TWI Frequently asked questions
There are three main groups of electrodes for MMA (SMA) welding according to the flux coating, namely cellulosic, rutile and basic. There are no special power source requirements for any particular groupof electrodes although some power source manufacturers claim superior running characteristics with cellulosic electrodes. If recommended by the manufacturer, particular brands of electrodes can be operated with AC or DC power sources.Note, not all DC electrodes can be operated on AC but AC electrodes can usually be used on DC.
For AC operation, the no-load voltage is important and some electrodes, particularly the basic types, may require 70 to 80V (AC).
The welding current level is determined by the size of the electrode and the operating range for typical electrode sizes are given in Fig.1. As a rule of thumb, when choosing a suitable power source of adequate capacity, anelectrode will require approximately 40A per millimetre (diameter). As an example, for the commonly used electrode size of 4mm, with an operating range of 140 to 180A, the power source should have a current capacity of at least 200A atan appropriate duty cycle ie, 20%, 35% or 60%, depending on the type of work.
Fig.1 Welding current for a range of electrode diameters
Extracted from an article entitled 'Let's get technical - choosing an arc welding power source' by W Lucas and G Melton, published in Welding & Metal Fabrication, Vol.67, No.4, May 1999, pp.18-21. (Publishers - dmg BusinessMedia Ltd, Redhill, Surrey, UK - http://www.dmg.co.uk/dmgbm/ )
For further information contact arc@twi.co.uk
