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EP-3872982-B1 - CONTROL UNIT AND CONTROL METHOD FOR CONTROLLING A MOTOR

EP3872982B1EP 3872982 B1EP3872982 B1EP 3872982B1EP-3872982-B1

Inventors

  • PEDERSEN, NIELS

Dates

Publication Date
20260513
Application Date
20200228

Claims (14)

  1. A control unit (200) configured for controlling a motor (215) driving a piston compressor pump (216) below a selected rotational speed value, wherein the motor (215) is driven by a frequency converter comprising a rectifier unit (212) being operatively connected to an inverter unit (214) via an intermediate DC circuit (213), and wherein the inverter unit (214) provides a multi-phase AC power drive output to the motor (215), the control unit (200) comprising a) a motor speed calculation unit for calculating, in the digital domain, the rotational speed of the motor based on measured BEMF voltages from the motor, b) a speed controller unit for calculating, in the digital domain, a stator current reference based on the calculated rotational speed and a rotational speed reference, c) a D/A converter unit for converting the calculated stator current reference from the speed controller unit to an analog stator current reference, wherein the analog stator current reference sets an upper value for the stator current, d) a comparator unit for comparing, in the analog domain, the analog stator current reference and a measured current in the intermediate DC circuit of the frequency converter, and providing a comparator unit output in response to this comparison, and e) a PWM unit for providing PWM control signals to a phase commutation unit in response to the comparator unit output, said phase commutation unit providing a switch pattern to the inverter unit based on the measured BEMF voltages from the motor.
  2. A control unit according to claim 1, wherein the PWM control signals from the PWM unit comprise square-wave signals, and wherein the duty-cycles of the square-wave signals are based on the comparator unit output.
  3. A control unit according to claims 1 or 2, wherein the calculated stator current reference from the speed controller unit is a 12 bit register value having a value in a range between 0 and 4095.
  4. A control unit according to any of the preceding claims, further comprising an A/D converter unit for converting the measured BEMF voltages from the motor to digital BEMF signals, said digital BEMF signals being provided to the phase commutation unit and the motor speed calculation unit.
  5. A control unit according to any of the preceding claims, wherein the inverter unit comprises a B6 inverter bridge, and wherein the inverter unit is configured to provide a three-phase AC power drive output for driving the motor and the piston compressor pump.
  6. A control unit according to any of the preceding claims, further comprising an application controller unit for providing the rotational speed reference based on a user defined set point and a measured load related feedback signal.
  7. A control unit according to any of the preceding claims, wherein the selected rotational speed value of the motor is below 4000 rpm, such as below 3500 rpm, such as below 3000 rpm, such as below 2800 rpm, such as below 2600 rpm, such as below 2400 rpm, such as below 2200 rpm, such as below 2000 rpm, such as below 1800 rpm, such as between 1200 rpm and 1800 rpm.
  8. A refrigerator system comprising a control unit according to any of the preceding claims, a piston compressor pump, a motor driving the piston compressor pump, and a frequency converter comprising a rectifier unit being operatively connected to an inverter unit via an intermediate DC circuit, the inverter unit providing a three-phase AC power drive output to the motor and the piston compressor pump.
  9. A refrigerator system according to claim 8, wherein the motor driving the piston compressor pump is a brushless DC motor/synchronous permanent magnet motor.
  10. A method for controlling a motor (215) driving a piston compressor pump (216) below a selected rotational speed value, wherein the motor (215) is driven by a frequency converter comprising a rectifier unit (212) being operatively connected to an inverter unit (214) via an intermediate DC circuit (213), and wherein the inverter unit (214) provides a multi-phase AC power drive output to the motor (215) and the piston compressor pump (216) the method comprising the step of a) calculating, in the digital domain, a rotational speed of the motor based on measured BEMF voltages from the motor, b) calculating, in the digital domain, a stator current reference based on the calculated rotational speed and a rotational speed reference, c) converting the calculated stator current reference to an analog stator current reference, wherein the analog stator current reference sets an upper value for the stator current, d) comparing, in the analog domain, the analog stator current reference and a measured current in the intermediate DC circuit of the frequency converter, and providing a PWM control signal in response to this comparison, and e) controlling a switch pattern to the inverter unit based on measured BEMF voltages from the motor.
  11. A method according to claim 10, wherein the selected rotational speed value is below 4000 rpm, such as below 3500 rpm, such as below 3000 rpm, such as below 2800 rpm, such as below 2600 rpm, such as below 2400 rpm, such as below 2200 rpm, such as below 2000 rpm, such as below 1800 rpm, such as between 1200 rpm and 1800 rpm.
  12. A method according to claim 10 or 11, wherein the PWM control signals comprise square-wave signals, and wherein the duty-cycles of the square-wave signals are based on the comparison of the analog stator current reference and the measured current in the intermediate DC circuit of the frequency converter.
  13. A method according to any of claims 10-12, wherein the calculated stator current reference from the speed controller unit is a 12 bit register value having a value in a range between 0 and 4095.
  14. A method according to any of claims 10-13, wherein the inverter unit comprises a B6 inverter bridge, and wherein the inverter unit is configured to provide a three-phase AC power drive output for driving the motor and the piston compressor pump, and wherein the switching frequency of the six controllable switches of the B6 inverter bridge is within the range 5-20 kHz, such as within the range 5-15 kHz, such as within the range 5-10 kHz, such as within the range 6-8 kHz, such as within the range 6-7 kHz, such as approximately 6.6 kHz.

Description

FIELD OF THE INVENTION The present invention relates to a control unit and an associated control method for controlling a motor driving a piston compressor pump in the low-speed range. In particular, the present invention relates to current control of a motor in the low-speed range in order to reduce electrical losses and increase a coefficient of performance of a refrigeration system including the motor and the piston compressor pump. BACKGROUND OF THE INVENTION Power optimization, including power loss minimization, has always been an issue in the field of power electronics. Within the field of power amplifiers for driving motors it has thus always been of interest to comply with a given load on the motor by feeding as little power as possible to the motor in order to increase the coefficient of performance of for example a refrigeration system including the motor and a piston compressor pump coupled thereto. US 2013278189 A1 relates to an inverter control system comprising a voltage-source PWM inverter, a current sensor for detecting the input current of the inverter and a speed-control controller for controlling the rotation of a motor. The speed-control controller includes an analog sensorless circuit for a sensorless phase detection and an arithmetic unit, such as a CPU, wherein the arithmetic unit comprises a speed detector, a speed control section and a current control section. US 5,592,058 relates, in general, to a system for conditioning air in a space by heating or cooling the air to change its temperature. A thermostat located within the space generates a two-state temperature signal having a cyclic parameter which corresponds to the temperature of the air in the space as it rises and falls. A compressor supplies refrigerant to a heat exchanger by means of which changes are made in the temperature of the air. A variable speed motor drives the compressor in response to a motor control signal. A controller responds to the temperature signal and senses the cyclic parameter of the temperature signal. The controller generates the motor control signal as a function of the sensed cyclic parameter whereby the motor control signal is provided to the motor to control the torque or speed of the motor. The system of US 5,592,058 is disadvantageous due to its lack of power efficiency in relation to piston compressor pumps being operated at low rotational speeds. It may be seen as an object of embodiments of the present invention to provide a control scheme that reduces electrical losses when a motor is driving a piston compressor pump. It may be seen as a further object of embodiments of the present invention to provide a control unit and a control method that reduces electrical losses when a motor is driving a piston compressor pump at low rotational speeds. DESCRIPTION OF THE INVENTION The above-mentioned object is complied with by providing, in a first aspect, a control unit configured for controlling a motor driving a piston compressor pump below a selected rotational speed value, wherein the motor is driven by a frequency converter comprising a rectifier unit being operatively connected to an inverter unit via an intermediate DC circuit, and wherein the inverter unit provides a multi-phase AC power drive output to the motor, the control unit comprising a) a motor speed calculation unit for calculating, in the digital domain, the rotational speed of the motor based on measured BEMF voltages from the motor,b) a speed controller unit for calculating, in the digital domain, a stator current reference based on the calculated rotational speed and a rotational speed reference,c) a D/A converter unit for converting the calculated stator current reference from the speed controller unit to an analog stator current reference, wherein the analog stator current reference sets an upper value for an associated phase stator current,d) a comparator unit for comparing, in the analog domain, the analog stator current reference and a measured current in the intermediate DC circuit of the frequency converter, and providing a comparator unit output in response to this comparison, ande) a PWM unit for providing PWM control signals to a phase commutation unit in response to the comparator unit output, said phase commutation unit providing a switch pattern to the inverter unit based on the measured BEMF voltages from the motor. Thus, the control unit of the present invention is configured to operate the motor and the piston compressor pump below a selected rotational speed value, i.e. at low rotational speeds. The control scheme provided by the present invention is advantageous in that it reduces electrical losses. It has in this respect been estimated that the electrical losses may be reduced by approximately 5-8% due to reduced current levels in various power electronic devices, such as in the frequency converter and the motor. The rotational speeds at which the electrical losses may be reduced may be below 4000 rpm, such as b