Detection of stator short circuits in inverter-fed induction motors
Lucia Frosini, Ezio Bassi, Luca Girometta
Dipartimento di Ingegneria Industriale e dell’Informazione, Università di Pavia [email protected]
Abstract-In this paper, an experimental investigation about the use of non-invasive instruments, like stator current and external leakage flux analysis, to detect inter-turns short-circuits in stator windings of low voltage induction motors supplied by inverter is reported. The results are encouraging and the technique here proposed can be evaluated as a tool to detect these faults in their early stage in motors which are more stressed with respect to those supplied by the mains, due to the IGBT PWM feeding.
I. INTRODUCTION
It is well known that the stator winding insulation of inverter-fed motors is more stressed and, then, more subject to premature deterioration with respect to the insulation of motors supplied by the mains [1]. This situation is common to motors fed by a wide range of voltage values, even if the ageing process of their insulation is different according to their winding type. For this reason, two different technical specifications have been developed by IEC, in the attempt to reduce the number of failures associated with adjustable speed drives: IEC 60034-18-41 for Type I insulation systems (rated voltage <700 V) and IEC 60034-18-42 for Type II insulation systems (rated voltage ≥700 V) [2]. Such technical specifications report the qualification and quality control tests for these electrical insulation systems used in rotating electrical machines fed from voltage converters, but they do not consider the tests that could be performed during the normal operations of these machines, aimed at detecting possible failure on the stator windings in their early stage.
In this paper, an experimental investigation about the use of non-invasive instruments, like stator current and external leakage flux analysis, in order to on-line detect inter-turns short-circuits in Type I stator windings supplied by inverter is reported and compared with the results obtained in previous researches by other authors.
II. STATOR WINDING PROBLEMS IN INVERTER-FED MOTORS
For motors fed at voltage less than 700 V (Type I), the winding stator is usually in random-wound form, i.e. consisting of insulated copper conductors that are wound continuously through slots in the stator core to form a coil. Each turn could be placed randomly against any other turn in the coil, therefore the first and last turns of one or more coils may be adjacent [3]. Due to the low level of voltage, this type of winding insulation is not expected to experience partial
discharge (PD) activity during its normal operation, if supplied from a sinusoidal waveform.
On the contrary, machines operating at higher rated voltages (Type II) are provided with form-wound coils, which are organized in distinct packages consisting of a number of stranded conductors. Unlike random-wound windings, the turns in form-wound windings always occupy a defined location within the coil. These windings, due to the high level of voltage, may experience PD, even if the voltage has a sinusoidal waveform. For this reason, the principal characteristic of these insulation systems is the use of mica and inorganic filler materials treated with VPI or resin-rich pressing operations to minimize voids and to resist degradation by PD [4]. In practice, when the voltage is sinusoidal, in Type I systems the ageing mechanisms are thermo-mechanical rather than electrical, whereas in Type II systems the principal ageing mechanism is electrical [5].
The main stresses due to the feeding by IGBT PWM inverters are: high frequency switching (the frequency can reach 20000 pulses per second), short rise time (the inverter output can increase from 0 to 690 V in one–millionth of a second), transient voltage spikes (each rectangular pulse begins with a spike of overvoltage nearly twice the DC bus voltage), reflected wave voltage (long cables between inverter and motor can amplify three or four times the rated voltage at the motor terminals), additional heat. These stresses can accelerate the ageing of Type I insulation systems, due to the vulnerability to PD attack of the very thin organic insulation on the wires. In random-wound stators, there are often small air-gaps inter turns, between turn and ground and between phases. The high valued and brief voltage pulses, due to PWM, can cause the electrical stress in these small air gaps to exceed the dielectric breakdown strength of air (about 3 kV/mm), resulting in a PD. Repetitive PD will eventually erode the organic insulation film on the magnet wire or even the ground and phase insulation barriers. If PD occurs as a result of impulses from an inverter in a stator winding not designed to withstand such discharges, the insulation will eventually fail [6-8].
III. THE STATE OF THE ART
The possibility to detect inter-turns short-circuits in induction motors with Type I insulation, by means of both the motor current signal analysis (MCSA) and the external leakage flux analysis, has been investigated by different authors in the last twenty years. A review of the literature on
this topic is reported in [9]. In most cases, the authors have considered only motors fed by the mains, but some studies have been devoted to inverter-fed drives, e.g. [10],[11], even in an industrial environment [12].
In fact, when the motor is part of a closed-loop system, some effective techniques for the mains-connected motors may be totally inappropriate, while others may constitute an adequate diagnostic tool. In [11], the monitoring of the 3rd harmonic of one phase current was found to be an effective method to detect stator faults in case of a DTC induction motor drive. The drawback of this technique is that the possible inherent asymmetries of the motor may lead to the appearance of the 3rd harmonic component in the supply current, even if no faults are present.
In [10] the authors prove that the analysis of the external leakage flux is more reliable than the MCSA, especially when the number of shorted turns is small. The stray flux sensor is built around a circular air-coil and placed near the machine body, parallel to its shaft. The diameter of the air coil has to be much less than the total height of the machine body, in order to measure the flux in an area covering the width of several stator slots. Then, the dimension of this coil is related to the machine size. A variable gain amplifier is connected at the terminals of this flux sensor and a low-pass anti-aliasing filter is implemented in order to set the frequency bandwidth to a correct range for spectrum analysis. The tests have been carried out on an 11 kW, 4 poles, squirrel cage, induction motor supplied from both the mains and inverter, with six turns short-circuited in one phase; in all cases the motor is at standstill. The authors prove that the 3rd and the 9th are the more excited flux harmonics of the supply frequency, for both types of supply (mains and inverter), and the variation between the healthy and faulty conditions of these harmonics is about 20 dB.
A different approach has been proposed in [13],[14], where the idea is to use the inverter to perform the standard off-line insulation tests (capacitance, dissipation factor, insulation resistance) whenever the motor is not operating.
None of the previous papers has developed a 3D-finite element analysis showing the distribution of the leakage flux around the motor, even if in [15] a diagram representing the regions of flux is reported. A common consideration found in the literature is that the external leakage flux distribution is complex to model, but not really necessary, because the diagnostics by means of the leakage flux could be based on the relative changes of given harmonic components, without determining their absolute amplitude values. On the contrary, the idea of placing a flux probe inside a low voltage induction motor is impracticable, at least for two reasons: the small length of the air-gap (about 1 mm) and the increase of the cost of the motor due to the addition of this device during its construction.
IV. THE EXPERIMENTAL TEST-BENCH
The experimental test-bench is made up of a three-phase cage induction motor (rated power 1.5 kW, 4 poles, 36 stator
slots, 46 rotor slots), supplied by a VSI inverter with PWM, equipped with IGBT. The motor has been tested in two conditions, i.e. at no-load and at rated load. In the latter case, it is coupled with a brake.
The stator winding of the motor has been realized on purpose, to simulate inter-turns short circuits within one phase: on the supply terminals, three wires derived from one phase are accessible and allow to short-circuit the 5%, 10% or 15% of the total number of turns (294) of that phase.
Two different strategies have been implemented to simulate the short circuit. The first one is similar to that implemented in [10] and in most of the literature: two of the additional external wires have been connected to form a short circuit inter 15 turns, about 5% of the turns of the whole winding of one phase. In these short circuited turns a fault current can flow, due to the electromotive force induced from the rotor magnetic field. In the present work, the choice was to limit the fault current to the rated value of the current of the motor, by inserting a variable resistance between the shorted wires; in this way, the fault current is not harmful for the machine during the time required by the measurements and the supply voltage can be maintained at its rated value.
With the second strategy, not mentioned previously in the literature, the supply was provided directly on the first additional wire and downstream the first 15 turns, by reducing in this way of 5% the overall impedance of this phase.
The following types of measurement have been carried out: - current of a given phase (always the same, chosen
between the two healthy phases), collected by a commercial probe Tektronix TCP305, with its amplifier TCP305 and a filtering stage realized in laboratory;
- axial leakage flux, collected by a commercial flux probe Emerson M-343F-1204, which is a circular air-coil positioned in front of the motor, on the opposite side to the load;
- axial and radial leakage flux on the body of the motor and radial leakage flux on the end-winding (on the load side of the motor), collected by means of an experimental flux probe (Fig. 1), realized in laboratory and already presented in [9] [16].
Both flux sensors have been used together with an amplification and filtering stage, designed and realised on purpose in laboratory (Fig. 2, Fig. 3): its characteristics and functionality are explained in detail in [9].
The measurement of the leakage flux in different positions allows obtaining an experimental evaluation of the 3D distribution of the magnetic flux around the motor.
In the healthy operating condition, twenty consecutive acquisitions were carried out for each type of measurement, while maintaining a constant value of load, the same position of the experimental flux-probe and the same supply frequency. For the measurements in the faulty operating conditions, the number of acquisitions has been limited to ten, in order to avoid over-stress on the machine, since there were no significant deviations from a statistical point of view.
The sampling parameters were: sampling frequency 10 kHz; sampling period 50 s; number of samples 500000; FFT step 0.02 Hz.
Fig. 1. The experimental flux probe radially positioned on the motor.
Fig. 2. The amplification and filtering stage.
Fig. 3. The scheme of the amplification and filtering stage.
V. EXPERIMENTAL RESULTS
In this paragraph, the synthesis of the experimental measurements is reported, with the aim to compare the flux and the current spectra with normal operation of the machine and in case of inter-turns short-circuit, by analyzing the variations of the harmonic components multiple of the fundamental (which is equal to 30, 40 or 50 Hz). In the following tables, which summarize the experimental results for each probe or different arrangement of the probe, are pointed out the maximum variations between the absolute values of the considered harmonics in healthy and faulty conditions. In red color are highlighted the harmonics for which this variation exceeds five times the standard deviation of the harmonics collected in the healthy condition (Δ > 5s) for both the type of short-circuit simulations (by resistance
and by supply). In orange color are shown the harmonics for which Δ > 5s only for one type of fault simulation, whereas for the other one Δ > 2s. The choice to evaluate the variation of the harmonics between the healthy and faulty conditions with respect to the standard deviation of the measurements allows to obtain a rather “robust” identification of the diagnostic indicators by avoiding measurement errors.
From the no-load measurements (tables from 1 to 5), it is possible to observe that:
- the experimental probe radially arranged on the body of the motor does not provide particularly useful information to detect this fault; moreover, the experimental results are not coherent for all the considered supply frequencies (Table 1);
- the informative content associated with the experimental probe axially positioned is more consistent, for diagnostic purposes, with respect to the previous arrangement; however, a certain discrepancy among the three considered supply frequencies can be noticed (TABLE 2);
- the experimental probe in radial position on the end-winding provide extremely significant and very homogeneous results at the three supply frequencies: it is apparent that this position is more suitable to reveal possible anomalies related to inter-turns short-circuits within one stator phase (TABLE 3);
- as for the Emerson probe, although it does not give particularly useful results for this diagnostic activity (TABLE 4), nevertheless it provides stronger evidences with respect to the experimental probe radially arranged on the motor body;
- the 3rd, 7th and 13th harmonics of the stator current behave as significant indicators of this fault at the three supply frequencies (TABLE 5).
As a conclusion, at no-load, the harmonic spectra of the current and of the flux collected on the end-winding give a sound informative content for diagnostic purposes: in particular, both signals show remarkable variations in the 7th and 13th harmonics at the three supply frequencies.
Similar observations can be extrapolated from the measurements with the motor at rated load:
- the radial flux on the motor body is more significant at rated load with respect to the no-load condition: in particular, the 3rd harmonic is index of misoperation common to the three supply frequencies. Nevertheless, there is a certain inhomogeneity among the results at the three different supply frequencies (TABLE 6);
- the harmonic spectra obtained from the experimental probe in axial position on the motor body give a rather satisfying information, which is definitely better with respect to the no-load condition (TABLE 7);
- as in the no-load condition, the flux probe on the end-winding provides a diagnostic content higher than for the other positions (TABLE 8): the results at rated load are in any case more significant with respect to the no-load condition;
- the harmonic spectra obtained by the Emerson flux signal do not contain any harmonic component which is a common fault indicator to the three supply frequencies (TABLE 9);
- the MCSA at rated load does not show particularly significant results and a correspondence with the end-winding flux analysis similar to that highlighted in no-load condition (TABLE 10 is in fact lacking).
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A full analysis of the measurements highlights that the flux sensor more sensible to detect the inter turns short circuits is the experimental one, when it is located on the end-winding on the load side of the motor. The transducer here positioned has revealed significant variations between the faulty and the healthy conditions, for both values of load (no-load and rated load) and for the three considered supply frequencies (30, 40 and 50 Hz). Note that the order of magnitude of these variations is similar to that found in [10], i.e. 20÷30 dB, even if the harmonics more excited in faulty condition do differ in general with respect to those revealed in [10]. The other two positions in which the experimental flux probe has been located have provided inhomogeneous results, highlighting more sensitivity when the motor is loaded.
The commercial flux probe, here used for comparison, has shown a much lower informative content. It is necessary to underline that this probe is typically used on motors with a frame diameter much larger than its own diameter (which is 200 mm internal, 230 mm external); in the case here analyzed, the opposite is true (diameter of the motor lower than diameter of the probe). Then, it is possible to ascribe to this situation the poor sensitivity of this transducer for this fault. By disregarding this last observation and by supposing that both flux probes are equally sensitive to the fault, it is apparent that the experimental one allows to carry out measurements on different points of the stator frame, thus enabling the identification of the location which results more significant for diagnostic purposes. On the contrary, the commercial probe, from this point of view, has more limits.
The comparison with the current signal analysis has provided further corroborations to the possibility of employing the analysis of the harmonic spectrum of the leakage flux as a technique to recognize the incipient faults in the stator winding of an inverter-fed induction motor. In fact, two important hints are provided by the experimental measurements shown in the tables:
i) at no-load, there is coherence between the information yielded by the harmonic spectra of the flux collected with the experimental probe on the end winding and of the current, since there are two common harmonics which vary consistently at the three different supply frequencies (the 7th and 13th);
ii) at rated load, the leakage flux is more sensible with respect to the current of one phase, resulting as a more reliable indicator; this is probably due to the fact that, at rated load, the inter-turns short-circuits give rise to substantial disturbs on the symmetry of the machine, which are most reflected in a flux variation, instead of a current variation.
VI. CONCLUSIONS
The experimental analysis reported in this paper brings to the following conclusions. A precise and accurate technique for the acquisition and elaboration of the harmonic spectra of the signals provided by different measurement transducers has been defined. Besides, a methodology of statistical analysis which allows verifying and quantifying significant variations of specific harmonics has been identified. The
possibility to employ the leakage flux analysis as a diagnostic indicator for inter-turns short-circuits has been confirmed; this method seems more incisive for inverted-fed motors rather than for motors connected directly to the mains.
The method here presented is not yet suited to be applied in the industry, since further corroborations on its efficiency in the identification of the stator faults have to be achieved, however the results here presented are encouraging. This is a preliminary study requiring further works focused both on the finite element analysis of the distribution of the external leakage flux around a motor and on experimental measurements on motors in industrial environment.
ACKNOWLEDGMENT
The authors wish to thank Mr. Andrea Albini for his valuable help in carrying out the experimental measurements.
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