When sizing a Reduced Duty Cycle transformer, there are some important facts to consider:

1.) The On-Off cycle must be considerably shorter than the thermal time constant of the transformer. Thermal time constants for transformers are usually expressed in hours. A 50 KVA, 100% duty cycle transformer has a thermal time constant of approximately 6-8 hours. With an On-Off cycle of less than 60 seconds, you can consider taking advantage such a design.

2.) The Duty Cycle is calculated as follows:
Duty Cycle (As a Percent) = [Cycle On Time / (Cycle On Time + Cycle Off Time)] x 100
Example: On-Time = 6 seconds, Off-Time = 14 seconds, Duty Cycle = [6 / (6 + 14)] x 100 = 30 Percent

3.) The relative KVA size is calculated by multiplying the 100% Duty Cycle KVA size by the square root of the duty cycle.
Example (Single Phase Application):
If the Duty Cycle = 30.0 Percent and the load during the On-Time = 50 KVA,
the Design KVA = 50 KVA x SQRT(0.300) = 50 x 0.548 = 27.4 KVA Design.
For easy calculation, assume the On-Time load is 100 Volts and 500 Amps (50 KVA).
The "Continuous" (100% DC) Design Current is 27.4 KVA / 100 Volts = 274.0 Amps.
Summarizing, a transformer designed to deliver a continuous current of 274.0 Amps
can perform well in providing 500 Amps, operating at a 30.0 percent duty cycle.

If a user is not aware of using a reduced duty cycle transformer, they may inadvertently increase the duty cycle beyond the capability of the transformer. We recently had an automotive parts customer who had been using a 30 percent duty cycle transformer successfully for many years on an induction heating machine. In designing an additional production line, the user desired to reduce the cycle-time and increase the thru-put of the new line. By increasing the duty cycle to 50 percent, they catastrophically destroyed two transformers before determining the root cause for the failure.

Calculations showing the impact of the application are:
"Design" Duty Cycle: 30 percent, On-time = 6 seconds, Off-time = 14 seconds
"Actual" Duty Cycle: 50 percent, On-time = 6 seconds, Off-time = 6 seconds
Design Equivalent Continuous Current (30% Duty) = 500 x SQRT(0.30) = 274.0 Amps
Actual Equivalent Continuous Current (50% Duty) = 500 x SQRT(0.50) = 353.5 Amps

Since copper losses vary with the square of the current, running a transformer at 353.5 amps rather than 288.5 amps results in increased copper losses by (353.5 / 274.0)^2 = 1.66 times.

By increasing the power dissipated in the transformer windings by 1.66 times (66% increase), the result is typically an significantly overheated transformer. If the design temperature rise is 115 degrees Celsius, and the actual rise is 1.66 x 115 = 191 degrees Celsius, the life of the insulation system is seriously compromised.

When properly applied, a reduced duty cycle transformer does save size and money. But when misapplied, the results can damage your equipment. Don´t let inappropriate transformer designs waste your time and money.