Brushless Direct Current (BLDC) motors are increasing in popularity in consumer and industrial electronics because of their compact size, controllability, and high efficiency. For instance, this type of motor can eliminate automotive belts and hydraulic systems for improved functionality and fuel economy. Their compact size is a result of continued improvements in magnets, rendering improvements in efficiency. Additionally, BLDC motor systems are experiencing a reduction in magnetic and electronic costs.

You can implement BLDC motors into systems that are sensor or sensorless. If you choose to implement a sensorless BLDC motor application, you eliminate the cost of Hall-Effect or optical sensors along with support electronics. A sensorless approach to BLDC motor systems is also an advantage in applications where temperature extremes occur. For example, the heat generated from the compressor control in the refrigerator of a HVAC system may accelerate Hall-effect sensor failures. As long you understand the disadvantages of a sensor-based system, the cost advantages of sensorless BLDC motor control are very attractive.

The preferred implementation to determine the position and speed of a sensorless BLDC motor uses Back Electromotive Force (BEMF) "zero-crossing" (see Figure 1). Figure 1 illustrates the voltage changes of the three wires to the BLDC motor. The motor's BEMF waveform is a function of position and speed. Simple resistor dividers and operational amplifiers determine these motor characteristics. This system detects instances when the BEMF of an inactive phase is zero.

Fig. 1: By using the motor leads of a BLDC motor, the "zero-crossing" of the BEMF signal appears in sectors 0 through 5. Each sector corresponds to a 60 portion of the electrical cycle. There is a 30 offset between the BEMF zero-crossings.

The BEMF zero-crossing detection system is suitable for a wide range of motors. To facilitate ease of design, you can use Y and connected 3-phase motor theory in your design. The BEMF zero-crossing does not require detailed knowledge of motor properties. Since the zero-cross BEMF technique is looking for an instance where the rise and fall of signals cross a threshold voltage, this method is insensitive to motor manufacturing tolerance variations. Finally, it works with voltage- or current-control circuits. A primary disadvantage is that the motor must generate sufficient BEMF by moving at a minimum rate. A secondary disadvantage is that abrupt changes in the motor load can cause the BEMF loop to go out of lock. A lock condition can sometimes be overcome in the Digital Signal Controller (DSC) software.

Figure 2 shows a hardware example of a sensorless BLDC system. In this figure, a six-channel PWM register from Microchip Technology's dsPIC30F2010 DSC drives the BLDC through a 3-phase inverter. The PWM portion of the DSC generates multiple, synchronized outputs. The PWM module has six PWM I/O pins with three duty-cycle generators. The PWM counter has up to 16-bit resolution and you can perform "on the fly" frequency changes.

Components description
A 10-bit A/D Converter monitors the BEMF of the BLDC motor. The A/D Converter inputs AN12, AN13, and AN14, simultaneously sampling the three legs of the BLDC motor and testing for bus voltage. The 10-bit A/D converter also measures the "zero-crossing" voltage (VDC) for BEMF testing. Additionally, a current-feedback circuitry that uses an amplifier/comparator network (not shown) connects to a PWM fault protection pin, FLTA. If a PWM fault is detected, the motor is shut down.

The dsPIC30F2010's onboard 10-bit high-speed ADC module uses a Successive Approximation Register (SAR) architecture, and provides a maximum sampling rate of 1MS/s. The ADC module has up to 16 analog inputs, which are multiplexed into four sample and hold amplifiers. The output of the sample and hold is the input into the converter, which generates the result. The analog reference voltages are software selectable to either the device supply voltage (AVDD/AVSS) or the voltage level on the (VREF+/VREF-) pin.

This application uses 276 Bytes of data memory storage, six PWM output-channel drives (16 kHz PWM), four input channels for simultaneously sampled 10bit ADCs of bus current, bus voltage, demand pot and phase voltage samples synchronized to the PWM module, and three timers. The six-channel PWM register drives the BLDC motor.

In software, there are four different ways of controlling the motor speed. The software also implements the BEMF zero-crossing routine. Motor efficiency and an extended speed range is enhanced with a 30 phase advance, commutation scheme. The software consumes approximately 5 MIPS and requires approximately 16KB of program memory.

Block diagram

Fig: Hardware block diagram of a sensorless BLDC motor control circuit

References:
Electric Motors and Drives, A. Hughes, Heinemann Newness, ISBN 0750617411 "Using the dsPIC30F for Sensorless BLDC Control", Charlie Elliott, Steve Bowling, AN901, Microchip Technology Inc.

"Brushless DC Motor Control Made Easy", Ward Brown, AN857, Microchip Technology Inc.

"Sensorless BLDC Motor Control Using dsPIC30F2010", Stan D'Souza, AN992, Microchip Technology Inc.