Santerno DCREG4.150 - 6.10 Electromagnets Application

15P0059B3 DCREG2
USER MANUAL DCREG4
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6.10 ELECTROMAGNETS APPLICATION
6.10.1 DRIVE POWER CONNECTIONS AND PROTECTING DEVICES
DCREG converter can be used to power very inductive loads, such as electromagnets.
Applications problems due to this type of load - that can be compared to an ohmic resistance series-
connected to a very strong inductance - have been solved by a control algorithm specially developed.
The application featuring a DCREG4 will be considered first, then the application featuring a DCREG2 will
be considered; the limits of that application compared to the one featuring a DCREG4 will be highlighted.
With very inductive loads, safety problems may occur in case of power failure due to faults in the system
or to unsafe installations. Power failure, in conjunction with the high inductive value of the magnet, can
cause instant over voltage up to some thousands of Volts. The only way to protect the converter from
those over voltage conditions consists in taking special measures when installing the equipment (see
sections below).
Over voltage depends on how quick magnet power failure is:
dt
dI
LV 
Because L value is very high (approx. 1 Henry), the voltage value can reach instant values up to some
thousands of Volts.
To protect the converter, the most effective measure consists in installing clamping unit CU400, which
protects a DC converter when it is subject to dangerous over voltage conditions generated when the
current conduction mesh of the magnet unexpectedly opens.
Energy stored in the magnet is obtained as follows:
2
2
1
ILE 
Energy in the magnet is absorbed and stored in an RC-type clamping circuit, where over voltage is limited
by a capacitor and energy is dissipated by a resistance, which are both integrated in the clamping circuit.
Clamping unit CU400 must be connected directly to the converter output by means of fuses equipped with
a micro switch detecting power failure.
To ensure proper clamping, unit CU400 must be connected directly to the converter DC side. To suppress
the first current peak generated by the capacitor, this is precharged when connecting the main voltage of
the mains (typically 400 Vac) to the relevant terminals.
(Wiring diagram and terminal board are shown in Figure 1).
If the current conduction mesh on mains side opens (due to sliding contacts or other), or wires connecting
the magnet to the converter are torn, the electric arc generated when current is cut off is limited by the
clamping circuit, ensuring that over voltage does not exceed safety values.
This setting is essential for electromagnets installed on bridge cranes where 3-phase supply
voltage L1-2-3 is derived from the mains through sliding brushes that could accidentally open.
Multiple clamping units CU400 can be parallel-connected to the output of a unique converter. Each
clamping unit is indicated for a rated DC current of approx. 150A as a maximum allowable value.
For more details, please refer to CU400 User Manual (P/N 15P0068B1).