issue: June 2003 APPLIANCE Magazine Engineering Washer Drives Three-Phase A.C. Motor Drive and Controller for Clothes Washers	Share  Printable format  Email this Article  Search
by Andrea Bianchi and Mr. Luciano Buti, MagneTek S.p.A. Because a complete washing cycle for domestic clothes washers consists of two phases - tumble-wash and spin-dry - it calls for very different motor characteristics.

This paper describes a clothes washer controller with integrated DSP-based, three-phase a.c. motor drive. The results and estimates included herein are based on a front-loading 230-V a.c. clothes washer, but they are equally indicative for top- and front-loading 115-V a.c. clothes washers.

The paper will describe how, when compared with single-phase 50/60-Hz a.c. induction motors or universal (brush-type) motors, using this controller to drive a three-phase a.c. motor results in improved washer efficiency, less water consumption, less acoustic noise and electromagnetic interference (EMI), and enhanced reliability.

Typical Washing Cycles

A typical front-loading clothes washerÕs duty cycle is shown in Figure 1. Generally, there are two sequential phases: tumble-wash and spin-dry.

Figure 1. Two-Speed Washing Cycle

Figure 2. Example of Tumble-Wash Period.

During the spin-dry phase, the washer drum turns at high speed, typically 400 to 600 rpm, for brief intervals. Between these high-speed intervals, the drum turns at low speed for longer intervals. In the example shown in Figure 1, the drum turns for 1 min at 400 rpm, then turns at low speed for 4 to 5 min. Then there is the second 400-rpm spin (2-min long) followed by another low-speed spin for 4 to 5 minutes. Finally, there is a high-speed spin 3 to 4 min long.

When a variable speed motor is used, the whole washing cycle can become more sophisticated, enabling better washing performance and improving energy efficiency. An example of such a washing cycle is shown in Figure 3.
The tumble-wash phase in Figure 3 can have different speeds (two or more), and the drum can turn at slower speeds than the typical 40 rpm. The spin-dry phase has three different speeds, and all of them are higher than the spin-dry phase of two-speed washers.

Figure 3. Improved Washing Cycle

As noted above, there are two approaches widely used by washing machine manufacturers to provide the performance requested by consumers. The first and older method uses a two-speed a.c. motor driven by an electro-mechanical commutator on a relay board. The second approach uses an a.c. universal motor (brush-type) electronically controlled with a triac.

Two-Speed Motor

In the European washing machine market, only 10 to 20 percent still use two-speed, single-phase a.c. motors. These motors are cheaper than universal motors, so they are sold primarily to the low-price market segment. However, the market in general now demands improved washer performance. Typically, the motor is a single-phase permanent split capacitor (PSC) type that can run in both directions in low-speed operation, which can be easily controlled by electro-mechanical commutators or relays. During the tumble-wash phase, when the drum reverses direction (CW, OFF, CCW, OFF, and so on), the motor accelerates to running without any ramp-up. This means that, even if you are using a high-efficiency PSC motor, the total tumble-wash phase efficiency cannot be high.

Spin-dry speed also is reached without any ramp-up. This results in high power consumption during acceleration and, since there is no load-balancing, spinning consumes a great deal of energy due to unbalanced loads, and maximum spin speed cannot be attained.

Universal Motor

In the European market, 80 to 90 percent of all washing machines use universal motors with controlled circuit boards containing triacs. These boards are microprocessor-based and usually control the whole washing cycle. The electro-mechanical commutator is gradually disappearing from the market.
Universal motor speed can be controlled easily with a triac using feedback from a speed sensor, so the motor turns the drum at variable speeds, enabling improved tumble-wash performance. And higher spin speeds are possible due to load-balancing ramp-up capability.

Universal motors are more complicated and expensive than two-speed a.c. motors, but their variable-speed capability has made them successful in the European market. The biggest drawbacks of universal motors are their limited speed range, low top-speed, acoustic noise, and the brush wear. In order to improve motor performance, d.c. ÒchopperÓ motor drives are sometimes used as well.

Three-Phase A.C. Motors

Some washing machines also use three-phase a.c. motors, but these motors are used only in very expensive models, and due to their high cost, their market share is low. Three-phase motors themselves are more simple and less expensive than two-speed, single-phase a.c. motors, or universal motors.
The motor drive is more complicated, sophisticated, and expensive with respect to the others. This technology allows for good performance in terms of motor efficiency, maximum speed, and acoustic noise.

With respect to a three-phase motor washing machine, the motor efficiency during the washing phase could be further improved by using DSP-based, field-oriented control algorithms.

Washing Controllers

Most washing machines available on the European market have electronic controllers inside. They work like electro-mechanical commutators, controlling the water-heating resistor relay, the water pump relay, and several valve triacs, while on-board microprocessors improve washing performance based on sensor feedback.

The proposed washing machine controller accomplishes that and more. A dedicated microprocessor controls all washer functions, user interface displays, and actuators. A motor drive DSP carries out real-time calculations in order to maximize motor efficiency. Since the microprocessor controls the entire washing cycle, it works as a master in the serial link, and the DSP runs the motor according to the microprocessorÕs requirements.

Figure 4 is a block diagram of the integrated board control logic. The hardware arrangement is shown in Figure 5.

Figure 4. Integrated Washing Machine Control Logic

Figure 5. Integrated Washing Machine Hardware Arrangement

EMI Filters

Looking at Figure 5, there are two EMI issues to addressÑhigh-frequency electromagnetic conducted noise and low-frequency input current harmonics. In compliance with European standards, a small EMI filter is used on the washerÕs a.c. input bus, and a choke is used to remove input current harmonics. The input choke also is useful in reducing ripple current in the inverter bulk capacitor.

Three-Phase Inverter

As shown in Figure 6, the inverter d.c. bus is obtained by rectifying a.c. input power with a diode bridge and an electrolytic bulk capacitor. The three motor output voltages are generated using six electronic switches (IGBT).

In order to apply field-oriented control (FOC), the motor currents must be known. Three shunts are used because this method yields the best performance in the fast spin-dry phase. The six electronic switches are controlled by means of space vector pulse-width modulation (SVPWM).

Inverter efficiency in excess of 95 percent is attained in the spin-dry phase because the motor is applying high power to the load. Efficiency declines at low drum speeds because the motor is applying less voltage and more current to the load, and inverter losses are determined by motor current.

The motor drive power supply also powers the microprocessor, DSP, insulated sensors, and user interfaces, so it must be a multi-output switch mode power supply (SMPS) providing three different sets of outputs for the following components:

• Motor drive. Since the microprocessor drives triacs and relays, it calls for both 5 V and 12 V d.c. Since the triacs have to be driven with a negative gate signal, the microprocessor positive supply input is connected to the line and the microprocessor negative input is connected to the -5 V. The relays are connected between -5 V and +7 V.
• Microprocessor and DSP. The DSP requires 3.3 V, and the three-phase inverter requires 15 V. Both return paths are connected to the negative d.c. link (or ÐDC link, as shown in Figure 6).
• Sensors and user interfaces. The insulated sensors and the user interfaces require a 5 V SELV supply voltage (8-mm clearance).

Triacs and Relays Section

The microprocessor uses the relays and triacs to control the water pump, valves, and heating resistors. The relays switch the larger loads (heating resistors, pump), and the triacs control the valves, fans, and other smaller loads.

Benchmarks

In the European market, washer performance parameters must be stated on the appliance label based on a standard 60¡C washing cycle. Test results are indicated with letters from A to G (A = best). The main parameters are energy consumption, washing performance, and spinning efficiency. In addition, the appliance producer must indicate the acoustic noise level in decibels.
The energy consumption parameter is a measurement of kWh during the standard 60-degree tumble-wash phase related to water heating and motor operation. The energy required for drying the clothes is not included in this measurement.

Washing performance is the more important parameter. By improving washing characteristics, it is possible to save both water and energy. In order to optimize washing performance, it is also important for the motor to operate efficiently below 40-rpm drum speed (15 to 25 rpm).

Spinning efficiency is the measurement of the water content still in the clothes after the spin-dry phase. The faster the drum speed, the less water is left in the clothes. The better the spin-dry phase, the more energy is saved in the dryer.

Since so much energy is consumed by the heating resistors, using a more efficient motor by itself does not solve the overall efficiency problem. However, by using an efficient motor and controlling its operation correctly, we are able to attain excellent washing performance at low water temperatures. In order to do this, the motor must be able to work efficiently across a broad range of speeds. The maximum-minimum speed ratio may be greater than 100:1.

Performance Comparison

Table 1 shows a cost/performance matrix comparison between different motors. There are two rows regarding the three-phase motor because of two different control methods. As previously mentioned, the field-oriented control method improves the energy efficiency of the motor because the inverter feed always has the optimum voltage. Since the minimum speed is reduced, the washing performance is improved as well. Moreover, thanks to the tight torque control, efficient ramp is possible and, therefore, the highest drum speed can be achieved.

Table 1. Cost/Performance Matrix Comparison

This paper has described a cost-effective integrated washing machine controller. By correctly driving a three-phase a.c. motor, it is possible to improve washing performance and reduce water and energy consumption. Acoustic noise is reduced as well.

Maximum efficiency gain is reached when the washer drum turns slowly during the tumble-wash phase, meaning that the motor controller algorithm enables the motor to run efficiently even well below the nominal speed.

As noted, although the spin-dry phase has no great impact on the total energy consumption, very high drum speed during spin-dry is mandatory in order to minimize the dryer power consumption. It is important to control in-rush current during drum acceleration as well.

References

1. Lipo, Novotny. Vector control and Dynamics of AC Drives. Oxford science publication, 2000.
2. P. Vas. Sensorless Vector and Direct Torque Control. Oxford science publication, 1998.
3. Aengus Murray, AE. Dash DSP Simplifies Washing Machine Control System.
4. Frattesi, R. Petrella, M. Tursini. An Efficient Induction Motor Vector Controller for Washing Machine Applications. Energy Efficiency in Household Appliances and Lighting. Naples, 2000.

This is an edited version of a paper presented at the 2003 International Appliance Technical Conference, held March 10-12.