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issue: October 2006 APPLIANCE Magazine - Part 2: Motors & Air-Moving Devices

Motors and Air-Moving Devices
The Benefits of FOC Sensorless Motor Control


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Given that motor-driven equipment—including motors, pumps and fans—accounts for 64 percent of the electricity consumed in the U.S., improvements to motor systems could result in dramatic energy and cost savings. The brushless DC (BLDC) motor is used in both consumer and industrial applications, owing to its compact size, controllability and high efficiency. Examples include air-conditioner compressors, direct-drive washing machines and refrigerator compressors.
This article will describe how sensorless field-oriented control (FOC) of the permanent magnet synchronous motor (PMSM) type of BLDC motors can be implemented in home appliances such as washing machines. A washing machine features a drum unit that is comprised of a BLDC motor, a motor-controller board and a user-interface board with keypad, LCD and temperature sensors. Over a serial link, the user-interface board communicates the desired wash load, rinse speed and other commands to the motor-controller board. Acting on the commands received, the motor-controller board changes motor speed and torque. Given this scenario, any improvement to the PMSM controller board could yield considerable energy and cost savings.

Figure 1. An application block diagram of a direct-drive washing machine PMSM motor.

DSCs Enable FOC Implementation

Appliance designers can now take advantage of the latest digital signal controllers (DSCs) to improve motor systems. Where DSCs are used, the sensorless FOC algorithm is a natural fit for PMSM motor control (see Figure 1). This is because DSCs and their on-chip peripherals enable the designer to execute FOC algorithms efficiently in order to implement a sensorless method for rotor-position sensing in BLDC motors. The FOC algorithm generates a three-phase voltage as a vector to control the three-phase stator current as a vector. By transforming the three-phase time- and speed-dependent system into a 2-dimensional rotating coordinate system aligned with the rotor (using Park and Clarke transforms), the torque and flux components become time-invariant-allowing control with conventional techniques such as Proportional and Integral (PI) controllers, as with a DC motor.
When a sinusoidal input current is applied to the stator, it causes a rotating magnetic flux to be generated. The speed of the rotor is directly related to the rotating flux vector. The flux vector must be kept in alignment with the rotor magnetic poles at all times, so that the motor can produce the maximum torque.
The entire process of coordinate transformations, PI iteration, transforming back, and generating pulse width modulation (PWM) is illustrated in Figure 1, which describes the functions required for FOC control. The process starts out by measuring the three-phase motor currents. Through a series of coordinate transforms, the time-invariant (under steady-state load conditions) values of torque and flux can be indirectly determined and controlled with classic PI control loops. Error signals are formed using Id, Iq and reference values for each. The Id reference controls rotor-magnetizing flux.
The Iq reference controls the torque output of the motor. The error signals are input to PI controllers. The outputs of the controllers provide Vd and Vq, which is a voltage vector that is sent to the motor. A new coordinate transformation angle is calculated based on the motor speed, rotor electrical time constant, Id, and Iq. The FOC algorithm uses the new angle to place the next voltage vector, in order to produce an amount of slip for the present operating conditions. The Vd and Vq output values from the PI controllers are rotated back to the stationary reference frame using the new angle. This calculation provides quadrature voltage values va and vb. Next, the va and vb values are transformed back to three-phase values va, vb and vc. The three-phase voltage values are used to calculate new PWM duty-cycle values that generate the desired voltage vector.
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. The motor vendor supplies the motor’s winding characteristics, although they can be measured using the inductance and resistance values of the windings. Rotor position calculation is performed by indirectly measuring back electromotive force (EMF) based on a motor model. Back EMF is extracted from the motor’s model by inferring the estimated currents, which equal the measured current.

Advantages of FOC

Taking the FOC approach yields many benefits to PMSM motor power management. For instance, FOC improves the dynamic response of PMSM motors, benefiting appliances like washing machines that need to quickly respond to speed changes involving both the agitation and spin processes. FOC makes this possible by allowing the most optimal torque production, which uses less current, since it controls the amplitude and phase of the currents in order to keep the stator and rotor magnetic fields at 90 degrees. Also, since FOC allows for controlling the currents of the motor every PWM cycle, the current is inherently limited.
Traditional control methods for BLDC motors drive the stator in a six-step fashion, generating oscillations on the produced torque. In this scenario, a pair of windings is energized until the rotor reaches the next position, then the motor is commutated to the next step. Using FOC, the motor is continuously commutated in a sine-wave fashion with controlled phases of the currents. So, torque production is kept linear as the motor rotates. Since torque production is continuously commutated, the torque ripple is removed-thus reducing mechanical oscillations on the motor. This reduction of mechanical oscillations reduces audible noise significantly-allowing designers to build appliances, such as washing machines and refrigerators, that feature quieter washer and compressor operations.
By using DSCs in the design of appliances, engineers can develop software-based motor systems that allow the rapid customization of newer models to address multiple markets. Some appliance makers are trying to design a common platform for their products that use PMSM motors. For example, using DSCs and FOC algorithm motor control, a common platform can handle different models of washing machines. Likewise, air-conditioners and refrigerator compressors could be addressed by a single platform. The only changes needed in these cases are FOC algorithm parameters-otherwise no modifications are required in the algorithm core, the control-board hardware or the DSC device.

References

1. Microchip’s Online Motor Control Design Center, www.microchip.com/motor

This information provided by Jorge Zambada, applications engineer, Digital Signal Controller Division, Microchip Technology Inc.

Suppliers mentioned in this article:
Microchip Technology Inc.
 

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