Enlarge this picture Fig. 1. Inside view of single rotary compressor and twin rotary compressor.

Power conversion is being used in many everyday appliance products, including microwave ovens, washing machines, air conditioners and refrigerators. With many more sophisticated applications arising in conjunction with energy saving and cost reduction, more advanced control technologies like field-orientated control or load-adaptive control for motors have been developed. Performance for these advanced controls are generally unattainable via standard 8-bit microcontrollers due to lack of either signal-processing capabilities or suitable peripherals. Recently, however, this situation has changed dramatically through utilization of DSP-based controllers. The architecture of a Digital Signal Controller (DSC), which includes both digital signal processor (DSP) and microcontroller (MCU) functions with sophisticated on-chip peripherals, was designed to reduce component usage and system cost as well as advance processing power.

Today, refrigeration systems typically make up the majority of electrical energy consumption in both residential homes and office buildings. The number of installation units has been steadily increasing over the past 10 years, particularly in Asian countries. The majority of units are controlled by an on/off switch to keep the temperature within a set range. For heavy load conditions, a fixed-speed compressor will operate at higher efficiency. However, such compressors will operate at less efficiency with lighter load conditions. In normal operation, the most heavily loaded condition presents itself at compressor start up, because heat needs to be extracted from the cooling area to bring the temperature within the desired level. This maintains the temperature at the desired level, causing the compressor to operate at a much lighter load level.

To reduce the indirect contribution to global warming, efforts to improve the efficiency of refrigeration systems are being sought in both compressor design, as well as implementations of sophisticated electronics control. To achieve both quiet operation and energy savings, variable-capacity compressors have been increasingly used in refrigeration systems to vary cooling capacity as desired. The output of a variable-capacity compressor can be controlled by electronic-frequency modulation. Since a consumer refrigeration system is a cost-sensitive and labor-intensive application, low-cost and high-integration control systems implementing sophisticated control algorithms benefit both manufacturers and consumers.

The continuous improvement in efficiency has also been achieved by the implementation of input power-factor correction (PFC) to reduce reactive energy usage, which translates to more than 15 percent energy savings. PFC for appliances is now mandated in many countries because excessive reactive current and high-order current harmonics drawn by electronic load will require extra capacity for the power grid and may affect grid stability.

Choosing a drive

Enlarge this picture Fig. 2. Sensorless BLDC compressor control with PFC.

A number of types of compressors are designed for refrigeration applications, including: single rotary compressors (SRC), twin rotary compressors (TRC), scroll compressors and linear compressors.

An eccentric, circular cam-like rotor inside a larger, circular cavity comprise an SRC. Pressure cycling and the eccentric weight distribution of the rotor create mechanical vibration and a cyclically varying motor load. Two symmetric eccentric circular cam-like rotors comprise a TRC. Its advantages include more efficiency and a more balanced load for the motor rotor. Although SRC has relatively less operational efficiency and a more unbalanced load, it is being used in low-cost refrigeration systems because of its simple mechanical configuration.

The newest trend for refrigeration systems is the gradual reduction in usage of AC induction motors and increasing usage of Permanent Magnet Synchronous motors (PMSM), including Brushless DC motors (BLDC) and high-efficiency interior permanent magnet motors, which were generally favored because of their compact size, high efficiency, and good torque performance.

There are four key issues we are facing today within refrigeration motor control applications: quiet operation, low cost, high-energy efficiency systems, and elimination of high-cost sensors. In most compressor designs, the motor is encased with coolant and lubricant materials in a sealed unit, operating in a high-temperature environment. This eliminates the rotor position sensors that must be a part of the design and makes the mechanical design simpler and more reliable. A sensorless control method that provides variable speed for the motor without using rotor-position sensors eases the concerns of cost, reliability and system efficiency.

Today, compressor control has started adopting field-oriented control. But BLDC control is still attractive because its algorithms can be very easily implemented using a low-cost programmable controller and general 3-phase power stage (from a modeling perspective it looks very similar to a DC motor.) The difference is the BLDC control adopts an electronically controlled commutation system, instead of a mechanical commutation system. Physically, the two motors are completely different. The easy to create rectangular DC voltage shape of the applied voltage makes controlling and driving the motor simple. However, the rotor position must be known at certain angles to be able to align the applied voltage with the rotor. Alignment of the rotor with commutation events is very important; when this is achieved, the motor behaves as a DC motor and runs at maximum efficiency. Thus, simplicity of control and high performance makes the BLDC motor the best choice for low-cost and high-efficiency applications.

For the operation of a BLDC motor, only two of the three phases are energized, with one un-energized at any moment. The motor windings are selected in pairs with a six step commutation sequence resulting in rectangular winding current in the powered phases and trapezoidal back Electromotive Force (EMF) induced on the phases. Sensorless BLDC control measures the back EMF on the un-powered phase to infer rotor position, eliminating the need for rotor-shaft position sensors. The most common, low-cost rotor-position detection is sensing the zero-crossing of the back EMF voltage in the un-powered motor phase. The major advantages of sensorless BLDC control are added simplicity and less dependency on motor parameters.

Although this control method can be implemented by an 8-bit MCU, the noise issue is never resolved. The sources of noise come from mechanical vibration due to an unbalance load, which presents as load torque ripples on the motor shaft and delays in winding current commutation, introducing unwanted torque spikes. The output torque of a BLDC motor is directly proportional to the current feed in its motor windings. Without a current control loop plus proper torque forward compensation, the acoustic noises cannot easily be eliminated. It is very difficult to add a current control loop into an 8-bit MCU control system because intensified math calculations and high loop bandwidth are needed. The reductions of acoustic noise, compressor cost and complexity of both control circuit and algorithms can be solved by using a highly integrated Digital Signal Controller (DSC).

Digital signal control

Enlarge this picture Fig. 3. On-chip comparator interconnection.

For many price-sensitive refrigeration systems, low-cost rotary compressors and BLDC motors are still popular. Designers can take advantage of the DSC to improve the performance of overall drive systems without noticeable system cost increasing through implementing current feedback and forward-load compensation. The DSC is a specialized microprocessor whose architecture contains a core engine capable of competitively performing both MCU and DSP functionalities:

• Core processing capability applicable to many types of system solutions.
• Common basic features: MAC, single instruction cycle allowing several memory accesses, address generation units, algorithms for efficient looping.
• Specialized low-cost, high performance on-board interfaces utilized in implementing embedded control applications: PWM; multifunction timer; high speed ADCs; DACs; Comparators; SCIs (UART); SPIs; CANs and I2Cs, etc.
• Embedded nonvolatile memory: Flash memory, ROM or EEPROM.

Fig. 2 shows the control block diagram of a rotary compressor driven by a trapezoidal Back EMF BLDC motor, controlled by a MC56F8025 DSC, a member of Freescale’s 56F8000 series.

In this application, the BLDC motor with controllable torque is driven by a voltage source, current-controlled 3-phase inverter. The dynamic expression of the BLDC motor can be represented by equations below:

At the heart of the system is the closed-loop current controller, or current control loop, enclosed by the dotted lines. The purpose of the current control loop is to make the actual current flow in the motor terminals, whose magnitude, id is proportional to the motor shaft torque, follow the current reference signal (iref), the output of the closed-loop speed controller. Because only two of the three motor terminals are powered at any moment, current flow in the motor terminals is equal to the DC-link current id.

So the current measurement is sampling the voltage drop on a single low-cost current shunt connected in the DC-link. Motor output torque is directly proportional to current without delay (or microsecond delay due to winding leakage inductance) shown in Equation 4, so the current controller is in effect also a torque controller.

From Equation 2, rotor speed can also be affected by load torque after a 10 ms delay due to the sum of the inertia of both load and rotor. Consequently, for a system without a current control loop, motor response to torque change will be much slower. To respond to instant torque change, the current control loop calculation must be done in less than a few microseconds. Rotary compressors have an unbalanced load. Torque compensation function adds additional value to current reference signal iref according to the rotor position, which is detected by back EMF detection circuits. So the unbalanced load can be compensated by the inner current control loop before it affects rotor speed.

Another importance of a current control loop is to prevent excessive inverter currents from flowing. As long as the current control loop functions properly, the motor current can never exceed the reference value, and the motor can constantly output maximum torque without damaging both inverter and motor.

Rotor position signals are derived from zero crossing of the back EMF voltage in the un-energized motor windings. Resistor networks connected to motor terminals extract back EMF voltage and feed them to on-chip comparator inputs. Only one comparator is used to detect a zero-crossing signal. Internal analog multiplexing connects only the un-energized winding, through a signal attenuation resistor network, to the comparator’s input, in the sequence defined by commutation table. The internal 12-bit DAC acts as voltage reference on the comparator’s negative input. Sensed zero-crossing signals are then processed to derive the rotor speed for the speed control loop and phase commutation signal. Uni-polar PWM switching patterns applied to the inverter greatly reduce switch losses and current ripples, which help further reduce acoustic noise.

The input power factor is also controlled by the same DSC device. The purpose of the PFC circuit is to:

• Force the input current to have sinusoidal wave shape and zero phase-shifting with respect to the input voltage waveform.
• Stabilize the DC-link voltage.

However, analog PFC controllers can only force the input current to follow the input voltage shape. So, any spikes or harmonics on the input voltage waveform will be reflected on the input current with the same spike or harmonic shapes because the input voltage waveform is the reference for the PFC current feedback loop. Digital PFC will eliminate this problem by generating a digital sine waveform using a table.

Now, the DSC only detects input-voltage zero-crossing via an on-chip comparator with another internal DAC as voltage reference. It then locks the phase of the digital sine waveform with the phase of input voltage. As a result, the DSC generates a glitch-free sine waveform as current reference to feed into the PFC current controller. Using a Timer PWM output feature, the output of the current controller controls the PWM duty cycle of the Timer output that drives the Transistor of the PFC circuit to force input voltage and current in phase. The switching frequency of PFC is normally between 40 KHz and 75 KHz.

Freescale’s 56F8000 16-bit fixed-point DSC series is an example of a cost-effective controller with on-chip DACs, analog comparators and a relaxation oscillator reducing external component usage. It offers very high-performance peripherals, 32 MIPS core performance, and variety of selections. The core supports program execution from internal memories and two data operands can be accessed from the on-chip data RAM per instruction cycle. The watchdog is running on a clock independent from the CPU, which provides additional safety for motor-control applications.