GB2096770A

GB2096770A - Testing mechanical components of electrical machines

Info

Publication number: GB2096770A
Application number: GB8111815A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-04-14
Filing date: 1981-04-14
Publication date: 1982-10-20
Also published as: SE8202319L

Abstract

A system for testing for wear in mechanical components of an electrical machine 2 driven by an electrical power supply 1, without the need for dismantling or removing the machine from its operating position. A wattmeter 4 which derives an electrical signal proportional to the instantaneous electrical power dissipation in the machine is connected between the power supply and the machine. The output of the wattmeter is applied to a spectrum analyser 5 to produce a power frequency spectrum which is related to the condition of mechanical components of the machine, such as bearings, that affect the performance of the machine and allows inspection of component condition to be made. <IMAGE>

Description

SPECIFICATION Testing electrical machines This invention relates to methods of testing electrical machines and to apparatus for use in such methods.

A machine may include components, such as bearings, which are liable to wear and thus affect the performance of the machine. Various methods have been proposed to monitor the condition of such components. One such method, vibration analysis, is frequently used for bearings. The vibration analysis technique uses an accelerometer operated over its linear response range to provide an electrical signal proportional to acceleration.

This signal is passed through a spectrum analyser, enabling frequency components corresponding to various vibration sources to be identified.

Changes in the spectrum indicate wear effects and possibly incipient failure.

A variant of this technique, known as demodulated resonance analysis, or envelope detection, uses the natural resonance frequency of a transducer to highlight high frequency signals produced by flaws in rolling element bearings.

Acoustic emission is another phenomenon utilised. High frequency vibration produced by bearings is analysed to monitor condition. This method is very effective since high frequency signals are more influenced by bearing defects.

An object of the present invention is to provide an alternative method of testing electrical machines having components such as bearings which are liable to wear and adversely affect the performance of the machine, and apparatus for use in such in a method of testing.

According to one aspect of the present invention there is provided a method of testing mechanical components of an electrical machine driven by an electrical power supply, the condition of which mechanical components affects the performance of the machine, comprising the steps of deriving an electrical signal proportional to the instantaneous electrical power dissipation in the electrical machine and applying the signal to a spectrum analyser whereby to obtain a power frequency spectrum which is related to the condition of the mechanical components.

According to another aspect of the present invention there is provided apparatus for use in testing mechanical components of an electrical machine driven by an electrical power supply, the condition of which components affect the performance of the machine, the apparatus comprising means to derive an electrical signal proportional to the instantaneous electrical power dissipation in the electrical machine and means to obtain a power frequency spectrum from the electrical signal, which spectrum is related to the condition of the mechanical components.

According to a further aspect of the present invention there is provided an electrical wattmeter including electronic multiplying means, means to derive an electrical signal proportional to the instantaneous current through a load and apply it to one terminal of the multiplying means, and means to derive an electrical signal proportional to the instantaneous voltage across the load and apply it to be another terminal of the multiplying means, the multiplying means serving to multiply the two derived signals and produce an electrical signal proportional to the instantaneous electrical power dissipation in the load.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows, schematically, an harmonic power analyser connected to test the condition of an electrical machine according to the present invention; Fig. 2 shows a block diagram of a wattmeter connected to a la̲d for single phase power measurement; Fig. 3 shows a block diagram of a wattmeter for connection to a load having a three phase power supply; Fig. 4 shows a block diagram of one part of a wattmeter for connection to a load having a polyphase power supply, there being such a part associated with each phase.

We have found that mechanical noise/vibration frequencies in electrical machines, such as pump drives, may be related to peaks in the electrical input power spectrum of the machine, and similarly the bearing friction loss is related to the electrical input power spectrum. The noise/vibrations and other effects in an electrical motor dissipate power with components at certain dominant frequencies, additional to the main supply of power absorbed directly in driving the shaft of the motor against a steadly load.

Harmonics in the machine, speed fluctuations and vibrations, all account for some of the energy supplied to the motor. This produces a noise spectrum with components at certain dominant frequencies. Each component has an associated energy loss that will be dissipated with that particular frequency.

In order for the frequency spectrum of the input power to be obtained it is necessary to be able to measure the instantaneous power dissipation or produce a signal that is proportional to the instantaneous power, which signal can be applied to a spectrum analyser to produce a frequency spectrum plot therefrom.

Fig. 1 shows a power supply 1 connected to a load 2, an electrical motor, via an harmonic power analyser 3. The harmonic power analyser 3 comprises a wattmeter 4 capable of producing an instantaneous power signal P, that can be applied to a spectrum analyser 5, which may be onventional and which produces a power spectrum, features of which are related to the condition of bearings, for example., of the motor 2.

In the case cf d single phase power supply, the Nattm~ier for obtaining the required instantaneous power signal P may be electronic and simply take the form illustrated in Fig. 2. A current monitoring resistor R is connected in series with the motor 2 between the terminals L, N comprising the input terminals for mains supply or a motor controller, for example. The resistor R is of such a value to provide an input voltage for a multiplier 6 which is of a suitable magnitude for the multiplier and subsequent electronics and is proportional to the instantaneous load current V.

Resistors RV1 and RV2 comprise a potential divider, to bring the line voltage down to a suitable value for the electronics, which provides an input voltage Vv for the multiplier 6 which is proportional to the instantaneous load voltage.

The output Vp of multiplier 6 is thus proportional to the instantaneous power P1 and can be applied directly to a conventional spectrum analyser. For a 240V main supply, and with a 100 watt rating for motors up to 10 amps, typically R=1 ohm Rev1=47 K ohms, 2 watts, Rev2=1 K ohms (variable for ease of scaling) and the multiplier is a type AD 435J or equivalent.

For a load with a three phase power supply, two such single phase multiplier arrangements may be connected together to measure the instantaneous power dissipated in the load, in a manner analogous to the conventional "two wattmeter" method of power measurement for three phase systems.

Fig. 3 shows, schematically, a three phase wattmeter including two multipliers 7 and 8.

Termainals Rl, Y and B, are provided for input from the mains or a controller, for example, and terminals R0,Y0 and B0 are the output terminals for connection to a load (not shown) such as a three phase motor of a pump drive.

Resistors RR and Ry are current monitoring resistors, whereas resistors R2, R3, R4, R5, R6, R7, R8, Rg R10 and R11 comprise five potential divider networks to bring the line voltage signal leveis down to suitable vaiues as in the Fig. 1 embodiment.

Differential amplifier A1 thus has an input signal proportional to the line current IR and differential amplifier A2 has an input signal proportional to the RB line voltage.

Multiplier 7 has an output signal V7 proportional to the product of those input signals to amplifiers A1 and A2.

V7=(kplRRRkAl) (kp VRB KA2) where kp=potential divider ratio and kA amplifier gain Thus RV RB Similarly V6=KlyVy9 The signals V7 and Vs are combined in a summation circuit 9 to provide an output signal V which is proportional to the total power at any instant.

Vp=K(lRVRB+IYVYB) Vp=K(IR)VRVB)+iy(Vy +l,(VyV5)) =K(IRVR+IYVY VB(IR+IY)) =K(lRVR+IYVv+lBVB) Typically for a mains supply of 41 5 volts (line) RR=Ry=O.OS ohms, R2=Rs=R6=R9=R11=47K ohms, R3=R4=R7=RB=Rg=1 K ohms, the amplifiers A1, A2, A3 and A4 can comprise general purpose op-amps, for example type 741 with appropriate gain setting resistors, the multipliers 7 and 8 can each comprise a type AD 435J and the summer 9 can comprise a summing amplifier such as a general purpose op-amp, such as type 741.

The arrangement illustrated in Fig. 3 can be modified to provide improved accuracy and to make it applicable to polyphase systems in general. Associated with each phase is a respective multiplier and network as illustrated in Fig. 4, the outputs of the various multipliers being combined in a summation circuit 10 to provide a signal, proportional to the total power at any instant, which can be applied to a spectrum analyser. The wattmeter described with reference to Fig. 3 has limited accuracy if the current monitoring resistances R R and Ry need to be very small to minimise insertion effects, and the balancing of the potential dividers becomes very critical when attempting to measure a few millivolts difference in a few hundred volts of line voltage.These difficulties can be overcome by first amplifying the current signals, using amplifiers such as A5, on each line, that take the line voltage Vph as their reference point and have a separate floating power supply as shown in Fig.

4.

The separate line amplifier A5 has the effect of improving the common mode rejection, which would be lower is the differential amplifier A6 alone were used. This makes the balancing of the potentil dividers less critical.

Differential amplifier A7 produces a signal proportional to the voltage difference between the phase voltage Vph and reference voltage Vref.

V ref will usually be the neutral or common point, but a separate reference voltage terminal is shown since it would be required in configurations requiring electrical isolation between phases. The phase current signal and the phase voltage signal (the outputs of A6 and A,) are the multiplied electronically in multiplier 11 and the resultant phase power signal is combined with those of the other phases in, for example, a summing amplifier 10.

In an alternative arrangement the line amplifier A5 of Fig. 4 could be replaced by an optically coupled isolation amplifier.

Whereas the wattmeters described with reference to Figs. 2, 3 and 4 were designed to be used in combination with spectrum analysers, they may alternatively be used as general purpose wattmeters. The instantaneous power signal provided thereby can be applied to a digital display and give a single reading of power averaged over a suitable period. Alternatively, the power signal can be applied to a "true r.m.s. to d.c." converter, and this allows the true r.m.s.

value of the power to be displayed. This overcomes the disadvantages of the moving coil type of wattmeter that is accurate only over a restricted frequency range and is calibrated for sinusoidal waveforms.

By comparing the spectrum analyser results obtained by vibration analysis and power analysis of the same bearing supporting a drive shaft of an electric motor, it was found that there was close correlation between the two. The power spectrum had particularly pronounced peaks at the lower harmonic frequencies, for example up to about 500 Hz. Comparison of the power spectra of bearings with varying degrees of wear showed that the height of the associated peaks likewise varied, with the result that it is possible to monitor the condition of a bearing by study of the power spectra, the overall envelope or peaks thereof, for example, obtained from the electric motor.

The particular advantage of this harmonic power analysis is that components of a machine, for example, bearings can be tested without having to strip the machine down, remove the machine from its operating position or even stop the functioning of the machine during testing.

This is particularly advantageous in the case of submersible pumps, such as pumps employed to pump sewage or the like while submersed therein. It is also possible to use harmonic power analysis in situations where alternative methods are non-applicable, for example large three phase motors. Whereas the method of the present invention has been described mainly with regard to bearing condition monitoring, it is also applicable to monitoring mechanical faults of rotors, coupling/shaft misalignment, pump impellor/fan faults etc.

Whilst the method of the invention has been specifically described with respect to electrical machines driven by an alternating current supply, it is also applicable to electrical machines driven by a direct current supply.

Claims

1. A method of testing mechanical components of an electrical machine driven by an electrical power supply, the condition of which mechanical components affects the performance of the machine, comprising the steps of deriving an electrical signal proportional to the instantaneous electrical power dissipation in the electrical machine and applying the signal to a spectrum analyser whereby to obtain a power frequency spectrum which is related to the condition of the mechanical components.

2. A method as claimed in claim 1, wherein the supply is an alternating supply.

3. A method as claimed in claim 1, wherein the signal proportional to the instantaneous electrical power dissipation is obtained by means of an electronic wattmeter coupled between the electrical power supply and the electrical machine.

4. Apparatus for use in testing mechanical components of an electrical machine driven by an electrical power supply, the condition of which components affect the performance of the machine, the apparatus comprising means to derive an electrical signal can be applied to a spectrum analyser to produce a frequency spectrum plot therefrom.

5. Apparatus as claimed in claim 4, wherein the electrical signal derivation means comprises an electronic wattmeter, and wherein for a single phase alternating electrical power supply the wattmeter comprises electronic multiplying means having two input terminals, first means for supplying an electrical signal proportional to the instantaneous load current through the machine to one input terminal and second means for supplying an electrical signal proportional to the instantaneous load voltage across the machine to the other input terminal.

6. Apparatus as claimed in claim 5 including first means comprising a current monitoring resistor in series with the electrical machine in use of the apparatus and the second means includes a potential divider in parallel with the electrical machine in use of the apparatus.

7. Apparatus as claimed in claim 4, wherein the electrical signal derivation means comprises an electronic wattmeter, and wherein for a threephase alternating electrical power supply including three supply lines, the wattmeter comprises a first and second electronic multiplying means each having two input terminals, first means for supplying an electrical signal proportional to the instantaneous load current through the machine on a first supply line to one input terminal of the first multiplying means, second means for supplying an electrical signal proportional to the instantaneous load voltage across the machine between a first and second supply lines to the other input terminal of the first multiplying means, third means for supplying an electrical signal proportional to the instantaneous load current through the machine on a third supply line to one input terminal of the second multiplying means, fourth means for supplying an electrical signal proportional to the instantaneous load voltage across the machine between the second and third supply lines to the other terminal of the second multiplying means, and a summation element, the outputs of the first and second multiplying means being applied to the summation element, the output of the summation element comprising the electrical signal proportional to the instantaneous electrical power dissipation.

8. Apparatus as claimed in claim 7, wherein the first electrical signal supplying means includes a first current monitoring resistor in series with the first supply line in use of the apparatus, and wherein the third electrical signal supplying means includes a second current monitoring resistor in series with the third supply line in use of the apparatus.

9. Apparatus as claimed in claim 4 and for use with a poly phase alternating electrical power supply, wherein the electrical signal derivation means comprises an electronic wattmeter, a summation element and wherein for each phase of the power supply there is provided an electronic multiplying means, a current monitoring resistor arranged in a respective supply line in use of the apparatus, means for amplifying a signal proportional to the instantaneous line current signal obtained from the current monitoring resistor and applying it to one input of the multiplying means, means for obtaining a signal proportional to the instantaneous line voltage signal applying it to another input of the multiplying means, the respective multiplier outputs being applied to the inputs of the summation element whose output is proportional to the instantaneous power dissipation in the electrical machine.

10. An electrical wattmeter including electronic multiplying means, means to derive an electrical signal proportional to the instantaneous current through a load and apply it to one terminal of the multiplying means, and means to derive an electrical signal proportional to the instantaneous voltage across the load and apply it to be another terminal of the multiplying means, the multiplying means serving to multiply the two derived signals and produce an electrical signal proportional to the instantaneous electrical power dissipation in the load.

11. An electronic wattmeter as claimed in claim 10 and substantially as herein described with reference to Fig. 2, Fig. 3, and Fig. 4 of the accompanying drawings.

12. A method of testing mechanical components of an electrical machine driven by an electrical power supply substantially as herein described with respect to and as illustrated in the accompanying drawings.

1 3. Apparatus for use in testing mechanical components of an electrical machine driven by an electrical power supply substantially as herein described with reference to Fig. 1, Figs. 1 and 2, Figs. 1 and 3 or Figs. 1 and 4 of the accompanying drawings.