GB2466472A

GB2466472A - Electric motor power sensor based on detecting motor vibrations

Info

Publication number: GB2466472A
Application number: GB0817086A
Authority: GB
Inventors: Stephen Russell; Keith Bradley
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 2008-09-18
Filing date: 2008-09-18
Publication date: 2010-06-30
Anticipated expiration: 2028-09-18
Also published as: GB0817086D0; GB2466472B

Abstract

A device for measuring the power output and input of an electric induction motor uses a vibration sensor and frequency analysis to measure the motor speed, and calculates the motor slip and output power from the motor speed and motor plate data. An accelerometer may be mounted on the motor casing and used to detect vibrations. The frequency spectrum of the vibrations includes peaks 2, 3 associated with the AC power supply and the motor rotation speed. Input power is derived from output power and efficiency, the efficiency being derived from motor supplier data or from data on similar types and sizes of motor. The vibration sensor also senses vibration due to the motor drive frequency, thus enabling the motor no load speed to be calculated. An embodiment of the invention applies sensors to a group of motors, and the sum of the input powers can be compared with the total input power for the motor group measured by conventional electrical means. The motor parameters can then be adjusted to minimise the difference between the electrical total power, and the sum of the individual motors under a range of load conditions.

Description

Electric motor power sensor This invention relates to a device for measuring the power delivered and consumed by an electric induction motor.

Alternating current (AC) mains powered electric induction motors are the prime movers for fixed machinery worldwide, and in this role, they consume a significant proportion of the world's electrical energy. Whilst most users of induction motors will monitor the electrical power of a site for revenue purposes, they may not know how much power is being used by individual motors. Suppliers of motor control equipment are beginning to include power monitoring facilities within their products, however there are a huge number of legacy motors which are not monitored for power consumption. For persons wishing to control their use of energy, the first step is to understand how much energy is used by electrical loads. The invention is designed to provide data on power output and input to an induction motor without any need to make electrical connections to the motor or its control equipment. Conventional methods of electrical power measurement require the fitting of current transformers, and this usually entails switching off motors with possible downtime for an industrial process, the need to isolate electrical panels and the services of a trained electrician.

Induction motors under zero load will rotate at a speed directly related to the alternating current supply frequency. The actual multiple is determined by the number of poles' in the motor, for example a 2 pole motor fed by exactly 50 Hz supply will rotate at 50 Hz or 3000 rpm. Nominal no load speeds of some common motors are listed below.

Number of poles No load rpm when driven from a 50 Hz supply 2 3000 4 1500 6 1000 8 750 600 12 500 When the motor is driving a load, the motor speed drops slightly, and the difference in speed between the no load speed and the actual speed is known as the motor slip.

All induction motors have a plate attached which gives the motor speed, the output power and the current under full load when fed from a 50 Hz and 60 Hz supply. The relationship between output torque and motor slip is close to linear, and can be used to obtain a good estimate of motor output power if the slip can be measured.

The motor slip requires two measurements to be made, the actual motor rotation speed, and the power supply frequency. The direct mains supply frequency in the UK is allowed by law to vary by � 0.4% from the nominal 50 Hz. Other countries have different standards, and some use 60 Hz as the standard supply frequency. This means that the no load speed of an induction motor will also vary and cannot be taken for example for a 2 pole motor to be exactly 3000 rpm. Many motors are driven by variable speed drives (VSD5) which generate their own supply frequency so that the no load speed for a motor can then vary over a wide range.

The invention uses a vibration sensor, typically an accelerometer, or velocity sensor though other sensors may be used, attached to the motor. By analysing the vibration data the motor rotation speed and the motor supply frequency can be extracted. The motor rotation speed could also be measured directly using a tachometer or other device where the motor shaft is accessible.

All motors will have an imbalance in their rotating parts, such that there will be a vibration of frequency equal to the rotation speed. By analysing the frequencies of vibration using Fourier analysis or some other means this peak in the vibration spectrum is always present and can be extracted.

Induction motors also vibrate due to their electrical excitation. There is a magnetic attraction force pulling the rotor and stator of the machine together. This magnetic attraction force pulsates at twice the frequency of the electrical supply. This is because the attraction force between the rotor and stator is dependent upon the square of the magnetic flux density in the air gap and is thus in the same direction irrespective of the sign (direction) of the flux. If the vibration from an induction motor is analysed by Fourier analysis or some other method, there will be a peak due to the motor supply at double the supply frequency, so for example using a 50 Hz supply the peak will occur at 100 Hz. Referring to figure 1, the time series data from the vibration sensor is transformed using Fourier analysis or other method into a frequency series (1) with a peak due to the rotor imbalance at the motor rotation speed (2) and a further peak (3) at double the motor supply frequency.

The vibration due to the shaft rotation and motor supply frequency may be sensed at any part of the motor, but the best location referring to figure 2, is generally on the motor casing (4) near the middle of the motor windings (5) at any convenient point around the circumference.

There will be other vibration frequencies present in any real motor, due to harmonics of the main rotation and supply frequencies and other parts of the machine driven by the motor. There may also be other vibration peaks due to machines in the vicinity of the motor whose power is to be measured. The two vibration frequencies to be extracted are related by the following relationship.

f x M/4> NA > f x M/4 -(N050 -NF) Where f is the supply frequency M is the number of motor poles NA is the motor speed in revolutions per second N050 is the motor no load speed in revolutions per second with a 50 Hz supply NE is the motor full load speed in revolutions per second with a 50 Hz supply Thus the number of poles, the full load speed and the supply frequency determine the motor rotation frequency within a narrow range of frequencies. This is of assistance in locating the two frequencies against a noisy background.

This relationship can be coded into an automated procedure, whereby the spectrum is scanned for pairs of corresponding peaks.

The motor plate data provides the motor full load speed (NE), current (IF) and power (PF). The vibration data provides the drive frequency (1D) and the rotation speed (NA).

The power delivered by the motor under these conditions can be estimated by the following steps: 1 m =_L * IF NF where TF is full load torque PF full load power NE full load speed 2 TF(NO-NA) * TA-(NO-NE) where TA is actual delivered torque TF full load torque N0 no load speed NA actual speed NF full load speed P0 TA* NA where P0 is (output) shaft power TA actual delivered torque NA actual speed In practice it is necessary to take account of the fact that for a motor driven directly from the mains, the mains frequency will not be exactly 50 Hz, so the no load speed and full load speed under these conditions will change slightly.

For variable speed drives the steps will be: 1 -N050* * NOf-50 where N01 is no load speed at frequency f N050 no load speed at 50Hz f measured drive frequency 2. T=--

FNF

where TE is full load torque PE full load power NF full load speed * TA=T0fN where TA is actual delivered torque TF full load torque N0f no load speed at frequency t N053 no load speed at 50 Hz NA actual speed NF full load speed Pout TA* NA where P01 is (output) shaft power TA actual delivered torque NA actual speed The shaft power is useful in its own right as the power used to drive the load and do work in the process, however for control of energy consumption, the electrical power input to the motor is also needed.

For an induction motor driven directly from the mains supply, the motor input power can be calculated from the output power and the motor efficiency.

Input power = Output power x 100 I %efficiency Efficiency varies with load so this must be taken into account in estimating efficiency.

Where available, efficiency data for a motor can be used to calculate input power.

Where no data is available, the EC motor efficiency class, (EFF1, EFF2, EFF3) and the motor size can be used to identify a similar motor whose efficiency data can be used instead.

An alternative method of estimating input power is to estimate it from a motor model, using the plate data parameters and using other parameters from the best match to the motor from a library of motor data.

For motors driven by variable speed drives some of the losses will change with drive frequency, others will not, therefore the motor model method is preferred for the estimation of input power.

Where motors are driven directly from the mains supply, it may be convenient to measure the mains supply frequency directly using a simple frequency counter, rather than by analysing the motor vibration data. This need not involve an electrician in isolating and opening electrical equipment panels, as the mains can be accessed using a power outlet socket to make the connection with the frequency counter.

The motor power also varies with the voltage applied, therefore it will improve the estimation of power if the voltage is measured.

A number of embodiments of the invention can be envisaged.

Firstly a portable handheld device would consist of an accelerometer or other vibration sensor connected by cable or wirelessly with battery power to a logger/control unit with a keypad. The user would attach the sensor to the motor, type in the plate data for motor power, current and speed, and the unit would record some vibration data, and then calculate an estimated output and input power for the motor.

In a second mode of use, the instrument could be set to log the power input and output over a period, and left to acquire data to find the energy consumed over a period.

A second embodiment is for a survey system which would use a set of clamp-on devices fitted to motors, all battery-powered and having accelerometers or other vibration sensors which would collect vibration data and send it wirelessly to a logger, either as time series data, or as the results of a frequency analysis of the time series data. At installation, the plate data from all the motors under test would be input to the logger. The logger would include hardware and software to compute the input and output power of each motor in use.

A refinement of the second embodiment is to combine the vibration measurements from a group of motors with electrical power data from the electrical panel which supplies the group of motors. Voltage and current connections to the supply for the entire panel would be made by an electrician, and the system left to log for twenty four hours, or longer. When the data analysis is performed, the estimates of input power of all the motors will be made from vibration and plate data as described, and the sum of the input powers compared with the actual total power supplied to the panel, calculated from the voltage and current measurements. On most industrial sites, different load conditions will exist at different times, and this enables numerous comparisons between the sum of the individual motor estimates and the actual total power supplied to be made. Parameters such as motor efficiencies can be varied to improve the estimates, either manually, or by automated software, so that the errors in estimation are minimised. The system software may also include useful additional information for the user, such as power trends for individual and combinations of motor loads. It may also incorporate analysis of the mains supply, such as power factor and estimates of energy wasted through phase imbalance and harmonics.

A third embodiment would be a permanent system which would use a set of accelerometers or other vibration sensors fixed to a group of motors, one attached to each motor, by bolt or adhesive or other means to form a rigid attachment. Vibration data as a time series, or in a processed form as the extracted frequencies would be communicated wirelessly to a logger in the electrical panel supplying the motors. The logger would optionally have three phase electrical inputs of current but would be expected to have a voltage input from the panel. As with the second embodiment, motor plate data would be input to the logger which would be equipped to calculate the input and output power of each motor. By sampling over a period, the system would provide data on trends in input and output power by individual and groups of motors. If the system is equipped with voltage and current sensors, the comparisons described for the second embodiment can be made and the estimates of motor input and output power optimised by using the naturally occurring range of load conditions.

Embodiment three would include means to transmit data to a central plant control room, to enable operators to manage the power in the process. The system may include alarms to be raised when the power moves outside set limits. Analysis of the mains itself may also be included, providing data such as power factor, phase imbalance or harmonic content.

Other embodiments may be made.

The accuracy of the power estimation depends crucially upon the accuracy of the measurement of motor shaft speed and the motor drive frequency. On large efficient motors, the motor shaft will be turning at hundreds of revolutions per minute, but the motor slip at full load may be less than 10 revolutions per minute. The invention therefore requires resolution of less than 1 revolution per minute to obtain good resolution of motor power. This may be obtained by sampling the data for long periods using Fourier methods, adaptive filtering methods or other digital signal processing method.

It is common practice to monitor the condition of machines by measuring vibration, typically in the 0 -1 kHz range. The invention may make use of the same sensor to produce motor power data, which operates in the 0 -200 Hz range, without need of an additional sensor.

When operating in a mechanically or electrically noisy environment, improved signal to noise for sensing the motor rotation frequency may be obtained by utilising two similar vibration transducers mounted exactly opposite each other around the body of the motor. The signals from the two transducers due to rotation will be 180 degrees out of phase, whilst signals from other sources will have different phase relationships.

By subtracting the time domain data from the two transducers, an improved signal to noise can be obtained. This procedure does not enhance the signal to noise for the electrical drive frequency, but by locating the rotation frequency, the region of spectrum for the drive frequency is greatly narrowed.

Claims

Claims 1. A device for monitoring the output power of electric induction motors using one or more vibration sensors attached to the body of the motor, the vibration data being used to determine the motor shaft speed and the motor electrical supply frequency, from which with the motor plate data the motor slip and motor output power can be calculated.
2. A device as in claim 1 in which the motor input power is estimated using the motor output power and the motor efficiency, the motor efficiency being derived from tables of similar size and type of motors.
3. A device as in claim 1 in which the motor input power is estimated using the motor output power and the motor efficiency, the motor efficiency being derived from a theoretical model of the motor whose values are available from published data on similar size and types of motor.
4. An embodiment of the invention in which the device is applied to a group of motors and the summed power inputs compared to the total power input of the group of motors measured using conventional electrical means, and the motor parameters optimised to obtain the best agreement under a range of load conditions.
5. A device as in claim 1 where the mains supply frequency is separately measured by direct electrical connection to the supply, rather than using the supply frequency derived form vibration data.
6. A device as in claim 1 where the mains supply voltage is separately measured by direct electrical connection to the supply and used to improve the power estimation.
7. A device as in claim 1 where the vibration data is analysed using Fourier analysis to extract the motor rotation speed and the motor supply frequency.
8. A device as in claim 1 where the vibration data is analysed using adaptive filtering to extract the motor rotation speed and the motor supply frequency.
9. A device as in claim 1 where the vibration sensor is attached to the motor case at or near to the centre of the motor windings.
10. A device as in all preceding claims fitted with keys to input the motor plate data.
11. A device as in all preceding claims which uses a wireless link to communicate the vibration data to a separate logger.
12. A device as in claim 10 in which the data is communicated as the processed results of frequency analysis of the vibration data.