Brochure

ESG
8.608.60
8.608.60
8.60
Three-phase motor reversal
(Fig. 17) (Fig. 17)
(Fig. 17) (Fig. 17)
(Fig. 17)
For ac SSRs can provide a reversing function for a three-phase motor, using the drive logic suggested. The half cycle
time delay before enabling the drive, in either direction, prevents make before break which would result in a line to
line short. Two opposing SSRs (nos. 1 and 4, or 2 and 3 ) could still mistrigger simultaneously due to dv/dt or high
voltage transients; therefore, resistors R1 through R4 are inserted to limit the resultant surge current. The sum of any
two resistors plus the source impedance should limit the shorting current to less than a peak one cycle surge rating
of each relay.
Semiconductor type fuses should be chosen to permit such a condition for one cycle and open as soon as possible
thereafter.
SSRs should have a transient (blocking) rating equal to twice the line to line voltage to withstand the combined line
and back EMF generated at the moment of reversal.
Phase A
Out
AC
SSR1
In
+–
Out
AC
SSR2
In
+–
+5 VDC
Direction
Logical 1 = forward
Logical 0 = reverse
Enable
8.3 ms
Time
delay
In
SSR3
AC
out
–+
In
SSR4
AC
out
–+
Motor
F1
F2
F3
Phase B
Phase C
R1 R2
R3 R4
A
CB
Fig. 17: Three-phase motor reversal