Engineering Motor Control
Brushless D.C. Motor Control for Refrigerator Applications

by Padmaraja Yedamale, senior applications engineer,Microchip Technology, Inc.

Evolving market needs and changing energy policies push designers to consider energy efficiency, audible noise, product lifetime, maintenance, and the cost of initial products and operation.

Today's Technology

Refrigeration performance largely depends upon the control capabilities of the compressor and cooling fan. The compressor is instrumental in cooling the refrigerant, while the fan blows cold air throughout the appliance. Traditionally, Permanent Split Capacitor (PSC) single-phase induction motors are used for the compressor. Shaded pole motors, which are also a single-phase induction motors, are used for fan control.

The PSC motors have a non-linear speed versus torque characteristic. For this reason, using a variable-speed drive does not give good torque control over the speed range, thus, the motors have low-energy efficiency and tend to be noisy compared to brushless d.c. (BLDC) motors. PSC motors are not ideal for energy-efficient or quiet refrigerators.

Most of today’s refrigerators switch on at full speed when the temperature increases above a trip temperature. The compressor is turned off when the refrigerator reaches the set temperature. This operation is noisy because the motor runs at full speed with a “bang” ON and “bang” OFF. Also, the compressor motor draws a large starting current. A typical curve, as shown in Figure 1, demonstrates the starting characteristics of the compressor motor. This may introduce a voltage dip in the main-power supply and cause a momentary brownout situation.

Using Brushless D.C. Motors

Brushless d.c. (BLDC) motors are gaining popularity due to their performance advantages over PSC and brushed d.c. (BDC) motors, including the following:

• BLDC motors have a relatively flat speed-torque characteristic (see Figure 2). This enables the motor to operate at lower speeds without compromising torque when the motor is loaded.

• The ratio of output power to frame size is higher in BLDC motors. This reduces the size and weight of the product. This also saves the cost of motor mounting and shipping expenses.

• The BLDC motors operate at higher-power efficiency compared to induction motors and BDC motors because they have permanent magnets on the rotor and there are no brushes for commutation.

• Brush inspection is eliminated, making them suitable for limited-access areas like compressors and fans. This also increases the life of the motor and reduces the service requirements.

• They operate much quieter compared to BDC motors since brushes make audible noise.

• BLDC motors have less electromagnetic interference (EMI) generation. With BDC motors, the brushes tend to break and make contacts while the motor is rotating, resulting in the emission of electromagnetic noise into the surroundings.

However, there are two concerns with BLDC motors. First, BLDC motors can be more expensive; however, the performance advantages override this concern. In addition, the cost delta is reduced as BLDC motors become more common. Second, BLDC motors need electronic commutation. The stator windings are commutated based on the rotor position. This requirement can be turned into an advantage. The same electronics used to control the commutation can also provide speed control. This paper discusses the control methodology for BLDC motors.

BLDC Motor Control

BLDC motors have alternate north (N) and south (S) permanent magnets on the rotor. The stator has stacked-steel laminations with windings placed in the slots that are axially cut along inner periphery. To rotate the BLDC motor, the stator windings should be energized in a sequence. Knowing the rotor position is important in order to follow the commutation sequence. Rotor position is sensed using Hall-Effect sensors embedded into the stator. Most motors have three Hall sensors embedded into the stator on the non-driving end of the motor. Whenever the rotor-magnetic poles pass near the Hall sensors, they give a high or low signal, indicating that the N or S pole is passing near the sensors. Based on the combination of the three Hall sensor signals, the exact sequence of commutation can be determined. Table 1 provides a typical commutation sequence with respect to Hall sensor input.

With every 60 electrical degrees of rotation, one of the Hall sensor outputs changes its state from high to low, or from low to high. Given this, it takes six steps to complete one electrical cycle. In a synchronous pattern corresponding to these 60 electrical degrees, the sequence of phase-voltage switching should be updated. However, one electrical cycle may not correspond to a complete mechanical revolution of the rotor. The number of electrical cycles that must be repeated in order to complete a mechanical rotation is determined by the rotor-pole pair. For each rotor-pole pair, one electrical cycle is completed. So, the number of electrical cycles per rotation equals the rotor-pole pair.

As Table 1 indicates, every sequence has two of the three phases connected to the power supply, with the third phase left open. Figure 3 shows a simplified BLDC motor control block diagram. The 8-bit Flash microcontroller (MCU) in the figure is used to control the power switches. Matching drivers are used to give the appropriate gate drive to the power switches. The MCU family has six Pulse-Width Modulated or Modulation (PWM) channels with a programmable PWM frequency and duty cycle. Phases A, B, and C are connected to the center point of each half H-bridge PWM0 to PWM5 control the power switches, and Q0 to Q5 are connected in a three-phase inverter bridge configuration. The MCU has three input-capture pins indicated as IC1, IC2, and IC3. The input-capture module has a mode in which Timer5 captured every transition on any of the input-capture pins. This mode is suitable for interfacing Hall sensors to the MCU. With every Hall sensor transition, an interrupt is generated, and the Timer 5 value is captured. This captured value corresponds to the motor speed.

The motor will run at a rated speed when the signals marked by PWM0 to PWM5 are switched ON or OFF according to the sequence (see Figure 3). This is assuming that the d.c.-bus voltage is equal to the motor-rated voltage, plus any losses across the switches. To vary the speed, these signals should be PWM at a much higher frequency than the motor frequency. As a rule of thumb, the PWM frequency should be at least 10 times that of the maximum frequency of the motor. When the duty cycle of the PWM frequency is varied within the sequences, the average voltage supplied to the stator decreases, thus reducing the speed.

Another advantage of having PWM is that when the d.c.-bus voltage is much higher than the motor-rated voltage, the voltage supplied to the motor can be limited to the motor-rated voltage by limiting the percentage of the PWM duty cycles corresponding to that of motor-rated voltage. This adds the ability to use the refrigerator control circuit in multiple countries with different a.c. inputs. The a.c.-voltage inputs are converted to d.c. using a diode-bridge rectifier, and the average voltage output by the controller is matched to the motor-rated voltage by controlling the PWM duty cycle.

There are different approaches for control. If the PWM signals are limited in the MCU, the upper switches can be turned ON for the entire time during the corresponding sequence, and the corresponding lower switch can be controlled by the required duty cycle on the PWM.

In Figure 3, the temperature is set by the user. The internal refrigerator temperature is measured using temperature sensors. Depending upon the size of the refrigerator, there may be more than one temperature sensor placed at different locations inside the refrigerator. The set temperature and actual temperatures are read using an on-chip Analog-to-Digital Converter (ADC). When the difference in temperature is more than a predetermined hysterisis (typically less than 2°F), the motor starts rotating at a lower speed. If the temperature difference is larger (i.e., the refrigerator door is left open), then the motor should run at a higher speed. A relationship between the temperature difference and speed can be derived based on the size of refrigerator. As the difference between set and actual temperature changes, the compressor and fan speed can be changed accordingly.

Sensorless Control of a BLDC Motor

Sensorless control of a BLDC motor calls for commutation based on the back EMF produced in the stator windings. Hall sensors are not required for this method. Sensorless control has two distinct advantages: increased reliability and lower costs. Inherently, systems with fewer components are more reliable. The compressor generates heat and an elevated temperature accelerates Hall sensor failures. The cost savings are achieved, not only with the piece part pricing of the Hall sensor, but also since the Hall sensor wires are not necessary. At least five wires can be eliminated.

Until now, we have seen the commutation based on the rotor position from the Hall sensor. Manipulating the back EMF signals, instead of the Hall sensors, can commutate the BLDC motors.

The amplitude of back EMF depends upon three factors: the angular velocity of the rotor, the number of turns in the stator winding, and the rotor-magnetic field. Once the motor is designed, the rotor-magnetic field and the number of turns in the stator windings remain constant. The factors that govern the back EMF are the angular velocity or speed of the rotor. The back EMF increases proportionally to the rotor speed. However, back EMF can be estimated for a given speed using the back EMF constant provided in the motor data sheet.

The relationship between the Hall sensors and the back EMF, with respect to the phase-voltage, is shown in Figure 5. As we have seen in the earlier sections, every commutation sequence has one of the windings positively energized, a second negatively energized, and a third left open. As shown in Figure 5, the Hall sensor signal changes state when the voltage polarity of the back EMF crosses from a positive to negative, or from a negative to positive, with a phase difference of 30 degrees. In ideal cases, this happens on zero crossing of the back EMF. However, there will be a delay due to the winding characteristics. This delay should be compensated by the MCU.

Another aspect to consider is very low-speed operation. Because the back EMF is proportional to the rotation speed, at very-low speeds, the back EMF would have very low amplitude with which to detect zero crossing. From standstill, motors need to be started in open loop. When sufficient back EMF is built in to detect the zero cross point, the control should be shifted to the back EMF sensing. The minimum speed at which the back EMF can be sensed is calculated from the back EMF constant of the motor.

Some motors may have the Hall-sensor magnets on the rotor, in addition to the main rotor magnets. These are used to simplify the process of mounting the Hall sensors onto the stator, a scaled down replica of the rotor. Therefore, whenever the rotor rotates, the Hall-sensor magnets give the same effect as the main magnets. The Hall sensors are normally mounted on a PC board and fixed to the enclosure cap on the non-driving end of the motor.

With a sensorless method of commutation, the Hall sensors, Hall sensor magnets, Hall sensor wires, and the PC board can be eliminated. This simplifies the motor construction, reduces costs, and increases reliability.