Electrical Test and Electric Motors

1. Based on construction:

Core type: Windings surround a considerable part of the core.

Shell type: Core surrounds a considerable portion of the windings.

2. Based on cooling:

Oil-filled self-cooled: Small- and medium-sized distribution transformers.

Oil-filled water-cooled: High-voltage transmission line outdoor transformers.

Air Cooled type: Used for low ratings and can be either of natural air circulation (AN) or forced circulation (AF) type.

3. Based on application:

Power transformer : These are large transformers used to change voltage levels and current levels as per requirement. Power transformers are usually used in either a distribution or a transmission line.

Potential transformer (PT): These are precision voltage step-down transformers used along with low-range voltmeters to measure high voltages.

Current transformer (CT): These transformers are used for the measurement of current where the current-carrying conductor is treated as a primary transformer. This transformer isolates the instrument from high-voltage line, as well as steps down the current in a known ratio.

Isolation transformer : These are used to isolate two different circuits without changing the voltage level or current level.

Some points about transformers:
• Used to transfer energy from one AC circuit to another
• Frequency remains the same in both the circuits
• No ideal transformer exists
• Also used in metering applications (current transformer, i.e., CT, potential transformers, i.e., PT)
• Used for isolation of two different circuits (isolation transformers)
• Transformer power is expressed in VA (volt amperes)
• Transformer polarity is indicated by using dots. If primary and secondary windings have dots at the top and bottom positions or vice versa in diagram, then it means that the phases are in inverse relationship.

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.