Transformer Inrush Current Calculator

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What is Transformer Inrush Current?

Transformers experience inrush current when powered. It lasts milliseconds to seconds and can be 5 to 10 times the full-load current. 

Inrush current is mostly caused by transformer core magnetization from residual flux or demagnetization.

The current flows quickly. Inrush currents rapidly heat equipment, puncturing coils, damaging insulation, and possibly causing failure.

Higher inrush current for rotating machinery than static. 

Ex: Motors draw more current than transformers for the same capacity.

Causes of Transformer Inrush Current

Transformer inrush current happens when a transformer is activated (turned on), particularly when there is residual flux in core. 

The primary causes are:

• Magnetism left in core after the last operation.
• Energizing the transformer at peak of AC waveform can result in a large magnetizing force.
• If the core rapidly saturates, the impedance decreases and the current increases.
• Low primary impedance in winding allows for higher surge current.
• Larger transformers have a higher inrush because of the magnetic core’s increased energy storage.

Effects of Transformer Inrush Current

1. High magnetizing inrush current in transformers needs the oversizing of breakers or fuses.
2. Distortion and noise are injected back into the primary power source.
3. Arcing (electric current jumping from the one connection to another) and the failure of circuit components such as switches.
4. The fuse’s action was interrupted.
5. The system suffers from current disturbance and harmonics, as well as poor power quality.
6. Irregular voltage distribution across transformer windings.
7. Mechanical & electrical vibrations occur along the transformer windings.

Formula

I_Inrush = (√2 x V)/R

I_Inrush = (1.414 x V)/R

Where

I_Inrush – Inrush Current (A)

V – Input Voltage (V)

R – Winding Resistance (Ω) 

Solved Example

A transformer has an input voltage of 230 V & a winding resistance of 0.5 Ω. Calculate the inrush current that would flow if the transformer is suddenly energized.

Given

Input Voltage V = 230 V

Winding Resistance R = 0.5 Ω 

Formula

I_Inrush = (√2 x V)/R

I_Inrush = (1.414 x V)/R

Solution

I_Inrush = (1.414 x 230)/0.5

= 650.44 A

Answer

I_Inrush = 650 A

How to measure Transformer Inrush Current?

A clamp meter can be used to determine the inrush current in the transformer. 

A clamp meter monitors current via a device without requiring physical contact. 

The non-contact feature provides a safe working environment for measuring extremely high currents, which would otherwise be dangerous to the user.

How to reduce Transformer Inrush Current?

• To reduce transformer inrush current, consider using controlled switching (point-on-wave switching).
• Use circuit breakers to energize the transformer at voltage zero crossing point.
• Helps to prevent flux saturation and reduce inrush.
• Temporary resistors introduced during energization lower the voltage applied between the transformer windings.
• To reduce the initial current on smaller transformers, utilize devices such as NTC thermistors (or) soft starters.
• Demagnetizing the core before switching can remove any residual magnetism. Some relays and controllers enable automatic core demagnetization.
• To limit the rate of rise of current, connect a small reactor (inductor) in series with a transformer.
• To reduce peak inrush in transformers with numerous windings or phases, activate each phase progressively.