PSC motor start system

Abstract: A motor winding system having a main winding and a start winding circuit with a start winding and a PTC resistor in series therewith wherein the resistor is electrically removed from the circuit at a predetermined time after motor starting and remains in this state until motor shut off. The start winding circuit also includes a bimetallic switch in series with the resistor and a series circuit of a capacitor and an electromagnet in parallel with the resistor and switch. At motor start up, current from the main power source is applied across the main winding and the start winding circuit heats up the resistor and causes an increase in the resistance thereof. Also, current through the switch and PTC heat causes the bimetallic element to snap or move when it has reached a predetermined temperature to open the start winding circuit. Meanwhile, low current passes through the capacitor and electromagnet winding, energizing the electromagnet and causing the bimetallic element to be attracted and latched to the electromagnet after predetermined movement. After cool down, the bimetallic element will not return to the closed switch condition, thereby completely removing the resistor from the circuit. The switch remains in the switch open condition until current is removed from the electromagnet at motor shut off with removal of power from the motor windings. The current passing through the capacitor is a small fraction of the current normally present in the resistor during normal motor operating conditions after start up.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to motors having an auxiliary start/capacitor run winding and, more specifically to a system for removal of the PTC motor starter from the circuit during motor operation. BRIEF DESCRIPTION OF THE PRIOR ART Permanent split capacitor (PSC) electric motors are generally provided with a stator winding circuit which includes a main or run winding and an auxiliary or start winding connected in series with a run capacitor which are both connected in parallel with the main winding. During motor starting and running, both the main winding and the start winding are connected across the motor power source to magnetically excite the rotor and cause rotation thereof. Sometimes a low resistance positive temperature coefficient (PTC) resistor starter is placed in parallel with the run capacitor to increase the starting torque of the motor. When the motor has reached a predetermined speed or after a predetermined time, it is advantageous to disconnect the PTC starter from the circuit to save the energy wasted keeping the PTC hot in its high resistance off state. In some of the older prior art, the start winding was removed from the circuit by some mechanical speed related device. More recent art, as exemplified, for example by Rathje Patent No. 4,161,681 and DeFilippis et al. Patent No. 4,706,152 minimized the effect of the start winding by placing a resistor in series with the start winding having a positive temperature coefficient of resistance. The PTC resistor self heats due to current therethrough, this heat causing an increase in the resistance thereof, thereby causing a gradual decrease in the current through the start winding to that current primarily through the run capacitor with continued motor operation. The resistor is designed to reach a predetermined maximum resistance in a predetermined time period which is related to the motor design. A problem with such PTC resistor controlled start windings is that the current through the PTC resistor is never completely terminated and wastes energy. This is readily apparent from the fact that, if current were completely cut off, the PTC resistor would start to cool off, thereby causing a decrease in its resistance and an increase in the current therethrough. It follows that an equilibrium low current through the PTC resistor is established during standard motor operation and after the initial start. This continual current through the PTC resistor causes unnecessary heating in the PTC resistor as well as undesirable power dissipation with concomitant loss of motor system efficiency. In US-A-3 737 752, there is described a circuit for a capacitor-run motor using a relatively small capacitor permanently in series with the start winding and connected in parallel with the capacitor either an additional capacitor in series with PTC elements or the PTC elements alone, so that a larger current is applied to the start winding during the starting phase of the motor. A small current is applied to the start winding all the time that it is running, the current being mostly due to the small capacitor but partly through the PTC elements in their high resistance state. In US-A-4 161 681, there are described circuits for both capacitor-start and capacitor-run motors, in which, to eliminate the continuous voltage stress during running on a PTC element used to provide the starting current, a relay is provided to open-circuit the current path through the PTC element in response to the fall in the current through the main winding of the motor when it is up to speed. SUMMARY OF THE INVENTION According to the present invention there is provided a system for use in conjunction with a capacitor run motor with a winding system having a start winding and a main winding, comprising: a supply circuit for the start winding having a first circuit and a second circuit connected in parallel, said supply circuit serially connected to said start winding; the first circuit including a resistor having a positive temperature coefficient of resistance connected in series with a switch, and the second circuit including an electromagnetic device for controlling the switch connected in series with the running capacitor of the motor, the electromagnetic device and the switch being arranged so that when current is initially passed through the supply circuit the switch is closed and a portion of the current flows through the first circuit whilst another portion of the current flows through the second circuit, and later the resistance of the resistor increases causing the portion of the current through the second circuit to increase, the switch opening in response to the portion of the current through the first circuit or through the second circuit and being held open by the electromagnetic device while the motor is running. In one embodiment there is provided a motor having a main winding and a start winding circuit in parallel therewith, the start winding circuit including the start winding which is serially connected to the PTC resistor and a bimetallic switch. In addition, a series circuit of a capacitor and an electromagnet are connected in parallel with the PTC resistor and the bimetallic switch element. In operation, at motor start up, current from the main power source is applied across the main winding and the start winding circuit. The current in the start winding circuit heats up the PTC resistor and causes an increase in the resistance thereof. In addition, the current through the bimetallic switch element and the PTC heat causes this switch element to snap or move when it has reached a predetermined temperature to open PTC circuit after it has heated to a high resistance state. Meanwhile, current of much smaller level than is required by the start winding during normal motor starting passes through the capacitor and electromagnet winding, energizing the electromagnet and causing the bimetallic switch element to be held attracted to the electromagnet. In this way, after cool down of the bimetallic element, it will not return to the closed switch condition. This results in the PTC resistor being completely removed from the circuit and carrying no current therethrough. The bimetallic switch element remains in the switch open condition until current is removed from the electromagnet, this taking place only at motor shut off with removal of power from the motor windings. Though relays have been used where the relay coil is in series with the main winding and the relay contacts are in series with the PTC to deenergize the PTC at a predetermined main winding current which is related to motor speed, it is preferable not to use main winding current because different current levels are required for different motor sizes. The current flowing through the capacitor and electromagnetic winding is essentially controlled by the value of the capacitor. This capacitor value is typically one or two fixed values for many sizes of refrigerator compressor motor and therefore makes it advantageous to use the capacitor run current to hold open the PTC versus the main winding current because one or two ratings can be used on the majority of PSC motor driven refrigerator compressors. In accordance with a second embodiment of the invention, the bimetallic switch element is replaced by a non-thermostatic switch element, which is multimetallic, monometallic or multipiece, in a low power motor starter which is movable and can be opened in response to the magnetic field of the electromagnet or relay coil. The non-thermostatic switch element can be, for example, a good electrical conductor to minimize power loss and a magnetic material responsive to the magnetic field of the electromagnet or relay coil, and may be any metal, alone, as mixtures, as alloys or as assembled combinations of metals, mixtures or alloys. In this embodiment, the electromagnet opens the switch contacts as the PTC element current decreases and the capacitor current increases. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a circuit diagram of a motor winding system in accordance with the prior art; and FIGURE 2 is a circuit diagram of a motor start system in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGURE 1, there is shown a motor winding system in accordance with the prior art. The circuit includes a power source indicated by lines L1 and L2. Incoming current passes through an overload motor protector circuit 17 which includes a bimetallic element 19 and a heater 21. When a combination of ambient temperature and excessive motor current passing through heater 21 and bimetallic element 19 raise bimetallic element 19 to a predetermined temperature, the bimetallic element 19 will move away from the contact 23 to provide an open circuit between the bimetallic element and the heater, thereby opening the circuit to the motor windings. The motor winding system includes standard run winding 1 and start winding 3 with a parallel circuit of PTC resistor 5 and capacitor 9 in series with the start winding 3. Referring now to FIGURE 2, there is shown a motor winding system in accordance with the present invention wherein like elements as in FIGURE 1 have like reference designations. The system includes the same motor protector circuit 17 described with reference to FIGURE 1 as well as a main winding 1 connected across a power line L1 and L2. Connected in parallel with the main winding 1 is a start motor circuit including serially connected start/auxiliary winding 3, PTC resistor 5 and normally closed bimetallic switch element 7 which contacts a contact element 15 with a contact element 13 thereon. Connected in parallel with the PTC resistor 5 and bimetallic switch element 7 is a series circuit comprising a capacitor 9 and an electromagnet 11 which acts as a holding magnet for the bimetallic switch element 7 as will be explained hereinbelow. In operation, upon motor start up, a voltage is applied across the power lines L1 and L2 to apply current to both the main winding 1 and the start/auxiliary winding 3. A portion of the current through the start winding 3 will pass through the PTC resistor 5, the contact element 15 and the contact element 13 on bimetallic switch element 7, causing the PTC resistor to heat up and provide increased resistance to the start motor circuit. Also, the current passing through the bimetallic switch element 7 causes heating therein. At some predetermined temperature, designed in known manner into the bimetallic switch element 7 and related to the motor to be controlled, the element 7 and contact element 13 thereon will move away from the contact element 15 and open the circuit through the PTC resistor 5. Meanwhile, another portion of the current through the start winding which is a relatively very low current constantly passes through the capacitor 9 and the winding of the holding electromagnet 11 to energize the electromagnet. The electromagnet 11 is designed to have insufficient magnetic attraction to attract the bimetallic switch element 7 thereto when the switch formed by the bimetallic switch element and contact elements 13 and 15 is closed. However, as the bimetallic switch element 7 moves toward the electromagnet 11, either gradually or by snap action, it is ultimately attracted to the electromagnet and latches thereto. Accordingly, the circuit through the PTC resistor 5 remains open and remains in that state until the bimetallic switch element 7 returns to the closed switch condition. This can only happen when current to the electromagnet 11 is removed, primarily by turning off the motor. It can be seen that the above described motor winding circuit provides the dual advantages of decreasing heat generated by the motor start winding system as well as providing increased motor system efficiency as compared with prior art systems. In accordance with a second embodiment of the invention, in low power motor starters, the switch 7 of FIGURE 2 can be replaced with a switch as defined above. In this embodiment, as the motor speed increases after start and the PTC element 5 heats up due to the passage of current therethrough, the resistance of the PTC element increases to cause a decrease in current therethrough. Accordingly, the current through the capacitor 9 and relay coil or electromagnet 11 will increase, thereby increasing the magnetic field strength provided by the electromagnet and causing the non-thermostatic switch 7 to open. The switch 7 will remain open as long as sufficient current is passing through the coil 11 to perform this function.

Claims: A system for use in conjunction with a capacitor run motor with a winding system having a start winding (3) and a main winding (1), comprising: a supply circuit for the start winding having a first circuit and a second circuit connected in parallel, said supply circuit serially connected to said start winding; the first circuit including a resistor (5) having a positive temperature coefficient of resistance connected in series with a switch (7), and the second circuit including an electromagnetic device (11) for controlling the switch connected in series with the running capacitor (9) of the motor, the electromagnetic device (11) and the switch (7) being arranged so that when current is initially passed through the supply circuit the switch (7) is closed and a portion of the current flows through the first circuit whilst another portion of the current flows through the second circuit, and later the resistance of the resistor (5) increases causing the portion of the current through the second circuit to increase, the switch (7) opening in response to the portion of the current through the first circuit or through the second circuit and being held open by the electromagnetic device while the motor is running. A system according to claim 1 wherein the switch (7) is normally closed and is opened by the electromagnetic device (11) when the current through that device reaches a predetermined level. A system according to claim 1 wherein the switch (7) is a normally closed temperature responsive bimetallic switch that opens when heated sufficiently by the current through it. A system according to claim 3 wherein the electromagnetic device (11) is responsive to predetermined movement of the switch to latch the switch in the open state. A system according to claim 3 or 4 wherein said switch (7) comprises a pair of contacts (13,15) and a bimetallic element movable in response to a predetermined temperature change, one of the contacts being disposed on and movable with the bimetallic element. A system according to claim 5 wherein said bimetallic element is a snap acting element.

Inventors: COOPER LAWRENCE E, JENNE RICHARD L, COOPER, LAWRENCE E., JENNE, RICHARD L.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1990
Main classification: H02P1
IPC codes: H02P1, H01C7, H02K17