FIELD OF THE INVENTION

This invention relates generally to electric motors and more particularly to systems for protecting such motors from adverse conditions such as over-temperature of the windings.

BACKGROUND OF THE INVENTION

Electronic motor protection systems have been in wide use for a number of years. Typically, such systems have temperature sensors, such as positive temperature coefficient of resistivity (PTC) thermistors embedded in the windings of a motor to be protected with the thermistors electrically connected to a circuit which is arranged to de-energize the motor whenever a winding heats up to a selected level, occasioned by overcurrent or the like, thereby causing the PTC thermistors to go into a high resistance state. A system of this type is the subject of U.S. Pat. No. 4,281,358, assigned to the assignee of the instant invention. This system employs a transformer for an isolated power supply with different transformers used for different input voltage levels such as 115 and 240 volts. Subsequently a new electronic circuit, the subject of U.S. Pat. No. 5,585,990 also assigned to the assignee of the present invention, was developed which employs an integrated power supply in which an integrated circuit reduces the voltage to a selected level, e.g., 24 volts, providing a lower cost circuit, however, in some applications, particularly in some parts of the world where step down transformers to attenuate line voltage are not used and where power is connected across a phase to neutral, the IC power supply is vulnerable to transients, electrical noise and the like thereby resulting in premature failure in some cases.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved motor protection system which is low in cost yet robust in design, one in which the temperature sensors are electrically isolated from line voltage and one in which there is greater commonality in parts among all voltage ratings, without the need for jumpers or additional circuit connections.

Briefly, in accordance with the invention, an electronic motor protection system employs temperature responsive thermistors, i.e., sensors, which are adapted to be embedded into respective windings of a motor. An electrical signal is fed from the sensors to a comparator circuit which controls the energization of a transistor switch serially connected to the coil of a low current relay which in turn controls the energization of a contactor which switches current to the motor being protected. According to a feature of the invention, a transformer is used to provide an isolated power supply which is compatible with either 120 or 240 VAC input voltage. In one embodiment, line voltage is inputted to a transformer and the combination of a zener follower network and a series regulation network are used to simultaneously provide independent DC voltage supply levels for control circuitry, thereby allowing for logic operation and voltage detection over a wide range of input AC voltage. According to a second embodiment, line voltage is input to a dual-primary transformer whose coil orientation is controlled by a double pole, double throw relay which switches the primary from a 240 volt series connection to a parallel connection when exposed to 120 VAC line input. The relay is controlled on the secondary side of the transformer by a comparator circuit. On the secondary side, after the transformer, a full wave bridge is followed by a simple regulation arrangement, such as a zener follower regulation network, to provide DC voltage to the logic circuitry. According to a feature of the invention, the protection system includes comparator circuits to detect whenever the AC input voltage drops a selected amount below nominal and thereupon turn off the output relay. According to another feature, a dead-band voltage range is provided between the 120 and 240 VAC modes of operation so that when in the dead-band voltage range the output relay contacts will not close to thereby prevent the possibility of excessive voltage being applied to 120 volt motors. According to yet another feature of the invention, an adjustment circuit is provided to compensate for the change in load conditions whenever the output relay is de-energized to thereby stabilize current in the circuit.

Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with the description, serve to explain the objects, advantages and principles of the invention. In the drawings:

FIG. 1 is an electrical schematic of an electronic motor protection system made in accordance with a first embodiment of the invention;

FIG. 2 is an electrical schematic of the power supply portion of a system made in accordance with a second embodiment of the invention; and

FIG. 3 is an electrical schematic of the output portion of the FIG. 2 embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a power supply section 10 comprises a transformer T1 connected to AC line voltage through terminals L1, L2. Transformer T1 steps down the voltage approximately 5 or 6:1, depending upon input voltage, current and temperature, although other ratios could be provided, if desired. The secondary of transformer T1 is connected to a full-wave bridge BR1 which is followed by filter capacitor C1 to provide a ripple voltage line VR which is directly proportional to line voltage. A first voltage rail VR1 is provided by a zener follower circuit comprising serially connected zener diodes ZD1, ZD2 connected to resistor R9 across line VR and ground. An NPN transistor Q1 has its base connected between resistor R9 and zener diode ZD1 and its collector connected to line VR so that its emitter provides approximately a 10, 12 volt rail VR1 to power the coil KA of the output relay to be discussed below. A second voltage rail VR2 is provided by a precision voltage reference U1 such as TL4311, available from Texas Instruments Incorporated. Regulator U1 provides a stable, relatively temperature independent output. Reference U1 is serially connected to resistor R1 across line VR and ground. A capacitor C2 is coupled between the input and output of U1. A bias resistor R2 is connected between the junction of resistor R1 and U1 and the base of NPN transistor Q2 whose collector is connected to line VR and emitter is in turn connected to the base of NPN transistor Q3. A resistor R11 is connected to the base of transistor Q3 and to the output of U1 through resistor R10. The input of series regulator U1 is connected to ground through resistor R12. Resistor R11 is also coupled to the emitter of transistor Q3 providing voltage rail VR2 of 8 volts for control voltage. Capacitor C10 is connected between line VR2 and ground.

Over-temperature protection section 12 comprises temperature responsive sensors in the form of PTC thermistors S1, S2, S3 which are adapted to be embedded in respective windings of a motor to be protected. The sensors are connected externally of the protection system and form voltage dividers with resistors R23, R24, R25, respectively, between voltage rail VR2 and common. The junction of each voltage divider is connected through a respective diode D6, D7 and D9, and through an attenuating resistor R38 to the negative input of a comparator U3A, connected to provide a selected hysteresis. A reference voltage is connected to the non-inverting input through voltage dividers comprising resistors R44, R26 along with diode D3 connected between voltage rail VR2 and ground. Feedback resistor R34 is connected between the output of the comparator, the non-inverting input and the cathode of diode D3. Filter capacitors C3 and C4 are respectively connected between the inputs of the comparator and ground.

The output section 14 comprises an NPN transistor Q5 whose base is connected, through resistor R37, to the output of comparator U3A. The collector of transistor Q5 is connected to coil KA of a 12 volt relay with diode D4 coupled across the coil. Power is supplied from voltage rail VR1 with the emitter of transistor Q5 connected to ground. Thus, when a winding of the motor gets too hot the respective PTC sensor will abruptly increase in resistance from a nominal low level to 10 K ohms or higher which will drive the output of comparator U3A to ground thereby turning off transistor Q5 and in turn preventing the flow of current in coil KA. This opens the contacts of the relay (not shown), in turn de-energizing the contactor (not shown) and the motor being protected.

A time delay section 16 is provided to allow the motor to cool off. Time delay section 16 comprises a comparator U3B whose inverting input is connected through resistor R35 to the output of over-temperature comparator U3A. The non-inverting input of time delay comparator U3B is connected to an RC circuit comprising capacitor C5 and resistor R47 through resistor R36. The junction of the RC circuit and resistor R36 is connected to the cathode of diode D5 which in turn is connected through resistor R28 to voltage rail VR2. The output of time delay comparator U3B is connected to the junction of serially connected resistors R29 and R48 which in turn are connected between voltage rail VR2 and the inverting input of over-temperature comparator U3A. The output of comparator U3B is driven high whenever the output of over-temperature comparator U3A is low thereby maintaining the relay off until the voltage on the time delay capacitor C5 decreases below a selected level at which point the time delay comparator U3B is output is driven low. After the time delay, the over-temperature protection comparator U3A output will be driven high and the relay contacts will close only if fault conditions are no longer present, as will be discussed in further detail below.

Low voltage cut-out section 18 comprises first and second comparators U2C, U2D. Comparator U2C detects low voltage on the voltage ripple line VR for 120 volt applications at approximately 70% below nominal, i.e., approximately the equivalent of 84 volts. Reference voltage is provided by a voltage divider comprising resistors R17, R3 connected between voltage rail VR2 and ground, inputted to the inverting input of comparator U2C. The non-inverting input is connected through resistor R13 to the junction of voltage divider resistors R18, R4 connected between voltage ripple line VR and ground. The output of comparator U2C is tied to its non-inverting input through feedback resistor R15 to provide hysteresis and to voltage rail VR2 through resistor R5. The output of comparator U2C is connected to the cathode of diode Di which in turn connected to the output of over-temperature protection comparator U3A.

Similarly, comparator U2D detects low voltage on the voltage ripple line VR for 240 volt applications at approximately 70% below nominal, i.e., the equivalent of approximately 168 volts. Reference voltage is provided by a voltage divider comprising resistors R19, R6 connected between voltage rail VR2 and ground and whose junction is connected to the inverting input of comparator U2D. The non-inverting input is connected through resistor R14 to the junction of voltage divider resistors R20, R7 connected between ripple voltage line VR and ground. Hysteresis is provided by feedback resistor R16 connected between the output of comparator U2d and the non-inverting input. The output of comparator U2D is tied to voltage rail VR2 through resistor R8 and to the output of over-temperature protection comparator U3A through diode D2 with the cathode of diode D2 connected to the output of comparator U2D. A filter capacitor C9 is connected between the inverting input of comparator U2D and ground and filter capacitor C8 is connected between the junction of resistors R20, R7 and ground.

The output of comparators U2C, U2D are driven to ground when low voltage is detected which in turn grounds the output of over-temperature protection comparator U3A thereby turning off the relay.

Working in conjunction with low voltage cut-out section 18 is line voltage cut-out selector section 20 which provides a dead-band between selected voltages of approximately 150 and 170 VAC input. Section 20 comprises a comparator switch U2A. The detection voltage is provided by the junction of voltage divider resistors R40, R30 connected between ripple voltage line VR and ground, the junction being tied to the non-inverting input. A reference voltage, connected to the inverting input, is provided at the junction of voltage divider resistors R41, R31 connected between voltage rail VR2 and ground. The comparator serves as a switch with no hysteresis and has its output connected to the junction of voltage divider resistors R19, R6, which provide the reference voltage for comparator U2D. The output of comparator U2A is maintained low as long as voltages equivalent to under approximately 155 VAC are detected. A low output state results in the junction of resistors R19, R6 being connected to ground causing the reference voltage for comparator U2D to be 0 volts, and maintaining the output of U2D high. When voltages are detected above approximately 155 VAC, comparator U2A switches its output to high, thereby increasing the reference voltage to its intended value and allowing comparator U2D to perform its function, which is to detect voltages less than a selected level, e.g., 168 VAC, and to cause the relay contacts to open. Thus the two comparator stages, U2A and U2D, work in conjunction such that between approximately 155 VAC and 168 VAC the contacts of the output relay will never close. This prevents the possibility of chatter in 240 volt motors as well as excessive voltage being applied to 120 volt motors.

Adjustment circuit for relay drop-out section 22 comprises comparator U2B having a reference voltage connected to the inverting input provided by the junction of voltage divider resistors R42, R32 connected between voltage rail VR2 and ground. The non-inverting input is connected to the output of over-temperature protection comparator U3A. Comparator U2B is connected as a switch with no hysteresis and has its output connected to voltage rail VR1 through serially connected resistors R33 and R21. The junction of these resistors is connected to the base of a PNP transistor Q4 whose emitter is connected to voltage rail VR1 and whose collector is connected to ground through a resistor R43, sized to be of equivalent resistance to that of coil KA. Thus, whenever coil KA is de-energized the output of over-temperature protection comparator U3A is driven low which in turn lowers the voltage on the non-inverting input of comparator switch U2B causing its output to go high and turning on transistor Q4 thereby allowing current to flow through resistor R43 to provide an essentially constant load for the power supply. This avoids drastic change in the secondary voltage of the transformer.

Terminals X and Y are provided for testing purposes. When terminal X is connected to ground (C terminal) the time delay section 16 is nullified, a resistor is placed between the Y ad C terminals thereby shortening the time delay.

The following components were used in making a protection system in accordance with FIG. 1:

         S1      PTC Thermistor                 R1 3.92K
         S2      PTC Thermistor                 R2 33.2K
         S3      PTC Thermistor                 R3 60.4K
         K       Siemens T7N5501-12             R4 40.2K
         BR1     DF04M                          R5 20K
         C1      220 uF-63V                     R6 34K
         C2      .01 uF                         R7 80.6K
         C3      .01 uF                         R8 20K
         C4      0.22 uF                        R9 2.87K
         C5      100 uF                         R10 22K
         C9      1.0 uF                         R11 4.87K
         C10     0.1 uF                         R12 10K
         ZD1     BZX84C5V1LTI Motorola 5.1V 5%  R14 22K
         ZD2     BZX84C5V1LTI Motorola 5.1V 5%  R15 412K
         D1      BAS16LT1                       R16 442K
         D2      BAS16LT1                       R17 178K
         D3      BAS16LT1                       R18 40.2K
         D4      BAS16LT1                       R19 130K
         D5      BAS16LT1                       R20 20.5K
         D6      1N4007                         R21 6.8K
         D7      1N4007                         R22 5.11K
         D8      1N4007                         R23 8.06K
         Q1      BCP56T1 Motorola               R24 8.06K
         Q2      MPSA06                         R25 8.06K
         Q3      MPSA06                         R26 130K
         Q4      2N4403                         R28 15K
         Q5      MPS406                         R29 200K
         U1      TL431l Texas Instruments       R30 287K
         U2A     LM2901NA                       R31 200K
         U2B     LM2901NA                       R32 124K
         U2C     LM2901NA                       R33 27.4K
         U2D     LM2901NA                       R34 53.6K
         U3A     LM2903MA                       R35 1.82K
         U3B     LM2903MA                       R36 200K
                                                R37 12.1K
                                                R38 200K
                                                R40 42.2K
                                                R41 162K
                                                R42 42.2K
                                                R43 402K
                                                R44 42.2K
                                                R45 19.6K
                                                R48 412K