DE3405002A1 - Starting protection for an asynchronous rotating-field machine - Google Patents

Abstract

The starting protection of a rotating asynchronous rotating-field machine (1) has a current transformer (4), a voltage converter (5), a device for producing a speed-dependent variable from the values of the current (I) and of the supply voltage (U) detected by the current transformer (4) and voltage converter (5), as well as a control device. This control device is used for continuously testing the speed-dependent variable and, in the event of a value of the speed-dependent variable, which value signals that the machine has started within a starting time (tA') remaining absent, produces a command to switch the machine (1) off. This starting protection is intended to enable reliable monitoring of the entire starting phase in a simple manner. This is achieved in that the control device, which is constructed as a controlling and computing mechanism (9) having an associated memory (10), detects the real current (Iw) of the machine (1) as a speed-dependent variable, forms the differential real current ( DELTA Iw) from this and, when the starting of the machine (1) has been achieved, produces a suppression signal which results when the mathematical sign of the differential real current ( DELTA Iw) changes. <IMAGE>

Description

151/83

23-12.83

Ka / eh

Start-up protection for an asynchronous induction machine

The invention is based on start-up protection for asynchronous induction machines according to the preamble of claim 1.

With this generic term, the invention refers to a state of the art such as that in CH-PS 615 538 is described. With this start-up protection, the start-up process of an asynchronous induction machine is detected by means of detection a speed-dependent variable, such as the effective resistance, is monitored. A start-up process is then called judged to be successful if a specified value of the speed-dependent value is reached within a fixed, specified start-up time Large, such as the effective resistance, is achieved. With the well-known tarnish protection, however, not the entire Monitor critical area of an electrical machine during start-up, which lies between standstill and breakdown point. Only after passing through the tipping point, i.e. the one Point at which the torque of the machine decreases rapidly with increasing speed, can with certainty

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it can be stated that a start-up process was successful. Especially with machines with extreme material utilization Depending on the rotor design, a voltage drop before the tipping point is reached can have detrimental consequences.

The invention as characterized in the claims solves the problem of tarnishing protection of the generic type Art to create, which in a simple manner a safe monitoring of the entire critical area during the start-up process of the machine to be monitored enables.

The task is in connection with the features of the preamble according to the characterizing part of the claim 1 solved. The invention is characterized in that the entire critical area for the start-up process an asynchronous induction machine is monitored to ensure that dangerous voltage drops are not exceeded the tipping point of the machine can be taken into account, and that, in addition, a continuous correction the start-up time taking into account the during a start-up process acting supply voltage is enabled. Furthermore, the tarnish protection according to the invention offers the possibility by using different criteria for the start-up protection and an additional short-circuit protection to achieve complete fault selectivity.

The following is an embodiment of the invention explained in more detail with reference to the drawing.

Here shows:

1 shows a functional diagram of one implemented according to the invention Start-up protection,

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2 shows a diagram from which the dependency of the active current I "and the phase angle φ of the machine from the slip s, and

3 is a basic circuit diagram of a selective start-up and short-circuit protection, in which the invention

Appropriate anti-tarnish protection according to FIG. 1 is used will.

In the start-up protection shown in FIG. 1, 1 denotes an asynchronous induction machine, the supply of which is three-phase (R, S, T) takes place via a switch 2, which is controlled by an actuator 3, for example a magnet, can be opened or closed. Via a current transformer 4 and a voltage transformer 5, currents I and supply voltage U measured. The readings are

15 'via transducers 6 and 7 to an analog-to-digital converter

8 supplied. The digital current and voltage values are output by a control and arithmetic unit

9 generated and at differential time intervals At periodically repeating memory signal in one with the control and arithmetic logic unit 9 in operative connection Transfer memory 10. A further memory 11 is also operatively connected to the control and arithmetic logic unit 9, in which the values

Ij. and U _w of the nominal current and voltage of the induction machine,

I one indicating the starting current, one Mehrmax a?

times the nominal current I _M amount of the current I,

t. the nominal starting time of the induction machine 1 (i.e. the time within which the machine

has run up from standstill to nominal speed at a defined load and moment of inertia and at nominal voltage U _n ), and

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t, the time span within which the direct current element has subsided in the equalization process of the switch-on process,

are entered.

The control and arithmetic unit 9 for its part has outputs on, of which one with the actuator 3 and two more with inputs of an AND element 12 in operative connection stand.

The voltage U of the machine 1, measured analogously by the voltage converter 5, is also fed to a pulse converter 13, the output of which is connected to a further input of the AND element 12. The output of the AND gate 12 is connected to a control element of a bipolar switch 14 placed, which in turn via a burden 15 at the output of the transducer 6 and on the other hand via a Filter 16 connected to the input of the analog-digital converter 8 is.

This system now works as follows:

When switching on the induction machine at the time t = 0, current and voltage transformers continuously determine the values of current I and supply voltage U. In the start-up phase of the induction machine 1, the current I increases strong. The control and arithmetic unit 9 calls periodically after a memory signal has been output within a repeating time interval At the value of the current I just stored in the memory 10 at the time mAt, where m = 1, 2, 3,. ·., and those stored in memory 11 Threshold value I of the current and compares

Max / \

these values together. If the current exceeds I. at the time M at the threshold value I, so sets

m ο χ

the control and arithmetic logic unit 9 sends an activation signal A to an input of the AND element 12.

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The control and arithmetic unit 9 also has a timer, not shown, whose time information is continuous with the value of the decay time t called up from the memory 11 after each time period At. is compared. If the time specification of the timer exceeds the decay time t 1, the control and arithmetic unit 9 sends an activation signal A "placed on another input of the AND gate. The AND element 12 is now activated and the im Pulse converter 13 converted into square-wave pulses analog values the supply voltage U are now fed to the control element of the bipolar switch 14. That kind of thing The activated switch 14, together with the downstream filter 16, forms an active current I ,, = I-cosip proportional voltage signal, where ψ is the phase angle between current I and supply voltage U. This signal is digitized in the analog-to-digital converter 8 and corresponding to the digitized values of current I and Supply voltage U stored in memory 10.

The control and arithmetic unit 9 calls within the time period At that previously at the times m · At and (m + 1) At stored values I .. and I .. of the active current from the memory 10 and determined therefrom by forming the difference the differential active current.

AI - I ^{(m + 1)} - I ^{(m)
t0 üi} W " ¹ WW '

The sign of the differential active current AI ₁₁ determined in this way is continuously monitored for a change in sign in a device (not shown) provided in the control and arithmetic unit 9.

From the values U of the supply voltage retrieved from the memory 10 within each time span A t at the times mA t, where m = 1, 2, 3,..., And the values of t stored in the memory 11. and U _{n is} determined

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the control and arithmetic unit 9 continuously determines the start-up time t 'of the machine, which is dependent on the supply voltage U according to the algorithm

f = t ( ^Un > ² .

The value of the start-up time t!

the machine 1 is continuously monitored by the timing mechanism, not shown, of the control and computing unit 9. If the differential active current AI ₁ - changes. if its sign is within this start-up time t ', the machine has started up successfully. If, on the other hand, the sign does not change, the start-up process is disturbed, for example by blocking the rotor of the machine 1, and the control and arithmetic unit 9 issues a command A to the actuating element 3, by means of which the switch 2 is actuated to interrupt the current I. .

Here, the inventive start-up protection, the operation of an asynchronous rotary field machine shown in Fig. 2 in a simplified manner, takes advantage namely that the active current I ₁₁ η of the machine at start-up with increasing speed and decreasing slip s initially increases monotonically, when slip s, the maximum reached and then monotonically decreases. At nominal slip s _M , when the machine 1 is running in nominal operation and the phase angle φ between current I and supply voltage U passes through a minimum, the active current I. has already decreased again considerably, so that the differential active current ΔΙ., Long before that has changed its sign.

As can be seen from Fig. 3, the inventive Start-up protection interact in a simple manner with a short-circuit protection. The short-circuit protection provided here has three identical residual current transformers

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W _n , W_, W ₁ . with toroidal cores, which each against-K b I

It makes sense that one of three winding strands L _R , L-, L _{x of} the machine 1 are penetrated by the supply lines acting as primary windings. The secondary windings of the current transformers W _n , U-, W _x are not in sequence

K b I

The excitation elements shown are connected by identical differential relays Dp, D- and D _T , which act on the actuating element 3 of the switch 2 and on the start-up protection of the machine 1 labeled E. When a short circuit occurs, for example between the winding phases

L _n and L _r of machine 1, causes the differential current K b

converter W _n and W ^ uncompensated short-circuit current flowing through in the secondary windings of these current converters currents, which above a threshold value lying, for example, 0.1 to 0.2 times the rated current I of the machine to be protected, triggers the differential D _n and D ,, cause. These relays emit trigger signals A and A_ as well as locking signals V _n and ν _ς . The trigger signals cause the switch 2 to open and thus the interruption of the power supply to the electrical machine. On the other hand, the locking _{signals V n} and V ^ cause the start-up protection E to be blocked.

Claims (6)

- / - 151/83 Claims Start-up protection for a rotating asynchronous induction machine (1) with a current transformer for recording the current (I), a voltage transformer for recording the supply voltage (U), a device for generating a speed-dependent variable of the machine (1) from the recorded values of current (I ) and supply voltage (U) and a control device for checking this variable and for generating a command (A) to switch off the machine in the absence of a value of the speed-dependent variable signaling the successful start-up of the machine within a start-up time (t '), characterized in that _{the control device detects the active current (I 1.} ) of the machine as a speed-dependent variable and W ^ When the machine starts up, a signal that suppresses command (A) is generated when the value is exceeded of the tipping point (s,) of the machine within the Kp running time (t ') is caused by the active current (I _11). AW 2. Start protection according to claim 1, characterized in that the control device has a first memory (10) and a control and arithmetic unit (9) that in the memory (10) periodically determined values of the Active current (I ,,) of the machine are stored, and that the control and arithmetic unit (9) continuously the Difference (Δ Iy) of successively stored Values of the active current (Iy » ^I w" ^{1 + 1} ^ forms, and with a change in sign of such the differential active current (Λ I ₁₁ ) generated Suppression signal generated. 3. Start-up protection according to claim 2, characterized in that periodically determined in the first memory (10) 151/83 Values (U) of the supply voltage are stored, and that the control and arithmetic unit (9) continuously the Start-up time according to the algorithm determined, where t 'the continuously determined start-up time, t. the start-up time with defined load and mass moments of inertia as well as with a defined supply voltage (nominal voltage), U _n the nominal voltage of the machine, and U, the supply voltage at the time m · At with At as the period of the determined values and m = 1, 2, ...
mean. 4. start-up protection according to claim 3, characterized in that periodically determined in the first memory (10) Values (I) of the current are stored, which the control and arithmetic unit (9) continuously from the first Memory (10) retrieves and with a threshold value (I) des stored in a second memory (11) Max Current (I) compares. 5. start-up protection according to claim 4, characterized in that a timing element in the control and arithmetic unit (9) is provided, which after the threshold value is exceeded (I) and after a cooldown time (t.) max J. of direct current components, which occur when the machine is switched on, the device for generating the speed-dependent size of the machine is activated. 6. Start-up protection according to one of claims 1 to 5, characterized in that an input for a blocking signal (V _R , V-) is provided, which is emitted by a short-circuit protection of the machine when it responds