Abstract:
A low cost, low part content single phase AC induction motor control starting circuit is provided which is speed sensitive, load sensitive and AC line voltage fluctuation insensitive. A dual comparator chip (20, 22) senses and compares relative magnitudes of AC line and auxiliary winding voltages (48, 40) to de-energize the auxiliary winding (2) at cut-out speed, and automatically re-energize the auxiliary winding at cut-in speed to accelerate or restart the motor from an overload or stall condition. Hysteresis circuitry (20, 50, 52, 57, 80) provides a lower cut-in speed than cut-out speed. Simplified power supply and voltage detection circuitry is also disclosed.

Description:
BACKGROUND AND SUMMARY 
     The invention relates to disconnect switches for use with the start or auxiliary winding of a single phase AC induction motor. The invention particularly relates to improvements in further reduced cost and part content over the circuitry shown in commonly owned U.S. Pat. Nos. 4,622,506, 4,658,195 and 4,670,697, incorporated by reference. 
     The present invention provides simplified comparator circuitry, with reduced part content. The invention also provides simplified main and auxiliary winding voltage detectors and power supply circuitry of reduced cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a typical environment in which the preferred embodiment of the invention is used. 
     FIG. 2 is a circuit diagram of a motor control starting circuit in accordance with the invention. 
     FIG. 3 is a waveform diagram illustrating operation of the circuitry of FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     As known in the art, a single phase AC induction motor has a main winding for running the motor, and a start or auxiliary winding. The auxiliary winding is energized when starting the motor, and then disconnected at a given motor speed. The fields in the main and auxiliary windings are phase shifted, for example by capacitance, inductance, resistance or the like, to establish a rotating magnetic field for starting torque. 
     FIG. 1 shows main winding 1 and auxiliary winding 2 of a single phase AC induction motor connectable through a switch 3 to an AC power source 4. In capacitor start type motors, the start winding circuit includes a start capacitor 5. When the motor reaches a given cut-out speed, start switch 6 is opened to disconnect auxiliary winding 2 from AC source 4. The present invention provides control circuitry for start switch 6. 
     Referring to FIG. 2, a main voltage detector circuit including diode 10 is connected across AC source 4 for sensing the input AC line reference voltage. Voltage from AC source 4 is sensed through resistor 12 and halfwave rectifying diode 10, and filtered by capacitor 14. Varistor 16 provides transient spike protection. An NPN bipolar pass transistor 18 has its emitter-collector circuit connected between AC source 4 and node 19 which provides sensed voltage from AC source 4 and also provides a DC power supply for a pair of comparators 20 and 22, to be described. The base circuit of transistor 18 includes a zener diode 24 applying a limited voltage from AC source 4 to the base of transistor 18 to bias the latter into conduction. Base drive current is supplied through resistor 26. Resistor 28 is connected between zener diode 24 and the base of transistor 18. Resistor 28 develops an IR drop thereacross which varies with the varying voltage from AC power source 4 and applies varying bias to the base of transistor 18. Zener diode 24 limits the conductivity of transistor 18, and resistor 28 varies such conductivity according to the IR drop thereacross. This provides conductivity modulation to vary the voltage at node 19 at the emitter of transistor 18 to track the voltage from AC source 4 and provide line compensation. 
     An auxiliary voltage detector circuit including diode 30 is connected across auxiliary winding 2 for sensing auxiliary winding voltage. The auxiliary winding voltage is reduced by the voltage divider network provided by resistors 32 and 34 and sensed through halfwave rectifying diode 30 and resistor 36 and filtered by capacitor 38, to provide sensed auxiliary winding voltage at node 40. 
     Voltage comparators 20 and 22 are provided by a dual comparator chip having power supply voltage provided at line 42 from node 19, and are connected by line 44 to a common reference at 53. Noninverting input 46 of comparator 20 senses voltage from AC source 4 at node 48 as reduced from node 19 by the voltage divider network provided by resistors 50 and 52. Inverting input 54 of comparator 20 senses auxiliary winding voltage at node 40. Comparator 20 compares the auxiliary winding voltage against the voltage from AC source 4. When the auxiliary winding voltage increases as a function of motor speed to a predetermined cut-out value relative to the voltage at node 48 from AC source 4, comparator 20 outputs a turn-off signal at output 56 causing switch 6 to open and disconnect auxiliary winding 2 from AC source 4, to be described. When the auxiliary winding voltage decreases as a function of motor speed to a predetermined cut-in value relative to the voltage at node 48, comparator 20 outputs a turn-on signal at output 56 causing switch 6 to close and connect auxiliary winding 2 to AC source 4. The predetermined cut-in value corresponds to a voltage derived from rotationally induced voltage in the auxiliary winding during overload or stall of the motor after starting. 
     At initial energization of the motor, the auxiliary winding voltage is zero, or at least substantially smaller than the main winding voltage, and hence the voltage at comparator input 54 is less than that at comparator input 46, FIG. 3, and thus comparator output 56 is high, as referenced through pull-up resistor 57 to node 19. Comparator output 56 is connected to inverting input 58 of comparator 22. When comparator output 56 is high, comparator output 62 is low, which enables conduction from node 19 through resistor 64 and light emitting diode 66, which turns on switch 6. 
     Switch 6 is an optically triggered semiconductor power switch, including a power triac 68. LED 66 is optically coupled to light responsive SCR 70 to optically drive the latter into conduction, which in turn conducts current through resistor 72 and bridge circuit 74 to the gate of triac 68 to bias the latter into conduction. Resistors 76 and 78 improve dv/dt capability of SCR 70 and triac 68, respectively. Upon conduction of triac 68, current flows from AC source 4 through start capacitor 5 and auxiliary winding 2. 
     As motor speed increases, the sensed auxiliary winding voltage at node 40 increases. At a given cut-out value the voltage at comparator input 54 increases as a function of motor sped above that at comparator input 46. Comparator output 56 then goes low, which low state is supplied to comparator input 58, which in turn causes comparator output 62 to go high. The high state at comparator output 62 disables conduction through LED 66, which terminates the emmission of light to SCR 70 such that the latter turns OFF, which in turn removes the gate drive from triac 68, such that the latter turns OFF. Turn-off of triac 68 disconnects auxiliary winding 2 from AC source 4. 
     Comparator output 56 is connected through resistor 80 to comparator input 46 at node 48. When comparator output 56 goes low at the noted cut-out speed, the voltage at comparator input 46 is reduced through the connection provided by resistor 80, as shown at transition 82 in FIG. 3, i.e. the voltage at input 46 is pulled low by its connection through resistor 80 to low output 56. Output 56 is now at approximately the same potential as reference 53. The voltage at comparator input 46 is modified because resistor 80 is now effectively in parallel with resistor 52. Comparator output 56 will not transition high again until the auxiliary winding voltage at input 54 decreases below the lowered and modified voltage at input 46, as shown at transition 84 in FIG. 3. Comparator output 56 then goes high again, to turn on start switch 6 and reconnect auxiliary winding 2 to AC source 4. The connection through resistor 80 provides hysteresis such that the cut-in speed is always lower than the cut-out speed. 
     The connection through resistor 80 changes the voltage at comparator input 46 according to the voltage at comparator output 56 such that the auxiliary winding voltage at comparator input 54 is compared against different voltages at comparator input 46. A lower comparison reference voltage is provided at input 46 when output 56 is low and outputting a turn-off signal because resistor 80 is now effectively in parallel with resistor 52. A higher comparison reference voltage is provided at input 46 when output 56 is high and providing a turn-on signal. Auxiliary winding voltage must decrease to a cut-in value established by the new voltage divider ratio of resistors 50, 52 and 80 which is less than the cut-out value, whereupon comparator output 56 changes states to output the turn-on signal to comparator 22 to reconnect auxiliary winding 2 to AC source 4. 
     Resistor 80 is connected between comparator inputs 58 and 60 and provides a voltage drop therebetween. Comparator input 60 is connected to node 48. When comparator output 56 is low, the reference voltage from AC source 4 at node 48 is dropped across resistor 80 in parallel with resistor 52 to low output 56, and the voltage at comparator input 60 is higher than that at comparator input 58. When comparator output 56 is high, the voltage thereat is dropped across resistor 80 to node 48 and through resistor 52 to common reference 53, and the voltage at comparator input 58 is higher than that at comparator input 60. 
     It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.