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Going Green: The Trend Toward High-Efficiency Brushless DC Motors

In recent years, there has been a move toward using higher-efficiency brushless DC (BLDC) motors in many markets and applications. Many applications are using (or could be using) these motors to replace AC induction motors.

Some of the key benefits of using BLDC motors include higher efficiency (75 percent vs. 40 percent), lower heat generation, higher reliability (no brushes to wear out), and safer operation (no brush dust generated or arcing). Using these motors in key subsystems also can reduce the overall system weight.

Typical functional block diagram of a 24V brushless DC motor control.

Typical functional block diagram of a 24V brushless DC motor control.

And since the BLDC motor is totally commutated electronically, it is much easier to control the torque and RPM at much higher speeds.

Cross-section of BLDC motor.

Cross-section of BLDC motor.

A closer look
BLDC motors are synchronous motors that have permanent magnets integral to the rotor. Like the AC induction motor, they have coil windings in the stator. The windings produce magnet fields on the stator pole pieces, and these fields can be made to rotate (see left). Electrical terminals are directly connected to the stator windings, so there are no brushes or mechanical contacts to the rotor.

These motors use DC power and switching circuits to produce bidirectional currents on the stator windings. Switching circuits consist of a high-side and low-side switch for each winding. A total of six switches are needed for one BLDC motor. This switching action causes the stator's magnetic field to rotate.

Switching circuit using IGBTs.

Switching circuit using IGBTs.

Due to cost, reliability, and size issues, current motor designs use solid state switches such as MOSFETs or IGBTS instead of relays, depending on the voltage and speed of the motor. The switching currents produce the appropriate magnetic field polarity, which attracts opposite polarity and repels equal polarity. Magnetic forces rotate the rotor. Using permanent magnets on the rotor offers a mechanical advantage reducing size and weight. BLDC motors offer improved thermal characteristics compared to brush and induction motors, making them the ideal choice for the next generation of power-saving mechanical systems.

BLDC motors typically use three phases (windings), and each phase has a conducting interval of 120 degrees.

Six-step commutation.

Six-step commutation.

Since the current is bidirectional, each phase is broken into two steps per conducting interval. This is called six-step commutation. One commutation phase sequence option is AB-AC-BC-BA-CA-CB. Each conducting stage is called a step, and only two phases conduct current at any time, leaving the third phase floating. The unenergized winding can be used as a feedback control, which is the basis for sensorless control algorithms.

To keep the magnetic field in the stator advancing ahead of the rotor, the transition from one sector to another must occur at precise rotor positions for optimal torque. Maximum torque is achieved by switching circuit commutating every 60 degrees. All the switching control algorithms are embedded in the microcontroller unit (MCU). The MCU can control the switching circuit via MOSFET drivers that have the appropriate characteristics, such as propagation delays, rise and fall times, and drive capability such as gate drive voltage and current sync required to turn the MOSFET/IGBT to the on or off state.

The rotor position is the key to determining the right instant for commutating the motor winding. In applications where precision is required, hall sensors or tachometers are used to calculate the position, speed, and torque of the rotor. In applications where cost is a factor, back electromotive force (EMF) can be used to calculate position, speed, and torque.

Back EMF is the voltage generated in the stator winding by the permanent magnet when the rotor of the motor is turning. The back EMF has three important characteristics that can be used for control and feedback signals. First, its magnitude is proportional to the speed of the motor, so engineers use MOSFET drivers that can operate at least two times the voltage supply. Second, as speed increases, so does the signal slope. Finally, the signal is symmetrical around the crossing event, as shown below.

Details of the crossing event.

Details of the crossing event.

Detecting the zero-cross event accurately is the key to implementing the back EMF algorithm. The back EMF analog signal can be sent directly to the MCU via its mixed-signal circuitry. Generally, all that is needed is one or more high-voltage op-amps. These amplify and level shift (as needed) and send the control signals to the ADCs that are typically available in most modern microcontrollers.

When using the sensorless control, the start sequence is important, because the MCU doesn't know the initial rotor position. The first step initiates the motor by energizing two windings at a time while taking several measurements from the back EMF feedback loops until a precise position can be determined.

BLDC motors typically operate in a closed-loop control system, which requires a MCU. The MCU performs the servo loop control, calculations, corrections, PID controls, and management of sensors such as back EMF, hall sensors, or tachometers.

Block diagram of the servo system.

Block diagram of the servo system.

These digital controllers are typically eight bits or higher and require EEPROM to store the firmware that performs the algorithms required to set the desired motor speeds and direction and maintain motor stability. MCUs often have ADCs that allow for sensorless motor control architecture, which saves costs and board space. The MCU offers the ability to optimize the algorithms for the application. The analog devices offer the MCU an energy-efficient power supply, voltage regulation, voltage references, the ability to drive MOSFETS or IGBTs, and fault protection. Both technologies allow you to use three-phase BLDC motors efficiently and at a comparable price point to induction and brushed motors.

Around the globe, many governments are facing power deficits that are the direct result of insufficient electrical grids. Many areas now face blackouts during peak demand seasons. These governments are now giving or planning subsidies for more efficient use of BLDC motors. BLDC implementation is one of the many greening initiatives that can save our resources without impacting our way of life.

Carlos Ribeiro, a staff RF test engineer at Micrel, and John T. Lee, a director at Micrel, assisted in the preparation of this article.

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