bonitron M3460RD, Manual

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User’s Manual
23
If the circulation current shown above in Figure 7-2 is established and remains during
standby operation, the highlighted SCR/Thyristor in Figure 7-3 can remain on and
overheat due to the continuous current flow. This can occur if there is low enough
impedance between the AC input of the VFD and the AC input of the M3460.
If the circulating current situation is not corrected the SCR may be permanently
damaged and fail.
Figure 7-3: M3460R Typical Configuration with Current Flow with affected SCR
Highlighted
CHOPPER
BRAKING
INTERNAL
VFD
INVERTER
BRAKING
RESISTOR
DISCONNECT
DRIVE
DISCONNECT
RIDE-THRU
M3460R
CHO
PPER
+
-
+
-
DRIVE
BUS
STORAGE BUS
STORAGE BUS
AC
FEED
In order to prevent this situation, extra impedance should be added to the negative
power connection of the ride-thru to the drive. This prevents the circulation current
from being sustained after a sag event. The M3460RD unit adds roughly 5 Volts of
drop between the VFD and the M3460 by adding series diodes. Since this voltage drop
is not dependant on current flow, once the AC power returns to normal, the negative
power flow thr
ough the M3460 drops to zero, and the SCR’s turn off.
The M3460RD is not required in all installations, however determining the requirement
is difficult due to the variables involved with the physical installation. Using the
M3460RD does not reduce the Ride-Thru capacity for the system, and is
recommended for all installations.
Please note that this situation can only occur when there is an AC connection to the
input bridge of the M3460R unit. If the installation contains energy storage, such as
capacitors, the two input bridges are galvanically isolated from each other by the
Dwg: 140106 Rev: 20140527