What is Open Circuit Test or No-Load Test?

What is an open circuit or no-load test?

The Open circuit test is used to find the performance characteristics of the transformer at no-load condition. This test should be conducted at rated flux condition i.e, rated voltage and frequency.

Why does Open Circuit Test Need?

  • To find out the core loss or constant loss at rated voltage and rated frequency in the transformer.
  • To find out the shunt branch or no-load branch parameters such as Ro and Xm.
  • To find out the turns ratio of the transformer.

How to do Open Circuit Test:

An open circuit is carried out at a rated voltage on low voltage winding side with the open-circuited high voltage winding. It is carried out on a low voltage side because the rated voltage can be easily possible on low voltage side by varying the autotransformer.

open circuit test
open circuit test

A voltmeter, an ammeter and wattmeter are connected on a low voltage side. All three instrument readings are recorded. No-load or exciting current is only 2-5% of full load can be measure with the greater accuracy than the accuracy of high voltage side. Wattmeter reads core losses of the transformer because no-load losses are very less.

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Equations to find out the parameters:

Pi = Voc x Io x Cosφ


Pi = core loss

Voc=open circuit voltage

Io no-load current

Cosφ = No load power factor

Hence, the No-load Power factor Cosφ0  = Pi / (Voc x Io)

From the phasor diagram,

open circuit test phasor diagram
open circuit test phasor diagram

Ii = Iw

Here Ii = Io x cosφo

Im = Io x sinφo

Core loss resistance Rc = Voc / Ii

Magnetizing reactance Xm = Voc / Im

Open Circuit Test Video Explanation English:


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