GB2044940A - Testing of induction motors - Google Patents

Abstract

A method of and apparatus for testing induction motors is described which enables the condition of the motors to be monitored during normal operation. Any defect in the rotor circuit causes the torque to vary as the defective part interacts with the stator field and hence causes speed variations at twice the slip frequency. By means of transducer 12 (Figure 1), the time instants of rotor passage past a datum point are determined and, by using a ramp circuit 22 and a rotating time base sweep, an oscilloscope 20 displays the relationship between period of rotation and phase angle at the slip frequency and indicates the amplitude of the speed fluctuations at twice that frequency. <IMAGE>

Description

SPECIFICATION Testing of induction motors This invention relates to the testing of induction motors and is concerned more particularly with the detection of defects in the circuit (normally the rotor circuit) which co-operates with the electrically energised windings (usually the stator of the motor).

In considering this matter, it is convenient to assume that the energised windings are stator windings. The magnetic field produced by the alternating currents flowing in the stator windings induces currents to flow in a separate circuit, normally forming part of the rotor, and these currents produce a magnetic field which interacts with the originating field creating a torque. In an induction motor this torque causes the rotor to turn at a rate which is fractionally slower than that of the originating magnetic field. The small difference in rotational speeds, expressed as a fraction or percentage of the rotational speed of the originating magnetic field, is defined as the slip.

According to the present invention, a method of testing an induction motor comprises indicating speed fluctuations occurring at twice the slip frequency when the motor is energised with a constant frequency alternating supply. In the six pest form, the amplitude of the speed fluctuations at twice the slip frequency may be indicated or an indication may be given when this amplitude exceeds a predetermined level.

At any instant in time, only part of the rotor circuit will contribute significantly to the torque required to turn the rotor. This part of the rotor circuit will be the part which is then carrying significant current. If there is any defect in this part of the rotor circuit, then the torque produced will be altered, usually reduced, and the speed of the rotor will change.

However as the rotor circuit slips through the stator field (at the slip frequency), a different part of the rotor circuit will carry current and the torque will recover and the rotor speed will return to normal.

Because the rotor changes speed each time the defective part interacts with the field produced by the stator windings, and independently of the direction of the polarity of field, the rotor will exhibit speed fluctuations at twice the slip frequency.

The slip frequency is sf where s is the slip and f is the power supply frequency. The slip frequency sf is equal to f- p(j where p is the number of pairs of poles in the stator and co is the rotational speed. Thus (o = f/p (1 - s). For most motors under load and energised from a 50 Hz supply, the slip frequency is in the range 0.5 to 5 Hz.

The present invention makes use of these speed fluctuations at twice the slip frequency to detect any defect in the rotor circuit.

According to another aspect of this invention, a method of testing an induction motor comprises the steps of repetitively measuring the phase angle of an alternating energy supply when the rotor of the motor passes through a predetermined angular position or positions, and determining any irregularity at twice the slip frequency occurring in the rotational period of the rotor.

If the rotor speed is constant, the measured or sampled phase angle of the alternating supply, when the rotor passes through a predetermined angular position, will change gradually, corresponding to the slip. The rate of change of measured phase angle would thus be uniform if the rotor speed is constant.

If the rotor speed changes however then the measured changes of phase angle between each time the rotor passes through a predetermined angular position will change and the measured rotor period will change. As explained above, a defect in a rotor will cause such irregularities to occur at twice the slip frequency. This frequency is very much lower than the alternating supply frequency and hence such changes can be detected by a measurement made once in each rotation of the rotor. Conveniently therefore the phase angle of the alternating supply and the rotor period are measured once in each rotation of the rotor.This may readily be done using a sensor, preferably a non-contacting sensor such as magnetic or optical detector to determine each time the rotor passes through a predetermined angular position, this signal then being compared with the alternating supply to determine the phase angle of the supply at that instant. It would be possible however to make measurements at a plurality of positions during each rotation if so desired.

In many applications, a motor drives a load which may have cyclic or random variations. These cause the rotor speed to change and, in particular, to be non-uniform in each cycle of rotation. By detecting the changes at twice the slip frequency, however, it is possible to detect speed fluctuations indicative of a rotor defect whilst eliminating the cyclic or random speed fluctuations.

Preferably the rotor period is indicated visually.

For example using a cathode ray tube display, which may be arranged to show the change of rotor period during a cycle of change in the measured phase angle, or more simply on a meter in which the magnitude of the changes in rotor period at twice the slip frequency is displayed, for example, by a pointer. Alternatively the information may be processed digitally to provide a visual digital display indicating speed fluctuations. The display may be arranged to indicate the relationship between rotor period and phase angle of the supply frequency.

According to another aspect of the invention, apparatus for testing an induction motor comprises a sensor for giving an output signal when the rotor passes through a predetermined angular position or positions and means for determining the phase angle of an alternating energy supply for the motor at the time instants sensed by said sensor and for determining therefrom the rotational period of the rotor or changes therein and indicating means responsive to variations, at twice the slip frequency, in the rotational period of the rotor.

As indicated above, the sensor is preferably a non-contacting sensor for example a transducer magnetically or optically sensing a co-operating element or marking on the rotor to produce an output signal each time the rotor passes through a predetermined angular position.

In one simple form of construction, said indicating means comprises a cathode ray tube, means for producing a time base trace synchronised with said alternating energy supply and means for producing a deflection modulation of the time base after each sensor output signal, the modulation being of an amplitude dependent on the time interval between that sensor output signal and the preceding sensor output signal. Said means for producing a time base may be arranged to produce a polar time base, for example by applying phase quadrature signals derived from the alternating supply to orthogonal deflector plates or deflection coils of the cathode ray tube display; the radius of the polar time base may be controlled in accordance with the aforementioned deflection modulation.Preferably means are provided for applying the output signal from the sensor or a signal derived therefrom as a brightness modulation on the cathode ray tube whereby the display comprises a pattern of dots, occurring as the rotor passes through said predetermined angular position. With this arrangement, if the rotor speed remains constant, the cathode ray tube will show a circular display whereas if the speed fluctuates, the radial extent of the display will change. Assuming a rotor defect produces a decrease in speed, and if this decrease in speed produces an increased radial deflection, the angular positions at which the radius of the display on the cathode ray tube is a maximum will indicate the phase angle of the supply at which the defective part of the rotor is attempting to carry current at the instant of passage past the sensor.The appearance of the display immediately indicates the presence of a defect.

In another arrangement, an oscillator is provided with phase lock control means responsive to the sensor output for locking the oscillator to the mean rotor speed and wherein phase comparator means are provided to compare the phase of the sensor output with that of the oscillator to give a signal representative of speed fluctuations of the rotor and wherein phase sensitive detecting means are provided having a reference signal at twice the slip frequency for detecting variations in the output of the phase comparator means at twice the slip frequency. It is readily possible to ensure that the oscillator operates at a frequency corresponding to the average motor speed, by arranging the control circuit to smooth out any speed fluctuations which are synchronous with the slip or at higherfrequen- cies.The output of the sensor can then be used to pulse sample the oscillator output to give, for each instant of sensing, a signal indicating whether the rotor is running fast or slow compared with the mean rotor speed. If the rotor is defective, such a signal will fluctuate at twice the slip frequency and it may be compared with a reference signal at twice the slip frequency by phase sensitive detecting means to obtain an output representative of the phase and magnitude of the speed fluctuations. Two reference signals in phase quadrature may be employed. The quadrature phase reference signals at twice the slip frequency can conveniently be obtained by pulse sampling once per revolution and holding till the next sample a signal of twice the supply frequency.

It will be noted that the above-described forms of testing apparatus may readily be used for the monitoring of induction motors which are in normal operation. Scanning means may be provided for automatically connecting the testing apparatus in sequence to each of a number of induction motors, each induction motor having its own separate sensor. If an output is obtained representative of the amplitude of speed fluctuations, provision may be made for operating an alarm when the amplitude of the signal exceeds a predetermined amount.

The above form of apparatus may be used for example to detect the failure of the conductor bars in the rotors of squirrel-cage motors. It may also be used for motors having wound rotors which may be connected with external resistors via slip rings; in this case defects in any part of the rotor circuit would be detected.

The following is a description of two embodiments of the invention, reference being made to the accompanying drawings, in which: Figure lisa diagram of apparatus for testing an induction motor and having a cathode ray tube display; Figures 2 and 3 illustrate display patterns obtained with the apparatus of Figure 1; and Figure 4 is a diagram illustrating testing apparatus arranged for testing a plurality of induction motors in sequence and operating an alarm and an indicator if a defective rotor is detected.

Referring to Figure 1 there is shown diagrammatically an induction motor 10 energised from a 50 Hz alternating supply 11. A transducer 12 co-operates opticaily with a stripe 13 on the motor shaft or magnetically with a magnetic protrusion or insert in the shaft to provide an electrical output on a lead 14 once in each rotation of the shaft. This output signal has a waveform as indicated at 17 and which is essentially a pulse occurring once in each revolution of the rotor. The output signal is applied to a Schmitt trigger circuit 15 to give a short duration rectangular pulse once per revolution as indicated at 16. This signal is used to trigger a monostable circuit 18 giving output pulses of uniform, predetermined duration, as indicated at 19, which pulses are applied as a brightness modulation on a cathode ray tube 20 having a long persistent display screen 21.These pulses also are applied to a timing and ramp circuit generator 22 to produce a ramp waveform starting at a predetermined time interval after each pulse from the monostable circuit 18 and terminated by the next pulse from that circuit. Changes in the time interval between the monostable pulses thus cause corresponding changes in the ramp waveform duration; by delaying the start of the ramp, changes which are small compared with the interval between the pulses can become a significant fraction of the ramp duration.

The 50 Hz signal from the supply 11 is fed through phase shifters 24, 25, giving phase shifts of + 45" and - 45" respectively, so as to provide phase quadrature outputs to multipliers 26, 27 providing X and Y plate deflection signals for the oscilloscope 20 on leads 28, 29 respectively. In the absence of any other input to the multipliers, these signals would produce a circular display trace on the cathode ray tube screen. The output from the monostable multivibrator brightens the trace once in each revolution of the rotor of the motor and hence would produce a bright dot on the display which would move around the circular trace at the slip frequency.

The timing and ramp circuit generator 22 produces a time base waveform increasing linearly, as shown at 30, during the latter part of each period between successive pulses from the monostable circuit 22.

This output waveform is applied to the aforementioned multipliers 26, 27 so that the distance of each dot from the centre of the screen is linearly related to the last measured period of revolution. The angular position of each dot is determined by the phase angle of the supply at the time at which the monostable output pulse occurs and thus is proportional to the angle between the stator field and a datum determined by the reference mark sensed by the transducer 12. It will thus be seen that, whilst a healthy rotor, in which the time intervals between successive outputs from the transducer 12 are uniform, will give a circular display of dots on the screen as shown at 31 in Figure 2, a defective rotor, in which these time intervals vary, will give a display that is non-circular.If there is one defective current conductor in the rotor, the display is like a figure eight (as shown at 32 in Figure 3) since the radial distance increases when a defective part of the rotor is attempting to carry current at the instant of passage of the mark past the sensor. Because the rotor speed will fall each time the defective part interacts with the field produced by the stator windings, the rotor will have speed fluctuations at twice the frequency of slip thereby resulting in the figure eight pattern. Random speed fluctuations will broaden the pattern of dots and hence will set a limit to the smallest slip synchronous speed fluctuations that can be detected. A circular graticule may be generated electronically and displayed in the display screen. A quartz crystal calibrator may be provided for this graticule for calibration of the display.

Figure 4 illustrates another embodiment of the invention. In Figure 4 there are shown four induction motors 40, 41,42, 43. Each has a non-contacting transducer 44 for producing an output pulse once in each revolution of the motor and these transducers 44 are connected in sequence by an automatic scanning switch 45 to an input lead 46 for the test apparatus. The switch 45 is operated relatively slowly, giving time for the display of the condition of each motor before switching to the next motor. The signals on lead 46 are essentially a pulse occurring each time the rotor of the motor being tested passes through a predetermined angular position. The pulse is applied to a Schmitt trigger circuit and monostable 47 to give output pulses of predetermined short duration.The motors are all energised from a common 50 Hz alternating mains supply indicated diagrammatically at 50. A phase-locked loop 51 is also energised from the supply to provide sine and cosine phase output signals synchronised with the supply but at a frequency of 100 Hz on leads 52, 53 respectively. This phase-locked loop comprises a voltage-controlled square wave oscillator 54 operating at 200 Hz. In this particular embodiment, this oscillator gives a square-wave output which is passed through two divide-by-two stages 55, 56 in series so providing a 50 Hz input which is compared in a phase comparator 57 with the phase of pulses from a trigger circuit 58 connected to the incoming mains.The output of the phase comparator indicative of any phase difference, after passing through a low-pass filter circuit 59 to remove any high frequency components, is applied as a control voltage to the oscillator 54. The 100 Hz output from the first of the divide-by-two stages 55 is utilised as a sine phase output on a lead 52. The output from this divide-bytwo stage 55 is also applied to an exclusive OR gate 60 fed also with a 200 Hz output so as to give on lead 53 a cosine phase output at 100 Hz. The sine and cosine phase signals on leads 52, 53 are squarewave signals in quadrature time phase. These signals are applied to respective latch units 63 triggered from the aforementioned Schmitt trigger and monostable circuit 47 so as to hold the sine and cosine phase for the period of each revolution of the rotor.In each revolution, the pulse applied to the latches is slightly later (in phase) compared with the previous pulse, by an amount depending on the slip frequency. Since sine and cosine phase signals are at twice the supply frequency, the outputs of these latches 63 on leads 64, 65 are sine and cosine phase signals at twice the slip frequency. These are used as reference signals in phase sensitive detectors 66, 67 to detect respective sine and cosine phase components at twice the slip frequency in signals on a lead 68 representative of detected variations of the rotor period from the mean rotor period.

A second phase-locked loop 70 includes a voltagecontrolled square-wave oscillator 71, giving an output of pulse form, as shown at 72, with the pulse period equal to the mean rotor period. These pulses from the oscillator 71 are compared in a phase comparator 74 with the signals from the Schmitt trigger and monostable circuit 47 to give pulse-width modulated output pulses on the lead 68 having a waveform as indicated at 76 with pulses of one polarity if the rotor is fast and pulses of the opposite polarity if the rotor is slow. The output of the phase comparator is also applied on lead 77 through a low pass filter 78 to give a smoothed analogue control voltage for controlling the oscillator 72. The low pass filter provides a smoothing to smooth out any fluctuations related to the slip frequency so that the oscillator operates at the mean rotor speed. The output pulses on lead 68 are applied to the aforementioned phase sensitive detectors 66, 67 which, as previously explained, have respective sine and cosine phase reference signals at twice the slip frequency. The outputs of these phase sensitive detectors are thus signals representative of sine and cosine components of the motor speed fluctuations at twice the slip frequency. The phase sensitive detectors 66, 67 may be considered as demodulating or multiplying the pulse train on lead 68 by the reference waveforms and give output pulse trains which are integrated to give analogue voltages.

Such integration may be effected by smoothing the pulse trains in low pass filters 80,81 to give analogue voltage outputs having amplitudes indicative of the amplitude of the sine and cosine components of the rotor speed fluctuations at twice the slip frequency. These signals are fed to a rectifying and weighting network 82 operating an indicator 83.

Mathematically, for strict accuracy, the network 82 should evaluate the square root of the sum of the squares of the two components. For practical purposes, however, adequate accuracy may be obtained by a simpler operation, for example adding the moduli of the two components and including an additional proportion ofthe larger modulus. This evaluation may thus readily be effected using operational amplifiers. In this particular embodiment, the indicator 83 is an analogue indicator having a pointer 84 moving over a scale 85 to indicate the amplitude of the speed fluctuations at twice the slip frequency. If the rotor is healthy, and the rotational speed is uniform there is no component at twice the slip frequency on lead 68 and hence there will be no output from the phase sensitive detectors 66, 67. The indicators 83 will indicate zero.Random speed fluctuations due perhaps to a changing load on the motor will give rise to output signals from the phase sensitive detectors 66, 67 but these signals will not pass through the filters 80, 81. If there is a defect in the rotor, there will be speed fluctuations at twice the slip frequency which are detected by the phase sensitive detectors 66, 67 and indicated on the indicator 83. The output from the rectifying and weighting network 82 also operates an alarm threshold unit 86 which, in this particular embodiment, is connected by a scanning switch 87 operated in synchronism with the aforementioned scanning switch 45 so that the appropriate one of a number of alarms 88, conveniently warning lamps is operated in accordance with the particular motor which is found defective.The alarm lamps 88 have latches 89 so that they remain lit until the latches are reset.

Although the apparatus described with reference to Figure 4 is partly analogue and partly digital in operation, it will be readily understood that wholly analogue or wholly digital techniques may be employed.

In the above description, it has been assumed that, as is the normal practice, the stators of the motors are energised with the alternating current. The rotors may be squirrel-cage rotors or they might be wound rotors, for example with external resistors connected via slip rings. Defects in any part of the rotor circuit would be detected by the above-described apparatus. It will be immediately apparent that, although reference has been made more particularly to the normal arrangement of an induction motor in which the stator is energised, the invention may equally well be employed if the rotor was energised.

The sensitivity of the technique would be limited by the incidental anisotropy of the core of the motor.

An iron cored rotor may have magnetic properties that vary with the direction of the field and this couid produce small slip synchronous speed fluctuations.

It will be particularly noted that the apparatus described may be used for monitoring the condition of motors whilst the motors are being used in normal operation. In plant using many induction motors, it is therefore particularly convenient to use a monitoring system, such as has been described with reference to Figure 4, in which the various motors are scanned so that each is periodically tested to determine if there are any incipient defects in the rotors.

Claims (24)

1. A method of testing an induction motorcom- prising indicating speed fluctuations occurring at twice the slip frequency when the motor is energised with a constant frequency alternating supply.

2. A method as claimed in claim 1 wherein the amplitude of the speed fluctuations at twice the slip frequency is indicated.

3. A method as claimed in claim 1 wherein an indication is given when the amplitude of the speed fluctuations at twice the slip frequency exceeds a predetermined level.

4. A method of testing an induction motor comprising the steps of repetitively measuring the phase angle of an alternating energy supply when the rotor of the motor passes through a predetermined angular position or positions, and determining any irregularity at twice the slip frequency occurring in the rotational period of the rotor.

5. A method as claimed in claim 4 wherein the phase angle of the alternating supply is determined each time the rotor passes through said predetermined angular position or positions.

6. A method as claimed in either claim 4 or claim 5 wherein variations in the rotor period are visually indicated.

7. A method as claimed in claim 6 wherein the relationship between rotor period and phase angle of the supply frequency is indicated.

8. A method as claimed in any of claims 4 to 7 wherein the time at which the rotor passes through a predetermined position or positions is determined by a non-contacting sensor.

9. Apparatusfortesting an induction motor comprising a sensor for giving an output signal when the rotor passes through a predetermined angular position or positions, means for determining the phase angle of an alternating energy supply for the motor at the time instants sensed by said sensor and for determining therefrom the rotational period of the rotor or changes therein, and indicating means responsive to variations, at twice the slip frequency, in the rotational period of the rotor.

10. Apparatus as claimed in claim 9 wherein the sensor is a non-contacting sensor.

11. Apparatus as claimed in claim 9 wherein the sensor is a transducer magnetically or optically sensing a co-operating element or marking on the rotor to produce an output signal each time the rotor passes through a predetermined angular position.

12. Apparatus as claimed in any of claims 9 to 11 wherein said indicating means comprises a cathode ray tube, means for producing a time base trace synchronised with said alternating energy supply and means for producing a deflection modulation of the time base after each sensor output signal, the modulation being of an amplitude dependent on the time interval between that sensor output signal and the preceding sensor output signal.

13. Apparatus as claimed in claim 12 wherein said means for producing a time base is arranged to produce a polar time base, the radius of which is controlled in accordance with the deflection modulation.

14. Apparatus as claimed in either claim 12 or claim 13 wherein means are provided for applying the output signal from the sensor or a signal derived therefrom as a brightness modulation on the cathode ray tube whereby the display comprises a pattern of dots, occurring as the rotor passes through said predetermined angular position.

15. Apparatus as claimed in any of claims 9 to 11 wherein an oscillator is provided with phase lock control means responsive to the sensor output for locking the oscillator to the mean rotor speed and wherein phase comparator means are provided to compare the phase of the sensor output with that of the oscillator to give a signal representative of speed fluctuations of the rotor and wherein phase sensitive detecting means are provided having a reference signal at twice the slip frequency for detecting variations in the output of the phase comparator means at twice the slip frequency.

16. Apparatus as claimed in claim 15 wherein, for deriving the reference signal at twice the slip frequency, means are provided for repetitively sampling and holding, once per revolution of the rotor, signals derived from and at twice the frequency of the mains supply.

17. Apparatus as claimed in claim 16 wherein means are provided for separately sampling and holding sine and cosine phase signals at twice the supply frequency to provide sine and cosine phase reference signals at twice the slip frequency for separate sine and cosine phase sensitive detectors which together constitute said phase sensitive detecting means.

18. Apparatus as claimed in claim 17 wherein means are provided for combining the outputs of said sine and cosine phase sensitive detectors to provide a single output from the phase sensitive detecting means representative of the magnitude of the speed fluctuations at twice the slip frequency.

19. Apparatus as claimed in any of claims 15 to 18 wherein said indicating means comprise a signal amplitude indicator responsive to the output of the phase sensitive detecting means.

20. Apparatus as claimed in any of claims 15 to 19 wherein said indicating means comprise an indicator or alarm operating responsive to the output of the phase sensitive detecting means when the amplitude of that output exceeds a predetermined magnitude.

21. Apparatus as claimed in any of claims 9 to 20 and having scanning means for sequentially scanning outputs from a plurality of sensors on separate motors.

22. A method of testing an induction motor substantially as hereinbefore described with reference to Figures 1, 2 and 3 or to Figure 4 of the accompanying drawings.

23. Apparatus for testing an induction motor substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings.

24. Apparatus for testing an induction motor substantially as herein before described with reference to Figure 4 of the accompanying drawings.