One of the most often misquoted and misunderstood terms in the welding industry is the term “duty cycle”. Hopefully we can clarify this term below
Industrial welders are rated on a duty cycle or duty factor basis. There may be some “hobby type arc welders” which are sometimes quoted in electrodes per period.
The duty cycle or duty factor is the ratio for a given time interval of the on load duration to the total time.
(The European norm EN 60974-1 states that the time period for one complete cycle is 10 minutes).
So in the case of a Jasic JM-352 which has a duty cycle of 350 amps at 40% this would mean the output of 350 amps could be maintained for 4 minutes after which a rest period of 6 minutes should be observed. This would represent a complete cycle. It should however be noted that using the machine for say four periods of 1 minute at 350 amps in the 10 minute period also represents the same 60% duty cycle.
Manufacturers that manufacture to the European norm EN 60974-1 often quote a number of duty cycle ratings on the rating plate.
For example, the Jasic MIG-500P is quoted at:
Duty cycle % Output Current
100 370A
60 500A
A 100% rating is the continuous rating for the machine and it can be seen that as the duty cycle % increases the current decreases.
Important points to consider when selecting a machine with a suitable duty cycle are:
1. The European norm EN 60974-1 states that welding power sources should be capable of delivering their rated output when the ambient air temperature is within the range of -100C to +400C. Consider that the normal temperatures in the UK fall into this band with good margins.
2. The thermal trip will not operate in accordance with duty cycle ratings.
3. When a machine is put into use it will be at ambient air temperature and may take several hours bring it to “working temperature” before encountering thermal trip problems. It is during this period that the duty cycle is often ignored and heat builds up within the machine.
4. The duty cycles achieved by most manual welding processes are often quite low, for example, MMA 10-20% MIG 30-40% TIG 20-25%. These figures should be considered when selecting equipment.
5. Often a machine is selected which requires operating parameters that are within the range of the machine. But care should be taken to also ensure the machine construction is of suitable design i.e. mechanical components, switching requirements etc.
Estimating duty cycles
To make a quick estimation of the duty cycle of a machine other than the stated rated outputs.
The following formula can be used:
Dn = ( I/In)2 x D
Where: Dn = Required Duty Cycle [%] I = Rated Current at the required Duty Cycle [A] In = Maximum Current at the required Duty Cycle [A] D = Rated Duty Cycle [%]
Example:
Using Power Source rated at 200A at 60% Duty Cycle. The power source is operating at 250A what will be the duty cycle % ? Dn = (200/250)2 x 60 Dn = (0.8)2 x 60 Dn = 38.4%
To make a quick estimation of the current of a machine at a specific duty cycleother than the rated one the following formula can be used:
Im = I x√(D/ Dn) Im = Maximum Current at the required Duty Cycle [A] I = Rated Current at the required Duty Cycle [A] D = Rated Duty Cycle [%] Dn = Required Duty Cycle [%]
Example:
The maximum output current at a duty cycle of 100% will be calculated by: Im = 200 x√(60/100) Im = 200 x 0.775 Im = 155A
Please note The resultant figure is approximate but will give a good guide. If the formula is used with inverter unit this formula may not provide accurate results due to the machine designs and constraints.