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issue: October 2005 APPLIANCE Magazine Part 2: Motors & Air-Moving Devices

Feature - Motors and Air - Moving Devices
Three-Phase AC Induction Washing Machine


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Introduction

A trend in washing machine design is to replace traditional drive systems with electronically controlled brushless drives. In the past, washing machine designs employed two widely used drive systems. The older designs used electromechanically controlled, two-speed, single-phase AC induction motors. According to some, this drive system is hardly used for new machines and if it is, it is only found in the low-end models. The majority of washers have universal brushed motors with Triode Alternating Current switch (TRIAC) control. However, with the advent of new electronic devices, drive controls are offering advanced benefits.

A new generation of washing machines will be designed with brushless three-phase motors. Good candidates for this kind of design are three-phase AC induction motors and permanent magnet sinusoidal motors. Both motors need sophisticated algorithms to perform control functions, and this requires microcontroller (MCU) based solutions. DSP-based devices are preferred because of the real-time signal processing demands from AC motor control applications.

Figure 1. Block Diagram of Indirect Vector Control Algorithm

Figure 2. Example of a Graphical Interface

Application Features

The three-phase AC induction washing machine drive is a response to new market demands for higher performing appliances. The aim is to provide maximum drive performance at a competitive price. The most important features include:

• Three-phase AC induction motor
• Cost-efficient tachogenerator on motor
shaft for speed sensing
• Indirect vector control algorithm
• Speed range 0 to 18,500 rpm
• Reconstruction of three-phase currents
from DC-bus shunt resistor
• Non-recuperative braking and decelera
tion control
• Slip-optimization control for higher
efficiency
• Over-current, over-voltage and under-
voltage protection
• Out-of-balance detection for spin dry

Control Algorithm

A wide range of speed operations is a typical requirement for washing machine applications. For the washing cycle the drum speed varies from -40 to +40 rpm. For spin-dry cycles the drum speed can go up to 1,900 rpm.

In addition to broad speed ranges, washing machines require very precise speed control. At washing speed, with a full load, the speed ripple should not exceed +/-3 rpm. High start-up torque must also be considered. These are tough requirements that call for high-performance control algorithms. An AC induction drive with features described above is based on indirect vector control algorithms. The control structure accommodates the AC induction motor’s ability to control torque and motor excitation separately. It also demonstrates a low sensitivity for motor parameters variation, which minimizes manufacturing costs (motor parameters can vary throughout manufacturing) and provides higher reliability during motor aging.

To execute an indirect vector control algorithm, the motor’s stator current must be established, preferably by using only one shunt current sensor to minimize system cost. The application then executes a reconstruction algorithm, which reconstructs three-phase stator currents from the DC-bus current. The reconstruction itself requires considerable processing capabilities from the MCU. Three analog-to-digital converter (ADC) samplings and a reconstruction algorithm are executed within one pulse wide modulation (PWM) cycle. The MCU unit must include such features as timer multi-triggering, fast ADC conversion, PWM to ADC synchronization, and fast interrupt processing to perform these computational intensive tasks.

Brake Control

Another important feature of the washing machine drive is the motor brake, which can significantly reduce the time needed to stop the drum. In order to keep the power circuit simple, and help maintain lower system cost, a braking resistor and a power insulated gate bipolar transistor (IGBT) cannot be used to brake the motor. Rather, non-recuperative braking, in which energy is not fed back to the DC-bus, is required. The AC induction drive implements such non-recuperative braking when energy is dissipated in the motor windings.

Out-of-Balance Detection

Drum spinning speed is another significant design consideration and important product differentiator. Today’s washing machines advertise spinning speeds of up to 1,900 rpm. Higher spinning speed translates to greater centrifugal forces, resulting in better water extraction from wet clothes, shorter spinning cycles and, ultimately, shorter drying times. High spinning speeds, however, require the clothes in the drum to be well balanced in order to prevent washing machine vibration. An out-of-balance detection algorithm can be executed directly, using a vibrometer, or indirectly, by observing speed error, torque ripple and motor currents to detect the unbalanced load.

The AC induction drive described above uses the indirect method in order to minimize external sensor requirements. If out-of-balance is detected, the drum speed is reduced to help reposition the clothes in the drum. This process continues until the load is balanced satisfactorily and the speed can be increased up to maximum spinning speed.

Slip Optimization
A primary design consideration in modern washing machines is keeping the energy consumption as low as possible. The AC induction drive described above implements a slip optimization algorithm to make sure the motor operates at highest efficiency at all times. By controlling the motor excitation, optimal rotor slip frequency can be achieved for each drive mode, saving energy at each washing cycle and ensuring high overall motor operating efficiency.

Control Interface
For the product developer implementing a three-phase AC induction drive as described in this article, mastering and debugging is performed through a control interface. This application development tool allows remote control of the target application through a user-friendly graphical environment running on a PC, providing real-time application monitoring through a real-time oscilloscope and high-speed recorder. Drive parameters can be easily modified, and the tools include a powerful run-time debugger.

A three-phase AC induction drive, using control algorithms with a DSP/MCU hybrid embedded controller, can provide high-end functionality with low-end power consumption. This technology can make a new generation of feature rich, top-class washing machines possible.

This information is provided by Petr Stekl, systems application engineer for the Transportation & Standard Products group of Freescale Semiconductor, Inc.

Suppliers mentioned in this article:
Freescale Semiconductor Inc.
 

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