Controls & Transformers

Questions and Answers

The transformer tap switch of a resistance welding transformer is installed on the windings of the primary side of the transformer. By changing the tap switch one is changing the turns ratio of the transformer. When the turns ratio changes it will change the voltage in the secondary coil. Tap 1 will produce the lowest secondary voltage and the highest tap will produce the highest secondary voltage and potentially the maximum KVA rating of the transformer.

See article:
DO I NEED TO INSTALL A SERIES BAR TO MEASURE THE TURNS RATIO OF A RESISTANCE WELDING TRANSFORMER?

Tap switch

                   PRESS WELDER TAP SWITCH

 

As a suggestion to get the best performance:

Tap Setting Rule of Thumb

To see other articles on this subject do a search on transformer taps in this blog.

 

References: RWMA – Resistance Welding Manual 4th Edition

This answer to this question is included in the answer of another article:
HOW DO YOU SIZE THE PRIMARY CONDUCTORS FOR AN AC RESISTANCE WELDING TRANSFORMER?
It builds upon the formula:

Power in vs Power Out

 

In this instance we have a 250 KVA AC machine transformer with a high tap of 25.8 volts and a low tap of 12.9 volts.
All of this depends upon not overheating the transformer Therefore we must operate at no more than 50% duty cycle. The above article explains how to insure the transformer is operated under safe conditions:
If the Duty Cycle or Weld Current are 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.

 

Wire is used in some non-resistance welding transformer applications. The design of transformers used in resistance welding can be very demanding and must be designed and built accordingly. This blog does not get involved with machine design and build subjects. Contact your local machine sources or distributors for information and assistance.

Reference: RWMA Resistance Welding Manual 4th Edition

Transformer turns refers to the ratio of the number of turns in the primary versus the secondary of the transformer. This is discussed in other articles in this blog listed below. Many transformers are designed with tap switches which allow one to change the effective number of turns in the primary side of the transformer. This changes the turns ratio either up or down depending upon the direction of the change (up or down). In turn the voltage increases or decreases which will increase or decrease the amperage in the secondary. The result is that the tap change has increased or decreased the total power available in the secondary depending upon the tap chosen.

 Transformer Tap Vs Voltage

 Tap Setting Rule of Thumb

Other articles on Turn Ratio in this Blog:

WHAT IS THE TURNS RATIO FOR A SPOT WELDER?
CAN CHANGING THE TURNS RATIO OF A RESISTANCE WELDING TRANSFORMER CHANGE THE WELDING CURRENT?
WHAT IS THE TURNS RATIO FOR A RESISTANCE WELDING TRANSFORMER?

Reference: RWMA - Resistance welding Manual 4th Edition

The answer to this question allows sizing conductors for the machine and the primary buss feed to the machine and transformer. The actual amperage out of an AC transformer depends upon the secondary itself. That will not be addressed here. Here we will address input for a given amperage output.

The first is called KVA Demand. The second consideration is Primary Effective Continuous Thermal Current (ECTC).

These topics are covered fully in two other articles:

WHAT IS KVA DEMAND?

HOW DO YOU SIZE THE PRIMARY CONDUCTORS FOR AN AC RESISTANCE WELDING TRANSFORMER?

 KVA Demand

 

The instantaneous KVA Demand is 196 KVA

The KVA Demand@ 50% Duty Cycle is 60 KVA

The transform is rated at 70 KVA and is adequate for this job.

Apply the above calculations to your situation with 64 KVA and calculate the primary conductor size for the finalized information.

 

Reference: RWMA Resistance Welding Manual 4th Edition

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