DUTY CYCLE RATING
Since, arc welding power sources are delivering high currents for welding; the components of such a system are bound to get heated. In order to have increased life of such components the heating should be limited. This is done by specifying what is called as "Duty Cycle". The duty cycle is explained as a percentage and at a particular current level.Example: 60 % duty cycle at 400 A This specification says that 400 A welding current can be used continuously upto 60 % of a base period. The base period for duty cycle calculations in many of the standards is five minutes.60 % of five minutes is three minutes. Which means 400 A welding can be done without break for three minutes and for two minutes the power source should be allowed to cool down. Fig. shows the duty cycle rating.
If the same power source is used to deliver 500 A welding current then to avoid over heating of power source components the duty cycle has to be reduced.
Since the heating is proportional (I2R) the duty cycle rate for different currents are related by this simple formula.
I^{2}_{1}T_{1}=I^{2}_{2}T_{2}
If we now want to calculate the duty cycle for 500 A then,
= (400 / 500)^{2} X 0.6 = (4 / 5)^{ 2} x 0.6 = (16 / 25) x 0.6 = 38.4 %
which means 500 A current can be used for 1.9 minutes without break and the power source should be allowed to cool for 3.1 minutes before next welding. For automatic welding applications the power source should be rated for 100 % duty cycle (continuous current) at the desired welding current.