FOUR-PHASE EXAMPLE

Figure 6 is typical of the circuitry used to commutate a four-

phase motor using the LM621 This application is seen to

differ from the three-phase application example in that the

LM621 outputs are utilized differently Four-phase motors

require four-phase power switches which in turn require the

commutator to provide four current-sinking outputs and four

current sourcing outputs The 18-pin package of the LM621

facilitates only three sinking and three sourcing outputs The

schematic shows the 3060 SELECT input in the 30-degree

select state (pin 8 high) and rotor-position sensor inputs

HS2 and HS3 connected together This connection trun-

cates the number of possible rotor-position input states to

four which is consistent with the 90-degree quadrature ro-

tor-position signals provided by four-phase motors With the

LM621 outputs connected as shown this approach pro-

vides the needed power-switch drive signals for a four-

phase motor Note that only four of the six LM621 outputs

(SINK

1 and

3 and SOURCE

1 and

3) are used

directly and that these are also inverted to form the remain-

ing four SINK

2 and SOURCE

2 outputs are not used

HALF-WAVE DRIVE EXAMPLE

The previous applications examples involved delta-config-

ured motor windings and full-wave operation of the motor

The application shown in

Figure 7 differs in that it features

half-wave operation of a motor with the windings in a Y-con-

figuration This approach is suitable for automotive and oth-

er applications where only low-voltage power supplies are

conveniently available The advantage of this power-switch-

ing scheme is that there is only one switch-voltage drop in

series with the motor winding thereby conserving more of

the available voltage for application to the motor winding

Half-wave operation provides only unidirectional current to

the windings in contrast to the bidirectional currents applied

by the previous full-wave examples

TLH8679 – 11

FIGURE 6 Commutation of Four-Phase Motor

10