Sensorless Motor Control Simplifies Washer Drives

Jun 1, 2006 12:00 PM
By Aengus Murray, Director iMOTION Products, Energy Savings Product Group, International Rectifier,

Washing controls have used power inverters to control motor speeds for years. Variable-speed control enables a significant simplification of the mechanical system by replacing a clutch and gearbox with a belt drive. Washers in North American commonly use a three-phase induction motor with tachometer feedback. The drive typically uses slip torque control during the washing cycle and field weakening for high-speed spinning. Compared to drives that do not provide field weakening, this simple mechanical system enables a higher spin speed that extracts more water from the clothes, which reduces the energy the dryer consumes.

To gain further energy savings, appliance engineers are now introducing laundry algorithms that minimize the hot water that the washing cycle consumes. However, the new algorithms require a higher dynamic response from the drive train so manufacturers seek to replace the belt-drive system with direct-drive permanent-magnet (PM) motors.

Compared to a three-phase induction motor, a direct-drive PM motor uses a larger number of motor poles to generate higher torque at lower speeds for the same power input. The ratio of the drum speed to the motor electrical frequency remains almost the same, but instead of stepping down the motor speed using, for example, a 10-to-1 pulley ratio, increasing the motor-pole count by a factor of 10 achieves the same effect.

Induction motors require a relatively large number of slots per pole and a small air gap that makes it difficult to build motors with high pole counts. PM motors provide much more construction flexibility and are the best choice for the direct drive. Fig. 1 shows a direct-drive motor with an external rotor that connects directly to the washer drum. Concentrated stator windings make it possible to design the motor with a “pancake” construction that easily fits into the washing machine's available space.

Though PM ac motors provide advantages over induction motors for washer applications, one critical issue to solve is rotor position sensing. Transformer action generates the rotor field in the induction motor, so the field is always synchronous to the stator current. However, the motor-drive circuit must measure the rotor position in the PM machine to synchronize the stator current with the rotor field. Given the rotor position, it is possible to drive the PM ac motor efficiently because the drive circuit can align the stator current to the optimum angle relative to the rotor field.

In the direct-drive motor shown in Fig. 1, Hall Effect sensors detect the rotor-magnet position. However, designs that eliminate these sensors alleviate the reliability issues that result from mounting the sensor electronics assembly within the motor housing.

One popular sensorless algorithm measures the rotor-flux position based on the stator winding's back EMF. This algorithm is suitable when driving the PM motor in six-step mode with rectangular winding currents. The algorithm is easy to implement but produces high torque ripple because of delays in current commutation due to the winding inductance.

One of the major drawbacks of this scheme is that the torque ripple causes unacceptably high acoustic-noise levels in the direct-drive motor, as the rotor acts like a loud speaker. Another important problem with the back-EMF-sensing algorithm is that it does not allow high-speed field weakening, which is required for high-speed spinning.

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