Dings Magnetic Group RECTIFIER - Manual (Page 2)

RECTIFIER INPUT OVER-CURRENT PROTECTION
All rectifiers contain a transformer. The transf
ormer has all the same component parts as a
motor. Like a motor, the transformer exhibits a current inrush when energized. This inrush
current is dependent upon where in the sine wave the transformer was when last turned off in
relation to where in the sine wave the transformer is when energized. The transformer inrush
could be up to 30 or 35 times full load current under a no load condition. Typically, it will be
the same as a motor, 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 that is used with a motor. If
using circuit breakers, select a breaker with a time delay, again the same as would be used with
a motor. If the time delay is not sufficient, “nuisance tripping” may be experienced. This is a
condition when the circuit breaker trips when energizing the transformer, but when tried again,
it works.
Fuses
Use a dual element time delay fuse with a maximum rating of 175 percent of the full load cur-
rent 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.
EXAMPLE:
Model 33 Electro Magnet
5 kW, NEMA 12 Rectifier
A/C input: 460 volts-3 phase-60Hz, 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, which is under the 225% maximum limit. USE A 12 AMP TIME DELAY
FUSE.
Circuit Breakers
6.7 amps x 250% = 16.75 amps
16.75is a non-standard circuit breaker size. The next standard fuse size is 20 amps. 20 amps is
299% of the full load amps, which is under the 400% maximum limit. USE A 20 AMP
INVERSE TIME CIRCUIT BREAKER.