A new spin on washing machine design

A growing global trend these days is the use of permanent magnet synchronous motors (PMSMs) in appliance applications. This is driven by a number of factors, ranging from raw material costs that shifted cost advantages away from induction motors, to the design and reliability benefits of a simplified mechanical structure.

Using variable-speed direct motor drives based on PMSM technology also offers advantages in terms of more efficient operation, smooth torque control, reduction in acoustic and electrical noise, and lower levels of vibration.

The advantages of variable-speed, PMSM washing-machine motors remain abundantly clear. But one barrier hindering the technology's rapid integration into applications is the need to obtain rotor position information. Typically, this is accomplished by using Hall-Effect sensors driven by the rotor magnet, which can add significant cost and complexity to the design. At the same time, the traditional trapezoidal current commutation can cause torque glitches at the switching points. These tend to be amplified by the external rotor structure, increasing acoustic noise.

A further significant issue is that the drive torque-speed curve isn't a good fit to a washing application that requires high torque at low speed, and low torque at high speed. These challenges (Fig. 1), combined with the benefits of PMSM solutions, have fueled the need for sensorless control, and they've driven the developments of integrated, semiconductor-based platforms such as the latest iMOTION technology from International Rectifier.

Sensorless control

Previously offering dedicated solutions that speed and simplify the design of air-conditioner applications, the concept of International Rectifier's iMOTION platform is to develop application-specific "building blocks" that can be quickly combined to deliver the functionality needed to implement variable-speed motor drives.

With the launch of the latest platform, this concept has now been extended to provide a sensorless control platform for direct-drive PMSM washing machines. The new platform, which features an embedded field-oriented-control (FOC) algorithm for high-dynamic torque control, improves washing cycles by consuming less hot water. All told, it can save up to 70%of energy usage.

This latest addition to the iMOTION family features a mixed-signal controller and companion intelligent power modules, as well as algorithms, development software, and design tools.

Combined in one platform, iMOTION for washers simplifies design of variable-speed motor control in both horizontal- and vertical-axis washing machines. The platform enables sensorless sinusoidal current control, completely eliminating Hall-Effect sensors and delivering smooth torque over an extended speed range.

Figure 2 illustrates the implementation of a washer application using the building blocks from the new iMOTION platform. Figure 3 shows a block diagram of the controller IC that lies at the heart of the system.

The IRMCF341 washer control IC integrates all control and analogue interface functions required for sensorless speed control of PMSM motors using dc link current measurements. The analogue functions on the IC include differential amplifier, dual sample-and-hold circuits, and a 12bit analogue-to-digital converter to sample the low-voltage signal across the dc link shunt. The digital control IC features IR's patented Motion Control Engine (MCE), which eliminates software coding from the motion-control algorithm development process through an embedded FOC algorithm. This algorithm optimises the output torque from interior permanent-magnet motors and maximises bus voltage utilisation in the high-speed field-weakening mode.

A co-integrated 60MIPS, 8bit, 8051 microcontroller enables application-layer software development, operating almost independently of the MCE, and it won't compete for system resources such as interrupts or internal registers. This streamlines application-software debugging, since the MCE can be treated very much like a hardware peripheral with a set of input and output registers.

The embedded Analog Signal Engine (ASE) integrates all signal-conditioning and conversion circuits required for single-current shunt, sensorless control of a PMSM motor. The IRMCF341 control IC comes in a 64pin QFP package with all of the required analogue inputs and PWM outputs to control the washing-machine motor.

The companion IRAMS06UP60B and IRAMS10UP60B inverter power modules feature an advanced high-voltage, gate-drive integrated circuit for three-phase inverter control. The modules include the dc link shunt used for both motor current measurement and power module protection. Built-in overtemperature/overcurrent protection, along with short-circuit-rated IGBTs, an integrated undervoltage lockout function, and built-in temperature monitor provide a high level of protection and fail-safe operation.

Design Tools

In order to support the rapid implementation and customisation of washer applications based on the new iMOTION technology, the platform also includes the IRMCS3041 reference design and a variety of software tools.

Tools include communications software running on the 8051 microcontroller that gives the PCbased MCEDesigner software access to the control parameters and system variables in the shared memory. As a result, the controller set points and control loop gains, along with other constants, can be modified without needing to modify or compile software. MCEDesigner tools generate wash-cycle profiles and plot motor speed and current, accelerating rapid evaluation of direct-drive motor performance with real washing loads.