Electrical Test and Electric Motors

A transformer is a device that transforms voltage from one level to another. They are widely used in power systems. With the help of transformers, it is possible to transmit power at an economical transmission voltage and to utilize power at an economic effective voltage.

Transformer working is based on mutual emf induction between two coils, which are magnetically coupled.

When an AC voltage is applied to one of the windings (called as the primary), it produces alternating magnetic flux in the core made of magnetic material (usually some form of steel).

The flux is produced by a small magnetizing current which flows through the winding. The alternating magnetic flux induces an electromotive force (EMF) in the secondary winding magnetically linked with the same core and appears as a voltage across the terminals of this winding.

Typically, the coil connected to the source is known as the primary coil and the coil applied to the load is the secondary coil.

SINGLE PHASE TRANSFORMER

A single-phase transformer consists mainly of a magnetic core on which two windings, primary and secondary, are wound. The primary winding is supplied with an AC source of supply voltage V1.

The current I, flowing in the primary winding produces flux, which varies with time. This flux links with both the windings and produces induced emfs.

The emf is also induced in the secondary winding due to this mutual flux. The magnitude of the induced emf depends on the ratio of the number of turns in the primary and the secondary windings of the transformer.

The ratio of the primary potential to the secondary potential is the ratio of the number of turns in each and is represented as follows:

N1/N2 = V1/V2

A step-up transformer increases the output voltage by taking N2 >N1 and a step-down transformer decreases the output voltage by taking N2 <N1.

When the transformer is loaded, then the current is inversely proportional to the voltages and is represented as follows:

V1/V2 = I2/I1 = N1/N2

EMF equation of a transformer:
rms value of the induced emf in the primary winding is:

E1 = 4.44 × f × N1 × Ø_m

rms value of the induced emf in the secondary winding is:

E1 = 4.44 × f × N2 × Ø_m

Where:
N1 = Number of turns in primary
N2 = Number of turns in secondary
Ø_m = Maximum flux in core and
f = Frequency of AC input in Hz.

IDEAL TRANSFORMER

• No loss or gain of energy takes place.
• Winding has no ohm resistances.
• The flux produced is confined to the core of the transformer, which links fully both the windings, i.e., there is no flux leakage.
• Hence, there are no I ²R losses and core losses.
• The permeability of the core is high so that the magnetizing current required to produce the flux and to establish it in the core is negligible.
• Eddy current and hysteresis losses are negligible.