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BLDC motor construction compared to induction and brushed DC motors

Read Time:4 Minute, 25 Second

Electrical equipment typically has at least one motor for rotating or moving an object from its initial position. There are a variety of motor types available on the market, including induction motors, servo motors, DC motors (both brushed and brushless), and more. Depending on the application requirements, specific motors can be selected. However, the current trend is that most new designs are moving towards brushless DC motors, commonly known as BLDC motors.

This article will focus on the following aspects of BLDC motor design:

BLDC motor structure

Operation of BLDC Motors

 Torque and Efficiency Requirements

Comparison with induction and brushed DC motors

 Selection criteria for BLDC motors

Motor Control – Speed, Position, and Torque, covered in the second part of this article.

Building a BLDC motor shares many similarities with an AC induction motor and a brushed DC motor in terms of structure and working principle, respectively. Like all other motors, BLDC motors have rotors and stators.

Stator

Similar to induction AC motors, BLDC motor stators are made of laminated steel, which is stacked to carry windings. The windings in the stator can be arranged in two ways; a star pattern (Y) or a delta pattern (Δ). The main difference between the two modes is that the Y mode provides high torque at low rpm, while the delta mode provides low torque at low rpm. This is because, in the delta configuration, half of the voltage is applied to the undriven windings, increasing losses, which in turn increases efficiency and torque.

The steel laminations in the stator can be slotted or slotless, as shown in Figure 2. A slotless core has lower inductance, so it can operate at very high speeds. Since there are no teeth in the lamination stack, the requirement for cogging torque is also reduced, making it ideal for low speeds as well (when the permanent magnets on the rotor and the teeth on the stator are aligned with each other, because they interact, resulting in undesired cogging torque and speed fluctuations). The main disadvantage of the slotless core is the higher cost as it requires more windings to compensate for the larger air gap

rotor 

The rotor of a typical BLDC motor is made of permanent magnets. Depending on application requirements, the number of poles in the rotor may vary. Increasing the number of poles does give better torque, but at the cost of reducing the maximum possible speed.

Another rotor parameter that affects maximum torque is the material used to make the permanent magnets. The higher the magnetic flux density of the material, the higher the torque.

Working principle and operation

BLDC motors work on the same basic principle as brushed DC motors; that is, internal shaft position feedback. In the case of brushed DC motors, feedback is achieved using mechanical commutators and brushes. With the built-in BLDC motor, multiple feedback sensors can be used. The most commonly used sensors are Hall sensors and optical encoders. Note: Hall sensors work according to the Hall effect principle, i.e. when a current-carrying conductor is exposed to a magnetic field, the charge carriers experience a force based on the voltage developed across the conductor.

If the direction of the magnetic field is reversed, the resulting voltage will also reverse. For Hall effect sensors used in BLDC motors, whenever a rotor pole (N or S) passes near the Hall sensor, they generate a high or low-level signal that can be used to determine the position of the shaft.

In a commutation system—one based on the motor position identified using feedback sensors—two of the three electrical windings are energized at a time, as shown in Figure 4.

In Figure 4(A), the green winding marked “001” is energized to the north pole, and the blue winding marked “010” is energized to the south pole. Due to this excitation, the south pole of the rotor is aligned with the green winding and the north pole is aligned with the red winding marked “100”. To move the rotor, the “red” and “blue” windings are energized in the directions shown in Figure 4(B). This results in the red winding being the north pole and the blue winding being the south pole. This movement of the magnetic field in the stator creates torque that moves the rotor in a clockwise direction due to the development of repulsive forces (red winding – north – north alignment) and attractive forces (blue winding – north-south alignment).

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This torque reaches a maximum when the rotor starts to move, but decreases as the two magnetic fields align with each other. Therefore, in order to maintain torque or build rotation, the magnetic field generated by the stator should be constantly switching. In order to catch up with the magnetic field produced by the stator, the rotor will continue to rotate. Since the magnetic fields of both the stator and rotor rotate at the same frequency, they belong to the category of synchronous motors.

This switching of the stator to establish rotation is called commutation. For three-phase windings, there are 6 steps to commutation; that is, 6 unique combinations in which the motor windings will be energized.

Also Read: 4 Essential Home Renovation Tips Everyone Should Adopt.

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