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How can I estimate the size of equipment needed for a resistance welding application?

 

Frequently Asked Questions

The main factors to consider in estimating equipment size are mechanical, such as the force capacity, and electrical, such as the kVA rating.

Mechanical factors

In terms of electrode force, the actual force required depends on the materials, their thickness and the weld size required. Recommended values are available in standards and published information for a wide range of applications of both spot and projection welding, but there are some additional factors to remember.

  • Higher forces are required for higher strength steels and coated steels compared to uncoated low carbon steel, for a given material thickness.
  • Higher forces are required for applications where component fit-up is poor and some of the force is required to close gaps between pressed parts.
  • The required force should be achievable at an air pressure, which does not exceed the minimum air line pressure, so that the regulated pressure is not subject to variation due to changes in line pressure.
  • The machine should be sufficiently strong and rigid to avoid undue deformation and electrode skidding under load.
  • Avoid using machines of too great a capacity so that the air pressure must be set at a low value. In such a case, small variation in air pressure would have a proportionally greater effect on the electrode force. Also, the machine characteristics such as friction and head follow-up may affect weldability.

Electrical factors

It is not easy to estimate the kVA rating, which is required for a particular application. The main requirement is to achieve the welding current needed. This must be established by trials with guidance from guideline data or experience on similar applications.

The kVA rating of the transformer is a thermal rating at 50% duty cycle. This defines the power which can be drawn over a long period, with current flowing intermittently for 50% of that time, without overheating a transformer.

Higher power may be drawn for short periods, such as typical weld times, as the duty cycle is usually relatively low. However, the peak current available depends on the impedance of the secondary circuit and the open circuit voltage available at the transformer. The maximum short circuit current may be known for a particular machine, but the welding current available will be much lower. This is because of the added resistance of the component being welded and the associated electrode or tooling. In simple terms, Ohms law applies, so that the voltage available V=IR, the current which can be drawn (I) multiplied by the circuit resistance (R).

The secondary circuit impedance is also influenced by an inductive component when using AC power supplies. This is related to the area within the machine throat. Current is reduced if this area is increased, and even more so if there is steel within the throat. Thus, the arm spacing and routing of jumpers or flexible connections is important in order to minimise the losses. Inductive losses are virtually eliminated when using DC current, such as with inverter welders.

These factors are not easy to calculate but a long reach machine with a relatively high secondary circuit resistance could easily require a transformer of twice the kVA rating, and possibly higher secondary open circuit voltage compared to a compact, small throat machine, to give the same welding current.

When estimating the current required, at least 20% must be added to allow for minor adjustments to the welding condition and to allow for automatic heat control adjustments in controllers which provide constant current control or automatic mains voltage compensation. The maximum available current from a transformer is based on normal mains voltage. The welding transformer voltage would be reduced if the high voltage mains transformer, or the supply lines to the machine, are of low capacity.

The machine manufacturers can normally provide guidance on kVA ratings required based on other application experience.

Further information

Resistance welding of sheet metals - a guide to best practice

For more information please email:


contactus@twi.co.uk