BLDC Motors

Brushless DC Motors were a new type of electrical motor, made possible due to a transistor switch. In this way, an electronic commutator is used to facilitate rotation instead of a mechanical commutator with brushes.

Major components

• A stator with windings that create a magnetic field when energized

  • Three classifications of a BLDC motor: Single-phase, two-phase, and three-phase

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    Simplified BLDC motor diagrams

    A single-phase has one stator winding to produce 4 magnetic poles. A three-phase motor has three windings, as shown. Each phase turns on sequentially to make the rotor revolve

• An armature or rotor made of permanent magnets

  • A rotor consists of a shaft and a hub with permanent magnets arranged to form multiple pole pairs that alternate between north and south poles

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Types of Motors and Controllers

• Inrunner motor - Rotor is internal, stator is on the outside of the motor. These have a more lightweight construction and a better rotational speed because of their smaller rotating diameter.

• Outrunner motor - Rotor is external, so permanent magnets spin around the stator together with the motor’s case. These have a higher torque because of the longer arm and greater EMF applied to the rotor.

Three phase BLDC motors can have different winding types:

• wye (Y) or star connection (windings meet at the center forming a Y

• delta connection (windings are connected in series forming a triangle)

Operational Motor Theory

Motor operation is based on the attraction or repulsion between magnetic poles. For reference, a three-phase motor is shown below. The process starts when current flows through one of the three stator windings and generates a magnetic pole that attracts the closest permanent magnet of the opposite pole. The rotor will move if current shifts to an adjacent winding. Charging each winding by turn will cause the rotor to rotate. The torque depends on the current amplitude and the number of turns on the stator windings, the strength and size of the permanent magnets, the air gap between the rotor and the windings, and the length of the rotating arm.

Motor Control

BLDC motors use electric switches to realize current commutation, and thus continuously rotate the motor. The electronic commutator sequentially energizes the stator coils that generates an electric field that drags the rotor around with it. These electric switches are generally connected in an H-bridge structure for a single-phase BLDC motor, and a three-phase bridge structure for a three-phase BLDC motor shown below. The high-side switches are controlled using PWM, which converts a DC voltage into a modulated voltage, which easily and efficiently limits the startup current, control speed, and torque.

BLDC motor controllers differ according to the method they use to detect the rotor’s position.

• One method involves the use of sensors such as Hall effect sensors, rotary encoders, optical sensors etc. For a three phase motor, three Hall-effect sensors are embedded in the stator to indicate the relative positions of stator and rotor to the controller so that it can energize the windings in the correct sequence and at the correct time. The Hall sensors are usually mounted on the non-driving end of the unit. When the rotor magnetic poles pass the Hall sensors, a high (for one pole) or low (for the opposite pole) signal is generated. The exact sequence of commutation can be determined by combining the signals from the three sensors.

• The other method requires no sensors. All electric motors generate a voltage potential due to the movement of the windings through the associated magnetic field, known as EMF. This gives rise to a current in the windings with a magnetic field that opposes the original change in magnetic flux. Basically, it resists the rotation of the motor and is thus called back EMF. For a given motor of fixed magnetic flux and number of windings, the EMF is proportional to the angular velocity of the rotor. By monitoring this back EMF, a microcontroller can determine the relative positions of the stator and the rotor without need for the Hall effect sensors; the closer the rotor’s magnet, the higher the back EMF. However, a stationary motor generates no back EMF, making it impossible for the microcontroller to determine the position of the motor parts at start-up. The solution is to start the motor in an open loop configuration until sufficient EMF is generated for the microcontroller to take over motor supervision.

Taking a closer look at the circuit design (three phase BLDC motor) -

A typical BLDC motor controller has a half-bridge or half-H bridge circuit. Unlike an H-bridge, this circuit configuration has only two switches - one high side and one low side transistor.

The stator has three-phase windings located at 120 degrees to each other. The BLDC motor controller Hall sensors identify the rotor’s position. Upon receiving the sensor data, the power MOSFETs switch the current, injecting it into the correct winding. In a high power BLDC motor controller, IGBTs and GaN switches can replace MOSFETs.

Either integrated or discrete gate drivers can control the transistors. The drivers of a brushless motor controller act as intermediaries between the switches and a microcontroller (MCU).

The three-phase BLDC motor controller circuit includes six steps necessary to complete a full switching cycle (that is to energize all the three windings of the stator). By turning the high-side and low-side transistors on and off, the current flows through the stator windings in sequence.

There are different approaches to current switching - sinusoidal, trapezoidal, and field oriented control methods for commutation.