Method and circuit for detecting motor winding over temperature

A hardware-based detection system includes, among other things, a signal-generating circuit for generating a signal which is functionally related to current in a motor winding, a reference current, and a duration of time. The system may also include a comparator circuit for comparing the generated signal to a reference signal, and for thereby detecting an over-temperature condition in the motor winding. If desired, a compensating circuit may be used to generate a variable reference signal as a function of ambient temperature. A method of operating a detection system is also disclosed. If desired, the detection system may be completely implemented in hardware using an uncomplicated analog circuit architecture.

This application claims priority to India Patent Application No. 201841032510, filed Aug. 30, 2018. The entire disclosure of India Patent Application No. 201841032510 is hereby fully incorporated herein by reference.

BACKGROUND

Electrical motor drives are used across many applications, including, but not limited to, home appliances, industrial machinery, telecom devices, medical devices, building automation devices, and automotive devices. When designing a motor drive for certain applications, it may be desirable to ensure that the device can be safely and reliably operated. Safety and reliability have different aspects, which include human/operator safety and the safe operation of the device itself. In particular, it may be important, or even necessary, to protect the operator, or the device or the device surroundings, from an over-heated surface or a fire. Thus, it may be desirable to monitor an electrical motor drive so that it does not develop an over-temperature condition within one or more of its windings or the stator or the rotor including the shaft or the motor body.

Indeed, one of the potential hazards associated with electrical motor drives is fire or a hot surface due to overheating of one or more motor windings. For some devices, it is desirable to continuously monitor the temperature of motor windings to ensure that there is no overheating or associated fire, especially in case of an over-load or other abnormal condition or fault. Many safety standards mention the maximum allowed temperature for different classes of motor windings. The designer may wish to make sure that the winding temperature will stay within such limits.

SUMMARY

The present disclosure overcomes the disadvantages of the prior art to a substantial extent. The present disclosure relates to a hardware-based detection system which includes, among other things: a signal-generating circuit for generating a signal which is functionally related to current in a motor winding, a reference current, and a duration of time; and a comparator circuit, coupled to the signal-generating circuit, for comparing the generated signal to a reference signal, and for thereby detecting an over-temperature condition in the motor winding.

The present disclosure also relates to a detection system which includes, among other things: a signal-generating circuit for generating a signal which is functionally related to current in a motor winding, a reference current, and a duration of time; a comparator circuit for comparing the generated signal to a variable reference signal, and for thereby detecting an over-temperature condition in the motor winding; and a compensating circuit for generating the variable reference signal as a function of the reference current and an ambient temperature.

The present disclosure also relates to a method of operating a hardware-based detection system. The method includes: causing a signal-generating circuit to generate a signal which is functionally related to current in a motor winding, a reference current, and a duration of time; and causing a comparator circuit to compare the signal to a reference signal, and thereby detect an over-temperature condition in the motor winding.

DETAILED DESCRIPTION

Referring now to the drawings, where like elements are designated by like reference numerals and other characters throughout, there is shown inFIG. 1an I2t curve110for a typical motor winding. The instantaneous energy dissipated in a motor winding is proportional to the square of the winding current IP, and the temperature in the winding depends on the time duration t of the energy dissipation. The motor may be designed to continuously operate at a nominal or rated current INwithout exceeding an allowed temperature. Any load current IPabove the nominal current INcauses excessive loss, which cannot be removed by the motor's cooling device(s). As a result, when the load current IPis greater than the nominal current IN, the motor temperature increases and eventually exceeds the rated temperature.

When the load current IPis only slightly greater than the nominal current IN, the time t required for the motor temperature to exceed the rated temperature may be relatively long, but the rated temperature eventually will be exceeded. When the load current IPis much greater than the nominal current IN, which is the situation illustrated inFIG. 1, the time t required for the motor temperature to exceed the rated temperature may be relatively short. When the load current IPis less than the nominal current IN, the motor temperature should not exceed the rated temperature regardless of the time duration t.

In general, the energy dissipated by a motor winding under nominal conditions ENOM, when the load current IPis equal to the nominal current IN, is as follows:
ENOM=IN,RMS2×R×t,
where IN,RMSis a root-mean-square (RMS) value of the nominal current IN, R is the resistance of the winding(s) carrying the current, and t is the time duration of the energy dissipation.

When the load current IPis greater than the nominal current IN(the situation which is shown inFIG. 1), then excess energy EPis generated by the motor winding as follows:
EP=(IP,RMS2−IN,RMS2)×R×t,
where IP,RMSis a root-mean-square value of the load current IP.

Since R may be considered constant (neglecting the variation of R with variation in temperature), the latter equation may be simplified as a proportional relationship as follows:
EPα(IP,RMS2−IN,RMS2)×t.

FIG. 2is a block diagram which illustrates a hardware-based detection system10coupled to a motor drive system12. The motor drive system12may be, for example, configured to operate a household appliance (not illustrated). The detection system10is configured to detect an over-temperature (OT) condition in the motor drive system12. In the illustrated configuration, the motor drive system12has a three-phase motor14coupled to a three-phase inverter16, and corresponding multiple motor windings electrically connected to lines15,17,19, and corresponding current sense elements18,20electrically connected in series with the winding connection lines15,17. The motor drive system12could be a multiphase system where a multiphase motor is driven by a multiphase inverter. The motor drive system12illustrated here senses the current through two motor windings out of three windings of the three phase motor14, using the sense elements18,20. An example of a current sense element could be a current sense resistor or a Hall effect sensor or any other suitable sense element but not limited to these.

The illustrated detection system10has in-line sensors22,24for sensing (monitoring) respective currents in the windings connected to the lines15,17using the sense elements18,20, and for outputting current signals on respective conductive lines26,28. The sensors22,24are electrically coupled to opposite ends of the sense elements18,20by suitable electrical connections30,32,34,36. In the illustrated configuration, the number of in-line sensors22,24(2) is one less than the number of phases of the motor14(3). This disclosure should not be limited, however, to the systems and devices shown in the drawings and described in this detailed description. In general, for an N phase motor, no more than N−1 current sensors may be required, especially where the drive system has suitable earth leakage insulation or protection. In general for an N-phase motor, N-current sensors may be required especially where the drive system has no earth leakage insulation or protection or each motor phase winding is independent without an electrical connection between the phase windings.

Further, as illustrated inFIG. 2, the detection system10has RMS calculator circuits38,40for receiving the current signals on lines26,28, and for outputting corresponding root-mean-square (RMS) signals on conductive lines42,44. The RMS calculator circuits38,40calculate the respective RMS values (IP,RMS) of the sensed winding currents. The RMS signals correspond to respective load currents IP,RMSsensed within the windings connected to lines15,17by the respective sensor elements18,20and sensors22,24.

Following the RMS calculator circuits38,40, first multiplier & subtractor circuits54,56are provided for generating first difference signals (the values of which correspond to IP,RMS2−IN,RMS2) based on the respective RMS values, and second multiplier & subtractor circuits58,60for generating second difference signals (the values of which correspond to IN,RMS2−IP,RMS2) based on the respective RMS values.

In the illustrated example, IN,RMSis the rated nominal motor current for the motor drive system12. That is, IN,RMSis the motor current at which the temperature will not reach the over temperature (OT) threshold until an infinite time. When the motor drive system12draws less than the nominal current IN,RMS, the motor drive system12will cool to a temperature less than the rated temperature corresponding to the rated current of IN,RMS. Hence, a cooling curve is generated by calculating the second difference values (IN,RMS2−IP,RMS2) and the same are subtracted from the first difference values (IP,RMS2−IN,RMS2) on a time-integrated basis, in suitable integrator circuits62,64. As discussed in more detail below, the multiplier & subtractor circuits54,56,58,60may include suitable operational amplifiers.

In the illustrated configuration, the integrator circuits62,64receive the difference signals and generate time-integrated signals. Comparator circuits66,68are provided for comparing the integrated signals to a reference signal on line70. The reference signal on line70may correspond to a predetermined fixed I2t Reference value, or, as discussed in more detail below, the reference signal on line70may correspond to a variable I2t Ref_comp value which changes as a function of ambient temperature. Thus, the detection system10has a first signal-generating circuit22,38,54,58,62for generating a first signal on line80. The first signal (on line80) is functionally related to current in a first motor winding (IP,RMS, associated with line15), a reference current (IN,RMS), and a duration of time (t). The detection system10also has a first comparator circuit66, coupled to the first signal-generating circuit22,38,54,58,62, for comparing the first signal to a reference signal I2t Ref_comp, and for thereby detecting an over-temperature condition in the first motor winding. Moreover, the detection system10has a second signal-generating circuit24,40,56,60,64for generating a second signal on line82. The second signal (on line82) is functionally related to current in a second motor winding (IP,RMS, associated with line17), a reference current (IN,RMS), and a duration of time (t). The detection system10also has a second comparator circuit68, coupled to the second signal-generating circuit24,40,56,60,64, for comparing the second signal to the reference signal I2t Ref_comp, and for thereby detecting an over-temperature condition in the second motor winding.

As illustrated inFIG. 2, by way of example, the difference signals may be output from the multiplier & subtractor circuits54,56,58,60and input to the respective integrator circuits62,64on conductive lines72,74,76,78, and the time-integrated signals may be output from the respective integrator circuits62,64and input to the respective comparator circuits66,68on other conductive lines80,82.

In operation, one of the comparator circuits66,68(or both) issues (or issue) an OT Fault signal on a respective conductive line84,86whenever the value of a respective integrated signal exceeds the value of the I2t Reference signal (or the variable I2t Ref_comp value). Outputs from the comparator circuits66,68are merged at node90, such that an OT condition warning signal is generated on a conductive line92whenever at least one of the comparator circuits66,68issues an OT Fault signal. If desired, the conductive line92is operatively connected to a suitable operator interface (not illustrated) to warn an operator that there is an OT condition. Alternatively, the conductive line92may be operatively connected to a suitable electrical or mechanical controller (not illustrated) for automatically taking a corrective action, for example, by disconnecting the motor drive system12from a power source (not illustrated).

As illustrated inFIG. 3, when the load current IP,RMSof one of the windings is continuously greater than the nominal current IN,RMSby an amount x, the integrated value [(IP,RMS2−IN,RMS2)×t] represented by the output signal from the respective integrator circuit62,64increases over time. As discussed in more detail below, when a motor winding connected to lines15,17has operated at the load current IP,RMSfor a period of time greater than tOT, the output signal generated by the respective integrator circuit62,64exceeds the I2t Reference signal by an amount y, which causes an OT condition warning signal to be generated on the merged conductive line92.

When the actual motor current is more than the nominal current, the respective motor winding(s) will heat. When the load current IP,RMSis greater than the nominal current IN,RMS, but only by a relatively small amount, then the amount of time tOTit takes for the detection system10to generate the OT condition warning signal is relatively long. When the load current IP,RMSis greater than the nominal current IN,RMS, and by a relatively large amount, then the amount of time tOTit takes for the detection system10to generate the OT condition warning signal is correspondingly short. When the load current IP,RMSis always less than the nominal current IN,RMS, in all of the windings connected to lines15,17(a condition which is not shown inFIG. 3), then the illustrated detection system10does not generate the OT condition warning signal.

According to one aspect of this disclosure, an appropriate value for the nominal current IN,RMSmay be obtained from information provided on the manufacturer's name plate (not illustrated), which may be attached to the motor drive system12. If the name plate indicates, for example, that the motor drive system12is configured to carry a 2-A winding current (specified up to 25° C. ambient), wherein the motor winding will heat to the rated temperature (for example 100° C.) at infinite time (steady state temperature) then IN,RMS=2 A. This would mean that the motor windings connected to lines15,17can each carry2A continuously at an ambient temperature of 25° C. However, the present disclosure should not be limited to the configurations and numerical values described herein, which are meant to characterize non-limiting examples of the present disclosure.

FIG. 4illustrates a circuit level implementation of a portion of the detection system10, where the voltage equivalent to the current sensed in the first winding, connected to line15, is available at IP, and is supplied to a non-inverting input100of a first operational amplifier102, through a suitable resistor104. The signal IPis an equivalent signal at the output of the first inline sensor22at the signal output line26ofFIG. 2. The motor windings typically carry alternating (bipolar) current. While sensing alternating current, the output of the sensor22is normally bipolar (having both positive and negative polarity). To make the sensor circuit and further circuits simpler, typically a unipolar output is used wherein the operational amplifiers can be operated with unipolar power supply VCC referred signal ground. To achieve this, the outputs of the sensors22,24are level shifted by a voltage VCC/2 (not illustrated here), and the level shifted signals are available at lines26,28ofFIG. 2. The signal IP(FIG. 4) is the level shifted (by VCC/2) voltage signal available at line26(or line28) ofFIG. 2, equivalent to the current flowing in the motor windings connected to lines15,17ofFIG. 2. When the winding current is zero, the corresponding IPsignal voltage is VCC/2, when the winding current is positive (current flows into the motor winding), the corresponding IPsignal voltage is more than VCC/2, and when the winding current is negative (current flows out of the motor winding), the corresponding IPsignal voltage is less than VCC/2. The operational amplifier102is an element of the first RMS calculator38(FIG. 2). The output of the first operational amplifier102is available on conductive line101(FIG. 4) transmitted through a suitable diode106, supplied to the inverting input108of the operational amplifier102through a suitable resistor111, coupled to ground by the resistor111and a suitable capacitor112, supplied to the non-inverting input114of a second operational amplifier116through resistor111and another suitable resistor118, and coupled to ground by resistors111,118and another suitable resistor120.

Thus, the circuit illustrated inFIG. 4includes a peak detector which includes, but is not limited to, the first operational amplifier102, the diode106, and the associated resistor111and capacitor112. In operation, the peak detector tracks the peak of the output of the first current sensor22(FIG. 2). Meanwhile the second operational amplifier116calculates motor winding heating by approximately calculating the first difference value (IP,RMS2−IN,RMS2). The approximation may be accomplished using, for example, an opamp gain of 1.2V/V over a suitable range (which may be from IPto IN). However, as noted above, the present disclosure should not be limited to the configurations and numerical values described herein. The output of the heating amplifier circuit, on conductive line128, is connected to the non-inverting input130of an integrator/third operational amplifier140.

INOM, the level shifted (by VCC/2) voltage equivalent to the nominal current IN,RMS, is applied, on a conductive line122, to the inverting input124of the second operational amplifier116, through a suitable resistor126. The output of the second operational amplifier116, on conductive line128, is applied to the non-inverting input130of the third operational amplifier140through a suitable resistor142, and to the inverting input124of the second operational amplifier116through another suitable resistor144. In operation, the first RMS calculator circuit38(FIG. 2) and the difference value (IP,RMS2−IN,RMS2) calculator54ofFIG. 2include the first operational amplifier102(FIG. 4) (operating as a peak detector) and the second operational amplifier116(operating as a gain stage). The non-inverting input130of the third operational amplifier140is electrically coupled to ground through a suitable capacitor145.

The current signal that is transmitted through the diode106is also supplied to the inverting input146of a fourth operational amplifier148through resistor111and another suitable resistor150. INOM, the level shifted (by VCC/2) voltage equivalent to the nominal current IN,RMS, is applied, on conductive line122, to the non-inverting input152of the fourth operational amplifier148, through a suitable resistor154, and is electrically coupled to ground through resistor154and another suitable resistor156. The output of the fourth operational amplifier148, on conductive line158, is applied to the inverting input160of the third operational amplifier140through suitable resistors162,164, and to the inverting input146of the fourth operational amplifier148through a suitable resistor166.

In operation, when motor current is less than the nominal current, the respective motor winding will cool to a temperature less than the rated temperature corresponding to the rated current of IN,RMS. The fourth operational amplifier148calculates the cooling by approximately calculating the second difference value (IN,RMS2−IP,RMS2). The approximation may be achieved by using, for example, an opamp gain of 0.48V/V over a suitable range (which may be from INto IP). The output, on line158, of the cooling amplifier circuit is connected to the inverting input160of integrator/third operational amplifier140.

If desired, the time constant of the integrator RC circuit may be tuned based on the value of the nominal I2t value. For that purpose, capacitor145and resistor142, and capacitor170and resistor164form the RC time constants. The capacitance C of capacitors145,170may be equal to each other, and the resistance R of resistors142,164may be equal to each other, in which case the integrator gain equals 1/RC. Meanwhile, resistor172receives an FB signal on line314to imitate the motor pre-heating when the motor current is less than the rated current IN,RMSfor a long time before the abnormal operation happens, wherein the abnormal motor current is more than the nominal current IN,RMS.

The output of the third operational amplifier140is coupled to the base of an NPN bipolar junction transistor (BJT)168. The collector of the BJT transistor168is coupled to the inverting input160of the third operational amplifier140through capacitor170. The capacitor170is connected in parallel to a resistor network172,164. The emitter of the transistor168is electrically coupled to ground through a suitable resistor174.

The collector of the BJT transistor168is also coupled to the inverting input190of a fifth operational amplifier (that is, the first comparator)66through a suitable MOSFET194and conductive line80. The MOSFET194is coupled to a power supply VCC by a suitable resistor196and a suitable capacitor198. The non-inverting input200of the fifth operational amplifier/first comparator66receives the variable reference signal I2t Ref_comp (or a fixed reference signal I2t Reference) on conductive line70. When the signal applied to the inverting input190is greater than the reference signal, the comparator circuit66generates the OT Fault signal on line84. The OT Fault signal is transmitted through a suitable resistor202and conductive line84, which is coupled to ground through a suitable capacitor204. The BJT168(at the output of the third operational amplifier140) and the MOSFET194(associated with the fifth operational amplifier66) may be used, if desired, to ensure that the integrator capacitor170does not discharge in the event of a power supply failure, but maintains charge in the detection system10, to thereby ensure a capacitive holding-charge effect that is similar to the motor holding-heat until it dissipates. The values of the RC circuit made up of elements170,172,164are selected such that the RC time constant of this circuit is equivalent to the cooling time constant of the motor.

In operation, the gain of the second and third operational amplifiers116,140is adjusted together to obtain a linear approximation of a squaring circuit in the vicinity of the winding current IP. Thus, the first, second, and third operational amplifiers102,116,140operate in combination to implement the desired RMS and squaring functions, for the first difference signals. The second operational amplifier116is an element of a first difference calculating circuit54(FIG. 2), and the fourth operational amplifier148(FIG. 4) is an element of the corresponding second difference calculating circuit58.

The foregoing description has assumed that the motor drive system12starts with current=IP, and the initial temperature when the motor drive system starts is equal to the ambient temperature. However, if the motor drive system12was running at slightly less than rated current for a long time and then suddenly an abnormal condition happens to increase the motor current more than the nominal current, the initial temperature from which overheating starts is not ambient temperature, but close to the OT threshold and hence the time to reach the OT threshold is less. On the other hand, if the motor drive system12was running a current very much lower than the rated current for a long time, then the initial temperature is close to ambient temperature and the time required to reach the OT threshold is more.

To address the above-mentioned circumstances, the circuit illustrated inFIG. 4may also have sixth and seventh operational amplifiers300,302coupled to suitable resistors304,306,308,310,312for setting an initial condition in the integrator circuit140when the load current IPis less than the nominal current IN. In particular, signal FB on line314(electrically connected to resistors172,162,164) is generated to set an initial condition in the integrator circuit140when the load current IPis lower than the nominal current IN. In operation, a VIPEAK signal is applied, on line316, through resistor304, to the non-inverting input of the sixth operational amplifier300. The non-inverting input of the sixth operational amplifier300is coupled to ground through resistor310. Meanwhile, a VCC/2 signal is applied through resistor306to the inverting input of the sixth operational amplifier300, which has feedback resistor308.

The output of the sixth operational amplifier300is applied to the non-inverting input of the seventh operational amplifier302, and a VINT signal is applied, on line80, to the inverting input of the seventh operational amplifier302. The VINT signal is the same as the signal applied to the inverting input of the operational amplifier66. The output of the seventh operational amplifier302is applied to resistor312and a MOSFET316to generate the FB signal on conductive line314. The operational amplifier300is a difference amplifier where VIPEAK minus VCC/2 signal is generated. The VCC/2 voltage signal corresponds to zero current and hence the output of the operational amplifier300can be considered as a voltage equivalent to the peak value of winding current IPor IP,RMSwith a suitable gain. The operational amplifier302is configured as a comparator. When the noninverting input of the operational amplifier302is greater than VINT signal at the inverting input of the operational amplifier302, the output of the operational amplifier302becomes high (equal to VCC) and the MOSFET switch316turns on, to make the FB voltage approximately equal to zero volts. When IP,RMSis less than IN,RMS, the output of the second amplifier116at line128is zero volts, and the VINT signal at line80remains constant when the FB signal is pulled to ground by turning on the MOSFET316.

If desired, at least the second, third, fourth, and fifth operational amplifiers116,140,148,66may be formed on or within a single chip. If desired, at least the second, third, fourth, fifth, sixth, and seventh operational amplifiers116,140,148,66,300,302may be formed on or within a single chip.

The system illustrated inFIG. 4has multiple stages for implementing an RMS calculator, squaring circuits, integrator, and comparator, and for responding to an OT condition in the first winding connected to line15(FIG. 2) by generating an OT Fault signal on line84. Another circuit (not shown, but essentially the same as the system illustrated inFIG. 4) may be coupled to the second winding connected to line17(FIG. 2) and the output node90, for responding to an OT condition in the second winding connected to line17by generating an OT Fault signal on line86.

FIG. 5is a block diagram of a compensating circuit69for generating variable (compensated) reference signal I2t Ref_comp on conductive line70. The value of the compensated reference signal changes as a function of the ambient temperature, whereas the value of a fixed I2t Reference signal may be a constant for the motor drive system12in a given working environment. The compensating circuit69illustrated inFIG. 5generates the variable reference signal as a function of ambient temperature

As illustrated inFIG. 5, one or more analog temperature sensors402may be provided for generating a temperature signal on line404representative of the ambient temperature, and a scaling circuit406for generating a scaling signal on line408corresponding to the temperature signal. A compensating circuit410applies the scaling signal to the fixed I2t Reference signal, to thereby generate the compensated reference signal on line70. In the illustrated configuration, the compensating circuit410adds or subtracts the scaling signal to or from the I2t Reference signal.

In general, when the ambient temperature sensed by the one or more ambient temperature sensors402increases, the value of the compensated reference signal on line70decreases, and vice versa. As a result, when the ambient temperature increases, the load current-based signal on line80(FIG. 2) which causes the OT warning condition to be generated decreases, and vice versa. The term “ambient temperature” means temperature in the vicinity of the motor drive system12, at a location which is not affected by heat generated by the motor windings electrically connected to lines15,17. The placement of the sensor(s)402may be performed, if desired, without any complex assembly requirements. A relatively uncomplicated, low-cost sensor can be used for such purpose.

The devices and methods described herein, which involve a hardware implementation for detecting a motor-winding over-temperature condition, have many advantages. The circuit architecture may be based on well understood and readily available amplifiers and comparators, and therefore may be easy to implement. They involve indirect determination of winding over-temperature, and therefore do not require temperature sensors configured for direct measurement of the temperature of the windings.

The detection system10may have an uncomplicated analog chip architecture, and does not require a separate printed circuit board (PCB). The system10may advantageously be implemented on the same PCB on which the motor drive system12is supported without any complex assembly requirements, and may consume less space because the sensors22,24may be constructed in an in-line configuration. As a result of these features, the illustrated systems may be produced at reduced cost compared to known systems and still have the desired accuracy. Also, since the solution architecture may employ relatively common operational amplifiers, improved integration may be possible.

Another advantage of the systems10described herein is that they may not be adversely affected by ageing. In contrast, known devices with direct temperature sensors are prone to ageing problems like dislocation of sensors, and are subject to improper or impractical maintenance.

What have been described above are examples. This disclosure is intended to embrace alterations, modifications, and variations to the subject matter described herein that fall within the scope of this application, including the appended claims. As used herein, the term “includes” means including but not limited to. The term “based on” means based at least in part on. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements.