Variable speed control of a compressor motor significantly reduces both the peak and average energy consumption. However, since the standards for air conditioners require a reduction in the seasonal average energy consumption, even the energy consumed by the evaporator and condenser fans is important. The focus in the washing standard is on hot-water consumption in washing, and water extraction in the spin cycle, to minimize the energy consumed by the water heater and dryer. In some regions of the world where drought is a major problem, the standards call for a reduction in the total water consumption. Standards are also in force for lower power appliances such as refrigerators and pumps because their almost continuous operation consumes a significant amount of energy.

Appliance manufactures are facing major design challenges to meet the new energy efficiency standards and to cope with significant increases in the cost of raw materials such as steel, aluminum and copper. Variable speed control of a compressor motor significantly reduces both the peak and average energy consumption. However, since the standards for air conditioners require a reduction in the seasonal average energy consumption, even the energy consumed by the evaporator and condenser fans is important.

The focus in the washing standard is on hot-water consumption in washing, and water extraction in the spin cycle, to minimize the energy consumed by the water heater and dryer. In some regions of the world where drought is a major problem, the standards call for a reduction in the total water consumption. Standards are also in force for lower power appliances such as refrigerators and pumps because their almost continuous operation consumes a significant amount of energy. They also need to add new product features to compete in the market place without simply reducing prices.

Manufacturers of appliances such as washing machines and air conditioners are looking to permanent-magnet synchronous motors (PMSM) because of their high efficiency and minimal iron and copper content. Cost-effective application of variable speed control of a PMSM demands a control approach that avoids the use of rotor-position sensors which are typically used in industrial-drive applications. A sensorless, field-oriented control algorithm enables cost-effective variable speed control of PMSM, based on current measurements alone.

The design challenge is to optimize the algorithm to the meet performance requirements in many different applications. Energy efficiency is the most important requirement in compressor speed control but minimizing acoustic noise is critical in evaporator fan control. High dynamic response is not required in the control a hot-water circulation pump for heating, but is required for control of cold-water feed pumps to maintain water pressure with rapid changes in flow. Washing-machine motor control demands high dynamic control of torque at low speeds to improve wash cycles, but also requires very high speed control for the spin cycle.

Sensorless motor control
Field oriented control (FOC) is a very common technique used in the control of permanent-magnet ac motors in high-end industrial-drive applications. The control approach provides good dynamic control of torque and maximizes the efficiency of the motor. The motor currents are sinusoidal, thus producing smooth torque, which minimizes acoustic noise and mechanical vibration.

While this performance is very desirable, appliance manufacturers cannot justify the cost of the high-resolution rotor-position sensor typically found in servo motors. However, by making a small compromise in low-speed performance, field oriented control can be implemented without any position sensors. The sensorless FOC algorithm, Figure 1, derives position from the rotor flux, which it calculates by integrating of the winding's back EMF.

Figure 1: Sensorless field oriented control (FOC) algorithm
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It can achieve good control at medium to high speeds, but performance degrades at low speeds when circuit and measurement noise swamps the back EMF signal. This is not a significant problem here since appliance drives do not require full torque control down to zero speed.

The controller calculates the winding's back EMF from the motor -ircuit model using the applied winding voltages and measured currents. Direct measurement of the motor winding current is relatively expensive because it requires an isolated measurement due to the high common-mode voltage. The more cost-effective approach is to reconstruct the motor currents by sampling the current in the dc -ink shunt. The sample timing derives from the pulse width modulator which determines the inverter switching states. Each PWM cycle has two active states which allow sampling of two different phase currents. The third phase is then easily determined since the winding currents all sum to zero.

A two-phase circuit model simplifies the flux estimator calculations since the back EMF terms are sine and cosine functions. The two-phase equivalent voltages and currents are readily available signals within the FOC current loop. Equations 1a and 1b model the winding back EMF, resistance and inductance.

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Integrating these equations yields Equations 2a and 2b, with terms that are sine and cosine functions of the rotor flux angle.

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The angle and speed phase-locked loop (PLL) forces the angle estimate to track the rotor flux angle and yields both position and speed information. The PLL uses the Park vector rotation to calculate the sine of the angle estimate error, which goes to zero when it locks on to the rotor angle. While starting, the flux estimator signals are unreliable, so the PLL runs in an open loop mode. A motor and load model estimates the rotor speed, which drives the angle integrator. The PLL switches to closed loop when the motor reaches some minimum speed.

Sensorless PMSM control design platforms
A digital control IC implements the sensorless field oriented control algorithm using a dedicated motion control processor. The Motion Control Engine (MCE) has a sequencer that links motor control ASIC functions from the MCE library. This combines the flexibility of programmable system and speed and efficiency of a dedicated ASIC. The control IC integrates the analog amplifiers and A/D converter needed for the motor phase current measurement.

The PWM modulator automatically generates the sample timing signals without software intervention. The third element in the control IC is an 8-bit microcontroller core for the appliance application functions. The 8-bit processor is the system master and manages load switching, speed profiles and external communication. It communicates with the MCE using a block of shared RAM so the motor control algorithm operates almost independently from the 8-bit processor just reacting to changes in set point or control parameters.

The design platforms include digital control ICs and an integrated power module targeting specific appliances such as air conditioners, washers or pumps. The design kit includes a ready to use application board with ac input rectifier, power supply, control IC and power module as shown in the schematic, Figure 2.

Figure 2: Reference-design kit schematic
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The control IC firmware includes the complete sensorless control algorithm and an 8051 based debug agent that allows the user easily customize control parameters to match the application requirements.

Appliance application development challenges
The control algorithm has various features targeting specific appliance applications. Air conditioning compressor motors use interior permanent-magnet (IPM) motors because of their higher efficiency. IPM motors can produce about 15 percent more torque per amp than surface-magnet motors due to the reluctance torque contribution. Equation 3, for the IPM motor torque, includes a magnet torque and a reluctance torque term that varies with the square of the motor current and is sine function of twice the rotor angle.

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The plot of the torque function, Figure 3, shows that you need to advance the phase angle as a function of current to maximize the torque per amp.

Figure 3: Internal permanent-magnet (IPM) motor-torque function
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The sensorless controller includes this angle advance function with a gain determined by the motor saliency (which is the preferred direction of magnetization)

Washing machine control is a very challenging application because of the very wide speed range required. High torque is required at low speeds for washing and so the motor design should maximize the rotor flux. However, the motor must also run at more than twenty times washing speed in the spin cycle, which requires a very low rotor flux. The field oriented controller manages this compromise by injecting a direct current component to appose the rotor flux once the back EMF reaches the inverter voltage limit. The field weakening current calculation is quite a complex since it varies with both speed and load. The field weakening controller avoids this calculation using a feedback control loop that monitors that the stator voltage magnitude and increases the field weakening current as the magnitude comes close to the inverter limit.

Another challenge in washing machine control is to detect out-of-balance in the wash load before entering the spin cycle. Improvements in load-imbalance detection enables savings in the mechanical dampers typically used to counter any vibrations caused by the imbalance. The controller decouples the motor currents into field-controlling and torque-producing currents and so provides information on the load. Washing machine control engineers have built their own load imbalance filters using functions from the MCE library. The washing machine cycle software running on the 8051 processor reads the filter output from the shared memory and makes decisions on spin-speed selection.