TWI Frequently asked questions
The duty cycle is the fraction or percentage of a ten-minute cycle that a power source may be used without overheating. On the rating plate, Fig.1, there is normally a table of duty cycle against weldingcurrent, with the corresponding conventional load voltage. The load voltages are standardised to give maximum and minimum values which should allow the equipment to be used at the rated current for most consumables and shieldinggases.
Light duty equipment, such as for occasional repair and maintenance, will typically specify duty cycles of between 8% and 60%. As an example of the importance of knowing the likely duty cycle for the equipment, the maximum currentfor a light duty MIG (GMA) equipment can vary from 150A (8%) to 55A at 60% duty cycle.
Industrial MMA (SMA) and MIG (GMA) equipment should be rated at a minimum of 60% duty cycle, but for light duty TIG (GTA) and MMA (SMA) welding, a lower duty cycle will normally be sufficient.
With small hobby type MMA (SMA) transformers, a duty cycle figure is not given. Instead, the rating is based on the number of electrodes that can be burnt in one hour, both when welding from cold and when the equipment has warmedup.
Fig.1 Rating plate
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
