Dings Magnetic Group Stationary Electro Overhead Magnets - Manual (Page 8)

Revised January 19, 2011
RECTIFIER INPUT OVER-CURRENT PROTECTION
All rectifiers contain a transformer. A transformer has all the same component parts as a
motor, and like a motor, exhibit a current inrush when energized. This inrush current is
dependent upon where in the sine wave the transformer was last turned off in relation to where
in the sine wave the transformer is when you energize it. Although transformer current inrush
could run up to 30 to 35 times full load current under a no load condition, it typically will be the
same as a motor at about 6 to 8 times the full load current. For this reason it is important to
use a dual element time delay type fuse, the same type of fuse you would use with a motor. If
using circuit breakers, select a breaker with a time delay, again the same type you would use
with a motor. If the time delay is not sufficient, you may experience “nuisance tripping”, a
condition where the circuit breaker trips when energizing the transformer but when you try it
again, it works fine.
Fuses
Use a dual element time delay fuse or a circuit breaker with a time delay having a rating of 175
percent of the full load current to protect the input side of the rectifier. If 175% of full load
amps does not correspond to a standard fuse size, select the next larger standard fuse size
(but not exceeding 225%).
Circuit Breakers
Use an inverse time circuit breaker with a maximum rating of 250% of the full load current. If
250% of full load amps does not correspond to a standard circuit breaker size, select the next
larger size (but not exceeding 400%).
Caution:
Always follow the latest edition of the National Electric Code and any other applicable codes
for over-current protection of 600-volt class transformers to determine the proper over-current
protection for rectifiers. One special consideration not listed in the above codes is to be careful
of the type of fuse or circuit breaker used to protect the input side of the transformer.
EXAMPLE:
Model 33 Electro Magnet
5 kW, NEMA 12 Rectifier
AC input: 460 VAC/3phase/60 Hz, 6.7 amps
Time Delay Fuses
6.7 amps x 175% = 11.73 amps 11.73 is a non-standard fuse size. The next standard fuse
size is 12 amps. 12 amps is 179% of the full load amps, but is under the 225% maximum limit.
USE A 12 AMP TIME DELAY FUSE.
Circuit Breakers
6.7 amps x 250% = 16.75 amps. 16.75 is a non-standard circuit breaker size. The next
standard circuit breaker size is 20 amps. 20 amps is 299% of full load amps, but under the
400% maximum limit. USE A 20 AMP INVERSE TIME CIRCUIT BREAKER.
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