Refrigerator Compressor Design Achieves High Energy Efficiency Ratings Using a Digital Signal Controller (DSC)

Refrigerators and other kitchen appliances present significant challenges to off-grid energy systems because of their high-energy demands. Improved refrigerator compressors are now driven by brushless DC motors or permanent magnet synchronous motors (PMSM) to meet high energy efficiency ratings. This high energy efficiency can be achieved by using an inverter-based variable speed drive for the brushless motor.

Older generation refrigerators used single phase induction motors which were not self-starting. This was fixed by adding an auxilliary winding or pole shading. However, both of these methods waste energy after starting as they serve no further purpose. Another challenge for an ACIM motor is that the torque output is very low until it reaches its target speed.

PMSM based compressors on the otherhand are very efficient and operate much more quietly. They can also provide a lot more torque at startup and at low speeds when you really need it. For these reasons, a PMSM or an Internal Permanent Magnet (IPM) motor is becoming the preferred solution for new refrigerators.

The compressor motor control software is especially challenged to provide reliable startups due to high coolant backpressure during a stop and quick restart, and to balance out the piston movement felt in every mechanical revolution. To solve these challenges, our dsPIC33 Digital Signal Controller (DSC) based refrigerator compressor reference design implements a unique algorithm for PMSM and IPM motors which is designed to ensure a reliable startup everytime. To reduce noise and vibration, a torque compensation algorithm automatically adjusts the motor speed for the piston movement. 

Using a PMSM motor provides another way to improve energy efficiency by implementing a variable speed (frequency) drive (VFD). This is not possible to implement using single-phase AC induction motors. A VFD allows operation of the compressor at the optimal speed required to maintain a constant temperature inside the refrigerator, thus saving energy.

Using a Field-Oriented Control (FOC) algorithm provides a VFD and other advanced motor control features like on-the-fly startup and stall detection with auto-recovery. Along with FOC, a single-shunt motor current sensing technique is utilized which lowers the overall BOM cost.

Microchip’s refrigerator compressor reference design promotes rapid prototyping and development of a cost-effective and innovative design using a dsPIC33 DSC. This design works with a wide variety of refrigerator compressor motors, supporting both Interior Permanent Magnet Synchronous Motors (IPMSM) and Surface Mount Permanent Magnet Synchronous Motors (SPMSM). Software algorithms ensure reliable startup of the compressor with high back pressure and low standby power consumption. It implements sensorless FOC VFD using a single-shunt current sensing technique. This design supports a range of advanced control techniques that help to improve efficiency. It offers over-current protection, over and under-the voltage protection, speed error and in-rush current limit features for reliable operation. 

See Figure 1. Major Circuit Components for the dsPIC33 DSC Reference Design

See Figure 2. The Refrigerator Compressor Development Board has the Same Form Factor as the Boards Used in Actual Refrigerators

The dsPIC33 DSCs have many features such as advanced motor control PWMs, integrated high-speed ADCs, op amps and high-speed analog comparators to help implement a cost-effective and high-performance FOC drive for a PMSM. The high level of peripheral integration helps to reduce the overall system BoM cost.

Functional safety has become a critical factor in the industry to ensure safe and reliable operation to protect end-user well-being. Our “functional safety ready” dsPIC33 DSCs offer many safety hardware features, functional safety collateral and VDE- and UL-certified IEC 60730 Class B safety diagnostic libraries that simplify meeting functional safety standard compliance for reliable and robust operation.

The following are some of the benefits of using the dsPIC33 DSC Refrigerator Compressor Reference Design:

• Speeds development time with a turnkey example design that includes rich protection features, motor control and application source code, a user guide and multiple communication ports

• Provides smoother torque and better start and stop characteristics using FOC, which enables quieter compressor operation and enables a longer lifespan

• Reduces system-level BOM cost with its high-performance dsPIC33 core and control peripherals for enabling sensorless FOC, DC-DC control for the auxiliary power supply and single-shunt current sensing with on-chip op amps

For more information, go to Refrigerator Compressor Reference Design. Please contact your local Microchip representative to obtain more information about this reference design. Check out other popular dsPIC33 DSCs and motor control solutions.