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An Introduction to Brushless DC Motor Control




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.

DC Motor
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.

 

For more detail: An Introduction to Brushless DC Motor Control

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