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issue: April 2006 APPLIANCE Magazine

Motor Technology
Sensorless Motor Control


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A motor control platform for air-conditioning applications enables variable-speed sinusoidal current control without position sensors.

A new air-conditioning platform from El Segundo, California, U.S.-based IR features a proprietary algorithm for interior permanent magnet synchronous motors in the compressor and the fan, and includes power factor correction.

The new iMotion integrated design platform was created by International Rectifier (IR) to deliver variable-speed control technology to air-conditioner manufacturers at no additional system cost. According to Aengus Murray of IR’s Digital IC Design Center, this meant putting a complete appliance control system, including power factor correction (PFC), onto one integrated circuit (IC). “We can manage the compressor motor; we can control the speed of the fan motor; and we can control the PFC on one piece of silicon,” he says.

The company began by creating the platform around permanent magnet (PM) synchronous motors, which Murray says are more efficient than induction motors in variable-speed applications.

The new platform also enables the use of a cost-effective single rotary compressor while still offering smooth torque and low noise. “With the integrated design platform, you can manage the torque delivered to the motor by controlling the current. So you can offset some of the torque fluctuations you get from a single rotary compressor,” he explains. “Smoother torque means a quieter compressor.”

Another fundamental design requirement was sensorless control since most compressors don’t have the space for sensors nor the wiring they require. IR engineers designed a platform that uses DC link current sensing and combines the company’s high voltage integrated circuit (HVIC) technology with digital, analog and power building blocks.

Murray says the DC link current sensor is key to achieving both cost-effective and reliable motor control. “It’s one resistor that measures the current going into the power block,” he explains. “By measuring that current at specific times, we can measure the current going into the motor winding. That’s how we control the speed of the motor.”

The platform’s digital block consists of the IRMCF3xx family of digital control ICs, which contains three main elements. The first is IR’s patented Motion Control Engine (MCE) that implements the sensorless PM synchronous motor algorithm in hardware, eliminating software coding from the development process. The second element is an embedded analog signal engine (ASE), which integrates all signal conditioning and conversion circuits. The final element is an 8-bit application layer processor that defines the operation of the air-conditioning system independently from the MCE controlling the fan and compressor motors. “This makes it very easy for the appliance engineer to use the system because he can write his software on the 8-bit microcontroller to manage the appliance…and he doesn’t have to worry about what’s happening in the motor control system,” Murray says.

The analog interface consists of the IRS2136D family of three-phase analog driver and protection ICs. According to Murray, the interface’s separate power and signal ground connections make it much easier to measure the current through the DC link. Cross-conduction protection further increases inverter reliability by preventing accidental shoot-through.

The platform’s power block uses the company’s next-generation of trench IGBTs. “A new power switch construction gives you lower on-voltage and lower switching loss to improve inverter efficiency,” Murray says. “This cuts down on losses, but it also allows you to fit the inverter into a smaller package, saving space and cost of the overall inverter.”

Suppliers mentioned in this article:
International Rectifier
 

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