Re: Potter & Brumfield Sensitive Plate Circuit Relays
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Hi ALL,
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I have amongst my Henry Amps a 3K-A. (My 3K-A was manufactured in 1972 and still works well.)? It has the plate current relay in the B- lead to trip on over current above 1A.
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5.2? ADJUSTMENT OF THE OVERLOAD RELAY
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A low voltage, variable, metered DC supply of 1 amp is needed for this adjustment. The 3K-A should stay of f during this adjustment.
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Connect the DC supply to the slider of R103 and to ground and increase the current of the supply to 1 amp.?? RYlOl should trip.?? If the relay does not trip, adjust
Rl03 until the relay trips at about 1 amp. When adjusting the overload relay, reset and check the overload current several times to get an average
setting.? RI03 should be about 3-4ohms.
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If you download a copy of the 3K-A schematic and manual it explains how the relay works and how to calibrate the trip point.
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In my opinion it would have been better served as a trip for the grid current.
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Other amps and devices may have also used this method for protection.
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73
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Walt
KR4HV
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