Transformer Current Ratings


Magnetic Particle Inspection (MPI) equipment including bench units and power packs work by generating large currents, normally in the range of 1,000 to 10,000 Amperes. The objective is to use the primary current to produce strong magnetic fields.

To achieve this objective, the primary current is typically used to either:

  • Apply current through components directly which induces a magnetic field in component - This is commonly called a ‘Current Flow’ technique. It is a contact method where the primary current is passed directly into the component. This is normally performed using a bench unit where current is applied between a pair of headstocks or using two pistol prods attached to a power pack.
  • Apply current through a central (or parallel) conductor. This is normally bar which is insulated from the component which the current passes through - This is commonly called a ‘Threader Bar’ or ‘T-Bar’ technique. A magnetic field is induced in the bar and a secondary magnetic field is induced in the component. This is normally performed on a bench unit with a Copper or Aluminium bar.
  • Apply current through a coil (or solenoid). There are various coil designs but the most common are helically wound rigid coils on bench units or performed by wrapping high current carrying cable around a component from a power pack, sometimes described as a flexible coil. There are also Magnet Flow (MF) coils that are mounted inside a headstock and differ from the other coils in that they are typically much lower currents (mA range) but with several or even hundreds as many turns.

Inspection standards and procedures normally detail the current needed for a correctly performed inspection, so achieving the right current with different techniques is critical.

This sounds easy so far - So what is the problem?

When using or purchasing MPI equipment it is common to see equipment specifications that make reference to a maximum current. The mistake is to not appreciate what that rating really means and to not understand the conditions in which that maximum is achieved.

It turns out there are many factors which limit the maximum outputs:

Transformer rating is no reflection of the current obtainable. Due to variation in component length, geometry and electrical inductance as well as heating effects this can limit the output practically available.

The layman’s thought experiment - Small unit with infinite power? No.

Imagine a small bench unit like a Johnson & Allen SBU-1500 that is listed as having a 1500A maximum output. Because the standard SBU-1500 only has a maximum headstock separation of 300mm it will not accept components longer than this normally. For components of this length, achieving 1500A when using a Current Flow technique is realistic.

But now let’s imagine I have a huge crank shaft that is 10 meters long and we extend the SBU-1500 to accept that component. Do you expect the same transformer to achieve 1500A even with the unit turned up to full power? It’s not going to happen…

There are similar thought experiments you can do by imagining a huge diameter coil (with many turns) on a small bench unit or impractically long cables coming out of a small Power Pack.

In fact coils almost always have a lower operating currents than Current Flow methods because the current has to go through much more material (if you can visualise the coil in its un-wound state - It is normally much longer than the component) And because coils naturally have a higher electrical inductance than a similar length straight conductor (where inductance is the property of a material to want to oppose the change in current going through it).

So what information is missing?

A transformers maximum current rating is only half the story - The open circuit Voltage (output) is a measure of the transformers ability to ‘push’ a current through a component in layman’s terms. And this is considered when we design and recommend different MPI equipment to organisations.

So for example, if we compared two 5000A transformers, one witch an open circuit Voltage of 10V and the second with an open circuit Voltage of 15V - The 15V transformer would be more likely to achieve 5000A through a long component or through a large diameter coil.

While the rule does hold true that more Voltage means more ‘push’ there are health and safety regulations that cap the maximum open circuit Voltage at 24V.

Discuss your requirements with our team

There is no one size fits all unit and we hope you can appreciate with so many build variables, selecting a unit with a suitable specification can be difficult - This is why Johnson & Allen’s Technical Sales team are happy to discuss and advise which unit is right for your organisation and their requirements.

So if you have an enquiry or a question please do not hesitate to contact our head office by phone or through our website.

Is bigger really better?

Find out more

Amperes and Ampere-Turns

Find out more