Dual-motor control technology simplifies energy-efficient appliance design

Reducing energy consumption has become a major concern for governments around the world. The production of electrical energy consumes large amounts of coal and oil, while also generating large amounts of carbon dioxide emissions that accelerate the process of global warming. Even clean energy sources used to generate electricity - such as hydropower and wind power - have their own environmental impacts. The increasing demand for electricity also requires huge investments in power plants and transmission lines. For these reasons, governments in many countries and regions, including China, South Korea, Japan, the United States, and Europe, are constantly raising energy conservation standards for domestic and commercial users. At the beginning of the last century, when commercial transmission and distribution of electricity first became possible, the use of electricity in the home was limited to lighting. However, in modern homes, lighting currently accounts for only 10% of the electricity consumed. Household appliances such as electric fans, air conditioning systems, refrigerators, washing machines, and various entertainment facilities - such as televisions, audio-visual systems, etc., have replaced them as the main electricity consumers in the home.

In household appliances such as electric fans, air conditioning systems, refrigerators, etc., electric motors are the main electricity consumers. Take refrigerators for example. Although the average power consumption of these appliances is quite low, they are in continuous operation, which means that daily power consumption accounts for a large share of the total energy consumption. Small appliances such as fans and water pumps usually use single-phase shaded pole motors or permanent capacitor induction motors, which have an efficiency of only 25%. If the energy losses caused by motor efficiency and the energy losses in the power generation and transmission process are taken into account, the results are very surprising. A small fan motor with an efficiency of 25% consumes 120 watts of electricity, of which only 30 watts of energy is converted into mechanical energy output to rotate the fan blades. If we now assume that the energy losses in the power transmission process are 7% and the power generation efficiency of a modern thermal power plant is 35%, the heat energy consumption is 370 joules per second. This means that the energy wasted in the thermal power plant, the power transmission cables and the motor is 11 times more than the energy wasted in rotating the fan blades. Large single-phase induction motors used in appliances such as air conditioning systems and refrigerator compressors are slightly more efficient, usually exceeding 65%. This can reduce the energy loss factor to less than 4. However, the load on the compressor during normal operating cycles and in standby mode is very low, because the cooling power is only required when the air conditioning system, refrigerator, etc. are turned on. A large amount of energy loss occurs during the compressor startup phase, which accounts for a large part of the total energy consumed by a compressor running at a fixed speed. Studies have shown that using a variable speed compressor can reduce average energy consumption by 40%. This improvement is entirely possible because the compressor speed is better matched to the cooling requirements, so the compressor can be under higher load and run at the high efficiency operating point for a longer time.
Over the past decade, Japanese manufacturers have continued to improve the efficiency of household appliances using variable speed permanent magnet motors. The control system uses an electric power converter to change the frequency of the motor winding voltage and thus the motor speed. In some household appliances (such as electric fans), they use Hall sensors to detect the position of the rotor and synchronize the switching of the windings with the position of the rotor magnets to maximize efficiency and simplify the startup process. The advantage of this method is that only very simple electronic circuits are required to achieve the control requirements. However, a sealed compressor cannot be equipped with Hall sensors, so a sensorless algorithm is required. A popular sensorless algorithm for 6-step permanent magnet motor drive systems uses zero crossings of the winding back EMF to detect the rotor position. This control algorithm usually uses an 8-bit microprocessor to manage the phase advance and start sequence. One disadvantage of this 6-step system is that it generates a torque disturbance when the motor current switches between windings (changes direction). In many fan and pump applications, this torque disturbance can cause an annoying noise, especially at low speeds when the fan blades are almost silent. To help reduce this noise, the rotor of this motor is equipped with surface-mounted magnets to reduce the inductance of the windings and minimize the period of current change direction. However, the ideal solution is to drive the motor with sinusoidal current, which can completely eliminate this torque disturbance. This type of control also makes it possible to use another motor design using interior permanent magnet (IPM). This interior permanent magnet (IPM) design can produce 15% more torque than permanent magnet motors and has the potential to further improve efficiency. The efficiency of IPM compressor motors can exceed 90%, which greatly reduces energy waste compared to the 65% efficiency of single-phase induction motors. This means that a compressor that uses a 3kW single-phase induction motor will only require 1.75kW if using an IPM motor.

Recent advances in electronics hardware and control technology have made it possible to build more cost-effective drives for IPM motors. To drive an IPM motor with sinusoidal currents to maximize drive efficiency and minimize noise, a field-oriented control (FOC) algorithm is required. The sensorless algorithm must be able to detect the position of the motor rotor based solely on the motor current measurement. Finally, the control hardware must monitor the current in the motor windings without expensive isolation circuits. The next section describes the sensorless control algorithm and the current sensor hardware for sinusoidal control on permanent magnet AC motors. This algorithm allows compressors to use high-efficiency IPM motors and eliminates the Hall sensors for fans using surface-mounted magnet or IPM motors. The



control algorithm in the sensorless control algorithm contains all the control functions required to drive an IPM or surface-mounted magnet motor with sinusoidal currents without sensors. A key element of the algorithm is a field oriented control structure using a vector rotation module (ej) that simultaneously converts the AC motor winding current into two DC current components, one for torque generation and the other for flux control. The current input to the rotation module is initially converted from 3-phase to 2-phase currents using a Clarke transformation module. The rotor flux angle splits the current into D components, which are combined with the flux. The Q component is used to generate torque. The two current control PI compensators are tuned to match the RL time constants of the motor windings and do not need to change with changes in the AC winding current frequency. The forward vector rotation module (ej) converts the DC voltage output of the PI compensator to an AC voltage that matches the rotor frequency. The space vector PWM unit calculates the switching times of the power switching diodes based on the calculated AC voltage requirements. Space vector modulation automatically adds the third harmonics to produce a sinusoidal voltage modulation, maximizing the voltage available on the DC bus. A 2-phase modulation function is also included to minimize switching losses in the power converter.

Maximizing the motor torque output per ampere of current makes the motor most efficient. For surface mounted magnet motors, the controller maintains the flux component of the current (ID) at zero to maximize motor efficiency. However, due to the unique rotor structure of an IPM motor, an IPM motor produces an additional torque component called reluctance torque. When driving an IPM motor, the IPM control module increases the ID current from zero as a function of the IQ target to operate the motor at the most efficient operating point. In any case, the speed loop compensator calculates the torque current required to maintain the speed at the target value. In some applications, such as washing machines, a wider speed range is required. In this case, the field current reduction control function adds negative flux current (ID) to reduce the effective back EMF of the motor and allow the motor to run at a higher speed before the back EMF reaches the DC bus voltage limit.

Many industrial drive systems currently use the algorithm structure described here, but this algorithm structure requires the motor to have a decoder or encoder feedback. This algorithm structure has a unique feature that it can infer the rotor position and speed from the motor winding current without physical sensors. This sensorless algorithm infers the motor rotor flux state through the motor loop model represented by the following equation. The controller drives the stator voltage while the current reconstruction loop detects the total motor current. A simple rearrangement and mathematical combination of the terms of the equation can generate sine and cosine terms. A phase-locked loop continuously tracks this algorithm to infer angle and speed, similar to the type used in IC digital converter decoders.

The second feature of this algorithm is that the phase current transmitter unit infers the motor phase current from the inverter DC link current. As shown in Figure 2, for any operating inverter unit, there is always one winding connected to one bus rail and two other windings connected to the other bus rails. This means that there are two motor winding current values ​​available for each PWM cycle. The phase current transmitter unit includes a sampling timing generator that operates according to the SVPWM input, a sampling analog/digital converter, and a mathematical operation unit to calculate the three-phase current. The obvious advantage of this approach is that it does not require isolated current sensors and can make the application of sensorless algorithms in home appliances more cost-effective.

The sensorless control algorithm is part of an integrated design platform for home appliance motor control. A mixed signal control IC can implement this algorithm in hardware without any complex, lengthy and error-prone software development. This IC integrates the analog/digital converter and buffer amplifier required for current measurement. You can see additional hardware functions including fault detection functions and start-up sequence in Figure 1. Finally, the control IC also integrates an 8-bit microprocessor core with its own memory to perform other application functions specified by the home appliance engineer.