Controls & Transformers

Questions and Answers

KVA demand is a measure of the actual instantaneous demand for power placed on the electrical delivery system of the plant during the weld cycle. The KVA Demand in turn is used to calculate the KVA rating at 50% duty cycle. Transformers are commonly rated at 50% duty cycle.
Below is an example of a calculation of KVA demand for an application using a 70 KVA AC transformer to determine if it will be adequate?

Resistance welding transformers are very robust. They can operate for years without any problems. When they fail it is catastrophic. A short between the primary and secondary coils is one method of failure. This occurs when the insulation between these coils fails. It is a very thin separation between the primary and secondary coils. Any damage to this thin layer can lead to a catastrophic short between the two coils. To prevent this from on the replacement transformer the cause for the short must be sought and a plan to prevent in the future put into place.

Transformer with Labels                                             Transformer Sketch

Transformer Coils Assembled showing primary and secondary coils.                                Simpolified Sketch of The Power Components 

The transformer turns ratio in resistance welding reflects the number of secondary coil turns versus the number of primary coil turns. In AC transformers, there are many turns in the primary. The secondary has one turn. If there are 50 turns then the ratio is 50/1. Therefor the primary voltage will drop from 220 V and 100 amp input to 220/50 = 4 volts in the secondary. The equations for this are:

Duty cycle is a measurement of the % of the time that the transformer is conducting current during one minute. This value is used to insure that the electrical components are not operating above their thermal capability. Resistance welding transformers are rated at a 50% duty cycle. Each application may operate at different duty cycle up to and including 100% continuous.


Duty Cycle AC


Duty Cycle MFDC

Reference:  RWMA Manual Chapter 19

                   Roman Manufacturing Inc.

Resistance welding transformers can have very high demand for a very short period of time. Installation of correctly sized primary conductors is imperative for the machine to run properly. The proper procedure to size these conductors for AC equipment is:

Transformer Labeled Sketch

When the duty cycle of the machine is known
1. Calculate the duty cycle
2. Determine the weld current
3. Divide the weld current by the turns ratio of the weld transformer. This is the Primary Demand Current.
4. Multiply the Primary Demand Current by the square root of the duty cycle. This is the Primary Effective Continuous Thermal Current (ECTC).
5. Refer to the Ampacity Chart of the cable type to be used. Select the size based on the Primary Effective Continuous Thermal Current (ECTC).

If the Duty Cycle or Weld Current is not known or this is a general purpose machine:
1. Divide the transformer nameplate KVA by the primary voltage.
2. Multiply that result by the square root of 0.5 (RWMA transformers normally are rated at 50% Duty Cycle)
3. The result is the Primary Effective Continuous Thermal Current (ECTC).
4. Refer to the Ampacity Chart of the cable type to be used. Select the size based on the Primary Effective Continuous Thermal Current (ECTC).

Ampacity charts are based on the National Electric Code (NEC). There may be other state, local or company standards that must be followed.

Reference: National Electric Code
                   RWMA Manual Chapter -21.7

                  Roman Manufacturing Inc.

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