[Design Application] DSP Motor Control Boosts Efficiency In Home Appliances Intelligent Operation Helps Designers Satisfy Consumer Needs And Government Requirements While Maintaining Economically Competitive. Contributing Author ED Online ID #7629 |
May 25, 1998
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
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
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.
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