Electronic Design

White Goods See Significant Motor-Control Innovations

Cars are exciting, and appliances are boring, right? That depends. While you can’t take an air conditioner for test drives on a frozen lake to evaluate its dynamic response to regenerative braking in slippery conditions, as Greg Solberg did with the Tesla Roadster, there can still be challenges.

For example, cultural and economic differences in regional markets for white goods influence motor-control design. On the cultural side, to cite one case, China and Japan present a contrast in attitudes that affects the kinds of air-conditioner compressors favored in each market, which in turn affects the design of motor controllers. In a nutshell, Chinese consumers tend toward thriftiness, while Japanese consumers tend to think green.

In rotary compressors, an eccentric circular cam revolves inside a larger circular cavity. Pressure cycling and the eccentric weight distribution of the rotor create mechanical vibration and a cyclically varying motor load—as the shaft rotates, the motor needs to provide different levels of torque at different angular displacements.

Scroll compressors, which cost more to build than rotary compressors, have an eccentric helical rotor that orbits within a stationary helix. They’re quieter than rotary compressors because they represent a generally constant shaft load. That makes them more neighbor-friendly. They’re also more efficient than rotary compressors, so they do represent a lower carbon footprint. Unfortunately, they’re not so efficient that their lifetime operating costs significantly balance out their higher acquisition costs.

Added to those factors, a considerable difference still exists between the amount of income available for luxury goods in members of the growing Chinese middle class and Japanese “salarymen,” even factoring in the distressed state of the Japanese economy these days.

Equally significant on the economic side is the white-goods industry’s trend toward replacing induction motors with permanent-magnet synchronous motors (PMSMs). In recent years, induction motor prices have risen, largely due to a worldwide increase in steel and copper costs.  PMSM prices, on ther other hand, are seeing more modest rises. 

At price parity, PMSMs offer lower conduction losses, good low-speed torque, simple speed control, and compact size, making them an appealing choice. From a motor-control standpoint, PMSMs don’t have iron cores with hysteresis and saturation issues, which makes it simpler to design for them. Their speed depends solely on switching frequency.

In addition, they don’t require the controller to deal with signals from Hall-effect sensors to monitor phase currents. It’s possible to simply use sense resistors that can be built into the controller to sample the driver currents for the different phases.

That combination of forces presented a challenge to International Rectifier, which responded with its iMotion controller ICs in 2006. The concept was to create a motor-control subsystem that would enable air-conditioner OEMs to cut their total materials and assembly costs while accommodating cultural differences in different market areas.

For instance, in addition to dealing with the steady torque loads of scroll compressors, they could also easily deal with the potential for vibration and varying loads presented by rotary compressors by creating a more complex driving function than three phases of simple sinusoids.

The same products could also be applied to fan control. Fans need variable speed control as much as compressors if the air conditioner is to efficiently adjust its output to match conditions and maintain a steady temperature. Studies show that up to 40% efficiency can be gained in systems that allow the fan and compressor to run at variable speeds. Moreover, the controllers had to provide power factor correction (PFC) to meet various countries’ regulatory requirements.

The iMotion “integrated design platform” comprises a digital control IC plus a driver IC that integrates the required IGBTs and FETs for power switching. The control IC is a multicore design. One side is built around an 8051 microcontroller that handles the user interface. The other is built around a patented Motion Control Engine (MCE) that implements a complex, sensor-less, PMSM-control algorithm in hardware.

The cores communicate via a RAM register block that provides set-points for the MCE and diagnostic feedback to the MCU. While the MCU is programmed conventionally, the MCE algorithm is implemented via a graphical drag-and-drop approach using a library of analog functions.

In addition to the MCE, the chip integrates a mixed-signal block called the embedded analog signal engine (ASE) that integrates signal-conditioning and sensor-less motor control using those built-in current shunts. Implementing the MCE in hardware provides faster loop response than a DSP, fast enough to provide simultaneous control of the fan and compressor motors.

IR has kept the iMotion product line up to date. Most recently, the company introduced the IRMCK171 controller and the IRAM336 integrated power module for appliance-motor control (see the figure). The IRMCK171 is a motor controller on a chip. On the MCE side, it comprises control elements, motion peripherals, a motion-control sequencer, and a dual-port RAM to map internal signal nodes. It includes an unusual single-shunt current reconstruction circuit in addition to a two-leg shunt-current sensing circuit that enables a direct shunt resistor interface to the IC.

Motion control is programmed via iMotion’s graphical compiler, which is integrated into the Matlab/Simulink development environment. On the control side, sequencing, user interface, host communication, and upper-layer control tasks are implemented on that 8051. The production IRMCK171 offers 32 kbytes of one-time-programmable (OTP) program ROM. The IRMCF171 development chip includes 64 kbytes of flash. They come in a 48-pin quad flat pack (QFP).

The IRAM336-025SB integrated power module for appliance motor control is a multichip hybrid that contains high-voltage MOSFETs and a three-phase driver in a thermally enhanced package. Bootstrapped power supplies for the high-side drivers can be generated using internal bootstrap diodes, eliminating the need for isolated power supplies. It will handle motors up to 250 W, operating from 85 to 253 V ac. A reference design incorporates both devices.

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