SE447685B - DEVICE FOR HEATING PROTECTION OF AN ELECTRIC ENGINE - Google Patents

Description

To this end, the device according to the invention is characterized by a company. and a second time constant circuit, to which said represented voltage is applied, and which are connected. to a first. respectively a second comparator, to which a first and a second separate reference voltage are supplied, e wherein the time constant at the second circuit is selected to be equal to the tertiary time constant of the motor specified by the manufacturer, and the second reference voltage is selected so. that it corresponds to the overload on. a few percent above the rated motor current, while the time constant in the first circuit is selected to correspond to the cold start time of the motor for a current equal to six times said rated current, and that the first reference voltage has a value twice as large as with the first, and by means for directly regulating the voltage at the terminals of the capacitor in the second circuit, when the latter tends to exceed the former.

According to a preferred embodiment, the invention comprises a third time constant circuit, which feeds the second comparator, and in which the time constant is chosen to be five times that of the second circuit, and means are provided for generating a signal indicating that a motor is running or stands still and to couple the second and third time constant circuits as a function of said signal.

Other special features as well as the advantages of the invention appear from the background of the following description and accompanying drawings.

Fig. 1 is the circuit diagram of an overheating protection device according to a preferred method of carrying out the invention.

Fig. 2 shows curves representing the permissible overload times for a motor according to the current it consumes.

In Fig. 1, three amplifiers 1-2-3 are shown connected. as integrating circuits resistors 100-101, 200-201, 500-301 and capacitors 102-202-302, which are advantageously supplied by Rogowski probes (not shown) emitting low current signals proportional to the derivative of the current 447 685 following in the respective phase capacitors connecting an asynchronous motor with the three-phase power source. The output of each amplifier is connected to its non-signal receiving input by a resistor 105-105-305 respectively and by a capacitor 104-204-304 respectively connected to ground by means of a resistor 105-205-505 respectively and by an input on a second amplifier 4-5-6 respectively. The second input of the second amplifier is connected to ground by means of a resistor 400-500-600 respectively and to its output by a diode 401-501-601 respectively. The outputs of the second amplifiers are together connected to a third amplifier 7 through the diodes 402-502-602 respectively, the anodes of which are connected to the said second inputs of the second amplifier through the resistors 402-505 -605 respectively.

At their output, amplifiers 1-2-3 emit alternating voltages proportional to the above-mentioned phase currents, which are rectified by diodes 401-402.

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

The anodes of the diodes 402-502-602 are connected together to an input of a comparator 8, the second input of which is connected by a resistor bridge 901-902 to the output of the comparator 9. The latter on the one hand receives the voltage at point A between resistor 704 and capacitor 705 and on the other hand a constant reference voltage provided by a source V + to resistor bridge 905-904. This reference voltage is, for example, that which corresponds to the voltage at A for a current flowing in the motor equal to 10% of the nominal IN. In other words, as long as the motor is running, the comparator must supply it with a digital level 1 (positive voltage) at its output and cannot supply a 447,685 level. 0 (negative voltage) at its output except when the motor is stopped.

It follows that only when the engine is running will the comparator 8 deliver a level. 0 (negative voltage) at its output whenever the output voltage from amplifiers 4-5-6 passes through zero, in other words, in the absence of a phase.

In this case, a capacitor 802 charged with a positive voltage (V +) is discharged through a resistor bridge 800-801 whenever the output voltage from the amplifiers 4-5-6 passes through zero and, after a certain number of these zero transitions, the voltage at its socket is low enough to trigger a comparator 10, on which one connection receives the voltage at the terminals of the capacitor 802, while the other input contact is supplied with a reference voltage determined by a current source (v +) and a resistor bridge 1000-1001.

The output signal from the comparator 10 is first fed, through a diode 1002, to one of the components. 1100 in a tripping circuit 1100-1101 which at its output 1102 supplies a pulse circuit to the relay (not shown), which supplies a control signal to the conventional motor protection contactor, and secondly to one of the components 1200 in a tripping circuit 1200-1201, which triggers an LED, not shown, connected to the output 1202.

It may be pointed out that in comparison with the circuit for detecting phase loss described in French patent 79.26102 filed October 16, 1979 by the applicant regarding "Electronic device for protecting an electric motor against overload and loss of a phase in the three-phase alternating current supply" , the present device for detecting phase loss is simplified in that the means for filtering the three-phase voltage, which constitute 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 Rogox skisonderxxa provides a sufficiently low voltage to be able to use the amplifier without saturation. It is then sufficient that these have a very low output impedance for their output signal to pass through zero in the event of phase loss and as a result be used directly by the circuit for detecting phase loss. 447 6-85 As can be seen from Fig. 1, the signal at point A is fed through the respective resistors 1500-1400-1500 to an input of each of the three amplifiers 15-14-15, the second input of which is connected to ground, and whose outputs are connected to said input by inverted diodes 1501-1401-1501 respectively and by resistors 1502-1402-1502 respectively. The anodes of each of these diodes are connected to said input by a diode 1505-1405-1505 respectively. The said input also receives for each of the amplifiers 14 and 15 a negative voltage V- through a resistor of 1404 and 1504, respectively. Such an arrangement, well known per se, gives at its output V a voltage proportional to the square of the current corresponding to the highest of the three values of the phase currents.

Point B is connected through resistors 1600 and 1700 to two switches 16 and 17, respectively, controlled by repeated pulses produced by two monostable tripping or trigger circuits 1900-1901 and 2000-2001, respectively. These two trigger circuits are initiated by a master oscillator consisting of two NOR circuits 2100-2101. A third monostable trigger circuit 2200-2201 can also control the chopper 17, which receives pulses from the trigger circuit 2000-2001 or the trigger circuit 2200-2201 according to the state of a logic circuit formed by three N (EP circuits 2500-2501-222. The circuit 2501 is controlled of the signal from the Wstopped "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 stationary.

The switches 16 and 17 charge two capacitors 24 and 25, respectively, whose potential is supplied through the resistors 2600 and 2700, the amplifier 26 and two amplifiers 27 and 28, respectively. The amplifiers 26-28 are built up of field effect transistors and therefore have a very high input impedance.

The output of the amplifier 26 is connected to an input of an amplifier 29 whose second input is supplied with a positive voltage V4 by means of a resistor bridge 2900-2901. The output of the amplifier 29 is connected through a diode 2902 to the component 1100 in the trigger circuit 1100-1101.

Similarly, the output of amplifier 28 is connected to a 447 685 input of amplifier 50 whose second input is supplied with a voltage V + through a resistor bridge 5000-5001 and whose output is connected through a diode 5002 to component 1100.

The output of the amplifier 27 is connected at the common point of the resistor 2600 and the capacitor 24 via a diode 2701 of the type having a leakage current of one pA followed by a resistor 2702. Since a diode of this type has practically no back current, the capacitor 24 has no possibility of discharging this path.

The amplifier 27 always prevents the voltage at the contacts on the capacitor 24 from becoming less than that at the contacts on. the capacitor 25. Should such a tendency occur, the discharge current from the capacitor 25, transmitted via the amplifier 27, the diode 2701 and the resistor 2702, would strive to charge the capacitor 24 until the voltages at the terminals of the two capacitors are equal.

The output of the amplifier 28 is connected to an input of an amplifier 51, the second input of which is supplied with a voltage V- through a resistor bridge 5100-5101. The output of the amplifier 51 is connected partly to said second input by means of a resistor 5102 followed by a diode 5105 and partly to the control electrode of a switch 52 connected between earth and the common point of the switch 17 and the capacitor 25.

The output voltages from the amplifiers 29 and 50 are supplied through the diodes 2905 and 5005, respectively, to the component 5500 on a trigger circuit 5500-5501, which triggers an LED (not shown) connected to its terminal 5502.

If the motor current tends to zero, capacitors 24 and 25 tend to be discharged to ground due to the presence of false capacitance in the circuit. When these capacitors have reached a zero potential, they could even strive to be recharged to a negative potential. Should this occur, the negative voltage at the terminals of the capacitor 25 supplied to an input of the amplifier 51 through the resistor 2700 and the amplifier 28, would be compared with the negative voltage supplied to the other input of. the amplifier 51, arranged as a comparator, with the result that an aperture signal would be transmitted to the 447 685 switch 52, the effect of which would be to cause a rapid discharge of the capacitor 25 down to zero potential. As explained above, the voltage at the terminals of the capacitor 24 would follow that at the terminals of the capacitor 25 and would therefore also be quickly brought to zero. The importance of this is explained below. A circuit consisting of a switch which charges a capacitor during the periodic intervals when closed to a voltage representing an image of a current which produces overheating at a load is well known per se, and it is also known that it enables simulation of the heating or cooling from the load, without any temperature measurement being made, and is particularly useful when the thermal time constant of the load is high, as is the case with an electric motor.

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

In the described arrangement, an initial circuit of this type (trigger circuit 2000-2001, switch 17, capacitor 25) is arranged to give a time constant GL, which is representative of the thermothermal behavior of the motor, when it operates at average temperature. This time constant is the time constant for conventional engine cooling specified by the manufacturer (eg 0L = 950 seconds). The first circuit provides overheating protection for the motor, when it operates between small overloads, in the following way. The constant reference voltage emitted by the resistance bridge 3000-3001 to an input of the amplifier 50 is selected so that it corresponds to an overload of a few percent (e.g. 7 to%), i.e. the amplifier 50, arranged as a comparator, supplies the trigger circuit 1100-1101 with a signal for interrupting the power supply to the motor as soon as the current I is 7 ä 8% larger than its normal value IN. Such a value was selected by drawing the curve as a solid line in Fig. 2, representing the allowable overload time ts, given by the manufacturer (expressed in seconds) as a function of the I / IN ratio, when the engine is hot . If this curve is assumed to be asymptotic to a straight line parallel to the vertical axis and passing through the abscissa 1.07 or 1.08, it can be observed by calculating the start-up times given by the system for different values of I / IN (simulation), that they in practice coincide with the permissible times given by the manufacturer.

A second circuit / for generating a time constant consists of the monostable trigger circuit 2200-2201, the switch 17 and the capacitor 25. The circuit is calculated so that this time constant QLA is equal to 5 GL. When the engine is stopped, it obviously cools down much less quickly than when it is running (because the fan is stopped). If the motor is restarted, it is important that the capacitor 25 be initially charged to a potential which forms an image of the temperature it had when it was "stopped", a temperature which apparently 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 GC defined by this circuit represents the start time that the motor can accept. As an example, it is equal to 175 seconds for a cold start time of 10 seconds for a current of 6 IN. It can be assumed that this value of the starting time is suitable for determining GC by simulation, since the starting current of an asynchronous motor is of the order of 6 IN, and it is important to protect a motor against too long or too frequent starts ooh, more generally , against large overloads. Nevertheless, the user of the circuit is given the option, if the start time is given for a start current for the motor other than 6 IN, to correct the value of OC so that it corresponds to the start time at 6 IN. GC is regulated, for example, by changing the value of the resistor 1600.

The two portions of the curve in Fig. 2 shown in dotted lines represent the permissible overload times when the motor is cold as a fi mlction of I / IN. given by the manufacturer.To follow up these points through an approximation A number of points on these curves are based on data 447 685 mative exponential Simulation corresponding to the charge of a capacitor in a time constant circuit, it can be observed that two values of the time constant must be used , ie a "short" value GC which corresponds to large overloads and a "long" value GL which corresponds to small overloads. The asymptote of the part of the curve corresponding to DC defines a reference voltage (which will be the constant voltage given by the resistor bridges 2900 and 2901 at the input of the amplifier 29), which is twice the reference voltage applied to the input of the amplifier 30, which suggests the assumption that, if the engine is hot, if it is stopped and then started, the copper temperature can reach a value twice the normal for the engine, without the motor housing exceeding the allowable temperature. Heat transfer from copper to iron takes place quickly and the overheating of the copper therefore has a short duration and can be tolerated by the motor.

As stated above, the circuit is designed so that the voltage at the terminals of the capacitor 24 can never be less than the voltage at the terminals of the capacitor 25. This condition reflects the fact that in a motor the copper can never be colder than the iron.

As explained, the voltage at the terminals of capacitors 24 and 25 can never be negative. This means that the circuit must never simulate copper or iron temperatures below ambient temperature.

It is obvious that various modifications can be made in the circuit described and shown without departing from the spirit of the invention.

Claims (7)

447 685 10 Patent claims An apparatus for overheating protection in an electric motor comprising means (1, 2, 3) for supplying to at least one time constant circuit a voltage which is a representation of the current consumed by the motor (this voltage being preferably proportional to the square of said current), means (29, 30) for comparing the voltage at the terminals of the capacitor (24, 25) which said time constant circuit comprises with a constant reference voltage and for supplying a signal which controls the shut-off of the motor power supply, whenever said voltage at the capacitor terminals, said reference voltage exceeds, characterized by a first (1900, 1901, 16, 24) and a second (2000, 2001, 17, 25) time constant circuit supplied with said mapped voltage and connected to a first (29) and second (30) comparator, to which a first and a second reference voltage, respectively, are supplied, the time constant in the second circuit representing the conventional cooling none of the motor indicated by the manufacturer, and the second reference voltage is selected to correspond to an overload of a few percent compared to the rated current of the motor, while the time constant in the first circuit is selected to correspond to the cold start time of the motor for a current equal to six times said nominal current and the first reference voltage has a value twice the value of the second, and of means (27, 2701, 2702) for regulating in a non-reversible manner the voltage at the connections to the capacitor in the first time constant circuit to the voltage at the connections to the capacitor in the second circuit, when the latter tends to exceed the former. Device according to claim 1, characterized in that it comprises a third time constant circuit (2200, 2201, 17, 25) which supplies the second comparator (30) and whose time constant is selected to five times that of the second circuit, and that means (9) are arranged to generate a signal indicating that the motor is running or stopped and to couple the other and the trvqj »LidkonntunLkf ~ L fl ~ n as fn Inkiíon of said» signal. 447 685 11 Device according to claim 1 or 2, characterized in that each of said time constant circuits comprises a switch (16, 17) controlled by repeated pulses. Device according to one of Claims 1 to 3, characterized in that the means for non-reversible control comprise an amplifier (27) to one input of which the voltage is supplied at the connections of the capacitor (25) in the other time constant circuit. whose second input is connected to the terminals of the capacitor (24) in the first time constant circuit via a resistor (2702), and whose output is connected to the capacitor - (24) of the first time constant circuit by a diode (2701) having a leakage current of the order of a pA followed by the resistor (2702). Device according to claim 2, characterized in that the means for generating a signal indicating whether the motor is running or stopped comprises a comparator (9) which compares the voltage, which is an image of the current consumed by the motor, with a reference voltage equal to a few percent of that corresponding to the rated current. Device according to claim 3, characterized by a comparator (31) which receives on the one hand the output voltage from one of the capacitors (25) in the time constant circuits and on the other hand a negative reference voltage through a further switch (32) controlled by the output voltage from said comparator (31) and arranged to prevent a negative charge on said capacitor (25). Device according to any one of claims 1-6, characterized in that said voltage, which is an image of the current, is applied by signal detectors (100, 101, 102, 200, 201, 202, 300, 301, 302) for low current connected to amplifiers (4, 5, 6) and, after rectification and filtration, fed to a comparator (9), which also receives a positive reference voltage supplied by a device indicating whether the motor is running or stopped and causes successive discharge of a capacitor (802) in case of loss of a phase, the voltage at the connections to said capacitor (802) being supplied to a comparator (10), which gives a signal for shutting off the power supply.