Dual Variable-Speed Drives for Air Conditioners – Reference Design

March 25, 2020 by Paul Shepard

Air conditioner (A/C) designs need to meet stringent energy efficiency ratings and power factor specifications to reduce overall power consumption. High energy efficiency is achieved by using an inverter-based variable speed drive for the brushless motor-based compressor and condenser fans.

The variable speed drive and the sensorless field-oriented control (FOC) of brushless motors allow an appliance to operate at an optimal power setting compared to a fixed speed single phase ac induction motor (ACIM) drive. Power factor correction (PFC) is required for all high-power appliances to connect to the electrical grid.

Microchip's Air Conditioner reference design, based on the dual core dsPIC33CH family of digital signal controllers, demonstrates efficient control of the condenser fan, compressor, PFC and the overall application logic implementation. The high-performance dsPIC33C DSCs (Dual core dsPIC33CH DSCs and Single core dsPIC33CK DSCs) offer the right set of peripherals that is designed for dual motor control and enables replacing three controllers with one for a cost advantage in your design.

This reference design has proven power stages for high power PFC (220V, 1.8kW), condenser fan (120W) and compressor (1.34kW) motor control. Additional features like temperature sensor interfaces, field communications and valve drives will enable you to develop on this platform for a wide variety of air conditioner systems like window A/Cs and the outdoor unit of split A/Cs.

Air conditioner reference design block diagram (click on image to emlarge)

Air Conditioner Reference Design Features

  • Turnkey solution for Air Conditioner designs like Window A/Cs and the Outdoor Unit of Split A/Cs
  • 220V, 1.8 KW input power stage, with Digital Interleaved Boost PFC with an efficiency of greater than 90% across the operating range, 0.991 PF and <5% THD
  • Compressor Motor:
    • Supports PMSM or IPMSM
    • Up to 1.4kW Inverter Stage
    • 4- to 20-kHz Switching Frequency
    • Variable Speed Motor Control
    • Sensorless FOC
    • Torque Compensation
    • Field Weakening
    • PLL Estimator
    • Maximum Torque per Ampere (MTPA) for IPMSM
    • Stall Detection
  • Condenser FAN:
    • Supports PMSM or IPMSM
    • 150W Power Stage
    • 20kHz Switching Frequency
    • Variable Speed Motor Control
    • Sensorless FOC
    • Wind Milling
    • PLL Estimator
    • Maximum Torque per Ampere (MTPA) for IPMSM
    • Stall Detection with Recovery
  • Field communication support with the Indoor Unit
  • Optional support to use an ACIM or a built-in drive BLDC motor for the Condenser Fan
  • Four temperature sensor interfaces for measuring: Ambient Air, Inlet Air, Outlet Air and Condenser Temperature
  • Solenoid Valves and Electronic Expansion Valve (Stepper Motor) Control interfaces
  • Fly-back dc-dc to generate 15-V, 12-V, and 3.3-V power rails