and converted to DC. The DC voltage is then used to drive the normally closed solid state switch (NCSS) 24 to an open state, which in turn inactivates the triac connected to the start winding 11 in the motor. Both ends of an EW are connected to a voltage doubler since it is desired to have as few windings as possible in the EW. A doubler, consisting of two Schottky diodes 20, 21, is used to maximize the DC voltage from the AC voltage created by the EW. The output of the doubler charges two capacitors 22, 23 of sufficient size to maintain the required DCV during the 60 Hz cycle for powering the opto-isolated NCSS 24. The switch time of the NCSS 24 is controlled by resistors 25, 35 and capacitor 26 connected to the gate of a FET 27 (instead of a FET, different switching devices can be used with more or less accuracy in the switch point, such as a Schmitt trigger inverter from TI, such as part number, SN74AUC1G14DCDR. In one embodiment, a FET is used such as Fairchild, FDV303N.) The output of the FET is used to drive the NCSS 24 via the DC power generated in the output capacitors 22, 23, of the voltage doubler. The length of time interval or period is determined by the values of resistors 25, 35 and capacitor 26. For a motor designed to drive a water well pump for example the cut-out speed is reached in approximately 0.5 seconds.

[0028] One advantage with using an RC 25, 35, 26 in conjunction with a FET 27 as opposed to more complex logic is that all the power being generated is used to drive the triac switch opto-isolator (except for the smallest amount of power used to overcome the discharge resistor connected to the logic level FET 27 that drives the NCSS 24).

[0029] While the auxiliary coil 31 may be inductively coupled with the start winding 11 as shown in FIGS. 1 and 4, or with the main winding 10 as shown in FIG. 2, or in a position in between as shown in FIG. 3, the position illustrated in FIGS. 1 and 4 is preferred for most motor applications. In the event that the motor is stalled, the main winding 10 will continue to be energized but the rotor will not turn and voltage will no longer be induced in the coil 31. As soon as the voltage stops being induced in EW 31, a reset function is performed by the back-facing diode 28 and resistor 29, which discharges the timing and storage capacitors 22, 23 and 26. This allows the NCSS 24 to activate the triac 16 and allows the start, winding 11 to be reenergized. Normally, the motor will start-up again and the capacitor 26 will recharge and time out the start winding 11. Of course, an overload device should be provided between the lines 12 and 13 to protect the motor against damage in the case where the motor continues to be stalled after the start winding is reenergized.

[0030] By the disclosed device the disadvantages of prior art systems are overcome. A more consistent system is designed for a consistent rate for restart is produced. The difficulties associated with the use of heating and cooling of the system, as are present with prior art systems, are overcome because of the use of the system to generate voltage. Further, the current system uses a solid state device which has much higher reliability.

[0031] It is recognized that various alternatives and modifications are possible within the scope of the appended claims.

COMPONENT NUMBER DESCRIPTION

[0032] 10 main winding [0033] 11 start winding [0034] 12 power supply line [0035] 13 power supply line [0036] 14 AC Power supply [0037] 15 Trigger Circuit [0038] 16 bi-directional electronic switch (triac) [0039] 17 triac first main or power terminal [0040] 18 triac second main or power terminal [0041] 19 tirac gate terminal [0042] 20 Schottky diode [0043] 21 Schottky diode [0044] 22 capacitor [0045] 23 capacitor [0046] 24 normally closed solid state switch (NCSS) [0047] 25 resistor [0048] 26 capacitor [0049] 27 FET [0050] 28 diode [0051] 29 resistor [0052] 30 resistor [0053] 31 exciter winding coil on axis of start winding [0054] 32 resistor [0055] 33 exciter winding coil on axis of main winding [0056] 34 exciter winding coil between main start winding [0057] 35 exciter winding coil connected to one side of start winding [0058] 36 resistor