GS153

DS93001A-page 2

© 2005 Microchip Technology Inc.

DRIVING SENSORED BLDC MOTORS WITH A

SINUSOIDAL VOLTAGE

When it is rotated like a generator, a BLDC motor

creates a sinusoidal voltage output (120 degrees apart)

in all three phase windings. So the “natural” drivers for a

BLDC

motor

are

three

sinusoidal

voltages

at

120 degrees apart. The six-step commutation normally

works very efficiently in most BLDC applications.

However, in some applications, the DC switching of the

PWM drive voltage used in six-step commutation some-

times causes a phenomenon known as torque ripple.

Torque ripple typically manifests as a low-frequency

rumble in some systems.

An alternative to the six-step method is to feed a PWM

driven sine wave to the three phases (at 120 degrees

apart) using a Space Vector Modulation (SVM)

technique. This method is just as efficient as six-step

commutation and delivers uniform torque to the load.

Microchip is developing an application note on this

technique.

Sensorless BLDC Motor Control

Sensors add cost to a BLDC motor application. Also,

sensors need to be adjusted during the manufacturing

process. In quite a few applications, however, the need

to find the exact position of the rotor is not necessary.

Fan blowers and compressor motors are typical appli-

cations which run at a constant or limited speed range.

In these applications, the back EMF detected on the

third unexcited winding can be used to switch the PWM

commutation of the motor windings.

Figure 2 shows a typical sensorless commutation

diagram. In this method, the back EMF voltage on the

winding that is not driven in each sector is monitored.

When this voltage crosses the imaginary “half-point” or

“zero-crossing” line, zero crossing is detected. The

algorithm now knows that it is in the center of the sector

and has 30 electrical degrees remaining to do the next

commutation.

The

time

taken

for

each

sector

(60 degrees) is known as, say T60. When the zero-

crossing point is detected, a timer is loaded with half

the value of T60. When this timer times out, an interrupt

is generated and the next winding commutation is

implemented.

This

method

of

control

is

called

sensorless control of a BLDC motor.

For example, in Sector 1, the Y winding is monitored for

zero crossing. When that transition occurs, the timer is

loaded with half the T60 time in a timer. When that timer

times out, the windings are commutated as described

earlier. That is, Y is driven high, B is kept at low and R

is not driven.

Microchip has developed two application notes on sen-

sorless BLDC control:

AN901, “Using the dsPIC30F for

Sensoreless BLDC Control” and AN992, “Sensorless

BLDC Motor Control Using dsPIC30F2010”.

FIGURE 2:

TYPICAL SENSORLESS

COMMUTATION

55

00

11

23

4

SECTOR

0

0

0

R

Y

B

T60

T30