AN857 データシートの表示（PDF） - Microchip Technology

部品番号

コンポーネント説明

メーカー

AN857

Brushless DC Motor Control Made Easy

Microchip Technology 

Sensorless Motor Control

It is possible to determine when to commutate the

motor drive voltages by sensing the back EMF voltage

on an undriven motor terminal during one of the drive

phases. The obvious cost advantage of sensorless

control is the elimination of the Hall position sensors.

There are several disadvantages to sensorless control:

• The motor must be moving at a minimum rate to

generate sufficient back EMF to be sensed

• Abrupt changes to the motor load can cause the

BEMF drive loop to go out of lock

• The BEMF voltage can be measured only when

the motor speed is within a limited range of the

ideal commutation rate for the applied voltage

• Commutation at rates faster than the ideal rate

will result in a discontinuous motor response

If low cost is a primary concern and low speed motor

operation is not a requirement and the motor load is not

expected to change rapidly then sensorless control

may be the better choice for your application.

Determining the BEMF

The BEMF, relative to the coil common connection

point, generated by each of the motor coils, can be

expressed as shown in Equation 4 through Equation 6.

EQUATION 4:

BBEMF = sin ( 

AN857

FIGURE 7:

V

BEMF EQUIVALENT

CIRCUIT

B

R

L

BBEMF

COM

ABEMF

A

CBEMF

L

R

C

EQUATION 5:

CBEMF = sin -2—3

EQUATION 6:

ABEMF = sin



-



—4

3









Figure 7 shows the equivalent circuit of the motor with

coils B and C driven while coil A is undriven and avail-

able for BEMF measurement. At the commutation fre-

quency the L’s are negligible. The R’s are assumed to

be equal. The L and R components are not shown in

the A branch since no significant current flows in this

part of the circuit so those components can be ignored.

 2002-2011 Microchip Technology Inc.

DS00857B-page 9

Share Link: