Leakage reaction

LEAKAGE REACTANCE

Whenever a voltage is induced in the coil a transformer winding by a changing flux through it, that induced voltage lags behind the flux by exactly 90 electrical degrees. Alsoelectrical degrees. lags behind the flux by exctly through o, since the flux and current are presumably in phase with each other, it follows that the induced voltage lags behind the current by 90 electrical degrees. 

EQUIVALENT RESISTANCE,

REACTANCE, AND IMPEDANCE

When regulation calculations are made for transformers, it is convenient to combine the resistance and reactance drops that actually occur on the primary and secondary sides into a single value of resistance and a single value of reactance.

EQUIVALENT CIRCUIT OF A

TRANSFORMER

Transformer performance under load has neglected the component of no-load current that flows in the primary winding only. This neglect is permissible in standard distribution and power transformers because current is, at most, but a small percent of the load current; when included in the calculations it adds very little accuracy of the final result.

It is also possible to represent a transformer as an ordinary series electric circuit that has three elements.The Equivalent ResistanceThe Equivalent Leakage ReactanceThe Load 

THE SHORT-CIRCUIT TEST

In order to determine experimentally the values of the equivalent resistance, impedance, and reactance, the so-called short circuit test is generally performed. This test is an accurate attempt to make the windings carry rated currents without requiring that he transformer deliver a load; the power input to the transformer will be then extremely low.  

THE OPEN-CIRCUIT TEST

When one side of a transformer is left open-circuited and the other side is connected to source of alternating current whose voltage is rated value, the current will be extremely low about 2 to 10 percent of rated load current.

This no-load current has two components, one producing the normal mutual flocks and the other taking care of the hysteresis and eddy-current loses in the core.

Two component of power loss are developed in the iron. They are:The Hysteresis Loss, which is purely magnetic, and results because the tiny magnetic particles produce a kind of molecular friction as they tend to change alignment with the rapid reversal of alternating current.The Eddy-current loss, which is electromagnetic in character and is caused by the flow of currents in the iron in exactly the same way as in the transformer windings.

REGULATION CALCULATIONS USING SHORT-CIRCUIT DATA

When the short-circuit test is performed upon transformer, it recalled that the impressed voltage is adjusted to permit rated current to circulate in the primary and secondary windings. Since there is no output voltage, it obvious that the voltage, used completely to overcome the equivalent impedance of the transformer, is actually the impedance voltage drop at full load.

EFFICIENCY CALCULATIONS USING SHORT-

CIRCUIT AND OPEN-CIRCUIT DATA

When the secondary of transformer delivers power to a load, an equivalent amount of power is supplied to the primary by the AC source; the power output is generally delivered to a voltage that is different from that of the source. In addition to this useful power input the is transferred to the load by transformer action, the AC source must supply the transformer with sufficient power to the loss

There are two kinds of losses in a static transformer:The copper losses in the primary and secondary windings.The Hysteresis and Eddy –current losses in laminated core. 

MAXIMUM EFFICIENCY

In the analysis of the operation of a transformer as a load increases from zero to rated output and above, it will be noted that the efficiency rises to a maximum and then proceeds to drop off; this is because efficiency calculations for progressively increasing values of load that rise linearly involved two kind of losses, namely, iron losses that are substantially constant and copper losses that increase as the square of the load