DSP Motor Control Boosts Efficiency In Home Appliances

Major home appliance manufacturers are always adding new features and simplifying the use of their products in order to maintain a competitive edge. New designs also are influenced by current and pending government regulations on energy efficiency and water usage. In many major appliances, advanced three-phase variable speed drive systems provide the performance improvements needed to meet these demands.

Designing fractional horsepower drives for home appliances such as refrigerators and washing machines presents some interesting technical challenges. As a result, manufacturers are turning to a digital-signal processing (DSP) control platform. The following applications show how DSP motor control designs are implemented in two different home appliances.

Home Refrigerator Control
A home refrigerator runs continuously and, therefore, consumes a significant amount of electricity. Since the main power consuming element is the compressor, appliance manufacturers are always looking to improve its cooling efficiency.

Designers can boost efficiency by reducing the speed of the compressor to match the cooling required for normal refrigeration operation. High-speed operation is reserved only for rapid cooling whenever the refrigerator is filled with food. The more simple control methods for single-phase induction motors result in a significant loss in efficiency of the motor. For fractional horsepower applications, the motor with the highest efficiency is an electronically controlled three-phase permanent magnet motor.

In domestic refrigeration systems, the compressor and motor are hermetically sealed within the same metal enclosure. The environment within the chamber is quite harsh, so Hall sensors can't be used. These sensors are typically used in other low-cost permanent magnet drives. As a result, a sensorless mode of operation where the motor acts as its own commutation sensor is essential.

Consequently, the target for a refrigeration application is to provide a drive for a 200-W compressor motor, without sensors, at minimum size and cost, and meeting all the regulatory requirements for electromagnetic compatibility (EMC) and safety.

Motor Control Strategy
A permanent-magnet motor is the most efficient ac motor type. It doesn't require rotor magnetizing current as does an induction motor. However, to run an ac motor efficiently, it is important to synchronize the frequency of the applied voltage with the position of the permanent-magnet rotor. An effective control scheme is to run the motor in a six-step commutation mode with only two windings active at any one time. In this case, the back emf on the unconnected winding is a direct indication of the rotor position. This position is estimated by matching a set of back emf waveform samples to the correct segment of the stored waveform profile. This technique averages the data from a large number of samples giving a high degree of noise immunity.

The control system has an inner position control loop (Fig. 1). This adjusts the angle (qs) of the applied stator field to keep the rotor synchronized. The integrator input tracks the motor velocity when the rotor position error is forced to zero. The outer velocity loop adjusts the applied stator voltage magnitude to maintain the required velocity. The controller can accelerate the compressor to its target speed within a few seconds and can regulate speed to within 1% of its target. The smooth running of the compressor reduces audible noise. The lower operating speed helps minimize the temperature cycles in the refrigeration compartment, and improves the quality of food refrigeration.

The complete drive system includes the EMI filter, the input rectifier, the control power supply, the DSP motor control circuit, the signal conditioning circuits, the power inverter, and gate drives (Fig. 2).

ADC Checks Currents, Voltages
Upon power-up, the internal program RAM inside the controller IC is loaded from an 8-pin external boot ROM via one of the serial ports. The control program performs initialization and diagnostics and then starts the motor in an open-loop mode. When the back emf reaches a minimum level, the motor is switched to normal running mode. During every PWM cycle the analog-to-digital converter (ADC) samples the motor back emf, the motor current, and the bus voltage.

An internal multiplexer selects the appropriate back emf signal to be converted. The DSP CPU calculates a new rotor position estimate and calculates the PWM duty cycle needed to apply the required voltage to the motor. At particular values of estimated rotor position angle, the CPU selects a new set of active motor windings by writing to the PWM segment selection register. The CPU also performs diagnostic functions and monitors dc bus voltage, motor current, and speed. In the case of overload conditions, the drive is shut down and a restart is attempted after a short time delay.

The drive power stage consists of a full three-phase MOSFET power inverter bridge and three integrated gate drive amplifiers. The rectifier common is connected to the control IC ground; so the PWM outputs are connected directly to the gate drive inputs. The back emf signal conditioning consists of three matched high-voltage resistive dividers and a passive RC filter. The current amplifier circuit is synchronized to the PWM sampling frequency in a way that it can determine the motor winding current from the dc bus current.

To reach the cost targets demanded by this application, the complete control hardware, including the processor core, memory, PWM, and ADC, was integrated into a single motor control IC. The ADMC330 DSP motor controller is an example of a single-chip DSP device for this application (Fig. 2, again). It has three independent computational units within the CPU section: an arithmetic logic unit (ALU), a multiply and accumulate unit (MAC), and a shifter unit.

The memory-mapped PWM controller requires only three register writes per PWM cycle to control the motor winding voltages. This minimizes the processor overhead in generating PWM signals. The ADC is synchronized to the PWM frequency, producing four updated samples every PWM cycle.

Major home appliance manufacturers are always adding new features and simplifying the use of their products in order to maintain a competitive edge. New designs also are influenced by current and pending government regulations on energy efficiency and water usage. In many major appliances, advanced three-phase variable speed drive systems provide the performance improvements needed to meet these demands.

Designing fractional horsepower drives for home appliances such as refrigerators and washing machines presents some interesting technical challenges. As a result, manufacturers are turning to a digital-signal processing (DSP) control platform. The following applications show how DSP motor control designs are implemented in two different home appliances.

Home Refrigerator Control
A home refrigerator runs continuously and, therefore, consumes a significant amount of electricity. Since the main power consuming element is the compressor, appliance manufacturers are always looking to improve its cooling efficiency.

Designers can boost efficiency by reducing the speed of the compressor to match the cooling required for normal refrigeration operation. High-speed operation is reserved only for rapid cooling whenever the refrigerator is filled with food. The more simple control methods for single-phase induction motors result in a significant loss in efficiency of the motor. For fractional horsepower applications, the motor with the highest efficiency is an electronically controlled three-phase permanent magnet motor.

In domestic refrigeration systems, the compressor and motor are hermetically sealed within the same metal enclosure. The environment within the chamber is quite harsh, so Hall sensors can't be used. These sensors are typically used in other low-cost permanent magnet drives. As a result, a sensorless mode of operation where the motor acts as its own commutation sensor is essential.

Consequently, the target for a refrigeration application is to provide a drive for a 200-W compressor motor, without sensors, at minimum size and cost, and meeting all the regulatory requirements for electromagnetic compatibility (EMC) and safety.

Motor Control Strategy
A permanent-magnet motor is the most efficient ac motor type. It doesn't require rotor magnetizing current as does an induction motor. However, to run an ac motor efficiently, it is important to synchronize the frequency of the applied voltage with the position of the permanent-magnet rotor. An effective control scheme is to run the motor in a six-step commutation mode with only two windings active at any one time. In this case, the back emf on the unconnected winding is a direct indication of the rotor position. This position is estimated by matching a set of back emf waveform samples to the correct segment of the stored waveform profile. This technique averages the data from a large number of samples giving a high degree of noise immunity.

The control system has an inner position control loop (Fig. 1). This adjusts the angle (qs) of the applied stator field to keep the rotor synchronized. The integrator input tracks the motor velocity when the rotor position error is forced to zero. The outer velocity loop adjusts the applied stator voltage magnitude to maintain the required velocity. The controller can accelerate the compressor to its target speed within a few seconds and can regulate speed to within 1% of its target. The smooth running of the compressor reduces audible noise. The lower operating speed helps minimize the temperature cycles in the refrigeration compartment, and improves the quality of food refrigeration.

The complete drive system includes the EMI filter, the input rectifier, the control power supply, the DSP motor control circuit, the signal conditioning circuits, the power inverter, and gate drives (Fig. 2).

ADC Checks Currents, Voltages
Upon power-up, the internal program RAM inside the controller IC is loaded from an 8-pin external boot ROM via one of the serial ports. The control program performs initialization and diagnostics and then starts the motor in an open-loop mode. When the back emf reaches a minimum level, the motor is switched to normal running mode. During every PWM cycle the analog-to-digital converter (ADC) samples the motor back emf, the motor current, and the bus voltage.

An internal multiplexer selects the appropriate back emf signal to be converted. The DSP CPU calculates a new rotor position estimate and calculates the PWM duty cycle needed to apply the required voltage to the motor. At particular values of estimated rotor position angle, the CPU selects a new set of active motor windings by writing to the PWM segment selection register. The CPU also performs diagnostic functions and monitors dc bus voltage, motor current, and speed. In the case of overload conditions, the drive is shut down and a restart is attempted after a short time delay.

The drive power stage consists of a full three-phase MOSFET power inverter bridge and three integrated gate drive amplifiers. The rectifier common is connected to the control IC ground; so the PWM outputs are connected directly to the gate drive inputs. The back emf signal conditioning consists of three matched high-voltage resistive dividers and a passive RC filter. The current amplifier circuit is synchronized to the PWM sampling frequency in a way that it can determine the motor winding current from the dc bus current.

To reach the cost targets demanded by this application, the complete control hardware, including the processor core, memory, PWM, and ADC, was integrated into a single motor control IC. The ADMC330 DSP motor controller is an example of a single-chip DSP device for this application (Fig. 2, again). It has three independent computational units within the CPU section: an arithmetic logic unit (ALU), a multiply and accumulate unit (MAC), and a shifter unit.

The memory-mapped PWM controller requires only three register writes per PWM cycle to control the motor winding voltages. This minimizes the processor overhead in generating PWM signals. The ADC is synchronized to the PWM frequency, producing four updated samples every PWM cycle.