BLDC Motors

• In order to make the operation more reliable, more efficient and less noisy, the recent trend has been to use brushless DC motors. They are also lighter compared to brushed motors with the same power output.

• The brusses in conventional DC motors wear out over the time and may cause sparking, thus the brushed DC motors should never be used for operations thatdemand long life and reliability.

• Let’s see how brushless DC motor works, the rotor of a BLDC is a permanent magnet, the stator has a coilarrangement as shown:

• By applying DC power to the coil, the coil will energizeand become an electromagnet. The operation of a BLDC is based on a simple force in reaction between the permanent magnet and the electromagnet.

• In this condition, when the coil A is energized, the opposite poles of the rotor and the stator are attractedto each other. As the rotor nears coil A, coil B isenergized.

• As the rotor nears coil B, coil C is energized. After that, coil A in energized with opposite polarity.

• This process is repeated and the rotor continues to rotate.

• An ancient humour helping remember it is to think of BLDC operation like the story of the donkey and the carrot, where the donkey tries hard to reach the carrotbut the carrot keeps moving out of reach.

• Eventhough this motor works, it has one drawback: youcan notice that at any instant only one coil is energized. The two dead coils greatly reduce the power output of the motor.

• Here is the trick to overcome this problem, when the rotor is in this position along with the 1st coil whichpause the rotor, you can energize the coil behind it in a such way that, it will push the rotor.

• For this instant, a same polarity current is passedthrough the 2nd coil.

• The combined effect produces more torque and power output from the motor.

• The combined force also make sure that a BLDC has a beautiful constant torque nature, with thisconfiguration, two coils need to be energized separatly.

• But by making a small modification to stator coil, wecan simplify this process. Just connect one free end of the coils together. When the power applied betweencoils A and B, let’s note the current flow through the coils. It’s just like the separately energized state.

• That’s how a BLDC works, how to know which stator coils to energize ? How to know when to energize it ? So we get a continuous rotation from the rotor.

• In a BLDC, we use an electronic controller for thispurpose. A sensor determines the position of the rotor, and based on this information the controller decideswhich coils to energize.

• Most often, a Hall effect sensor is used for this purpose.

• The BLDC design being discused for far is known as the out-runner type. In BLDC runner design, is also availablein market.

• We hope you get a nice introduction for the working of BLDC motors. Thank you.