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issue: October 2007 APPLIANCE Magazine - Part 2: Motors & Air-Moving Devices

Motor Control
FOC Algorithms Make Motors More Efficient


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by Padmaraja Yedamale, Principal Applications Engineer, Home Appliance Solutions Group, Microchip Technology Inc.

The mantra of appliance makers is to enhance efficiency and reduce the audible noise of their products— with minimum addition to the system cost.

The mantra of appliance makers is to enhance efficiency and reduce the audible noise of their products—with minimum addition to the system cost. Impetus for these twin objectives has been brought about by stringent government regulations and consumers preferring “greener” appliances. Thus, there is an urgent need for efficient motor-
control techniques in appliances using motors.

For example, the motor in a washing machine is the component that draws the largest amount of power. Therefore, improving motor control will lead to greater energy and cost savings. Deploying advanced motor-control techniques, such as a field-oriented control (FOC) algorithm, meets this goal. Utilizing FOC, the motor-controller board adjusts motor speed and torque for the most efficient operation possible (see Figure 1).

 

Figure 1: Using an FOC algorithm, the motor torque can be controlled dynamically—keeping it constant within the rated speed range.

Why Use an FOC Algorithm?

FOC improves the dynamic response of motors in appliances such as washing machines that need to alter their speeds rapidly during both the agitation and spin processes. The most optimal torque production—using less current—is made possible through FOC, because it controls the amplitude and phase of the currents, and keeps the stator and rotor magnetic fields at 90 degrees to each other. In fact, by controlling the currents of the motor with every PWM (pulse width modulation) cycle, FOC ensures that the current is inherently limited.

In the FOC algorithm method, three-phase separated PWM signals are sine-wave modulated using space vector modulation (SVM) and applied to the motor’s three-phase windings. Using shunt resistors, the current in each winding is monitored and compared to an electrical model that is based on the motor’s characteristics. Generally, the motor vendor supplies the motor’s winding characteristics—although they can also be calculated using the inductance and resistance values of the windings. Rotor position can be estimated by indirectly measuring the back electromotive force from the motor current.

The block diagram in Figure 2 shows the power and control blocks of an appliance motor-control system. In the input-convertor section, a rectifier bridge converts mains power to dc voltage. Depending on the appliance, there could also be an EMI suppression block. On the output side of the diode bridge, the dc ripple is filtered out by a capacitor bank. The output-inverter section features a voltage-source inverter, comprising two power switches per phase, with freewheeling diodes connected across each switch. This connects to the motor winding. DC voltage obtained from the input convertor block is used by the output inverter to obtain a variable-voltage-and-frequency power supply, which is then used to control the motor.

 

Figure 2: System block diagram of a digital signal controller-based appliance.

Motor Control Made Efficient Using DSCs

From the above discussion, it is evident that a versatile controller is necessary for an efficient implementation of an FOC motor-control algorithm—which involves sensing the rotor position and measuring back EMF from the motor current signals—for cost and ease of development. Toward this end, some controllers, such as Microchip Technology’s dsPIC digital signal controllers (DSCs), feature on-chip peripherals that help to execute FOC algorithms efficiently, thus enabling a sensorless method for rotor-position sensing in permanent-magnet synchronous motors (PMSMs) and ac induction motors (ACIMs). This DSC’s analog-to-digital convertor (ADC) supports current sensing and offers flexible triggering options. For  example, ADC conversion triggered by the PWM module enables an economical current-sensing circuit by sensing inputs, periodically, where switching transistors allow current to flow through sense  resistors.

Capable of capturing multiple signals simultaneously, the ADC peripherals on most DSCs eliminate the delay in motor-current measurements between two-phase samples. As directed by an FOC algorithm, a DSC’s motor-control system determines the PWM duty cycle and pattern of output. The on-chip PWMs feature complementary channels with programmable dead time, and edge or center alignment. The appliance control system’s EMI can be reduced by deploying DSCs with center-aligned PWMs. The additional importance of a DSC’s versatile peripherals can be understood in the implementation of the power factor correction (PFC) block.

PFC Block Implementation

The input converter section features an active PFC block, which helps to meet the stringent energy regulations stipulated by many countries. This active PFC block comprises an inductor, a power switch and a diode. A DSC’s ADC can be used to measure the current and voltage values from the dc bus. Based on these inputs, the DSC controls the power switch using its PWM module. This on-chip PWM runs under a PID loop, to keep the PF value close to unity. Using the same DSC to implement FOC and the PFC block is a major cost saver, because non-DSC-based solutions need to use expensive ASICs, and fixed-function or discrete-based solutions. DSCs facilitate the efficient implementation of the PFC function, which optimizes the input current consumed by the appliance motor and inverter. By doing so, the PFC block reduces the stress on the local electric grid.

Figure 3 shows an example of a washing machine interface, where the DSC’s ADC channels can be used to measure the motor current, the motor temperature, and the water temperature in the drum. General-purpose inputs and outputs (I/Os) are used for interface switches and LCD or LED displays. In some applications, the system can use a single DSC to handle both motor and system controls. Serial ports on the DSC can also be used for system calibration and diagnosing any faults in the system.

 

Figure 3: A DSC as the system controller in a washing machine.

 

Securing Appliance IP

Motor control and PFC block algorithms are valuable intellectual property (IP) assets for their owners. Also, design teams work at different locations to craft a family of appliances, resulting in IP that is functionally unique. For example, the FOC algorithm for an appliance may originate from location A; location B might contribute front-panel design and electronics; while system integrators at location C could be doing the final assembly and testing of appliances. Security features (such as CodeGuard, offered on the dsPIC DSC family) enable the security of IP in collaborative designs.

The manufacturing of washing machines can be made more efficient if the different models are based on a common production platform. For example, using DSC-based designs, appliance makers can quickly offer a range of washing-machine models that use PMSM or ACIM motors and an FOC algorithm (or other control algorithms) for motor control. They can also respond to market requirements quickly, as these software-based motor control designs allow the rapid customization of new appliance models to address multiple markets.

Conclusion

Using DSCs, an efficient sensorless or sensored FOC algorithm can be economically implemented in appliance designs. By adopting FOC, higher efficiencies of up to 95% can be achieved in motor applications. Additionally, because FOC aids in controlling the stator current, torque ripple is greatly reduced, thus enabling the construction of quieter washing machines. DSC-based FOC also enables appliances to meet stringent energy regulations while providing greener appliance options to consumers.

Note: dsPIC and CodeGuard are registered trademarks of Microchip Technology Inc. in the U.S.A. and other countries. All other trademarks mentioned herein are property of their respective companies.

Suppliers mentioned in this article:
Microchip Technology Inc.
 

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