# Brushless DC Motors – Part I: Construction and Operating Principles

Pushek Madaan, Cypress Semiconductor -February 11, 2013

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating.  Since the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.

This switching of the stator to build up the rotation is known as commutation. For 3-phase windings, there are 6 steps in the commutation; i.e., 6 unique combinations in which motor windings will be energized.

Driving circuitry and waveforms for the implementation of a BLDC motor will be discussed in the second part of this article.

Torque and Efficiency

For the study of electric motors, torque is a very important term. By definition, torque is the tendency of force to rotate an object about its axis.

Thus, to increase the torque, either force has to be increased – which requires stronger magnets or more current – or distance must be increased – for which bigger magnets will be required.  Efficiency is critical for motor design because it determines the amount of power consumed. A higher efficiency motor will also require less material to generate the required torque.

Where,

Having understood the above provided equations, it becomes important to understand the speed vs. torque curve.

Following are the takeaways from the graph shown in Figure 5:

• With an increase in speed, the torque reduces (considering the input power is constant).
• Maximum power can be delivered when the speed is half of the “no load” speed and torque is half of the stall torque.

Applications

Single-speed – For single-speed applications, induction motors are more suitable, but if the speed has to be maintained with the variation in load, then because of the flat speed-torque curve of BLDC motor, BLDC motors are a good fit for such applications.

– BLDC motors become a more suitable fit for such applications because variable speed induction motors will also need an additional controller, thus adding to system cost. Brushed DC motors will also be a more expensive solution because of regular maintenance.

Position control
– Precise control is not required applications like an induction cooker and because of low maintenance; BLDC motors are a winner here too. However, for such applications, BLDC motors use optical encoders, and complex controllers are required to monitor torque, speed, and position.

Low-noise applications
– Brushed DC motors are known for generating more EMI noise, so BLDC is a better fit but controlling requirements for BLDC motors also generate EMI and audible noise. This can, however, be addressed using Field-Oriented Control (FOC) sinusoidal BLDC motor control.

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