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issue: October 2009 APPLIANCE Magazine

APPLIANCE Engineer - Motor Controls
Using Field-Weakening Motor Control in Washing Machines


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Patrick Heath, High Performance Microcontroller Division, Microchip Technology Inc.

Washing machine engineers can achieve high spin-cycle speeds by using a field-weakening motor control technique.

Figure 1. A typical split-phase ACIM-controlled washing machine with a belt-driven transmission.

Consistent with other home appliances, washing machine manufacturers are continuously looking for methods to improve operational efficiency while simultaneously reducing cost. The washing machine spin operation is a critical component of this goal. Ideally, the drum is spun at a very high speed, typically 1.5K to 2K rpm, to extract the maximum amount of water in the least amount of time. Additionally, a lower-cost motor (with a minimal number of poles) and low-cost control circuitry (e.g., sensorless field-oriented control) are desired for meeting manufacturing cost targets. An advanced control technique called field weakening is available to help meet both of these goals without any additional cost. This article will explore the field weakening concept, how it is implemented, and the advantages it can bring to washing machine operation.

Motor Considerations

Various types of motors have been used by washing machines, dictated by cost or design constraints. In the past, nearly all machines used split-phase, belt-driven ac induction motors (ACIMs)—with or without mechanical gearing—to minimize manufacturing cost (see Figure 1). With rising energy costs and environmental concerns, many new designs have switched to front-loading washing machines to reduce water consumption, and to permanent magnet synchronous motors (PMSMs) to improve torque generation. Additionally, these new designs often use a sensorless, three-phase field-oriented control (FOC) algorithm, which improves electrical efficiency and reduces cost over a sensor-based approach.

One common design methodology is to use pancake-type, low-profile, and wide-diameter motors that are attached directly to the back of the drum. However, due to the large diameter of the motor, up to 48 poles may be required, increasing the motor cost. Directly attached, pancake motors utilize one-to-one gearing, with the result that high spin speeds are only accomplished by designing the motor for high-speed applications only. This increases the cost and weight of the motor.

Another common approach is to offset the motor and use a belt to drive the drum. Belt-driven motors can be geared to increase the drum spin speed by using different-sized pulleys or gears. With this implementation, PMSMs with significantly fewer poles, even as few as four, can be utilized to reduce cost.

 

Benefits of Sensorless FOC

One of the two main benefits of FOC is lower cost. This is due to replacing the Hall-effect rotor position sensors, connectors, and wiring harnesses with shunt-resistor op amp circuits on the control board, which instead measure motor phase currents. The other main benefit is improved energy efficiency, since the stator field coils are excited in a precise manner that optimizes torque generation. An ancillary benefit is quieter control, since the rough on-and-off cycles of six-step trapezoidal control are replaced with the smooth PWM ramp ups and downs of sinusoidal excitation.

Executing a FOC algorithm does increase the processing requirements. A high-speed, efficient 16-bit processor with DSP capability, known as a digital signal controller (DSC), is ideal for this application. While 32-bit MCUs can also be used, they are really unnecessary, as all the sensor inputs are coming from a 10- or 12-bit A/D converter.

For tight, accurate control with FOC, a few critical A/D and PWM module features are required. The A/D converter must have multiple sample-and-hold channels. By taking a simultaneous snapshot of the currents in two or three phases, time-delay measurement errors are eliminated. Another important feature is the ability to synchronize the A/D sampling with the PWM. If the A/D takes the sample too late, then bad data are given to the control loop. Having the ability to determine when to sample each A/D on an individual PWM basis provides the optimal control-loop results.

Methods for Obtaining High Drum-Spin Speeds

Mechanical transmissions can be designed with two different gearing ratios, one for normal agitation speed and another for high spin speed. The switching system is usually mechanical or electromechanical, but hydro-mechanical systems also exist. However, the disadvantages of these systems are the extra cost and their complexity.

Another approach is to use oversized motors, such that the full rated speed is used for spinning, while slower speeds are used for normal agitation. The obvious disadvantage here is that the larger motor costs and weighs more, and provides a lot of torque capability at high speed that is not needed or used, which can make the total system cost less competitive.

Field weakening is a method for achieving high spin speeds without the cost of additional transmissions or oversized motors. Instead, the existing control circuitry is used. However, software calculations, including fractional multiplication and single-cycle multiply and accumulate (MAC) instructions, require the selection of a DSC that features a higher-speed 16-bit processor with DSP capability.

Field Weakening

Field weakening is a motor control technique that allows a motor to run faster than its rated speed. Using this control method should provide at least 1.5 times the rated speed, achieving even faster speeds with buried magnet rotor designs. Spinning the motor up to its rated speed is referred to as operating in the “constant-torque region,” where the motor’s available torque is constant as the speed is varied (see Figure 2). Faster operation moves into the “constant-power region,” where torque and speed are inversely proportional. In a washing-machine spin operation, due to momentum, little torque is needed to maintain or increase drum speed; therefore, diminished torque is an acceptable tradeoff.

To understand how field weakening works, it is beneficial to first understand the basic operation of both sensor-based six-step and sensorless FOC on a brushless permanent-magnet motor. In order to entice a rotor to turn, the motor stator coil that is slightly ahead of the rotor magnetic field is energized, drawing the rotor forward toward it. In six-step trapezoidal (also called 120-degree control), this is accomplished in a sequential manner—stepping forward in response to the Hall-effect sensor inputs identifying the rotor position. Speed is controlled by increasing or decreasing the PWM high-time (or duty cycle), thereby adjusting the power input to the motor. Even higher speeds can be obtained by commutating up to 60 degrees (in time) prior to receiving the Hall-effect sensor signal. This process is called phase advance.

In FOC operation (also called vector control), all motor stator coils are energized in a smooth, quiet sinusoidal fashion, with each of the three phases energized 120 degrees out of phase from each other. Three PI control loops regulate speed, torque (Iq), and flux (Id) (see Figure 3). Rotor position is determined by sensing one or more motor phase currents and matching the measurements to an electrical model of the motor. Speed is controlled by adjusting the voltage vector going to the motor coils. To produce the maximum torque, the stator field is energized 90 degrees ahead (by timing) of the rotor field, by setting the Iq reference to zero.

Field weakening is an optimized version of phase advance. Instead of simply energizing the stator fields to keep 90 degrees ahead of the rotor, field weakening also produces a stator field component that opposes the rotor’s magnetic field. This reduces the stator back EMF, which acts as a braking force. To accomplish this in FOC, the Id (flux) reference is set to a negative number, while the Iq (torque) reference is set to the desired torque value.

 

Figure 3. Sensorless FOC of a permanent-magnet synchronous motor with field weakening provides very high spin speeds and optimal torque output without expensive Hall sensors and cables.

Drum Braking

A high deceleration rate is also important. Due to momentum, the drum and motor can take several minutes to spin down and stop naturally. Spinning down quickly after the spin cycle is completed saves time. More critically, for safety reasons, when the lid or door is opened, the drum must be stopped quickly to avoid potential injuries.

To meet these deceleration goals, braking is often employed. This can be accomplished by a supplemental mechanical braking system, which adds materials cost and system complexity. Another approach is to slow the motor electronically using a large external brake resistor. In the latter case, a resistor is switched into the circuit and abruptly slows the motor by limiting the current flow from the motor to ground. Although this method works, the braking resistor and switch still add significant cost to the system. A lower-cost method is to implement software braking.

Software Braking

Software braking works by controlling the magnetic flux of the stator coils. To change from high-speed spinning to braking requires that the FOC Id reference (flux) be stepped down from a negative number back to zero (i.e., the constant-torque operation range). At the same time, the reference for the speed PI control loop must be set to zero. The Iq torque reference can also be changed to a negative number (implying negative torque). These software variable changes direct the FOC control algorithm to effectively slow both motor and drum spin, and should be implemented at a rate that optimizes deceleration without blowing up the power electronics. Experimenting to find this optimal rate can be an exciting part of the motor-control development process.

Conclusion

To further increase energy efficiency and improve torque control, the trend in washing machine motor control is moving from split-phase AC induction motors to three-phase permanent-magnet synchronous motors. At the same time, to reduce cost, the trend is going from sensor-based to sensorless FOC. The field weakening software control technique helps by providing much higher spin speeds to extract water more quickly, without the additional component costs.

 

About the Author:
Patrick Heath

 

 

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