Fig. 2. Phase-current reconstruction based on inverter switching states.

Recent worldwide trends are toward a more widespread use of permanent-magnet synchronous motors in appliance applications. Global raw-material-price increases in aluminum, copper, and steel have driven this trend and have shifted the cost advantage away from induction motors, which are less efficient and more bulky than permanent magnet motors. The application of direct-drive permanent-magnet motors in clothes washers improves the system dynamics and saves cost due to a simpler mechanical structure.

However, the control of the permanent-magnet motor requires rotor-position information, which can add cost and complexity to the motor design. Hall-Effect sensors driven by the rotor magnet can provide cost-effective feedback, but the traditional trapezoidal current commutation produces torque glitches at the switching points. The external rotor structure tends to amplify the effect of these glitches and generate unwanted acoustic noise.

A more significant issue is that this drive’s torque-speed curve does not match well with the washing application, which requires high torque at low speed and low torque at very high speeds. A recent innovation using interpolation between Hall switching points enables sinusoidal current control, which delivers smooth torque with low acoustic noise along with high-speed, low torque operation. However, the reliability of the Hall sensor subassembly in the field is a major issue, and so a sensorless approach is very desirable.

Design platform for washer

Fig. 3. Clothes washer design platform.

The hardware platform for the clothes washer, motor-control system appears in Fig. 3. The appliance-control IC integrates all control-interface and analog-interface functions needed for sensorless speed-control of a PMSM using DC-link current measurements. The analog functions on the IC include the differential amplifier, dual sample-and-hold circuits, and a 12-bit A/D converter that samples the low-voltage signal across the DC-link shunt.

The inverter power module integrates a high-voltage, gate-drive IC with the six IGBT switches. The module includes the DC-link shunt that serves both the motor-current measurement and power-module protection. The IC implements the motor-control algorithm in hardware using the dedicated Motion-Control Engine while the appliance-application software runs on an independent, co-integrated 8-bit processor.

Fig. 4. Sensorless, motor-control algorithm.

The digital-control algorithm integrates phase-current reconstruction and rotor-angle estimation with a field-oriented control algorithm controlling the motor current (See Fig. 4). Field-oriented control uses vector rotations to decouple the AC motor-winding currents into two DC components controlling torque (IQ) and flux (ID) (Citation 1). This simplifies the controller design since the current-loop tuning becomes independent of the motor speed. The outer speed loop calculates the torque reference command for the IQ loop based on the speed error. A RAMP function at the input to the speed loop limits acceleration to within specified limits, and a LIMIT function on the output limits the motor current.

An additional control function introduces phase advance to maximize the torque output when driving an interior permanent magnet motor. The flux reference is set to zero at low speeds to maximize the torque-per-amp, but can be set to a negative value to weaken the flux for high-speed operation. The field-weakening algorithm calculates the optimum ID reference current to maximize the use of the available inverter voltage.

Library-macro blocks from the control IC’s Motion Control Engine implement the digital controller. The MCE library includes PI compensators, limit functions, and vector rotations that are common in motor-control algorithms. Graphical editing tools customize the algorithm and eliminate the requirement for software coding. The algorithm can execute up to two-and-a-half times faster than a RISC or DSP because so many of the time-critical control calculations are performed using dedicated hardware.

The control parameters and system variables are stored in shared data RAM that the co-integrated 8-bit microcontroller can also access. This allows the appliance-application software to easily change control set points such as target speed, or to monitor control variables such as the torque current (IQ). However, the appliance-software development can use “C” on this independent 8-bit processor, which has its own memory area and dedicated peripheral set. This simplifies application-code development since the appliance engineer does not have to become an expert in the motor-control processor in order to develop his application.

Design tools

The integrated design platform includes a reference-design board and software tools to customize the algorithm to the application’s requirements. The MCE design tools include communication software running on the 8051 microcontroller that gives the PC-based MCE Designer software access to the control parameters and system variables in the shared memory. This allows an engineer to modify controller set points, control-loop gains, and other constants without having to modify or compile software.

A spreadsheet that calculates the digital control parameters based on motor-nameplate and system-performance specifications simplifies controller tuning. The MCE Designer tool also includes a trace function to plot control-loop-system variables. This not only helps in commissioning and evaluating the performance of the drive but can also examine system variables that could add appliance features such as load detection or out-of-balance monitoring.

Conclusion

The application of direct-drive, permanent-magnet motors in clothes washers improves the system dynamics and saves cost because of a simpler mechanical structure. However, the control of the permanent-magnet motor requires rotor-position information, which adds cost and complexity to the motor design. Current drive systems use Hall-Effect sensors to sense the rotor magnet but they suffer from field-reliability issues.

A sensorless control algorithm that uses measurements of the inverter DC-link current eliminates the need for the hall sensors and simplifies the motor design. An integrated design platform is available with the PMSM control algorithm embedded in the digital-control IC. Design tools allow evaluation of the algorithm in the application without having to develop software code. Moreover, the appliance application-code development can proceed on a separate co-integrated 8-bit processor core. This design platform simplifies the mechanical design, motor-control-system design and application-code development.

For more information email: LHarris1@irf.com

Citation 1: Bose, B.K. “Power Electronics and Variable Frequency Drives” IEEE Press 1997.