Specifications

MedWeld 3000 Catalog Revision 8
July 2002 Page 67 of 72
RULES FOR SIZING ISOLATION CONTACTORS
By Jack Farrow, WTC R&D Center, February 7, 2001 Copyright, 2001 by WTC
FOR WTC RESISTANCE WELDING CONTROLS
DISCLAIMER:
The purpose of this document is to assist in selecting the proper size isolation
contactor for a given welding application. This document does not constitute any
recommendation, warranty or obligation on the part of Welding Technology
Corporation or any of its employees, representatives or distributors. The user of this
information assumes all responsibility for proper use of the information. The
information in this document is presented as-is; no warranty of correctness or
suitability for any application is given or implied.
The calculations in this document have been simplified and are designed for the most
common situations. If the application is very unusual, for example a system with
large welding transformers above 400 KVA, a system with more than six welding
transformers, or a system with a very low welding duty cycle (below 1%), competent
engineering help should be sought to size the isolation contactor.
ASSUMPTIONS:
The process to select the correct size of an isolation contactor consists of two parts.
Part 1 calculates the average primary current draw of each welding transformer in the
system. Given the average primary current draw of each welding transformer in the
system and how they are connected, Part 2 selects the appropriate size isolation
contactor.
This selection guide assumes the following things:
The isolation contactors are rated in amperes by UL general purpose current, which is
a continuous-duty current.
The isolation contactors are not opened or closed while weld current is flowing.
The KVA ratings on the nameplates of the welding transformers are in accordance
with the RWMA standards for resistance welding transformers.
The power line voltage is between 420 and 500 VAC nominal.
All calculations here apply to SINGLE-PHASE AC welding transformers only.