The unsaturated synchronous impedance and value of the saturated synchronous impedance can be obtained from the open-circuit and short-circuit tests. The effect of saturation on the performance of synchronous machines is taken into account by means of the magnetization curve and other data obtained by tests on an existing machine. Only some basic test methods are considered.

In the case of a constant voltage source having constant impedance, the impedance can be found by dividing the **open-circuit** terminal voltage by the **short circuit** current.

## Open-Circuit Test

At the point when the impedance is a component of the open-circuit voltage, as it is the point at which the machine is soaked, the open-circuit trademark or polarization bend notwithstanding the short out trademark is required.

The unsaturated coordinated reactance is consistent on the grounds that the hesitance of the unsaturated iron is insignificant. The proportionate circuit of one period of a polyphase simultaneous machine is appeared in Fig for the open-circuit condition and for the short out condition. Presently E_{af} is the equivalent in the two situations when the impedance Z_s. Where is E_{af} the open-circuit volts per stage and I_{sc} is the short out current per stage.

**Open-circuit Characteristic**

To get the open-circuit trademark the machine is driven at its appraised speed without load. Readings of line-to-line voltage are taken for different estimations of field current. The voltage aside from in exceptionally low-voltage machines is ventured somewhere near methods for instrument likely transformers.

Two arrangements of scales have appeared one, line-to-line volts versus field current in amperes, and the other** per-unit open-circuit voltage** versus per-unit field current. On the off chance that it was not for the attractive immersion of the iron, the** open-circuit** trademark would be direct as spoken to by the **airgap line** in Fig. It is critical to take note of that 1.0 per unit field current compares to the estimation of the field current that would create the evaluated voltage if there were no immersion. Based on this show, the per-unit portrayal is, for example, to make the **air-hole lines** of every simultaneous machine indistinguishable.

## Short Circuit Test

The three terminals of the armature are **shortcircuited** each through a current estimating circuit, which aside from little machines is an instrument **current transformer** with an ammeter in its auxiliary. A chart of associations where the current transformers are excluded has appeared in Fig.

The machine is driven at roughly coordinated (evaluated) speed and estimations of armature hamper are made for different estimations of **field current**, for the most part up to and to some degree above appraised armature current.

**Short-Circuit Characteristic**

The short out trademark(**short circuit characteristics)** for example** armature cut off versus field current** is appeared in Fig. In traditional simultaneous machines, the short out trademark is for all intents and purposes straight line of the fact that the iron is **unsaturated **up to appraised armature current and to some degree past, in light of the fact that the attractive to make of the armature and the field for all intents and purposes agree (if the armature had zero opposition the attractive to make would be inaccurate arrangement), and the field and armature** mmfs **contradict one another.

## Unsaturated Synchronous Impedance Of Synchronous Machine

In Fig, the **open circuit and short-circuit** features are represented on the same graph. The **field current** “oa” on the air-gap line generates a **line-to-line voltage **“oc” which will be the **open-circuit voltage** if there is no saturation. The same field current value produces the “o’d” armature current, and the unsaturated synchronous reactance is provided by:

X_d=\frac { oc }{ \sqrt { 3 } o^'d } \Omega phse , for star connected armature

If the **open-circuit characteristic**, **air-gap line**, and **short-circuit characteristics** are plotted in per-unit, then the per-unit value of unsaturated synchronous reaction is equal to the **per-unit voltage** on the air-gap line resulting from the same field current value as the one-unit rated armature current.