The brushless DC (BLDC) motor is becoming increasingly popular in sectors such as automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial because it does away with the mechanical commutator used in traditional motors, replacing it with an electronic device that improves the reliability and durability of the unit.
Another advantage of a BLDC motor is that it can be made smaller and lighter than a brush type with the same power output, making the former suitable for applications where space is tight.
The downside is that BLDC motors do need electronic management to run. For example, a microcontroller – using input from sensors indicating the position of the rotor – is needed to energize the stator coils at the correct moment. Precise timing allows for accurate speed and torque control, as well as ensuring the motor runs at peak efficiency.
This article explains the fundamentals of BLDC motor operation and describes typical control circuit for the operation of a three-phase unit. The article also considers some of the integrated modules – that the designer can select to ease the circuit design – which are specifically designed for BLDC motor control.
The advantages of brushless operation
The brushes of a conventional motor transmit power to the rotor windings which, when energized, turn in a fixed magnetic field. Friction between the stationary brushes and a rotating metal contact on the spinning rotor causes wear. In addition, power can be lost due to poor brush to metal contact and arcing.
Because a BLDC motor dispenses with the brushes – instead employing an “electronic commutator” – the motor’s reliability and efficiency is improved by eliminating this source of wear and power loss. In addition, BLDC motors boast a number of other advantages over brush DC motors and induction motors, including better speed versus torque characteristics; faster dynamic response; noiseless operation; and higher speed ranges.1
Moreover, the ratio of torque delivered relative to the motor’s size is higher, making it a good choice for applications such as washing machines and EVs, where high power is needed but compactness and lightness are critical factors. (However, it should be noted that brush-type DC motors do have a higher starting torque.)
A BLDC motor is known as a “synchronous” type because the magnetic field generated by the stator and the rotor revolve at the same frequency. One benefit of this arrangement is that BLDC motors do not experience the “slip” typical of induction motors.
While the motors can come in one-, two-, or three-phase types, the latter is the most common type and is the version that will be discussed here.
The stator of a BLDC motor comprises steel laminations, slotted axially to accommodate an even number of windings along the inner periphery (Figure 1). While the BLDC motor stator resembles that of an induction motor, the windings are distributed differently.
At each step, two phases are on with one phase feeding current to the motor, and the other providing a current return path. The other phase is open. The microcontroller controls which two of the switches in the three-phase inverter must be closed to positively or negatively energize the two active coils. For example, switching Q1 in Figure 3 positively energizes coil A and switching Q2 negatively energizes coil B to provide the return path. Coil C remains open.
Designers can experiment with 8-bit microcontroller-based development kits to try out control regimes before committing on the design of a full-size motor. For example, Atmel has produced an inexpensive starter kit, the ATAVRMC323, for BLDC motor control based on the ATxmega128A1 8-bit microcontroller.4 Several other vendors offer similar kits.
Driving a BLDC motor
While an 8-bit microcontroller allied to a three-phase inverter is a good start, it is not enough for a complete BLDC motor control system. To complete the job requires a regulated power supply to drive the IGBT or MOSFETs (the “IGBT Driver” shown in Figure 3). Fortunately, the job is made easier because several major semiconductor vendors have specially designed integrated driver chips for the job.
These devices typically comprise a step-down (“buck”) converter (to power the microcontroller and other system power requirements), gate driver control and fault handling, plus some timing and control logic. The DRV8301 three-phase pre-driver from Texas Instruments is a good example (Figure 6).
Figure 6: Texas Instruments’ DRV8301 motor driver integrates a buck regulator, gate driver, and control logic in a single package.
This pre-driver supports up to 2.3 A sink and 1.7 A source peak current capability, and requires a single power supply with an input voltage of 8 to 60 V. The device uses automatic hand shaking when high-side or low-side IGBTs or MOSFETs are switching to prevent current shoot through.
ON Semiconductor offers a similar chip, the LB11696V. In this case, a motor driver circuit with the desired output power (voltage and current) can be implemented by adding discrete transistors in the output circuits. The chip also provides a full complement of protection circuits, making it suitable for applications that must exhibit high reliability. This device is designed for large BLDC motors such as those used in air conditioners and on-demand water heaters.
In summary
BLDC motors offer a number of advantages over conventional motors. The removal of brushes from a motor eliminates a mechanical part that otherwise reduces efficiency, wears out, or can fail catastrophically. In addition, the development of powerful rare earth magnets has allowed the production of BLDC motors that can produce the same power as brush type motors while fitting into a smaller space.