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27 March 2010
Author: Giorgos Lazaridis
How Brushless Motors Work (BLDC Motors)

This motor could be characterized as the modern kind of DC motor. The letters BLDC means Brush-Less Direct Current. So, these motors have no brushes. If you do not know what the brushes are, then you should first read the article about How Brushed DC motors are made and how they operate. It is better to start learning from the simplest motors.

How are the Brushless DC (BDLC) motors made?

PC fans use BLDCs for their silent operation and reliability

The controller circuit for PC fans is so small, it can fir on the back side of the motor!

The brushless motor, unlike the DC brushed motor, has the permanent magnets glued on the rotor. It has usually 4 magnets around the perimeter. The stator of the motor is composed by the electromagnets, usually 4 of them, placed in a cross pattern with 90o angle between them. The major advantage of the brushless motors is that, due to the fact that the rotor carries only the permanent magnets, it needs of NO power at all. No connection needs to be done with the rotor, thus, no brush-commutator pair needs to be made! This is how the brushless motors took their name from. This feature gives the brushless motor great increament in reliability, as the brushes wear off very fast. Moreover, brushless motors are more silent and more efficient in terms of power consumption.

A brushless motor has yet another major difference from the brushed motors. In the theory of operation of the brushed DC motors with permanent magnets, i explain how the commutator is made and how the coils changes polarity during rotation. But brushless motors have no commutator nor brushes. Thus, there is actually no way of knowing where each time the rotor is.

Well actually they do know. There are several ways to find out where the rotor is. Sometimes they use rotary encoders along with their controllers and they know exactly the angle that the rotor is. Others use pairs of Hall sensors while most of them use just one Hall sensor. You can learn more information about the Hall sensor in this page. The Hall sensor is placed in an appropriate position. It can sense if in front of it is the North or the South pole. The Hall sensor will then transmit this signal to the controller of the motor. The controller will then switch on or off the appropriate coils needed in order to provide the torque. And that's the way it goes...

As you understand, this is a major drawback of brushless motors. They need of a controller circuitry to operate. Yet when reliability is required, this motor is the most suitable. The following video demonstrates exactly how a typical (and very popular type) of a brushless motor is made:

How the brushless motors work?

The trick of operation in BLDC motors is the Hall sensor that is attached to the stator. It faces the magnets perpendicularly and can distinguish if the North or South pole is in front of it. The following image shows this Hall senor. The photo is taken from a PC fan (yes, PC fans do have BLDCs!):

If you want to learn how the PC fans operate, follow this link. To better understand the operation of the Hall sensor in respect to the rotor position, i will show you an animation with only 2 magnetic poles and 2 coils. The magnetic poles are both South poles:

The Hall sensor is this little component under the right electromagnet. When it senses the South pole, it keeps the coils turned off. When the sensor senses no magnetic field (or could be also the South pole), then it turns on the coils. The coils have both the same magnetic polarity which is North. So they pull the opposite pole and torque is then created.

If you put a probe to the Hall sensor and watch the signal, then you will discover that during a full rotation of the rotor, the Hall sensor is two times HIGH and two times LOW. The waveform on oscilloscope would be like this one:

Yet another great advantage for the brushless motors. This very signal that is used to control the coils, can be used as is for measuring the speed of the motor! It can also be used to see if the motor is functional or not! Actually, this signal is exactly the one that comes out from the third wire from the PC fans that have 3 (or 4 wires)! These fans do not have any extra circuitry to measure the speed of the motor. They use the signal from the Hall sensor. Each revolution will generate 2 pulses. With a simple frequency measuring circuitry, someone can measure precisely the rpm of the motor.

A real brushless motor has 4 coils

I explained above the operation principle of a brushless motor with 2 coils and 2 permanent magnets. Yet, in real life, BLDCs have usually 4 coils and 4 magnets. Also, the Hall sensor is able not only to see if a magnetic field is in front of it, but also to distinguish if this is the North or the South pole. This is how a real BLDC motor looks like:

Around the perimeter of the rotor, there are 4 magnets in N-S-N-S patten. Also, there are 4 coils. The windings of the coils are not all of the same direction. 2 neighbor coils can never have the same magnetic polarity. The coils are connected in pairs, either each one with it's opposite coils, or there in two pairs or neighbor coils like shown on the above drawing.

The simplest operation cycle is, according to the pole that is in front of the Hall sensor, the controller will turn on or off the appropriate coil pair. The following animation demonstrates this cycle of operation:

And when the Hall sensor is between the two poles?

I had this question myself. What will happen if the rotor is stopped in a position where the Hall sensor is exactly between two different poles? Look for example the following drawing:

It may happen... Now the Hall sensor cannot sense exactly which pole is in front of it. Well, this is actually not a big deal... Suppose that the sensor senses the wrong pole and gives power to the wrong coils. What will happen? For a fraction of a millisecond the motor will try to rotate the wrong way. But a few degrees of rotation will bring the correct pole in front of the Hall sensor and it will immediately change the coils. Thus, the motor will then turn on the correct rotation.

But what if the motor controls a critical load and this backwards rotation, even if it is under 5o must NOT occur? There is a solution for this, but it requires the use of another Hall sensor. The second sensor will be placed with 45o difference from the first one:

Now, even if the first Hall sensor cannot get a proper reading, the second Hall sensor can clearly distinguish the magnetic pole. The controller will accept as "correct reading" the one that comes from the sensor with the most intense signal.

The Sensorless BLDCs

Yet another variation of the brushless motors. Using a Hall sensor will result in an increase of the overall price of the motor. Moreover, there are situations that a sensor cannot be used, as for example in submersible pumps, or in applications where the wiring must be kept to minimum. In such applications, the sensorless BLDC can be used instead. The operation of such motor is based on the BEMF effect. The BEMF (Back Electro-Magnetic Force) is inducted by the movement of a permanent magnet in front of a stator coil.

There are two problems that must be solved for the proper operation of the motor. First of all, the rotation direction. As no sensor is used, the controller cannot know where the rotor is stopped at any time. Thus, the rotation direction that the motor will start is -at least for the first degrees of rotation- coin toss. The other problem is the zero detection. The controller does not know when to change the polarity of the coils, as there is no sensor to sense when the permanent magnet pole crosses a specific point.

There are special designed controller chips to solve these problems. The chips will use the characteristics of the BEMF and the voltage generated on the coils from the BEMF effect. For example, the current produced on a coil due to BEMF will change its polarity, if the rotation of the permanent magnet is changed. Also, the amplitude of the produced waveform is proportional to the speed of the rotors, and the phase of the waveform depends on the position of the permanent magnet in respect to the coil. Yet, this is not the proper article to discuss about sensorless BLDCs in details.