DK177685B1

DK177685B1 - Method of controlling motor enable signals

Info

Publication number: DK177685B1
Application number: DK201200766A
Authority: DK
Inventors: Keld Christensen; Rune Thomsen
Original assignee: Secop Gmbh
Priority date: 2012-12-03
Filing date: 2012-12-03
Publication date: 2014-03-03
Also published as: CN103856117A; CN103856117B; DE102013224427A1

Abstract

A method of motor enablement is described in which a capacitor is charged or discharged depending on whether or not the current flowing through an inverter driving the motor is above a reference current. When the motor is in an enabled state, the motor is disabled if the voltage across the capacitor rises above a first (upper) voltage threshold. Similarly, when the motor is in a disabled state, the motor is enabled if the voltage across the capacitor falls below a second (lower) voltage threshold.

Description

- i - DK 177685 B1

METHOD OF CONTROLLING MOTOR ENABLE SIGNALS FIELD OF THE INVENTION

The present invention concerns methods and circuits for 5 controlling motor enable signals. More particularly, but not exclusively, the invention concerns methods for disabling a motor (such as a motor for a compressor) to prevent damage under high current conditions.

10 BACKGROUND OF THE INVENTION

Motor control circuits in which a motor is driven by an AC supply provided by an inverter are well known. In such circuits, under certain operating conditions the motor can experience high levels of current. For example, the rotor 15 of the motor may be prevented from rotating, either accidentally during normal use or intentionally during testing. This will lead to the motor drawing a high current, generating excess heat and risking damage to the motor.

20 By way of example, the present invention may be used during some approval tests for compressors in which the rotor of the compressor motor is deliberately blocked and the compressor temperature monitored to determine whether it exceeds a threshold level during the test.

25 One method to prevent overheating of a motor is to measure the temperature of the motor and to disable the motor if the temperature exceeds a threshold. This requires the provision of a temperature sensor, which introduces an additional cost and additional circuit configuration 30 problems.

DK 177685 B1 - 2 -

An additional problem is that in some scenarios, (including, but not limited to, test scenarios), the rotor of a motor may be blocked for a period of time and then released. A system is desired in which the motor can be 5 . disabled when the rotor is blocked and in which the motor will restart when the rotor is no longer blocked.

The present invention seeks to address at least some of the problems outlined above.

GB 2,152,308 describes a fault detection apparatus for 10 an air conditioner that detects overcurrents occurring within a predetermined time period and counts the number of occurrences of overcurrents as a criterion of an actual fault. The apparatus causes the compressor to stop when the number of occurrences becomes two or more, thereby 15 unnecessary interruption of operation of the air-conditioner due to electrical noise or a simple overload is considerably reduced.

SUMMARY OF THE INVENTION

20 In accordance with an aspect of the invention there is provided a method comprising: comparing a first voltage signal indicative of a current flowing through an inverter used to drive a motor with a reference voltage to determine whether the current flowing through the inverter is above a 25 reference current; charging a capacitor when it is determined that the current flowing through the inverter is above the reference current and allowing the capacitor to discharge when it is determined that the current flowing through the inverter is below the reference current; when an 30 enabled state of the motor is selected, changing the selected state to a disabled state in the event that the voltage across the capacitor is above a first (upper) voltage threshold; when the disabled state of the motor is selected, changing the selected state to the enabled state 35 in the event that the voltage across the capacitor is below a second (lower) voltage threshold lower than the first voltage threshold; and providing a motor control signal indicating whether the enabled state or the disabled state of the motor is selected.

DK 177685 B1 - 3 -

In accordance with a further aspect of the invention there is provided a circuit comprising: a comparison circuit configured to compare a first voltage signal indicative of a current flowing through an inverter used to drive a motor 5 with a reference voltage to determine whether the current flowing through the inverter is above a reference current; a capacitor that is charged when it is determined by the comparison circuit that the current flowing through the inverter is above the reference current and is allowed to 10 discharge when it is determined by the comparison circuit that the current flowing through the inverter is below the reference current; and an output circuit configured to select a disabled state in the event that the inverter is in an enabled state and the voltage across the capacitor is 15 above a first voltage threshold and to select the enabled state in the event that the inverter is in the disabled state and the voltage across the capacitor is below a second voltage threshold lower than the first voltage threshold.

As the motor control signal is dependent on the voltage 20 across the capacitor, and the capacitor will discharge when the current driving the motor is below the reference current, the inverter is put into the disabled state only when the inverter current has been above the reference current for a significant proportion of recent time. The 25 inverter is therefore controlled based on the operation of the motor over a period of time, not based on an instantaneous inverter current. This may be important if the inverter current includes a number of spikes so that the current level is sometimes above the reference current and 30 sometimes below. As the current driving the motor is liable DK 177685 B1 - 4 - to undergo large but short-lived variations under normal conditions of use, the method of controlling the inverter does not affect operation of the motor under normal conditions.

5 Further, as the voltage across the capacitor is compared to the lower second voltage threshold when the inverter is in the disabled state than the higher first voltage threshold used when the inverter is in the enabled state, after changing to the disabled state the inverter 10 will remain in the disabled state for a period of time despite the fact that the current driving the motor will immediately become zero (as the inverter is disabled so the motor will no longer be operating). The means the motor will remain disabled for a period of time, allowing excess 15 heat caused by the previously high inverter current to dissipate. When the period of time has passed the motor will be enabled, without (for example) the inverter needing to be manually reset.

The invention may include obtaining or generating the 20 first voltage signal. For example, the inverter current may be passed through a measuring resistor, with the voltage across that resistor providing the first voltage signal.

In one form of the invention, in the enabled state, switches of the inverter are enabled, and in the disabled 25 state, the switches of the inverter are disabled.

Alternatively, or in addition, in the enabled state, an inverter controller may provide switching instructions to switches of the inverter and, in the disabled state, the inverter controller may not provide switching instructions 30 to the switches of the inverter. This prevents both the DK 177685 B1 - 5 - motor and inverter switches from overheating and being damaged by the high current level.

Thus, in the enabled state, an inverter is able to drive the motor and, in the disabled state, the inverter is 5 not able to drive the motor. The enable signal may be provided to the inverter (to enable/disable the inverter switches). Alternatively, or in addition, the enable signal may be provided to a microprocessor (or some other controller) that provides the switching signals to the 10 inverter. In one form of the invention, two control circuits are provided - one to provide an enable signal to the inverter, the other to provide an enable signal to the microprocessor/controller. This provides redundancy for added security.

15 The first voltage signal indicative of the current flowing through the inverter may be generated from the inverter current by a transistor and a second electronic component (generally in addition to a measuring resistor, as indicated above). A change in the operating characteristics 20 of the transistor due to a change in temperature may be compensated for by a corresponding change in the operating characteristics of the second electronic component due to the change in temperature. The operating characteristic may be the diode voltage drop of the transistor (e.g. a PNP 25 bipolar transistor) and second electronic component. The second electronic component may be a transistor (e.g. an NPN bipolar transistor).

In one form of the invention, the first voltage threshold is provided by first and second resistors 30 connected to form a voltage divider. The second voltage DK 177685 B1 - 6 - threshold may be provided by pulling down the mid-point of the voltage divider. By way of example, the mid-point of the voltage divider may be pulled down by a transistor that is turned on when the disabled state is selected. Providing 5 a single circuit that can provide both voltage thresholds simplifies the circuit construction as separate first and second voltage threshold generators and comparators are not required.

The present invention may be used to selectively 10 enable/disable a compressor, although it is also applicable to other motor applications.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present 15 invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS 20 Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

Figure 1 shows a motor control circuit in accordance with 25 a first embodiment of the invention;

Figure 2 is a circuit diagram of the enable control circuit of Figure 1;

Figure 3 is a graph demonstrating an aspect of the motor control circuit of the present invention; DK 177685 B1 - 7 -

Figure 4 shows measurements during an exemplary use of the circuit of the present invention; and Figure 5 shows measurements during an exemplary use of the circuit of the present invention in which 5 the rotor of a motor under test is blocked.

DETAILED DESCRIPTION OF THE INVENTION

A motor control circuit in accordance with a first embodiment of the invention is shown in Figure 1. The 10 circuit 1 is powered by a DC link voltage 2. An inverter 3, controlled by a microprocessor 4, converts the DC link voltage into a three-phase AC supply for a motor 5. Of course, the DC link voltage may be generated from an AC supply voltage using a rectifier, as is well known in the 15 art.

An enable control circuit 6 provides an enable signal to the inverter 3 and/or microprocessor 4, as described in detail below.

The enable control circuit 6 protects the motor against 20 overheating (for example) by signalling the inverter 3 and/or microprocessor 4 to stop the motor 5. Thus, the enable signal may be provided to the inverter 3 to indicate whether or not the inverter should provide the signals required to drive the motor. Alternatively, or in addition, 25 the control signal may be provided to the microprocessor 4 to indicate whether or not the microprocessor should provide the signals required to drive the switches of the inverter.

In one form of the invention, two separate (and almost identical) enable control circuits 6 are provided, with one 30 providing an enable signal to the inverter 3 and the other DK 177685 B1 - 8 - providing an enable signal to the microprocessor 4. This level of redundancy provides a safe solution. However, it will be clear that alternative forms could be provided in which a single enable control circuit 6 is provided. That 5 single control circuit could provide an enable signal to both the inventor 3 and the microprocessor 4.

Alternatively, the enable signal could be provided to only one of the inverter 3 and the microprocessor 4.

One instance of the enable control circuit 6 is shown 10 in more detail in Figure 2. The circuit 6 uses a positive 5V voltage supply (hereafter the "positive rail"), and a corresponding signal ground (hereafter simply "ground").

The current flowing through the inverter 3 (IGND) is connected to ground via a small measuring resistor R-cur.

15 In this way, a voltage signal is generated indicative of the current flowing in the inverter. As described below, this voltage signal is used to determine whether the motor should be in an enabled state or a disabled state.

As shown in Figure 2, a PNP transistor TR1 has a base 20 connected to the voltage signal indicative of the inverter current via a resistor R01. The emitter of the transistor TR1 is connected via a resistor R02 to the positive rail, and the collector of the transistor TR1 is directly connected to ground. An NPN transistor TR2 has a base 25 connected to the emitter of transistor TR1, an emitter connected via a resistor R04 to ground, and a collector connected via a resistor R03 to the positive rail.

A PNP resistor TR3 has a base connected to the collector of transistor TR2. The emitter of the transistor 30 TR3 is connected to the positive rail via a resistor R07, DK 177685 B1 - 9 - and to ground via a resistor R08. The collector of the transistor TR3 is connected to a resistor R06, which is connected to ground via a capacitor Cl. A resistor R05 also connects the resistor R06 to ground, such that the resistor 5 R05 is in parallel with the capacitor Cl.

An NPN transistor TR4 has a base connected via a resistor R09 to the connection between the resistor R06 and capacitor Cl. The emitter of the transistor TR4 is connected via a resistor RIO to the positive rail, via a 10 resistor Rll to ground, and via a capacitor C2 to the base of the transistor TR4. The emitter of the transistor TR4 is also connected to the collector of an NPN transistor TR5, the emitter of which is connected to ground. The collector of the transistor TR4 is connected to the base of a PNP 15 transistor TR6. The collector of the transistor TR6 is connected to the base of the transistor TR5, and via a resistor R14 to ground. The emitter of the transistor TR6 is also connected via a resistor R12 to the collector of the transistor TR4, and via a resistor R13 to the positive rail.

20 The emitter of the transistor TR6 also provides the enable signal MCES of the circuit, which, as indicated above, is provided to the inverter 3 and/or the microprocessor 4.

In use, the inverter current IGND is converted to a voltage input by the resistor R-cur, as described above.

25 This voltage signal provides an input to the base of the transistor TR1, which acts as a diode, thus allowing the transistor TR2 in combination with the resistors R03 and R04 to act as an inverting amplifier. The resistors R03 and R04 have resistances of 1.3kQ and 100Ω respectively, and so the 30 collector of the transistor TR2 thus provides to the base of DK 177685 B1 - 10 - the transistor TR3 an amplification of the input voltage signal with a gain of around -13. The current signal IGND may be very small and so the voltage signal at the base of the transistor TR1 may be very small. Thus, the provision 5 of an amplifier circuit with high input impedance and a suitable voltage gain is required.

As discussed above, the circumstances in which the enable control circuit 6 are used are such that substantial temperature variations are not uncommon, which results in 10 variations in the operating parameters of the components in the circuit, particularly in the biasing of transistors. An input voltage signal sufficient to turn on the transistor TR1 will tend to turn the transistor TR2 off (and vice versa). A voltage drop (Vbe) will occur across the base-15 emitter junctions of both the transistors TR1 and TR2 when the respective transistors are turned on. These base-emitter junction voltages are temperature-dependent, but can be expected to vary in a similar manner to one another. Accordingly, the effects of temperature on the base-emitter 20 voltages of the transistors TR1 and TR2 tend to cancel each other out, such that the voltage signal at the output of the inverting amplifier (i.e. at the collector of the transistor TR2) is not significantly affected by temperature. Tests have shown that the inverting amplifier shown in Figure 2 25 has superior performance with changing temperatures when compared with a simple op-amp based inverting amplifier.

The resistors R07 (1.5kQ) and R08 (12kQ) form a voltage divider, such that the voltage at the emitter of the transistor TR3 is held at around 4.4 volts (referred to 30 below as a "reference voltage"). The transistor TR3 will be DK 177685 B1 -lion only when the voltage level at the collector of the transistor TR2, which is acting as an inverting amplifier for the voltage input, is below the reference voltage. As described in detail below, when the transistor TR3 is on, 5 the capacitor Cl is charged: when the transistor TR3 is off, the capacitor Cl is discharged. Accordingly, the voltage across the capacitor Cl is dependent on the proportion of time in which the transistor TR3 .is on.

When the voltage at the base of the transistor TR1 is 10 small, the voltage at the base of the transistor TR3 (which is at the output of the inverter amplifier) will be close to the positive rail voltage. Thus, the transistor TR3 will be switched off. Conversely, when the voltage at the base of the transistor TR1 is higher, the transistor TR3 will be 15 switched on.

In the normal operation of the motor 5, the current through the inverter will be relatively small. This results in a small voltage at the input of the transistor TR1, which, when amplified by the transistors TRl and TR2 is 20 above the reference voltage such that the transistor TR3 will be off. In this state, the capacitor Cl discharges and so the voltage across the capacitor drops. This means the base of the transistor TR4 will be low and so the transistor TR4 will be off, the base of the transistor TR6 will 25 therefore be pulled high (via the connection via resistors R12 and R13 to the positive rail) and so the transistor TR6 will be off. The base of the transistor TR5 will be pulled low (by the connection via R14 to ground) and so the transistor TR5 will be off. As a result, the output signal 30 MCES will be high, indicating to the inverter 3 and/or the DK 177685 B1 - 12 - microprocessor 4 as appropriate that the motor 5 can be used (an enable signal).

When the rotor of the motor 5 is blocked (for example, during a test), the inverter current rises and spikes in the 5 inverter current will cause the voltage level to the base of the transistor TR3 to repeatedly drop below the reference voltage such that the transistor TR3 turns on and the capacitor Cl charges. If the inverter current spikes are sufficiently extensive, then the discharging of the 10 capacitor when the inverter current is below the reference current will not be sufficient to discharge the charge applied to the capacitor Cl due to the spikes. As a result, the voltage across the capacitor Cl will rise. Of course, when the rotor of the motor 5 is no longer blocked, the 15 capacitor will discharge and the voltage across the capacitor will fall.

The voltage across the capacitor is dependent on the proportion of time in which the transistor TR3 is turned on.

The voltage across the capacitor is therefore indicative of 20 the proportion of time that the inverter current is above the reference current.

When the transistors TR4, TR5 and TR6 are all turned off, the resistors RIO and Rll act as a voltage divider. As shown in Figure 2, those resistors have resistances of 8.2kQ 25 and 18kQ respectively, so that the voltage at the midpoint of the voltage divider is about 3.7 volts. This voltage (3.7 V) is provided at the emitter of the transistor TR4.

Thus, when the transistor TR4 is switched off, it will be switched on when the base voltage of the transistor TR4 30 rises above about 4.4 volts (assuming a base-emitter voltage DK 177685 B1 - 13 - of 0.7 volts - of course, the actual voltage may be different in different implementations). Thus, if the voltage across the capacitor Cl rises above 4.4 volts (hereafter referred to as the upper voltage threshold), then 5 the transistor TR4 will turn on, which, in turn, turns on the transistors TR5 and TR6. In this new state, the output signal MCES is connected to ground via the transistor TR5 and the transistor TR6, thus providing a low output signal indicating to the inverter 3 and/or the microprocessor 4 (as 10 appropriate) that the motor 5 should not be used (a disable signal).

When the transistor TR4 is on, the transistor TR5 will also be on, connecting the junction of the resistors RIO and Rll to ground via the transistor TR5. This will pull the 15 voltage level at the emitter of the transistor TR4 to a voltage equal to the collector-emitter voltage drop of the transistor TR5, which might be of the order of 0.1 volts.

Thus, when the output signal is low, the transistor TR4 will only switch off when the voltage across the capacitor Cl 20 falls below about 0.8 volts (0.1 volts above the base- emitter voltage of the transistor TR4) (hereafter referred to as the lower voltage threshold).

Figure 3 is a graph demonstrating an aspect of the use of the circuit 6. The graph plots the voltage across the 25 capacitor Cl against the control output voltage MCES.

Assume that initially, the voltage across the capacitor Cl is zero and the transistors TR4, TR5 and TR6 are all switched off. In this state, the signal MCES is pulled to the positive rail voltage by the resistor R13.

DK 177685 B1 - 14 -

As described above, in this state, the signal MCES remains high until the capacitor voltage rises above the upper voltage threshold (labelled vupper in Figure 3) . When the capacitor voltage rises above the upper voltage 5 threshold, the transistors TR4, TR5 and TR6 are all turned on and the control signal MCES is pulled low by the transistor TR5 (to a voltage of perhaps 0.7 volts). The signal MCES remains low until the capacitor voltage falls below the lower voltage threshold (labelled viower in Figure 10 3). Accordingly, as clearly shown in Figure 3, the control signal MCES exhibits hysteresis. This hysteresis ensures that when a motor is disabled, it remains disabled for a minimum period of time to allow excess heat to dissipate.

Refer back to Figure 1. When the enable control 15 circuit 6 is providing a disable signal (indicating to the inverter 3 and/or microprocessor 4 that the motor 5 should not be used), no power will be supplied to the motor 5. The inverter current will therefore become zero. This means that transistor TR3 will be turned off, and in due course 20 the capacitor Cl will discharge until the voltage across the capacitor falls below the lower voltage threshold. When this happens, the enable control circuit will provide an enable signal to the inverter 3 and/or the microprocessor 4 such that the motor 5 can once again be used.

25 Under normal operation of the motor 5, the inverter current will typically be sufficiently low that the voltage across the capacitor will not reach the upper voltage threshold. Accordingly, the motor will rarely be disabled.

Figure 4 shows current data, indicated generally by the 30 reference numeral 10, recorded during an exemplary use of DK 177685 B1 - 15 - the circuit of the present invention. The current data 10 includes a plot 12 of the current output by each of the three phases of the inverter 3. The current data 10 also includes a plot 14 of the overall inverter current. The 5 inverter current 14 is the current IGND that flows through the measuring resistor R-cur.

The current data 10 represents a high load scenario in which the inverter current is high, but not so high that the motor should be disabled. The inverter current 14 varies, 10 but does not include significant current spikes. The inverter current 14 is below the reference current for most or all of the duration of the current data 10.

Figure 5 shows current data, indicated generally by the reference numeral 20, recorded during an exemplary use of 15 the circuit of the present invention in which the rotor of a motor under test is blocked. The current data 20 includes a plot 22 of the current output by each of the three phases of the inverter 3. The current data 20 also includes a plot 24 of the overall inverter current. The inverter current 24 20 is the current IGND that flows through the measuring resistor R-cur.

When the rotor of the motor 5 is blocked, the inverter current 24 rises significantly and includes a large number of significant current spikes. Thus, the inverter current 25 is above the reference current for a significant proportion of time. As a result, the capacitor Cl is charged and, over time, the voltage across the capacitor increases.

In Figure 4, the capacitor voltage never exceeds the upper voltage threshold and so the motor remains enabled.

30 In Figure 5, the capacitor voltage will rise over time until DK 177685 B1 - 16 - the voltage exceeds the upper voltage threshold and the motor is disabled. The motor will remain disabled until the capacitor voltage falls below the lower voltage threshold.

Figures 4 and 5 include measurements of the maximum and 5 minimum voltages across the measuring resistor R-cur for the duration of the plot (labelled "P5:max(C4)" and "P6:min(C4) respectively). In Figure 4, the maximum and minimum voltages are 232 mV and -312 mV: in Figure 5, the maximum and minimum voltages are 957 mV and -659 mV. Thus, it is 10 clear that the voltage spikes are much larger when the rotor of the motor is blocked. This difference in voltage is used in the present invention to disable the motor when the rotor is blocked.

As discussed above, when the voltage across the 15 capacitor Cl exceeds the upper voltage threshold, the motor will be disabled. With the motor disabled, the inverter current will fall to zero and the capacitor Cl will discharge. When the capacitor voltage falls below the lower threshold voltage, the motor will be enabled and will 20 restart. If the rotor is still blocked, the inverter current will again be high and the motor will soon be disabled once again. If, however, the rotor is not blocked, the motor will be restarted and will run as normal.

Accordingly, the control circuit 6 not only provides a 25 mechanism for disabling the motor 5, there is also provided a mechanism for restarting the motor, for example when a blockage of the rotor is removed.

While the present invention has been described and illustrated with reference to particular embodiments, it 30 will be appreciated by those of ordinary skill in the art DK 177685 B1 - 17 - that the invention lends itself to many different variations not specifically illustrated herein. For example, the particular threshold levels described herein are provided by way of example only. Similarly, the circuit diagram is 5 provided by way of example only and could readily be modified (for example, the use of bipolar translators is not essential in all embodiments of the invention).

Claims

A method comprising: comparing a first voltage signal indicating 5 a current passing through an inverter (3) and operating a motor (5) with a reference voltage to determine whether the current passing through the inverter (3) is larger than a reference current; and supplying a motor control signal indicating whether an activated or deactivated state of the motor is selected, characterized in that: a capacitor (C1) is charged when it is determined that the current flowing through the inverter is greater than the reference current and discharging the capacitor (C1) if it is determined that the current flowing through the inverter is less than the reference current; if the activated state is selected for the motor, the selected state changes to the deactivated state if the voltage across the capacitor is greater than a first voltage threshold; and if the deactivated state is selected for the motor, the selected state changes to the activated state if the voltage across the capacitor is less than a second voltage threshold lower than the first voltage threshold.

A method according to claim 1, characterized in that the inverter contacts are activated in the activated state and deactivated in the deactivated state. 30 - 2 - DK 177685 B1

A method according to claim 1 or 2, characterized in that an inverter control (4) in the activated state delivers switching instructions for contacts in the inverter and in the deactivated state does not provide switching instructions 5 for contacts in the inverter.

A method according to any one of claims 1 to 3, characterized in that the first voltage signal indicating the current flowing through the inverter is generated by the use of a transistor (TR2) and a second electronic component (TRI). and that a change in the operating characteristic of the transistor caused by a change in temperature is compensated by a corresponding change in the operating characteristic of the other electronic component caused by a change in temperature. 15

A method according to any one of the preceding claims, characterized in that the first voltage threshold is provided by first and second resistors (R10, Ril) which are connected to form a voltage divider.

A method according to claim 5, characterized in that the second voltage threshold is supplied by subtracting the center of the voltage divider.

A method according to claim 6, characterized in that the center of the voltage divider is pulled down by a transistor (TR5) which is switched on when the deactivated state is selected.

A circuit comprising: a comparison circuit configured to compare a first voltage signal indicating a current running through an inverter (3) and used to operate a motor (5), with a reference voltage to determine if the current flowing through the inverter is greater than a reference current, characterized in that the circuit further comprises: a capacitor (C1) which is charged when the comparator circuit determines that the current flowing through the inverter (3) is greater than the reference current and discharged when the comparator circuit determines that the current flowing through the inverter is less than the reference current; and an output circuit configured to select a deactivated state when the inverter is in an activated state and the voltage across the capacitor exceeds a first voltage threshold and to select the activated state when the inverter is in the deactivated state and the voltage across the capacitor is less than the first voltage threshold.

A circuit according to claim 8, further comprising an inverter control (4), characterized in that the inverter control is configured to provide, in the activated state, switching instructions for contacts in the inverter and, in the deactivated state, not to provide switch instructions for the inverter contacts.

A circuit according to claim 8 or 9, further comprising an input circuit for supplying the voltage signal indicating the inverter current to the comparator circuit, characterized in that the input circuit comprises a transistor (TR2) and another electronic component (TRI). ), - 4 - DK 177685 B1 such that a change in the operating characteristic of the transistor caused by a change in temperature is compensated by a corresponding change in the operating characteristic of the other electronic component caused by the change in temperature. 5

11. A circuit according to claim 10, characterized in that the operating characteristic is the diode voltage drop on the transistor and the other electronic component.

A circuit according to claim 10 or 11, characterized in that the second electronic component (TRI) is a transistor.

A circuit according to any of claims 8 to 12, 15 further comprising a first and a second resistor (R10, Ril) connected in series to form a voltage divider, characterized in that the first voltage threshold is provided by a center of the voltage divider.

A circuit according to claim 13, characterized in that the second voltage threshold is supplied by pulling down the center of the voltage divider.

A circuit according to claim 14, characterized in that the center of the voltage divider is pulled down by a transistor (TR5) which is switched on when the deactivated state is selected.