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AN857

Brushless DC Motor Control Made Easy

Microchip Technology 

AN857

In this example there are three electromagnetic circuits

connected at a common point. Each electromagnetic

circuit is split in the center, thereby permitting the per-

manent magnet rotor to move in the middle of the

induced magnetic field. Most BLDC motors have a

three-phase winding topology with star connection. A

motor with this topology is driven by energizing two

phases at a time. The static alignment shown in

Figure 2, is that which would be realized by creating an

electric current flow from terminal A to B, noted as path

1 on the schematic in Figure 1. The rotor can be made

to rotate clockwise 60 degrees from the A to B align-

ment by changing the current path to flow from terminal

C to B, noted as path 2 on the schematic. The sug-

gested magnetic alignment is used only for illustration

purposes because it is easy to visualize. In practice,

maximum torque is obtained when the permanent mag-

net rotor is 90 degrees away from alignment with the

stator magnetic field.

The key to BLDC commutation is to sense the rotor

position, then energize the phases that will produce the

most amount of torque. The rotor travels 60 electrical

degrees per commutation step. The appropriate stator

current path is activated when the rotor is 120 degrees

from alignment with the corresponding stator magnetic

field, and then deactivated when the rotor is 60 degrees

from alignment, at which time the next circuit is acti-

vated and the process repeats. Commutation for the

rotor position, shown in Figure 1, would be at the com-

pletion of current path 2 and the beginning of current

path 3 for clockwise rotation. Commutating the electri-

cal connections through the six possible combinations,

numbered 1 through 6, at precisely the right moments

will pull the rotor through one electrical revolution.

In the simplified motor of Figure 1, one electrical revo-

lution is the same as one mechanical revolution. In

actual practice, BLDC motors have more than one of

the electrical circuits shown, wired in parallel to each

other, and a corresponding multi-pole permanent mag-

netic rotor. For two circuits there are two electrical rev-

olutions per mechanical revolution, so for a two-circuit

motor, each electrical commutation phase would cover

30 degrees of mechanical rotation.

Sensored Commutation

The easiest way to know the correct moment to com-

mutate the winding currents is by means of a position

sensor. Many BLDC motor manufacturers supply

motors with a three-element Hall effect position sensor.

Each sensor element outputs a digital high level for 180

electrical degrees of electrical rotation, and a low level

for the other 180 electrical degrees. The three sensors

are offset from each other by 60 electrical degrees so

that each sensor output is in alignment with one of the

electromagnetic circuits. A timing diagram showing the

relationship between the sensor outputs and the

required motor drive voltages is shown in Figure 2.

FIGURE 2:

SENSOR VERSUS DRIVE TIMING

+V

A Float

-V

+V

B Float

-V

+V

C Float

-V

H

Sensor A

L

H

Sensor B

L

H

Sensor C

L

Code

...1

6

5

4

3

2

1

6...

101

001

011

010

110

100 101

001

DS00857B-page 2

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