Mains power transformer non-idealities

They are heavy (because of their iron core) and can be very large. The power transformers are design to operate best with an almost constant load which is equal to their rating. The maximum efficiency being designed to be at full load.

Actually, all the transformers are the same that is same design formulae apply for small signal transformers to the power transformers. In case of power transformers, the designer needs to consider: voltage, current, operation frequency (50 Hz or 60 Hz normally), resistances on the coil wires and to remember that heat will need to be dissipated without any part over-heating.

Transformer inrush current describes a spike in current that occurs when you initially turn on your transformer. This spike can be normally up to 10 times higher than normal current. Inrush current in a transformer can cause several problems. Not only does it interfere with the operation of circuits, but it could result in detrimental effects to the transformer. The distortion of the volt or current waveform, known as harmonics, is another side effect of inrush current. If not properly managed, inrush current could lead to failure of circuit components, shorten the operating life of the transformer, or even cause damage.

Understanding Transformers Part 1: Inrush, Saturation and Fusing

We test a transformer using a Rigol 1054z scope and a 120 volt transformer to see what current inrush we get due to saturation, and explain transformer characteristics and fusing considerations

Understanding Transformers Part 2: Developing a Simulation Model

We show how to develop a simulation model of an iron core transformer by bench testing and measuring, as well as by analyzing physical characteristics. We develop a linear model in LTSpice and also a nonlinear model, including saturation effects, and compare simulation results with oscilloscope bench tests.

Understanding Transformers Part 3: Improving the Transformer Model with Bench Testing

We do some actual bench testing on our 120VAC transformer by applying up to 230VAC (using our Morphon variable transformer) and measuring the current flow using our Rigol DS1054Z oscilloscope and two multimeters. We then put that model into our transient simulator software (you can use LTSpice or Matlab & Simulink) to verify our model matches reality.

Transformer Inrush in 5 minutes

A brief description of transformer inrush.

What Is Inrush Current And Why Do I Care?

Fluke Clamp Meters: What Is Inrush Current And Why Do I Care?
Featuring Fluke 370 Series and Fluke 381

Transformer Inrush Current: Limiting a 40VA Transformer


  1. Tomi Engdahl says:

    01821 Workbench 101 #2 How does a Dim Bulb Current Limiter Work

    In the first part of the video I go over how a dim bulb current limiter works and why you do not want to make one out of resistors. In the second part I do some practical demos of how a dim bulb works, the amperage differing wattage of bulbs let in, the difference between dim bulbs in parallel vs series and how it acts as a limited surge suppressor.

  2. Tomi Engdahl says:

    01021 Workbench Safety 101 #1 Variac and Isolation Transformer Hookup

  3. Tomi Engdahl says:

    Let’s Experiment with an Electromagnetic Chain!

    Have you ever seen an electromagnetic chain? Its a series of electric and magnetic toroid cores linked in the form of a chain transferring power from electricity to magnetism and back again as it works it way along the device. Its a great hands-on illustration of Maxwell’s equations as well as an illustration of transformer action taken to extremes.

  4. Tomi Engdahl says:
    That’s a step down transformer.
    Yes you can do it but need to exercise some precaution: The LV winding that was intended by design to be the secondary winding, will serves as the primary & the value of the magnetizing inrush current actually will be greater than expected. When a transformer is reverse fed, the taps move to the output side and so their operation is reversed. Taps will control the output voltage so chances of over excitation will be there. This is not serious concern till the input voltage variation is within limit. OC voltage of the winding which is supposed to be the secondary will be higher than the nominal voltage. This is to allow for the drop in the winding so that the nominal voltage will be present when the transformer is loaded fully. The degree of change in the secondary voltage is expressed as the transformer’s regulation; the lower the figure the better. Using the transformer in reverse you need to increase the applied voltage to the secondary which will act as primary when reverse fed.

  5. Tomi Engdahl says:

    Custom Coils has a good explanation of different core materials and their characteristics:


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