JP2004112906A - Detection method for crack of rotor bar of ac motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately detect a disconnection of the rotor bar of an AC motor even in a light load. <P>SOLUTION: In this detection method for the crack of the rotor bar of the AC motor, the number of revolutions of a rotor 12 is made to slip (S) relative to the synchronous number of revolutions S0 by reducing the generated torque of the motor itself by lowering a power supply voltage, a pulsation current component (2 x S x f0) contained in a motor current generated accompanied by a torque ripple caused by the crack of the bar is separated from a power supply frequency f0, and detection sensibility is thus improved. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of detecting a broken rotor bar of an AC motor, and more particularly, to a method of detecting a breakage of a connection portion between a rotor bar and a short-circuit ring of a generally used cage induction motor (referred to as an AC motor). The present invention relates to a method of detecting a broken rotor bar of an AC motor, which can be detected.
[0002]
[Prior art]
As shown in FIG. 1, a rotor 12 of a commonly used AC motor 10 has a structure in which, for example, a copper bar 16 is short-circuited at both ends of the rotor by, for example, an end ring (referred to as a short-circuit ring) 18 made of copper. And the connection is silver-welded. This connection portion has a large heat shrinkage stress due to an excessive current at the time of starting and stopping, and progresses from disconnection between the bar 16 and the short-circuit ring 18 → bar protrusion → stator coil damage → ground fault burning. In FIG. 1, reference numeral 20 denotes an iron core.
[0003]
Here, as shown in FIG. 2, power is supplied to the motor 10 from a power supply, and a load is connected to the output side. In FIG. 2, reference numeral 22 denotes an ammeter.
[0004]
When a bar break occurs, vibration and current ripples occur due to torque fluctuations caused by the breakage, and a method of diagnosing this (including frequency analysis) has been used.
[0005]
At this time, in the frequency spectrum of the current, as shown in FIG. 3, a pulsation component (2 · S · f0) (also referred to as a sideband) is present on both sides of the power supply frequency component (f0) (also referred to as a fundamental wave), and the bar is broken. (See Intelligent Equipment Diagnosis, pp. 104-105, published by Ohmsha) by Taro Shimozato. Japanese Patent Application Laid-Open No. 2000-184658 describes a diagnostic device for an induction motor using this method.
[0006]
[Problems to be solved by the invention]
However, in the conventional method, as described in the above literature, it is necessary for diagnosis that the motor load is 50% or more, and there is a problem that it is difficult to determine when the load is small.
[0007]
That is, since the slip (S) does not occur much at a light load, the pulsation component (2 · S · f0) approximates to the power supply frequency component (f0) and is separated as a frequency spectrum as illustrated in FIG. There was a problem that it was difficult.
[0008]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and has as its object to enable accurate detection of a broken rotor bar of an AC motor even under a light load.
[0009]
[Means for Solving the Problems]
In the present invention, when detecting the break of the bar of the rotor of the AC motor, preferably, by lowering the power supply voltage to reduce the torque generated by the motor itself, the rotation speed of the rotor is compared with the synchronous rotation speed S0. The above problem is solved by slipping (S) and separating the pulsation component (2 · S · f0) included in the motor current generated due to the bar breaking from the power supply frequency f0 so that the level of the pulsation component can be measured. It was done.
[0010]
Further, the slip is set to 0.5% or more.
[0011]
According to the present invention, as a method of separating a pulsating current component (2 · S · f0) generated by a rotor bar breaking under a light load state from a power supply frequency (f0), a slip by a brake or a load device which is usually adopted is used. By varying the power supply voltage with respect to the generating means, the generated torque of the motor itself is reduced, and a slip is generated with a light load. Therefore, bar breakage can be detected relatively easily and accurately with no need for major equipment modification.
[0012]
The present invention utilizes the characteristic that the torque generated by the AC motor is proportional to the square of the power supply voltage. As shown in FIG. 4, when the power supply voltage is reduced from the rated voltage (V), the torque curve is reduced. And the balance point deviates even with a light load, and the actual rotational speed is delayed with respect to the synchronous rotational speed, that is, slip occurs.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0014]
As shown in FIG. 5, for example, a diagnostic device for implementing the present invention is provided with a power supply voltage varying device 30 for changing a power supply voltage applied to the AC motor 10, and is provided to the AC motor 10 from the power supply voltage varying device 30. For example, a clamp-type current detector 32 for detecting a primary current, a frequency analyzer 34 for performing frequency analysis (for example, FFT) of a current detected by the current detector 32, and an output of the frequency analyzer 34 A bar break diagnosis determining device 36 for diagnosing a bar break in response thereto, and a rotation speed detector 38 for detecting the rotation speed of the AC motor 10 are provided.
[0015]
Hereinafter, the operation of the diagnostic device will be described.
[0016]
The power of the AC motor 10 is supplied from the power supply voltage varying device 30, and the power supply voltage is reduced until the slip S of about 1% occurs while measuring the rotation speed with the rotation speed detector 38.
[0017]
The primary current detected by the current detector 32 at that time is used as an input, and the spectrum is analyzed by the frequency analyzer 34, and the pulsating current level and the pulsating current / power supply frequency level ratio (pulsating current ratio) are shown in FIG. Utilizing such a relationship, the presence or absence of bars and the number of bars are detected.
[0018]
【Example】
7 to 9 show the results of the bar breakage diagnosis of the AC motor. The rating of the motor is 7.5 kW, 400 V, 6 poles (6P), 975 rpm, and the slip (S) at the time of rating is 0.025 (2.5%).
[0019]
FIG. 7 shows a result of a diagnosis by a conventional diagnosis method, and shows a result of frequency analysis of a current when one rotor bar is cut off at a load ratio of 35% with a small load, Since the rotational speed is 999 rpm and the synchronous rotational speed (S0) is close to 1000 rpm and the slip is small, the pulsation component (sideband wave) is erroneously recognized, the level of the pulsation component is determined to be small, and it is erroneously determined that there is no bar break. I have.
[0020]
On the other hand, FIG. 8 is a frequency analysis of the current when the power supply voltage is gradually reduced according to the present invention and 50 V is applied to the rated voltage of 400 V. The rotation speed is 992 rpm and the slip (S) = 0. 0.008 (0.8%), and the sideband (50 ± 0.8 Hz) is clearly separated from the power supply frequency of 50 Hz. As a result, it is possible to accurately determine that one bar has run out.
[0021]
FIG. 9 shows an embodiment in which two bars are cut. When the power supply voltage is set to 50 V, the number of rotations drops to 990 rpm, the detection level of the sideband increases, and the two bars are accurately judged. is made of.
[0022]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, detection sensitivity is raised and the bar | burr break of a rotor can be detected correctly even with a light load, without being bound by the conventional diagnostic conditions of 50% or more of load factors.
[0023]
Also, in the case of diagnosis of the motor alone, since there is no current component due to online load or noise, accurate current analysis can be performed, and if a disconnection is about to occur, disconnection prediction, rotor imbalance, bearing failure, etc. Diagnose.
[0024]
That is, the bar of the rotor of the AC motor can be diagnosed in any state, and the early detection of the bar can reduce sudden accidents and sudden repair costs.
[Brief description of the drawings]
FIG. 1 is a perspective view showing the configuration of a rotor of an AC motor. FIG. 2 is a front view showing the entire configuration of the AC motor. FIG. 3 is a spectrum diagram showing the principle of bar break detection. FIG. FIG. 5 is a diagram illustrating a slip occurrence state when the power supply voltage is reduced for the sake of explanation. FIG. 5 is a block diagram illustrating an overall configuration of a diagnostic device to which the present invention is applied. FIG. FIG. 7 is a diagram showing an example of the relationship between the ratios. FIG. 7 is a spectrum diagram showing a diagnosis result obtained by a conventional diagnosis method. FIG. 8 is a spectrum diagram showing one bar bar being diagnosed according to the present invention. FIG. Spectrum diagram when a book is diagnosed [Explanation of symbols]
10 AC motor 12 Rotor 16 Bar 18 Short-circuit ring (end ring)
Reference numeral 30: power supply voltage varying device 32: current detector 34: frequency analysis device 36: bar break diagnosis / judgment device 38: rotational speed detector

Claims (2)

When detecting the burnout of the rotor of the AC motor,
By reducing the torque generated by the motor itself, the rotation speed of the rotor is slipped (S) with respect to the synchronous rotation speed S0,
The pulsation component (2 · S · f0) included in the motor current generated due to the bar breaking is separated from the power supply frequency f0,
A method for detecting a broken rotor bar of an AC motor, wherein a level of the pulsation component can be measured. The method according to claim 1, wherein the slip is 0.5% or more.

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