### 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.

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.

Equations to find out the parameters:

*P _{i} = V_{oc} x I_{o }x Cosφ_{}*

Where,

P_{i }= core loss

*V _{oc}*=open circuit voltage

*I _{o }*no-load current

*Cosφ _{}* = No load power factor

Hence, the No-load Power factor *Cosφ _{0 } = P_{i} / (V_{oc} x I_{o}) *

From the phasor diagram,

I_{i} = I_{w}

Here I_{i} = I_{o} x cosφ_{o}

I_{m} = I_{o} x sinφ_{o}

Core loss resistance R_{c} = V_{oc} / I_{i}

Magnetizing reactance X_{m} = V_{oc} / I_{m}