What is the turns ratio for a spot welder?

To answer this question I will demonstrate the calculation of the turns ratio for a AC 16 KVA resistance welding transformer.

The turns ratio for all resistance welding transformers is calculated in the same manner no matter the KVA size. Turns ratio is the ratio of the number of coil turns in the primary vs the secondary.

                                            TURN RATIO Voltage

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 220V and 100 amp input to:

220/50 = 4.4 volts in the secondary. The equations for this are:

                                            TURN RATIO Voltage

                               Vpri = Voltage on the primary                          Tpri = Turns in the primary coil
                               Vsec= Voltage on the secondary                     Tsec = Turns in the secondary coil

                                Transformer Labeled Sketch

                                     Power into the transformer will equal power out of the transformer

                                           TURNS RATIO POWER AMPERAGE 

Isec = Secondary amperage                 Amperage Ipri = Primary amperage

In the secondary using the above formula:
If the Ipri = 100 amperes
If the turns ratio = 50/1
Isec = (Ipri X Tpri)/ Tsec
Isec = 100 x 50/1
Isec = 5000 amperes

There are several additional articles written on turns ratio available in the “HOW TO RESISTANCE WELD” blog.

Enter “Turns Ratio” in the search box on the home page for additional information.

Reference: RWMA Manual Chapter 19




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