Welder Duty Cycle Calculations, and Thermal Stress

A Welders duty cycle rating is an assigned figure of merit for sustainability of output current. It is expressed as a percentage of weld time over a 10 minute period (where 100% Weld On time equals 10 minutes) at a given output current. A formula for calculating the Weld Output Current at different Duty Cycles is shown.

A Welders duty cycle rating (Specification) is an assigned figure of merit for sustainability of output current. It is expressed as a percentage of weld time over a 10 minute period (where 100% Weld On time equals 10 minutes) at a given output current. For example, let's consider the specification: 300A @ 40% duty cycle. A 40% duty cycle represents 4 minutes out of a 10 minute period that the Welder can operate. For the other 6 minutes, the Welder must be allowed to cool down. If not, the Welder will shut down and light an over temperature LED, or display a Thermal Error code. If this occurs the Weldor (a person who Welds Metal) has to stop production and wait for the Over temperature condition to clear. The more basic Welders (Transformer/ SCR based Welders) usually incorporate two bi-metallic strip type Thermostats (one on the Input Rectifier Heatsink Assembly, and the other on the Output Diode Heatsink Assembly). They are wired in series with either one being capable of open circuiting under heat duress. The more sophisticated Welders (Microprocessor controlled Inverter based Welders) usually incorporate two Thermistors (a type of Resistor with a Negative Temperature Coefficient) to achieve the same outcome, however a more complicated electronic circuit is now involved to measure the Thermal characteristics of the Welder while it is being operated.

When the Welders Specification is exceeded apart from shutting down and displaying a fault condition, the Welders internal Fan should operate in order to bring the temperature of the components Heatsink to a safe operating temperature. This normally occurs at around 158 degrees F (70 degrees C). This is the maximum tolerable temperature rating for most commercial grade electronic components. Mounted on the Heatsinks are either (or combinations of) Rectifier Diodes, SCR's, IGBT's and MOSFET's. If the Fan stops working or the Temperature Sense circuit fails, these components can be degraded or permanently damaged due to thermal stress.

Having to wait to complete a job due to a low Specification or an Over temperature fault is non productive and frustrating for all concerned. This is particularly so when on a project deadline, as time is money! When an electronic component fails due to thermal stress the Weldor's anxiety just increased by a tenfold factor. Now the Welder is totally inoperative until some trouble shooting can be performed. Once again...time is money!

When a Welders current is specified at 100% duty cycle the Weld Output Current can be maintained indefinitely, without the need for Welder cool down requirements.

Manufacturers publish specifications for their Welders in Operator Manuals, Service Manuals, and in Technical Sales Brochures. Unfortunately, the Weld Output current isn't always shown at a 100% DC rating. A conversion formula comes in handy under these circumstances.

To calculate the output current based on the duty cycle specification use the following formula:

I out = √ [((I rated) 2 × (DC Spec rated)) ÷ (DC Spec needed)]

From our previous example of 300 A @ 40% DC, we can now easily calculate the Weld Output Current for 100% DC, as follows:

I out = √ [((300) 2 × (40)) ÷ (100)]

I out = 189.74 Amps.

As you can see, the output current at 100% Duty Cycle is quite a bit smaller than the claimed 300 Amp rating of this Welder. Duty Cycle makes a difference!