The Autotransformer And Transformer Parameters

The Autotransformer

This differs from the ordinary transformer in that the winding of the autotransformer are electrically connected as well as being coupled by mutual flux.

There is a type of autotransformer called a Variac (VARIAC) which is used to regulate the voltage applied to a particular device from ‘0’ to a certain maximum value V_max . In the starting of induction motors, variacs are used to start IM (Induction Motors) of rating 5hp and above. This is in order to limit the high surge of current (Starting Current) which usually occurs when the motor is started direct-on-line. Induction Motors of Low horse powers hp<5 are usually started direct-on-line.

Determination of Transformer Parameters

a)      Open Circuit Test: The high voltage winding is on open circuit and the low Voltage winding is connected to a variable voltage supply, at normal frequency. The primary is assumed to be the low voltage winding.

The input current and power and the voltage across the open circuit winding are measured for a range of applied voltages up to 125% of the rated voltage.

Then at rated voltage,  

Iron Losses P_i = V²g ie g = p_i/V²;

Open circuit admittance y = I/V,

Magnetising Susceptance b = √(y² – g²)

b)      Short Circuit Test : The low voltage winding is short circuited through an ammeter and the high voltage winding is connected to a variable voltage supply at normal frequency. The high voltage winding is assumed to be the primary winding.

The input current and power and voltage across the high voltage winding are measured for a range of short circuit currents up to 125% of the rated current.

Cupper loss P_c = I²R, Total

Resistance R = P_c/I²

Short circuit Impedance Z = V/I

Total leakage reactance X = √(Z² – R²)