GB2081995A - Thermal simulators for protecting electric motors - Google Patents

Abstract

Overload protection for an electric motor is provided by two RC temperature-simulating networks each charged from a voltage representing the square of the motor current and each comprising a resistor 1301, 1700, a switch 16, 17 controlled by recurrent pulses, and a capacitor 24, 25. The voltage across each capacitor 24, 25 is compared (29, 30) with a respective reference voltage in order to detect overload. The time constant of the second network 1700, 17, 25 represents the conventional cooling of the motor, indicated by the manufacturer. The time constant of the first network corresponds to the cold starting time for a current equal to six times the nominal current. The second reference voltage corresponds to a overload of a few percent and the first reference voltage is greater than this. A cross-coupling network 27, 2701, 2702 prevents the voltage across the first capacitor 24 falling below that across the second (25). The pulses supplied to switch 17 are varied depending on whether the motor is being energised or not. <IMAGE>

Description

SPECIFICATION Circuit of thermoprotection of an electric motor including two time constant organs The invention relates to solid state thermal protection circuits for an electric motor.

For ensuring the thermal protection of an electric motor, it has already been proposed to simulate, using a time constant circuit (of the RC type) the thermal behaviour of the copper parts of the motor, and to cut off the power supply to the motor once the temperature simulated by a circuit of this kind reaches a predetermined threshold. It has also been proposed to introduce additional components into the protection circuit, in order to take account of the losses in the iron parts of the motor (losses due to eddy currents and hysteresis) and of the heat transfer between the copper and the iron.

A disadvantage of this type of protection circuit is its complexity and the difficulty of building it in such a way that it provides a realistic simulation of the thermal behaviour of the motor, having regard in particular to the fact that there is also heat transfer from the iron to the air and that the losses in the iron depend on the voltage. Moreover, it is difficult to set the temperature limits that should not be exceeded, while at the same time permitting hot starts, and it would be desirable for the protection system to permit a relatively large but temporary overload, so long as this is followed by a fall in load below nominal.

The invention proposes to resolve the different problems by means of a protection circuit which, without attempting to provide a perfect simulation of the thermal behaviour of the motor, will take account of the manufacturer's permitted tolerances with regard to intermittent regimes.

The invention comprises means of applying, to at least one time constant circuit, a voltage representing the current taken by the motor (this voltage being preferably proportional to the square of the set current) means of comparing the voltage at the terminals of the capacitor which the said capacitor comprises with a steady reference voltage and of providing a signal to cut off the motor power supply whenever the said voltage at the terminals of the capacitor exceeds the said reference voltage, characterized by a first and a second time constant circuit to which the said representative voltage is fed and connected respectively to a first and a second comparator to which are fed respectively a first and a second separate reference voltage, the time constant of the second circuit being selected to be equal to the thermal time constant of the motor indicated by the manufacturer, and the second reference voltage being selected so as to c )rrespond to an overload of a few percent over the nominal motor current, while the time constant of the first circuit is selected so as to correspond to the told starting time of the motor for a currant equal to six times the said nominal current, and that the first reference voltage has a value double that of the second, and by means of directly regulating the voltage at the terminals of the capacitor of the first time constant circuit by the voltage at the terminals of the capacitor of the second circuit, when the latter tends to exceed the former.

According to a preferred method of construction, the invention includes a third time constant circuit which feeds the second comparator and of which the time constant is selected to be five times that of the second circuit, and means are provided to generate a signal indicating that a motor is running or stopped, and to switch the second and third time constant circuits as a function of the said signal.

Other special features, as well as the advantages of the invention, will become clear in the light of the following description.

On the attached drawing: Figure 1 is the circuit diagram of a thermal protection arrangement conforming to a preferred method of making the invention and, Figure 2 shows curves representing the permissible overload times of a motor according to the current it absorbs.

On Fig. 1, three amplifiers 1-2-3 have been shown connected as integrating circuits (resistors 100-101, 200-201, 300-301 and capacitors 102-202-302) which will with advantage be supplied by Rogowski probes (not shown) providing low current signals proportional to the derivative of the current which follows in the respective phase capacitors connecting an asynchronous motor to the three phase power supply. The output from each amplifier is connected to its input not receiving the signal through a resistor (103-203-303 respectively) and through a capacitor (104-204-304 respectively) connected to ground means of a resistor (105-205-305 respectively), to an input of a second amplifier (4-5-6 respectively).

The other input of the second amplifier is connected to ground by a resistor (400-500600 respectively) and to its output by a diode (401-501-601 respectively). The outputs of the second amplifiers are connected together to a third amplifier 7, through the respective diodes 402-502-602, the anodes of which are connected to the other said inputs of the second amplifier by the respective resistors 403-503-603.

The amplifiers 1-2-3 provide, at their output alternating voltages proportional to the abovementioned phase currents, which are rectified by the diodes 401-402.

The amplifier 7 enables the highest of these three voltages to be set to a predetermined value when the motor is working at its nominal current IN For this purpose, its second input is connected to the anode of the diode 700 mounted at its output by an adjustable resistor 701 in series with a resistor 702. A resistor 703 connects the said second input to ground and a filter circuit comprises a resistor 704 connected to the anode of the diode 700, in series with a capacitor 705 connected to ground.

The anodes of diodes 402-502-602 are in addition connected together to an input of a comparator 8, of which the other input is connected, through a resistance bridge 901902, to the output of comparator 9. The latter receives on the one hand the voltage at the point A between the resistor 704 and the capacitor 705, and on the other hand, a steady reference voltage provided by a source V + and to the resistance bridge 903-904.

This reference voltage is for example the one which corresponds to the voltage at A for a current flowing in the motor equal to 10% of the nominal IN: in other words, so long as the motor is running, the comparator is bound to supply it with a digital level 1 (positive voltage) at its output, and cannot supply a level 0 (negative voltage) at its output except when the motor is stopped. As a result, only while the motor is running will the comparator 8 supply a level 0 (negative voltage) at its output whenever the output voltage from the amplifiers 4-5-6 passes through zero, in other words in the absence of a phase.

In this case, a capacitor 802, charged by a positive voltage (V +) through a resistance bridge 800-801, discharges whenever the output voltage from the amplifiers 4-5-6 passes through zero and, following a certain number of these zero transits, the voltage at its terminal is sufficiently low to trigger a comparator 10, one input terminal of which receives the voltage at the terminals of capacitor 802, while the other input terminal is fed with a reference voltage determined by a source (V +) and a resistance bridge 10001001.

The output signal from the comparator 10 is fed: first, through a diode 1002, to one of the components 1100 of a trigger circuit 1100-1101 which supplies, at its output 1102, an excitation circuit to the relay (not shown) which provides a control signal to the conventional motor protection contactor; secondly, to one of the components 1200 of a trigger circuit 1200-1201 which triggers an LED, no shown, connected to the output 1202.

It will be noted that in comparison with the loss of phase detection circuit described in French patent No. 79.26102 lodged on 16 October 1979 by the applicant for: "Electronic arrangement for protecting an electric motor against overloads and the loss of one phase of the three phase alternating power supply", the present arrangement for detecting phase loss is simplified, in that the means of filtering the three phase voltages which constitutes the image of the current in the phase conductors of the motor do not comprise an inductance at the input. This simplification is made possible by the fact that the Rogowski probes give a sufficiently low voltage for amplifiers to be used without saturating.It is then sufficient for these to have a very low output impedance for their output signal to pass through zero in the event of loss of phase and, as a result, to be usable directly by the phase loss detection circuit.

Returning to Fig. 1, the signal at the point A is fed through the respective resistors 1300-400-1500, to an input of each of the three amplifiers 13-14-15, and of which the other input is connected to ground of which the outputs are connected to the said input by reversed diodes (1301-1401-1501 respectively) and by resistors (1302-1402-1502 respectively). The anodes of each of these diodes are connected to the said input through a diode (1303-1403-1503 respectively). The said input also receives, for each of the amplifiers 14 and 1 5, a negative voltage (V -) through a resistor (1404 and 1 504 respectively). Such an arrangement, well known in itself, provides at its output V a voltage proportional to the square of the current which corresponds to the highest of the three values of the phase currents.

The point B is connected through resistors 1600 and 1700 respectively, to two switches 16 and 17 controlled by recurrent pulses provided respectively by two monostable trigger circuits (1900-1901) and (2000-2001) respectively. These two trigger circuits are excited by a master oscillator consisting of two Nj circuits (2100-2101). A third monostable trigger circuit (2200-2201) is also able to control the chopper 17, which receives pulses from the intermediate trigger circuit (2000-2001) or the trigger circuit (22002201) according to the state of a logic circuit made up of three Nj circuits 2300-23012302. The circuit 2301 is controlled by the signal from the "stopped" detector 9, in such a way that the trigger circuit (2200-2201) finally operates the switch 17 when the motor is running, while it is the trigger circuit (2200-2201) which operates the switch 17 when the motor is stopped.

The switches 16 and 1 7 charge respectively two capacitors 24-25 the potential of which is fed, through respective resistors (2600-2700 respectively) amplifier 26 and two respective amplifiers 27-28. The amplifiers 26 to 28 are made up of field effect transistors and have therefore a very high input impedance.

The output of the amplifier 26 is connected to an input of an amplifier 29 the other of which is fed with positive voltage V + by means of a resistance bridge 2900-2901. The output of the amplifier 29 is connected through a diode 2902 to the component 1100 of the trigger circuit 1100-1101. Similarly, the output of amplifier 28 is connected to an input of an amplifier 30 the other input of which is provided with a voltage V + through a resistance bridge 3000-3001 and the output of which is connected through a diode 3002 to the component 1100.

The output of the amplifier 27 is connected at the common point to the resistor 600 and the capacitor 24 by a diode 2701 of the type having a leakage current of the order of a pA, followed by a resistor 2702. Since a diode of this kind has virtually no reverse current, there is no possibility of capacitor 24 discharging by this route.

The amplifier 27 at all times prevents the voltage at the terminals of the capacitor 24 from being less than that at the terminals of the capacitor 25: if this phenomenon tended to occur, the discharge current of the capacitor 25, transmitted through the amplifier 27, the diode 2701 and the resistor 2702, would tend to charge the capacitor 24 until the voltages at the terminals of the two capacitors was equal.

The output of amplifier 28 is connected to an input of an amplifier 31, the other input of which is supplied with a voltage (V -) through a resistance bridge 3100-3101. The output of the amplifier 31 is connected, first to the said other input by a resistor 3102 followed by a diode 3103, and secondly to the control electrode of a switch 32 connected between ground and the common point of the switch 1 7 and the capacitor 25.

The output voltages from the amplifiers 29 and 30 are fed, through the respective diodes 2903 and 3003, to the component 3300 of a trigger circuit 3300-3301 which triggers an LED (not shown) connected to its terminal 3302.

If the motor current tends toward zero, the capacitors 24 and 25 tend to discharge to ground owing to the presence of spurious capacity in the circuit. These capacitors, having reached a zero potential, would even tend to charge again to a negative potential. If this h. ppened, the negative voltage at the terminals of capacitor 25, fed to an input of amplifier 31 through the resistor 2700 and the amplifier 28, would then be compared with the negative voltage fed to the other input of amplifier 31, arranged as a comparator with the result that an opening signal would be transmitted to the switch 32, the effect of which would be to cause the rapid discharge of capacitor 25 down to a zero potential.As explained above, the voltage on the terminals of capacitor 24 would follow that at the terminals of capacitor 25, and would therefore also be rapidly brought to zero. The importance of this will be explained below.

l,le circuit consisting of a switch charging a capacitor, during the periodic intervals when it is closed, to a voltage representing the image of a current which produces excess heating in a load, is well known in itself, and it is known that it enables the heating or cooling of the load to be simulated, without any temperature measurement being made, and is particularly useful when the thermal time constant of the load is high, which is the case of an electric motor.

The time constant defined by a circuit of this kind obviously depends on the values of the components and on the duration of the control pulses from the switches.

In the arrangement described, an initial circuit of this type (trigger circuit 2000-2001, switch 17, capacitor 25) is devised to give a time constant 0L representative of the thermal behaviour of the motor when it is operating at average temperature: this time constant is the time constant for conventional cooling of the motor given by the manufacturer (as example B,= 950 seconds).The first circuit ensures thermal protection of the motor operating between small overloads, in the following way: The steady reference voltage applied by the resistance bridge 3000-3001 to an input of the amplifier 30 is chosen such that it corresponds to an overload of a few percent (for example 7 to 8%), i.e. the amplifier 30, arranged as a comparator, will supply the trigger circuit 1100-11 01 with a signal to cut off the motor power supply once the current I is greater by 7 or 8% than its nominal value IN. Such a value was chosen by drawing the curve as a solid line on Fig. 2, and which represents the permissible overload time ts, as given by the manufacturer (expressed in seconds) as a function of the ratio I/IN when the motor is hot.If this curve is assumed to be asymptotic to a straight line parallel to the vertical axis passing through the abscissa 1.07 or 1.08, it is observed, by calculating the actuation times given by the system, for different values of 1/1N (simulation), that in practice they coincide with the permissible times given by the manufacturer.

A second circuit generating a time constant consists of the monostable trigger circuit (2200-2201), the switch 1 7 and the capacitor 25. The circuit is devised for this time constant 19LA to be equal to 58,. When the motor is stopped, it obviously cools down much less quickly than when it is running (the fan being stopped). If the motor is restarted, it is then important for the capacitor 25 to be initially charged to a potential which constitutes an image of the temperature it had when "stopped", a temperature which obviously depends on the time during which the motor remained stopped.

A third circuit generating a time constant consists of the monostable trigger circuit (1900-1901), the switch 16 and the capacitor 24. The time constant Ac defined by this circuit is representative of the starting time acceptable by the motor. As an example, it is equal to 1 75 seconds for a cold starting time of 10 seconds for a current of 6 IN. It can be assumed that this value of the starting time will be suitable for determining dc by simulation because, for an asynchronous motor, the starting current is of the order of 6 IN and it is a matter of protecting a motor against too long or too frequent starts, and, more generally, against heavy overloads.Nevertheless the user of the circuit is left the choice, if the starting time is given for a motor starting current other than 6 1N' to correct the value of so so that it corresponds to the starting time at 6 1N- O is adjusted for example by changing the value of the resistor 1600.

The two parts of the curve on Fig. 2 shown dotted represent the permissible overload times when the motor is cold, as a function of l/IN. A number of points on these curves are based upon data supplied by the manufacturer. In order to join up these points by an approximate exponential simulation corresponding to the charging of a capacitor in a time constant circuit, it is observed that two values of the time constant have to be adopted, i.e. a "short" value At which corresponds to high overloads and a "long" value which corresponds to small overloads.The asymptote of the part of the curve corresponding to Ac definies a reference voltage (which will be the steady voltage given by the resistance bridge 2900 and 2901 at the input of the amplifier 29) which is twice the reference voltage applied to the input of amplifier 30, which indicates the assumption that, the motor being hot, if it is stopped and then restarted, the temperature of the copper may then reach a value double of that normally for the motor, without the shell of the motor exceeding the permitted temperature. Heat transfer from the copper to the iron takes place rapidly and the overheating of the copper is therefore of short duration and can be borne by the motor.

As stated by above, the circuit is devised so that the voltage at the terminals of capacitor 24 can never be less than the voltage of the terminals of capacitor 25. This condition reflects the fact that, in a motor, the copper can never be cooler than the iron. As has been explained, the voltage on the terminals of capacitors 24 and 25 can never become negative. This means that the circuit must never simulate copper or iron temperatures below the temperature of the enviroment.

It is self evident that various modifications may be made to the circuit described and shown, without departing from the spirit of the invention.

Claims (8)

1. System for thermoprotection of an electric motor comprising means of applying, to at least one time constant circuit, a voltage that is an image of the current absorbed by the motor (this voltage being preferably proportional to the square of the said current), means of comparing the voltage of the terminals of the capacitor which the said capacitor comprises to a steady reference voltage and of supplying a signal controlling the cutoff of power supplies to the motor whenever the said voltage at the terminals of the capacitor exceeds the said reference voltage, and is characterized by a first and a second time constant circuits supplied with the said image voltage and connected respectively to a first and a second comparator to which are respectively fed a first and a second reference voltage the time constant of the second circuit representing the conventional cooling of the motor indicated by the manufacturer, and the second reference voltage being chosen to correspond to an overload of a few percent with respect to the nominal current of the motor, while the time constant of the first circuit is chosen to correspond to the cold starting time of the motor for a current equal to six times the said nominal current, and the first reference voltage has a value twice that of the second, and by means of regulating in a nonreversible fashion the voltage at the terminals of the capacitor of the first time constant circuit to the voltage of the terminals of the capacitor of the second circuit, when the latter tends to exceed the former.

2. System according to claim 1, characterized by the fact that is comprises a third time constant circuit which feeds the second comparator and the time constant of which is chosen to be five times that of the second circuit, and that means are provided to generate a signal indicating that the motor is running or stopped and to switch the second and third time constant circuits as a function of the said signal.

3. System according to either of claims 1 and 2, characterized by the fact that each of the said time constant circuits comprises a switch controlled by the recurrent pulses.

4. System according to either of claims 1 to 3, characterized by the fact that the said means of nonreversible regulation comprises an amplifier to one input of which is fed the voltage at the terminals of capacitor of the second time constant circuit of which the other input is connected to the terminals of capacitor of the first time constant circuit through a resistor and of which the output is connected to capacitor of the first time constant circuit by a diode with leakage current of the order of a pA, followed by the resistor.

5. System according to claim 2, characterized by the fact that the said means of generating a signal indicating whether the motor is running or stopped comprise a comparator which compares the voltage that is an image of the current absorbed by the motor with a reference voltage equal to a few percent of that which corresponds to the nominal current.

6. System according to claim 3, characterized by a comparator receiving, on the one hand the output voltage from one of the capacitors of the time constant circuits, and on the other hand a negative reference voltage and through an additional switch controlled by the output voltage of the said comparator, and devised to prevent a negative charge on the said capacitor.

7. System according to either of claims 1 to 6, characterized by the fact that the said voltage which is an image of the current is supplied by low current signal detectors associated with amplifiers and, after rectification and filtering is fed to a comparator which also receives a positive reference voltage supplied by a device which indicates whether the motor is running or stopped and causes the gradual discharge of a capacitor in the event of a phase being lost, the voltage at the terminals of the said capacitor being fed to a comparator which provides a signal to cut off the power supply.

8. A system as claimed in any one of claims 1 to 7 substantially as described herein with reference to the accompanying drawings.