GB2315885A - Controlling electric motors dependent on load - Google Patents

Abstract

A system of motors 6, such as the compressor motors of refrigeration units, varies the supply voltage to the motors 6 in steps in dependence on the loads on the motors. The loads on the motors 6 can be determined from the current demanded by each motor, sensed by a current sensor 11, or the temperature of the working fluid in a refrigeration unit, sensed by a temperature sensor 10. If the load on all of the motors is light, the supply voltage is reduced. However, if the load on any one motor increases beyond a threshold, the supply voltage is increased.

Description

Motor System Description The present invention relates to a method of supplying power to a plurality of motors and a motor system. The present invention has particular, although not exclusive, application in the operation of refrigeration units such as are found in supermarkets.

Many applications require continuous or substantially continuous operation of electric motors. One example of this is the refrigeration units found in supermarkets for storing frozen and chilled produce. The compressor motors of the refrigeration units are operated directly from the 3-phase mains supply at a nominal voltage in Europe of 220-240V rms (415V between phases).

The present inventors have determined that, for much of the time, the compressor motors can be operated with a reduced voltage supply with little or no loss of performance. Therefore, conventional operation results in unnecessarily large electricity bills.

According to the present invention, there is provided a method of supplying power to one or more motors, comprising the steps of: supplying power to every motor at a first voltage; sensing for every motor a parameter value indicative of the load thereon; and supplying power to every motor at a second higher voltage if the parameter is on a first side of a respective threshold value. The parameter may be, for example, motor drive current or a temperature dependent on the operation of a motor. The method may be performed on the basis of two such parameters simultaneously. The method is preferably employed to supply power to a plurality of motor.

Whilst in its broadest aspect, the present invention is concerned with boosting the supply voltage if performance becomes unacceptable, the present invention extends to reducing the supply voltage if reduced-voltage operation seems practicable. Therefore, preferably, a method according to the present invention includes performing, , if power is being supplied to every motor at the second voltage, the steps of: sensing for every motor said parameter value; and supplying power to every motor at the first voltage, if for every motor the sensed parameter value is on a second side of respective threshold values.

A method according to the present invention may switch the supply voltage between two, three or more voltage levels, all of which are less than the nominal mains supply voltage.

It would be undesirable for the supply voltage to be continually switching, not least because of the effect this would have on the useful life of the switchgear employed. Preferably, therefore, for each change in the voltage at which power is supplied to the or each motor, the relationship between the parameter and the relevant threshold must persist for a respective predetermined period. These periods are preferably seleaed on the basis of the characteristic of a particular installation. However, by way of example, 8 seconds would be appropriate in the case of control on the basis of motor drive current. On the other hand, 30 minutes would be appropriate for control on the basis of a temperature.

There is the possibility that a fault condition will occur in a motor, for instance a winding may go open circuit causing a fuse to blow.

Advantageously, a method according to the present invention includes the steps of: detecting a motor overcurrent condition; and supplying power to every motor at a maximum voltage and generating an alarm signal if an overcurrent condition is detected.

The main control method, defined above, is preferably used when the parameter values are within what would be considered to be normal working ranges. If a parameter value leaves its normal working range, a more profound response is preferred. Accordingly, a method according to the present invention may include the steps of: detecting the parameter value for one of the motors passing a limit value; and supplying power to every motor at a maximum voltage if a motor parameter passes the limit value.

The maximum voltage may be the nominal mains voltage. However, it may be less than the nominal mains voltage.

A plurality of refrigeration units may be advantageously operated according to a method embodying the present invention. However, the present invention is applicable to other installations such as water pumping systems and airconditioning systems. In the case of water pumping, the water output pressure would be a relevant load dependent parameter.

According to the present invention, there is also provided a motor system and a refrigeration installation operated according to a method according to the present invention. Such systems or installations will include sensor means for sensing the parameter and a controller responsive to the sensor means.

Preferably, a refrigeration installation according to the present invention has one or more refrigeration units, respectively including a heat exchanger for heating a working fluid and an expansion chamber, and temperature sensors located so as to sense the temperature of the working fluid between the heat exchanger and the expansion chamber.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a refrigeration unit; Figure 2 is a block diagram of an installation according to the present invention.

Figure 3 is a block diagram of one phase of a 3-phase variable voltage ac power supply; Figure 4 is a block diagram of the controller of Figure 2; Figure 5 is a block diagram of one phase of another 3-phase variable voltage ac power supply; and Figure 6 is a block diagram of one phase of a single-phase variable voltage ac power supply.

Referring to Figure 1, a refrigeration unit comprises a heat pump. The conventional elements of such a heat pump are a compressor 1 for compressing a working fluid, a first heat exchanger 2 coupled to receive the working fluid from the compressor 1, an expansion chamber 3 coupled to receive the working fluid from the first heat exchanger 2, and a second heat exchanger 4 coupled to receive the working fluid from the expansion chamber 3 and output it to the input of the compressor 1. The second heat exchanger 4 is located in a space 5 to be cooled. The compressor 1 is driven by an elearic motor 6. The electric motor 6 is toggled on and off by a control circuit 8 on the basis of a temperature signal from a temperature sensor 9, located in the space 5 to be cooled. Drive power for the motor 6 is supplied via a 3-phase power supply network 7.

Working fluid from the second heat exchanger 4 is compressed by the compressor 1. This raises the temperature of the working fluid which is then supplied to the first heat exchanger 2. In the first heat exchanger 2, the working fluid gives up heat to the atmosphere. The working fluid flows from the first heat exchanger 2 to the expansion chamber 3 where it passes through a nozzle and is able to expand. The expansion of the working fluid causes it to cool. The cool, low pressure working fluid then progresses to the second heat exchanger 4 where it absorbs heat from the space 5 to be cooled, thereby cooling the space 5.

The refrigeration unit of Figure 1 differs from the conventional, basic design by the provision of additional sensors. A temperature sensor 10 is positioned to sense the temperature of the working fluid between the second heat exchanger 4 and the compressor 1. The temperature sensed by the temperature sensor 10 gives an indication of the cooling load on the unit. A current transformer 11 senses the current flowing in one of the phases of the power supply to the motor 6.

The motor 6 is comprised in a motor unit 12. The motor unit 12 includes fuses for each phase of the motor's power supply and circuitry for detecting blown fuses and outputting a fault signal. This circuitry comprises means for sensing the voltages across the fuses. When a fuse is intact, only a small voltage will appear across it. However, if a fuse has blown, a large voltage will appear across it.

Referring to Figure 2, an installation according to the present invention comprises a control unit 13 and three refrigeration units 14a, 14b, 14c. The control unit 13 comprises three variable voltage ac power supply units 15a, 15b, 15c and a controller 16.

The power supply units 15a, 15b, 15c receive respective phases of a threephase mains supply 17 and output power at various voltages, under the control of the controller 16, to the 3-phase power supply network 7.

The controller 16 receives the outputs of the sensors 10,11 (Figure 1) from each of the refrigeration units 14a, 14b, 14c on separate lines. The controller 16 also receives the fault signals from the refrigeration units 14a, 14b, 14c.

However, the fault signals are supplied via a common line. The controller 16 controls the operation of the power supply units 15a, 15b, 15c independence on the sensor and fault signals received from the refrigeration units.

Figure 3 shows one of the power supply units 15a, 15b, 15c of Figure 2. The power supply has an input 21 for receiving one phase of mains power at 240V rms and an output 22 for outputting power at varying voltages.

A first path 23 links the live terminal of the input 21 to the live terminal of the output 22. This path includes a first circuit breaker 25.

A second path from the live terminal of the input 21 leads to a first terminal of a second circuit breaker 26. The second terminal of the second circuit breaker 26 is connected to an input terminal of an autotransformer 27. The common terminal of the autotransformer 27 is connected to neutral. The output tap of the autotransformer 27 is located so that the output voltage is 200V rms.

The output terminal of the autotransformer 27 is connected to one terminal of the secondary winding of a transformer 28. The other secondary winding terminal of the transformer 28 is connected to a first terminal of a third circuit breaker 29. The other terminal of the third circuit breaker 29 is connected to the live terminal of the output 22.

The terminals of the primary winding of the transformer 28 are connected to a switching circuit 30. The switching circuit 30 is also connected to the input terminal of the autotransformer 27 and to neutral.

The switching circuit 30 has three configurations as follows: (a) current is directed through the primary of the transformer 28 in phase with the current in the autotransformer 27; (b) the terminals of the primary of the transformer 28 are connected together; and (c) current is directed through the primary of the transformer 28 in antiphase to the current in the autotransformer 27.

The turns ratio of the transformer 28 is chosen such that 20V rms is induced across its secondary winding in configurations (a) and (c) of the switching circuit 30.

The controller 16, which will be described in more detail below, controls the state of the circuit breakers 25, 26, 29 and the configuration of the switching circuit 30. The controller 31 simultaneously controls the corresponding circuit breakers of the other phases.

With the circuit shown in Figure 3, it is possible to output power at four different voltages: 240V, 220V, 200V and 180V. A 240V output is produced by closing the first circuit breaker 25 and opening the second and third circuit breakers 26, 29. A 220V output is produced by opening the first circuit breaker 25, closing the second and third circuit breakers 26, 29 and placing the switching circuit in configuration (a). A 200V output is produced by opening the first circuit breaker 25, closing the second and third circuit breakers 26, 29 and placing the switching circuit in configuration (b). A 180V output is produced by opening the first circuit breaker 25, closing the second and third circuit breakers 26, 29 and placing the switching circuit in configuration (c).

Figure 4 shows the controller 16 of Figure 2 in more detail. The skilled reader will appreciate that Figure 4 is however considerably simplified and that power supply and signal conditioning circuits will also be required.

Referring to Figure 4, the heart of the controller 16 is a microcontroller 40, for example a Toshiba TMP90C840P. The microcontroller 40 includes a central processing unit (CPU) 41, a RAM 42, a serial port 44, a two-bit wide parallel port 45, a six channel analogue-toZigital converter (ADC) 46, an interrupt handler 47 and a one-bit wide port 57. An EEPROM 43 is coupled to the microcontroller 40 and stores program instructions for controlling the.

operation of the microcontroller 40 and constant data.

The ADC 46 receives as inputs the temperature and motor current signals from three refrigeration units as shown in Figures 1 and 2. The fault signals from the refrigeration units share a common path and are applied to the interrupt handler 47.

A binary to decimal converter 48 receives the output from the parallel port 45 and places a logic 1 on one of its outputs in dependence on the state of the parallel port's output. The outputs of the binary to decimal converter 48 control the gates of respective FETs 49, 50, 51, 52. When the FETs 49, 50, 51, 52 are turned on, they energise the solenoids 53, 54, 55, 56, 57 of respectively first, second, third and fourth relays.

An alarm circuit 58, including for example a sounder, is activated by a signal being output from the one-bit wide port 57.

Referring now also to Figure 3, when the first relay is energised, the first circuit breaker 25 is closed and the second and third circuit breakers 26, 29 are opened, otherwise the first circuit breaker 25 is open and the second and third circuit breakers 26, 29 are closed. When the second, third and fourth relays are energised, the switching circuit 30 is placed in configurations (a), (b) and (c) respectively.

The operation of the system described with reference to Figures 1 to 4 will now be described. The operation of the microcontroller 40 is determined by the program instructions stored in the EEPROM 43.

When the system is first switched on, the CPU 41 performs an initialisation and the outputs the binary value "00" from the parallel port 45. This causes the first relay to be energized and the variable voltage ac power supply outputs 240V rms on each phase.

Once power at 240V is being supplied to the refrigeration units, the CPU 41 repeatedly reads the temperature and current values from the ADC 46. These values replace old values in respective moving windows and the averages of the values within the windows are calculated. Before the values are stored in the moving windows they may be normalised. The refrigeration units may employ motors having different current demands, for instance they may have different power ratings. Therefore, the current signals are preferably multiplied by normalising factors. Temperature signals, on the other hand, are preferably normalized by the addition or subtraction of a correction factor. Differences in the temperature signals may arise because different refrigeration units may be required to cool spaces to different temperatures.

The EEPROM 43 is programmed with limit values for temperature and motor current. There is one value for temperature and an upper and a lower value for current. These limit values are chosen to be indicative of a refrigeration unit operating well outside its normal operating range. The CPU 41 compares the averages of the value in the moving windows with each of these values. If it is thereby determined that a refrigeration unit is operating well outside of its normal operating range, the CPU 41 ensures that the first relay is energised and outputs a signal via the one-bit wide port 57 to activate the alarm. The controller 16 remains in this state until it is reset by an operator.

The EEPROM 43 is also programmed with a temperature threshold value, a current threshold value, temperature increase and decrease delay periods and a current increase and decrease delay periods. If none of the temperature and current signals have gone beyond the limit values, the CPU 41 compares the temperature and current values with the temperature and current thresholds.

If a value has exceeded a threshold for a period longer than the relevant increase delay value and the parallel port 45 is not outputting binary values "00" or "01", the CPU 41 reduces by one, e.g. 10 to 01, the value output by the parallel port 45. This has the effect of increasing the voltage of the power supply output by 20V.

If the CPU 41 determines that the averages are all below the relevant thresholds, it determines whether this has been the case for all the temperature signals for the temperature decrease delay period and also for the current signals for the current decrease delay period. If all these conditions have been met, and the output from the parallel port 45 is not "11", the CPU 41 adds 1 to the parallel port output value.

It can thus be seen that under normal conditions, the power supply output will switch between 180V, 200V and 220V in dependence on the loads on the refrigeration unit motors 6.

The power supply only outputs 240V under three conditions, on start up, when a refrigeration unit operates well outside of its normal operating range and when a fault condition occurs. A fault condition is detected from a fuse in a motor unit 12 blowing. The signal thus produced is applied to the interrupt handler 47 of the microcontroller 40. The CPU 41 responds by setting the output of parallel port 45 to "00" and activating the alarm 58.

The limit and threshold values and the delay period are programmed into the EEPROM 43 using a PC via the serial port 44. Such programming is well known to one skilled in the art.

Referring to Figure 5, in which features in common with Figure 3 have the same reference numbers, an alternative power supply unit design has a simplified switching unit 30 at the cost of a more complex autotransformer 127.

The autotransformer 127 has an output tap at 180V coupled to the secondary of the transformer 28. It is also tapped at 80V and 160V. These taps are connected to the switching circuit 30. The switching circuit 30 includes relays, controlled by the controller 16, to connect respective ones of the 80V, and 160V taps to the primary of the transformer 28 such that the current in the primary of the transformer 28 is in phase with that in the autotransformer 127. The turns ratio of the transformer 28 is such that 80V on the primary induces 20V in the secondary and 160V in the primary induces 40V in the secondary.

The operation of an installation, including power supply units as shown in Figure 5, is the same as described above with reference to Figure 3, except in the circuit breaker/switching circuit configurations produced by the different binary values output by the parallel port 45 (Figure 4). These configurations are as follows: Binary Value Configuration 00 First circuit breaker 25 closed, second and third circuit breakers 26, 29 open.

01 First circuit breaker 25 open, second and third circuit breakers 26, 29 closed and primary of transformer 28 connected to 160V tap.

10 First circuit breaker 25 open, second and third circuit breakers 26, 29 closed and primary of transformer 28 connected to 80V tap.

11 First circuit breaker 25 open, second and third circuit breakers 26, 29 closed and primary of transformer 28 shorted.

Thus, the power supply unit outputs the following voltages for each value of the parallel port's output: Binary Value Output Voltage 00 240 01 220 10 200 11 180 Referring to Figure 6, in which features in common with Figure 3 have the same reference numbers, an further power supply unit design avoids the switching unit 30 at the cost of a more complex autotransformer 227.

One terminal of the autotransformer 227 is connected to the live terminal of the input 21. The output tap of the autotransformer 227 is located at the nominal 180V point and is connected to the live terminal of the output 22.

The other terminal of the autotransformer 227 is coupled to the neutral line via a first contactor 229 and via a second contactor 225 to the live terminal of the output 22. Second and third taps are coupled to the neutral line via respective contactors 228 and 229. The contractors 225, 226, 228, 229 are controlled by the controller 16.

The operation of the controller in the installation, including power supply units as shown in Figure 5, is the same as described above with reference to Figure 3. However, the different ac output voltages are produced by a different technique. The configuration of the contactors in Figure 6 is controlled by the controller 16 as set out in the table below, assuming 240V rms input supply.

Contactors Microcontroller 225 229 228 226 Output parallel port Voltage output (rms) 00 | closed open open open 240V 01 open closed open open 220V 10 open open closed open 200V 11 open open open closed 180V The skilled person will appreciate that many modifications can be made to the above-described embodiments. For instance, a single circuit as shown in Figures 3, 5 or 6 could be used in a single phase system. Three- and singlephase motors can be mixed in a single system. The microcontroller 40 could be programmed to output diagnostic data via the serial port, which itself may be connected to a modem.

Resetting of the system may be performed by operation of a simple switch on the controller, entry of a code via a key pad or by sending a command via the serial port 44 from a PC.

The skilled person will also appreciate that further fault conditions may be detected and the power supply controlled appropriately in response thereto.

In the above-described embodiment, the system operates normally at only three of the four available voltages. It will be appreciated that all four voltages could be used for normal operation. Furthermore, a greater or lesser number of voltages could be used.

The actual voltages appropriate for a given installation will depend on the characteristics of the motors used. A motor nominally for 220V operation has been operated successfully, in a system according to the present invention, down to 163V rms.

Claims (23)

Claims

1. A method of supplying power to one or more motors, comprising the steps of: supplying power to every motor at a first voltage; sensing for every motor a parameter value indicative of the load thereon; and supplying power to every motor at a second higher voltage if the parameter is on a first side of a respective threshold value.

2. A method according to claim 1, including performing, if power is being supplied to every motor at the second voltage, the steps of: sensing for every motor said parameter value; and supplying power to every motor at the first voltage, if for every motor the sensed parameter value is on a second side of respective threshold values.

3. A method according to claim 1 or 2, including performing, if power is being supplied to every motor at the second voltage, the steps of: sensing for every motor a parameter value indicative of the load thereon; and supplying power to every motor at a third voltage, greater than the second voltage, if a sensed parameter value is on a first side of a respective further threshold value.

4. A method according to claim 3, including performing, if power is being supplied to the motors at the third voltage, the steps of: sensing for every motor said parameter value; and supplying power to every motor at the second voltage, if for every motor the sensed parameter value is on a second side of said respective further threshold values.

5. A method according to any preceding claim, wherein for each change in the motor supply voltage, the relationship between the parameter and the relevant threshold must persist for a respective predetermined period for the change to be effect.

6. A method according to any preceding claim, wherein the parameter value is motor drive current.

7. A method according to any one of claims 1 to 5, wherein the parameter value is a temperature dependent on the operation of a motor.

8. A method according to any one of claims 1 to 5, wherein the parameter value is motor drive current or a temperature dependent on the operation of a motor.

9. A method according to any preceding claim, including the steps of: detecting a motor overcurrent condition; and supplying power to every motor at a maximum voltage and generating an alarm signal if an overcurrent condition is detected.

10. A method according to any preceding claim, including the steps of: deteaing the parameter value for one of the motors passing a limit value; and supplying power to every motor at a maximum voltage if a motor parameter passes the limit value.

11. A method of operating one or more refrigeration units, each unit having a compressor motor, wherein every compressor motor is supplied with power by a method according to any preceding claim.

12. A method of operating one or more refrigeration units, each unit having a compressor motor, wherein every compressor motor is supplied with power by a method according to claim 7 or 8, wherein the parameter value is the temperature of the working fluid, after absorbing heat and before expansion, compressed by a compressor motor.

13. A motor system comprising one or more electric motors, sensor means for sensing for every motor a parameter value which is dependent on the load thereon, and a controller for supplying electrical power to every motor, wherein the controller is responsive to the sensor means to perform a method according to any one of claims 1 to 5.

14. A system according to claim 13, wherein the sensor means comprises a current sensor for sensing a drive current of every motor.

15. A system according to claim 13, wherein the sensor means comprises, for every motor, a temperature sensor arranged to sense temperatures dependent on the operation of the motor.

16. A system according to claim 13, 14 or 15, including an overcurrent detector for detecting an overcurrent condition in every motors, wherein the controller is responsive to every overcurrent detector to supply power to every motor at a maximum voltage and generate an alarm signal if an overcurrent condition is detected.

17. A system according to any one of claims 13 to 16, wherein the controller is arranged to compare the values from the sensing means with respective limit values and to supply power to every motor at a maximum voltage, if a parameter value passes a respective limit value.

18. An installation comprising a plurality of refrigeration units and a system according to any one of claims 13 to 17, wherein the motors of said system are compressor motors of the refrigeration units.

19. An installation according to claim 18, wherein the refrigeration units include a heat exchanger for heating a working fluid and an expansion chamber, the system is in accordance with claim 15 and said temperature sensors are located so as to sense the temperature of the working fluid between the heat exchanger and the expansion chamber.

20. A method of supplying power to a plurality of motors, substantially as hereinbefore described.

21. A motor system substantially as hereinbefore described with reference to Figures 1, 2, 3 and 4.

22. A motor system substantially as hereinbefore described with reference to Figures 1, 2, 4 and 5.

23. A motor system substantially as hereinbefore described with reference to Figures 1, 2, 4 and 6.