DE3228954C2 - Protective device for asynchronous motors - Google Patents

Abstract

The invention relates to an electrical protective device for monitoring, in particular converter-fed asynchronous motors. The aim is to create a protective device that, regardless of whether it is used with mains-fed or converter-fed machines, reliably detects all malfunctions that require the motor to be switched off in the entire speed range and causes the motor to be switched off when they occur. This is achieved by a circuit that generates an actual electrical signal corresponding to the effective value of the respective phase current for each phase of the asynchronous motor, a first comparison circuit in which the actual electrical signals are compared with a preset limit value and a first switch-off signal is generated, if at least one actual signal exceeds the limit value, a second comparison circuit, which compares the actual signals with each other and generates a second switch-off signal if the actual signals differ from one another by a presettable difference, by a third comparison circuit, which is in the frequency range of the phase current below a preselectable frequency limit, the actual voltage applied to the motor in each case checks for correspondence with the setpoint voltage of the converter associated with the current frequency and generates a third switch-off signal when the actual voltage exceeds the setpoint voltage by a presettable difference load falls below, and by a device for switching off ..

Description

The invention relates to an electrical protective device for monitoring in particular UmrichUirgespuisten asynchronous motors according to the preamble of claim 1.

So far, three-phase asynchronous mutors that used in a network of constant voltage and frequency, through motor protection switches or else built-in temperature sensor with activated

Switching device (called TMS) against accidents »overcurrent«, »current imbalance« (e.g. »two-phase operation«) and "excessive heating in the stator and / or rotor" are protected.

Electrical protective devices with the features 1-4 mentioned in the preamble of claim 1 are known from DE-OS 23 54 594. They can be used to protect three-phase motors against the incidents of "overcurrent" and "current imbalance".
From DE-OS 26 25 279 it is known to switch off three-phase motors, which are monitored for overload by measuring the phase currents, with a time delay depending on the size of the detected overload, in order to prevent only brief overloads that occur before the expiry of the The time delay disappears by itself and the motor would only be endangered if the duration exceeded the time delay, triggering an unnecessary shutdown.

From DE-OS 30 32 790 a motor protection circuit for multi-phase AC motors has become known, in which, in addition to the phase currents, the actual voltages applied to the motor are recorded and a shutdown signal in the event of overvoltage or undervoltage or specified differences between the voltages is produced.

The older patent application according to DE-OS 33 12 288 discloses a power converter with a Protective device to protect a connected induction motor against overload, which the Rectifier to the inverter supplied current determined and a comparison value depending on the converter frequency forms, the characteristic of this dependence in adaptation to the thermally limited The service life can be preset in consideration of the load's own cooling effect and a switch-off signal is generated when the determined current exceeds the comparison value.

There are also protective devices known, which by built-in temperature sensors in the rotor winding

monitor the excess temperature of the rotor.

All of these protective devices are inverter-fed Motors are not effective enough as with motors that have static or rotating frequency converters The speed can be adjusted with a voltage proportional to the frequency. The reason for that is that the maximum current permitted by the converters is only slightly higher than the rated motor current. The detection of an impermissible operating state via the motor current alone is therefore not possible possible. Monitoring of the winding temperature and / or the rated motor current is also possible no longer sufficient for all speed control ranges or load ranges because the rotor is opposite the stand can assume different, sometimes significantly greater, heating.

Often, however, especially with explosion-proof motors, it is necessary for safety reasons all malfunctions in the entire speed range clearly monitor and shutdown if certain limit values or limit value combinations are exceeded bring about.

Accordingly, the object on which the invention is based is to provide a protective device for

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4. Determination of the permissible overcurrent, depending on the speed and type of cooling of the motor

For separately ventilated, converter-fed asynchronous motors the thermally highest permissible motor current is almost independent of the set one Frequency or speed. In the case of self-ventilated machines, however, it deteriorates with decreasing Speed the cooling effect. As a result, the thermally permissible overcurrent decreases with decreasing Speed or frequency. This behavior is predetermined by the machine design. This is why the current monitoring function specified under item 1 is carried out on the various Speeds or frequencies according to these known engine characteristics, for what this borderline is simulated. Exceeds any phase current at any speed this tripping characteristic, the shutdown specified under item 1 is also initiated

The invention makes it possible to use any asynchronous motor without installing sensors of the protective device against all

To protect judge-fed machines of the type mentioned at the beginning 25 essential incidents. This applies in particular to creating the all essential shutdown of the also for explosion-proof machines.
Motor-requiring malfunctions reliably detected in the entire speed range and causes the motor to be switched off when they occur

This object is achieved by the features specified in the characterizing part of claim 1 solved.

The protective device according to the invention fulfills the following

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The following functions:

The subclaims relate to features of the invention that improve the functionality and safety of the Increase the protective device even further and, if necessary, also promote its construction in terms of circuitry.

An exemplary embodiment of the circuit device according to the invention is described below with reference to the Block diagram explained:

The protective device is set up for a 3-phase system. It contains two current sensors 1 for detection of the actual motor current in two phases. The third, not directly measured phase current is derived from the the other two, measured, derived. This worried

The curve shape, not just the sinusoidal shape, monitors 40 the circuit la. Furthermore, by means of a tension becoming. This is done in that each phase converter 2 the actual motor voltage and by means of a

Summation current transformer, the sum of the three phase currents is measured.

The temporal values of the phase currents ι \ _(ι] , i _{2 (l)} , h _(l ) and the motor voltage Um ₂ (D are each converted by an electronic circuit 5 into an effective value proportional _{DC voltage UmM U u} U ₂ , Ui In the first comparison circuit 6, the motor current is compared with a pre-settable, frequency-independent limit value k ■ In . Furthermore, in this first comparison circuit 6, with a signal Ur output by the converter 13 and proportional to its frequency, a current limit Igrem ( which is variable and proportional to the converter frequency) is variable depending on the converter frequency m is generated and the actual current value is compared with the relevant frequency-proportional limit value.

For separately ventilated motors, the limit value k I _n alone is sufficient in the comparison circuit 6. It is also sufficient for self-ventilated motors in and of itself

3. Monitoring of the "blocked rotor" incident 60 the limit value I _grs "z (n). However, both limit values can be specified in the circuit 6 in order to then be used for the comparison with the motor current, depending on the type of motor ventilation.

Only one of the three phase currents is used for the above comparisons in the first comparison circuit 6 used. This is due to the asymmetry check and fault current determination discussed below sufficient. If the one with the same chip

Monitoring of the level of the motor current in all phases

With this function, currents can be set at any

current is detected. Exceeds the motor current compared with a predetermined adjustable If the limit value is exceeded, the motor is switched off.

2. Monitoring of current imbalances

In the event of a phase failure or similar incidents, despite compliance with the provisions described under item 1 Current limit value of the motor exceed the permissible overtemperature. For this reason the asymmetry of the individual phase currents is monitored. This is done through a comparison of the phase currents with each other. In the case of deviations in the level of a phase current the other phases of predetermined values, e.g. B. 5%, the motor is switched off.

The case of the blocked runner poses for everyone Converter practically represents a short circuit. This causes the output voltage of the converter drops to a much smaller value than corresponds to the frequency-proportional behavior. This deviation is recognized and used for shutdown.

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tion t / 3, the actual value of this phase current either exceeds the frequency-independent limit value k Is or the frequency-proportional limit value I _gre m (n) , the first comparison circuit 6 delivers a corresponding first switch-off signal, which triggers the switch-off of the motor on line 6a when the frequency-independent limit value is exceeded and on line 6b when the frequency-dependent limit value is exceeded.

In a second two subtracting circuits 14 and a comparator 14a having comparison circuit, a comparison to current balance carried out in such a way that the differences h - h and I _t -1 ₃ are formed and the respective greater difference compared in comparator 14 with an adjustable threshold value will. If the difference, which is 0 in the case of current symmetry, exceeds the threshold value, then the second comparison circuit 7 also delivers a corresponding second motor switch-off signal.

To monitor the motor operating state (blocked rotor), the difference between the actual motor voltage and the frequency-proportional direct voltage Ur output by the converter 13 is formed in a third comparison circuit 11. If the signal U _me ir, which is proportional to the motor voltage, falls by a preset limit value below the signal Ur, which is proportional to the converter output voltage, the third comparison circuit 11 supplies a third motor shutdown signal.

The summation current transformer 3 works together with an evaluation relay 4 in such a way that fault currents are detected and, when they occur, a motor shutdown signal is also triggered. A fault current is known to exist in a three-phase motor if the sum of the three phase currents, which is normally = 0 at any point in time, differs from the value 0. For this purpose it can be, for. B. come when a ground fault occurs in the motor when the current flows to earth. B. in partial load operation the absolute level of the phase currents does not exceed the maximum permissible value k ■ I _n or igrenz (n). if the above disturbance concerns phase 2, because Z ₂ ^ is not measured, but derived from i _Ul) and hro . In order to record these incidents with certainty, monitoring is also extended to possible fault currents by means of the summation current transformer 3 and the associated relay (4)

30 can be adjusted to suit the particularities of engine operation.

The faults that have occurred are recorded in the electronic memory 9 and displayed. The shutdown signals control a relay 10 via an OR link, which then switches off in the engine control unit intervenes.

The protective device has test buttons 12 for simulating accidents such as "overcurrent" and "asymmetry" and "blocked runner" on.

The output signal of current sensor 1 of phase 3 is changed to achieve the fault message »overcurrent« increased by a corresponding presettable value.

For the simulation of the disturbance "asymmetry" is used in the gain of the adder

(h (t) + Ίχή),

that is, the circuit la, an adjustable value changed Thus results in a modified relative to the other current actual value I _{2 e} -rr-

The The secondary side of the voltage converter 2 is disconnected to record the actual motor voltage. This becomes the Motor voltage monitoring device = 0 specified.

Every disturbance simulation also leads to a delayed shutdown of the motor. The protective device described can of course also be used with conventional protective devices such as TMS or motor protection switches, be combined.

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Before fault current monitoring it is possible to actually measure only two phase currents and to form the third from the two measured, because either the sum of all three currents at any point in time = 0 or there is a fault current; and it is sufficient to monitor only one phase for the value k ■ I _n or Ilimit (n), because either this value is exceeded in all three phases or there is asymmetry, or a fault current occurs in one or more phases Leakage current on.

The motor shutdown signals of the comparison circuits 6, 7 and 11 as well as the evaluation or fault current relay 4 are only forwarded via timing elements 8 when the fault has existed for a certain time, since more or less brief malfunctions do not endanger the motor. The time delay is preferred to this ί sheet drawings

Claims (7)

Patent claims: 1. Electrical protective device for monitoring inverter-fed asynchronous motors, especially with self-ventilation, with 1) a circuit which, for each phase of the asynchronous motor, corresponds to the rms value of the respective Corresponding electrical actual signal generated for the phase current, 2) a first comparison circuit, in which the electrical actual signals with a presettable Limit value are compared and a switch-off signal is generated if at least an actual signal exceeds the limit value, 3) a second comparison circuit, which the actual signals compares with each other and generates a switch-off signal if the actual signals are over differ from each other by a presettable difference, and 4) a device for switching off the engine when a switch-off signal occurs, characterized in that 5) there is a third comparison circuit (11) which, in the frequency range of the phase current below a preselectable frequency limit, ensures that the actual voltage (U \ / 2 (o) applied to the motor (M) corresponds to the target voltage associated with the current frequency (Ur) of the converter (13) checks and generates a switch-off signal if the actual voltage falls below the target voltage by a pre-settable differential amount, and that 6) the limit value (I _grC nz (n)) is generated in the first comparison circuit (6) as a function of the converter frequency and that the characteristic of this dependence can be preset to match the thermal conditions of the respective motor construction. 2. Protection device according to claim 1, characterized in that to form the respective phase currents (i \ ( _t ) I ₂ (Ij, k (t)) corresponding electrical actual signals \ U \, U ₂ , i / 3) two Phase currents (h (t), h (o) are measured, these two phase currents are summed and the third phase current (hit)) under the assumption that the sum of the three phase currents (ίι <·,> iy,), h ( _t j) is zero, is calculated, and that the sum of the three phase currents is measured and continuously monitored to determine whether the measured variable actually has the value zero. 3. Protection device according to claim 1 for three-phase motors, characterized in that to form the respective phase currents (i \ _(lj , i _{2 (l)} , ij (i)) corresponding electrical actual signals (6Ί, U ₂ , Ui ) two phase currents (ίφ), iy,)) and the sum of the phase currents (ΐφ), i _{2 (l)} , / ^ can be measured. 4. Protection device according to one of the preceding claims, characterized in that the actual signals (U \, U ₂ , Ui) corresponding to the respective phase currents (ίφ), h (, j, i ₃ (, / ) as equivalent signals ( Direct current, direct voltage) the. 5. Protection device according to one of the preceding claims, characterized by timing elements (3) to delay the motor shutdown after the occurrence of the respective acoustic signal! underlying incident. 6. Protection device according to claim 5, characterized in that for exceeding the phase current limit value the time delay is set up depending on the size of the overshoot. 7. Protection device according to one of the preceding claims, characterized in that in the comparison circuits (6, 7, 11) randomly instead of the actual values (actual signals or actual voltage) fault-simulating interference signals (h> kI _N ; I ₂ > / 1, / 3; U _mc rr> Ur) can be entered to check the functionality of the protective device (12).