bonitron M3460RD, Manual

Page: 21 / 28

User’s Manual
21
7.
S
AG
O
NLY
I
NSTALLATIONS
& C
IRCULATING
C
URRENTS
The topology of the M3460R Voltage Boost Module incorporates a six diode full
wave three phase rectifier bridge. When the M3460R is connected to the DC bus of
the variable frequency drive, the input rectifier bridge of the variable frequency drive
is in parallel with the M3460R input rectifier bridge. There are two diodes in parallel
from the DC bus to the AC line for each connection.
Figure 7-2: M3460R Typical Configuration without Energy Storage
CHOPPER
BRAKING
INTERNAL
VFD
INVERTER
BRAKING
RESISTOR
DISCONNECT
DRIVE
DISCONNECT
RIDE-THRU
M3460R
CHO
PPER
+
-
+
-
DRIVE
BUS
STORAGE BUS
STORAGE BUS
AC
FEED
Since current flows through the connection with the lowest resistance, “circulation
currents” may enter the DC bus of the M3460R through one bridge and return to the
AC source through the VFD input bridge. This can occur through any parallel bridge
arrangements if the impedances between the two do not prevent it.
When the system is operating normally, the AC is at the nominal level and the boost
converter is turned off. In this case, the VFD is powered only through the AC bridge
in the VFD, as the SCR’s within the M3460 are turned off.
During a sag event, the incoming AC line is lower than normal, and the M3460
begins to boost the rectified AC line power to a voltage level that keeps the drive
running. When the M3460 is boosting, all of the power is provided to the drive
through the M3460 bridge and boost converter, because the DC bus is held at a level
higher than the peak of the sagging AC input line. Therefore, the VFD input bridge is
reverse biased and turns off.
At the end of a sag event, the power flow returns normally through the VFD input
bridge and the boost regulator turns off. In general, the impedance in the DC power
Dwg: 140106 Rev: 20140527