# 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 Vmax . 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 Pi = V2g ie g = pi/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 Pc = I2R, Total
Resistance R = Pc/I2
Short circuit Impedance Z = V/I
Total leakage reactance X = √(Z2 – R2)

## 3 thoughts on “The Autotransformer and Transformer Parameters”

1. NAME NNAMANI CHIJIOKE
REG NO ESUT/2009/102203
DEPT. E.E.E
A transformer is a device which consists of two or more coils placed so that they are linked
by the same flux.
they are used to step up or step down voltage.
PROPERTIES OF AN IDEAL TRANSFORMER
permeability of the core is infinity
the resistance of the winding is zero
the flux varies sinusoidally in the core with infinite permeability of the core.
EFFICIENCY OF A TRANSFORMER
efficiency=output/input
the loss in a transformer is limited to the following..
-copper losses in resistance at windings which is variable with load current.
-DI-electric losses in the insulation
this is the insulating materials is applicable only in the high voltage transformers.

2. EARTHING Earthing is electric connection to general mass to the earth,whose dimension are very large in comparism to the electrical system being considered. THE FOLLOWING ITEMS SHOULD BE FARTHED; 1.all metal works of wiring system. 2.exposed metal works at all apparatus . 3.one part of the secondary winding and on current carrying part of the metal work of any transformer.PARTS OF FARTHING SYSTEM. 1.Earth continuity conductor:this is a cable or conductor which the metal work is to be connected and which itself is connected to the earthing leads by way of the consumer earthing terminal. 2.The consumers earthing terminal. 3.The earthing leads:this is the final conductor by which connection is made between the consumers earthing terminal and earth electrode or its equivalent. 4. Buried electrode:one or more hards drawn bare copper wrought are deriven vertical into the ground sorrounding the building as dimp as possible.thanks and God bless you for your fatherly advices and care.Esut/2008/94355

3. Name: ONAH KINGSLEY UCHENNA Reg: No Esut/200/94443 Dept Elect/Elect Engineerig Course 454 Topic There are 3 main types of Bues (1) Referece – bus is a bus in which other bus calculation are refer to it has all information on it. No calculation are carried out on it but all calculation are carrind out with reference to it. (2) Voltage controlled Bus- is a bus where the voltage is kept constant, there must be a generation connected to this bus to sustain voltage at there level. (3) Long Bus- They are numerios in o bus system. Long is anything that can be connected to a power system. We have two types of load (1) Static load (2) dynamic load. * Static load are load that are static. * Dynamic load are load that dynamic because it is resistive and inductive: