Introduction
World-wide interest in water and energy conservation as well
as the overall environmental friendliness of new products and
services definitely impacts the home appliance market. The
actual regulatory standards and recommendations, either
nationwide or multination (EU standards), enforce basic
demands for a new generation of home appliances, particularly
washing machines, dryers, dish washers and refrigerators.
To meet these demands while still reducing system costs,
enhanced microcontrollers equipped with appropriate software
must be employed.
For the home appliance industry, a digital signal controller
(DSC) based solution is proposed, combining the processing
power of a digital signal processor with the functionality and
ease of use of a microcontroller on a single chip. A flexible
set of peripherals allows the designers to realize numerous
functions, such as standard motor drive algorithms, advanced
control algorithms, sophisticated feedback signal sensing,
power factor correction schemes and communication with
external environments.
Employing variable speed motor drives gives the designer an
opportunity to use more sophisticated control programs. This
enhances appliance performance and increases overall energy
efficiency. Thus, home appliances equipped with variable
speed drives and intelligent controls outperform those with
uncontrolled, fixed-speed motor drives.
The majority of electric motor drives in home appliances are
controlled very simply by either fixing the motor speed at
pre-defined levels or by running it directly from the AC main
supply without additional control electronics. The alternating
current mains-supplied single-phase induction motor (ACIM)
is extensively used because it is low cost, robust and reliable.
However, ACIM solutions also have significant drawbacks,
such as lower efficiency and reduced effective speed control,
which hinder control improvements designed to enhance
product flexibility and meet regulatory guidelines as well as
customer expectations. In contrast to single-phase ACIM, the
variable speed drives address energy efficiency requirements
by maintaining precise torque control, thereby improving overall
appliance efficiency.
There are different motor categories that can drive home
appliances, such as ACIM, permanent magnet (PM) motors or
switched reluctance (SR) motors. The product of the electronic
drive efficiency and the electric motor efficiency determines the
overall energy efficiency of the system.
In general, to achieve variable speed with an ACIM, variable
frequency and voltage need to be supplied to the ACIM. This
is referred to as constant volt per Hz control. The ACIM speed
drive efficiency can be further improved by using vector control.
This requires accurate velocity information sensed by a speed
or position sensor attached to the rotor. However, the additional
sensor, connector and associated wiring increase the cost of
the motor drive.
PM motors use permanent magnets to establish the flux
instead of creating it from the stator winding. Replacing the
electromagnetic excitation with permanent magnets has
several advantages. Most obvious is the absence of excitation
losses, which means PM motors have higher power density
than comparable DC motors. PM motors have no mechanical
commutator, and power density exceeds the AC induction
motor because there is no flux oriented current. Overall
efficiency approaches 90 percent while single-phase ACIM
efficiency reaches only about 70 percent. Permanently excited
synchronous motors are very attractive solutions for home
appliances, however, they cannot be operated by applying
AC main supply to the stator winding.
Variable Speed Motor Drives
Experts say that the total electric power consumption can be
reduced up to 30 percent if electric motors are controlled to
optimize the supplied energy. The controller for a variable-speed
drive actually controls the rotational speed of an AC electric motor
by regulating the frequency of the electrical power supplied to the
motor. Electronic variable speed drives allow electric motors to
continuously operate over their full speed range.
As Figure 1 indicates, the complete system for a variable speed
drive includes the EMI filter, the input rectifier, the on-board DC
power supply, a DSC, the signal conditioning circuits, the power
inverter and gate driver.
Motor Control Strategy
The open loop scalar control represents the most popular
control strategy for variable speed drives predominantly used
with ACIM. As depicted in Figure 2, the scalar control is
based on the variation of supply voltage frequency. Voltage
magnitude is proportional to the voltage frequency and changes
according to the frequency variation. This technique requires
modest computational power that can be handled by an 8-bit
microcontroller.
The great advantage of this simple method is that it is a
sensorless mode, and the control algorithm does not need
information about the angular speed or actual rotor position.
Speed dependence on the external load torque, however,
is a big disadvantage, which results in reduced dynamic
performance. Because of this, electric motors under scalar
control must be oversized in order to deliver the required
torque during load transients. In addition, scalar techniques
result in an inefficient system, power factor degradation in the
utility's network and noisy operation. In this control approach
the energy efficiency might degrade down to the 50 percent
range from theoretical maximum. Motor model-based method
estimates an electromotive force in which the electrical position
information of machine is encoded. The estimator depicted in
Figure 3 consists of the stator current observer with knowledge
of RL motor parameters, fed by applied motor voltage and
measured current.
Yet, market demands require the highest possible dynamic
performance and operating speed range. Vector control (field
oriented control) of AC machines, as a novel approach in
electrical drives, provides very good performance in comparison
to scalar control. Vector control eliminates most of the
disadvantages of constant volt-per-hertz control.
Within a vector control system the synchronous frame current
regulators have become the industry standard for inverter
current regulation. The inverse Park (aß/dq) reference-frame
transformation function calculates the stator currents and
voltages in a reference frame synchronized to the rotating rotor
field. All the electrical variables have DC steady state values
when viewed in a rotating reference frame, enabling a simple
PI regulator to provide zero steady state error. Additionally, it
is possible to set up the coordinate system to decompose the
current vector into a magnetic field generator and a torque
generator. The structure of the motor controller is shown in
Figure 3.
The inner current loop calculates the direct and quadrature
stator voltages required to create the desired torque and flux
currents. The Park (dq/aß) functions transform these voltages
into three-phase AC stator voltage demands in the stationary
reference frame. The motor currents are sinusoidal, thus
producing smooth torque, which minimizes acoustic noise and
mechanical vibration. The outer velocity loop adjusts the applied
torque magnitude, which is directly proportional to quadrature
torque-current and enables maintaining the required angular
velocity. In order to effectively extend the operating speed range
above base speed, an additional flux-weakening loop is added,
which manipulates directional stator flux-current.
Sensorless Control
To run an AC motor in vector control mode, it is important
to synchronize the frequency of the applied voltage with the
position of the rotor flux coming from permanent-magnet in
the rotor. This results in a sensorless mode of operation where
the speed and position calculation algorithm replaces the
sensor. The sensorless control approach to AC PM motors is
an innovative motor drive feature that improves reliability and
maintains high performance levels without additional cost. The
system should be more accurate and efficient, smaller, lighter,
less noisy and should have more advanced functionality at a
lower cost. Sensorless algorithms can be broadly divided into
two major groups-those that utilize magnetic rotor saliency for
tracking rotor position and those that estimate rotor position
from calculated motor models.
In a refrigerator, conventional control turns the compressor on
and off to maintain the temperature in a predefined range. With
sensorless PM motor control, the controller can accelerate the
compressor to its target speed within a few seconds and can
regulate speed to within 1 percent of its target. The smooth
running of the compressor reduces audible noise, and the lower
operating speed helps minimize the temperature cycles in the
refrigeration compartment, improving overall efficiency.
Normally, a washing machine operates in two basic cycles-
tumble-wash and spin-dry. During the tumble-wash cycle,
the motor drive operates at low speed with high torque. During
the spin-dry cycle, the motor drive works at high speed for
short time periods. In newer models, the estimated speed
ripples and calculated load torque provide valuable information
on the washing load distribution. The speed ripple is used to
estimate the load imbalance before starting the spin cycle. A
variable speed motor can be used to regulate the clothes tub
rotational speed and direction to redistribute the clothes to
correct the imbalance.
MC56F80xx Family of DSCs
Freescale's 56800E core is an ideal solution for this particular
application. It processes all motor control functions, including
space vector modulation, vector control, current, flux and
velocity loop control. Digital control improves drive reliability
by reducing the number of discrete components found in early
designs and facilitates advanced algorithms for optimal motor
performance. Sensorless speed vector control discussed above
is implemented on the MC56F8025 DSC with all control routines
using C-callable, optimal assembler language with fractional
numerical representations.
The MC56F80xx family members provide these peripheral
blocks:
- One pulse width modulation (PWM) module with PWM outputs, fault inputs and fault-tolerant design with dead-time insertion, supporting both center-aligned and edge-aligned modes
- 12-bit analog-to-digital converters (ADCs), supporting two simultaneous conversions. ADC and PWM modules can be synchronized
- One dedicated 16-bit general-purpose quad timer module
- One serial peripheral interface (SPI)
- One serial communications interface (SCI) with LIN slave functions
- One inter-integrated circuit (I2C) port
- On-board 3.3 V to 2.5 V voltage regulator for powering internal logic and memories
- Integrated power-on reset and low-voltage interrupt module
- All pins multiplexed with general-purpose input/output (GPIO) pins
- Computer operating properly (COP) watchdog timer
- External reset input pin for hardware reset
- JTAG/On-Chip Emulation (OnCE™) module for unobtrusive, processor speed-independent debugging
- Phase-locked loop (PLL) based frequency synthesizer for the hybrid controller core clock, with on-chip relaxation oscillator
The application uses the ADC periphery to digitize analog
feedback signals (voltage, current) before being processed.
The ADC trigger is synchronized to the PWM reload flag, but
the sampling time instances vary as a function of the actual
PWM pattern. This configuration allows multiple conversions of
the required analog values for the DC-bus current and voltage
within the one PWM cycle.
The PWM module is capable of generating asymmetric
PWM duty cycles in the center-aligned configuration. This
feature enables three-phase current reconstruction in critical
switching patterns.
The quad timer is an extremely flexible module, providing
all required services relating to time events. The application
requires two of the quad timer's channels:
- One channel for PWM-to-ADC synchronization
- One channel for system velocity control loop (1 ms period)
The application allows communicating with the front-panel
master control over an isolated serial link. This allows speed
profile information to be downloaded to the controller and motor
speed and torque information to be uploaded to the home
appliance's master controller.
Conclusion
It is widely acknowledged that home appliance demands
drive technological advances and, in turn, the production of
next-generation products. In response to this trend, vector
control with sensorless functionality is a technique used for
this purpose, and since it eliminates mechanical sensors, it
enables a cost-effective solution. Even such demanding control
techniques can be realized using the low-cost DSC based on
the Freescale 56800E core, which combines both MCU and
DSP capabilities.
The results also indicate satisfactory performance for other
various motor drive applications, such as pumps, fans, blowers,
dryers, etc. MC56F80xx devices designed with a focus on
motor control applications are being integrated into many
home appliance product designs, particularly where national or
multinational regulations on power consumption are strict.
Freescale offers a comprehensive portfolio of embedded
chips, application software and development tools that are
optimized for electronic motor systems. These embedded
solutions are designed to allow customers to extract a high
level of functionality and performance out of highly reliable,
cost-efficient motors. A digitally controlled motor in a washing
machine is designed to provide a more efficient agitation
cycle so that less water is needed and may allow a short-high
speed spin cycle, resulting in drier clothes and less energy
consumption. The chips incorporate motor-specific functions
which are designed to enable the appliances and other
motor-driven products to operate more efficiently and reliably.
About the Author
Peter Balazovic is a system application engineer with the motor control and power efficiency group for the Consumer and Industrial
markets. He works closely with the DSC/MCU product line for motor control applications and designs motor control algorithms at the
Roznov Czech System Center. He joined Freescale Semiconductor in 2001.
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