Typical Applications
(Continued)
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 30/60 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
Y
1 and
Y
3, and SOURCE
Y
1 and
Y
3) are used
directly, and that these are also inverted to form the remain-
ing four. SINK
Y
2 and SOURCE
Y
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.
TL/H/8679–11
FIGURE 6. Commutation of Four-Phase Motor
10