Energy Efficiency of Power and Distribution Transformers

Power Transformer:- these are used at the sending and receiving end of a long, high voltage transmission line for stepping up or stepping down the voltage,they are manipulated to operate almost at or near their rated capacity (KVA rating). In view of this a power transformer is designed to have maximum efficiency at or near full load.(rated KVA). Hence choice of transformer should be based on full load efficiency.

Distribution Transformers are the one which are installed at consumer premises, where they are use to step down the voltage. The secondary winding of distribution transformers are directly connected to consumer terminals. The load on this transformer varies wide range during 24 hours day due to varying requirement ranging from zero to a maximum of its rated capacity. Note the primary of distribution transformer is constantly energized therefore core losses occur continuously.So  these transformer are designed to have minimum core losses, but for reduced core loss the maximum efficiency may occur at about one-half of its rated capacity. therefore choice of distribution transformer must be calculated as follows
Energy efficiency of a transformer is defined as the ratio of total energy output for a certain period to the total energy input for the same period

Relationship between Magnetism and Electricity

Whenever current flows through a conductor magnetic field is produced in the area surrounding the conductor (this phenomena was discovered by scientist Orested). So concluded that whenever the electrons are in motion magnetic field is produced vice versa is also true whenever magnetic field is in motion with respect to a given conductor electrons start flowing in that conductor, hence the current comes into existence (as proved by Faraday) . 

This phenomena whereby an e.m.f  and hence current is induced in any conductor which is cut across of is cut by magnetic flux is known as electromagnetic Induction.

Electromagnetic Induction is the basic phenomena on which every electrical machine works whether DC or AC including Transformers.

* intersecting fact

While the production of magnetic field due to flow of electrons was accidental discovery.

It took nine long years for Faraday to convert magnetic energy into electrical energy and all through these he use to carry magnets in his pockets so as to constantly remind him of the problem.

Rating of Transformers (why KVA and not KW)

Transformers have a name plate fixed on them on which rated output, rated voltages, the rated frequency, cooling method are recorded. The interesting fact is that its output rating is expressed in KVA (kilovolts ampere)rather then KW (kilowatt). This is due to the fact that the  transformer rated output is limited by heating and hence losses in the transformer. The two type of losses in a  transformer are core loss and ohmic losses.These  losses depend on transformer voltage and current and are almost unaffected by the load pf, therefore transformer rated output is expressed in VA or in KVA and not in KW

Transformers Noise

Transformers usually with ferromagnetic core produce a constant hum which leads to noise which can be very annoying especially in residential areas.The major cause of this noise is a phenomena called as magnetostriction. When the ferromagnetic core of transformers is magnetize, the core length along the alternating flux decreases and increases alternatively, with an alternative decrease and increase of its cross section. this phenomena of small change in the core dimensions is known as magnetostriction, alternatively the ferromagnetic core vibrates and humming is produced. this humming is further transfered to oil, transformer tank and then surroundings. The level of sound will increase with core flux density.

Transformers

The transformer is a device that transfers electrical energy from one circuit to another circuit through the medium of magnetic field and without change in the frequency. The electrical circuit which receives energy from the supply mains is called primary winding and the other circuit which delivers electric energy to the load is called the secondary winding.

This device is a electromagnetic energy conversion device. The electrical energy present in the primary winding is changed into magnetic energy and this magnetic energy is again converted back to electrical energy in  secondary circuit. The two circuits primary and secondary are magnetically coupled. Energy is transfered by this magnetic coupling between two cicuits  and no mechanical device is utilized therefore efficiency of transformers is as high as 98%.

Step Up Transformers: If the secondary windings has more number of turns then primary winding then the secondary voltage is higher then the primary voltage and the transformer is called as step up transformer.

Step Down Transformers: If the number of turns in the secondary side is less than the primary side then the voltage in secondary side is  lower then the primary voltage and the transformer is called as step down transformer.

There is  a third kind of transformer where number of windings in primary is equal to the number of windings in secondary these transformers are used for isolation purpose.

A.C. Machines

These are further classified as transformers, synchronous machines, induction machines and ac commutator machines.

1. Transformers. :- A transformer is not an electromechanical converter. It is used in almost all device energized from ac source. It usually steps up or step down electrical energy from one voltage level to another voltage level. However, it is also employed with unity turns ratio for isolation purposes.

In small sizes, single-phase are used. for large ratings, 3-phase transformers are more common.

2. Synchronous machines.:-In these machines, the field poles may be on stator or rotor. In bigger sizes, field poles are always on the rotor.The cross sectional view of a two pole synchronous machine is given below.

Field winding on rotor is excited with dc whereas stator winding handles three phase ac power. Rotor run at synchronous speed N(s). Frequency f of the emf generated in armature is given by

f = PN(s)/120 Hz

where P =number of field poles.

A synchronous machine is a doubly excited machine. It is employed as an alternator for the generation of three-phase power at all the generating stations. It is also used as a synchronous motor for special purpose.

3. Induction machines.:-These are of two types, three phase induction machines and single phase induction machines. Both types are singly excited in the sense that stator of three-phase ac source whereas stator of single phase induction motor is energized from 1-phase ac source.

(i) Three phase induction machines:-The stator winding of three phase induction motor, connected to 3 phase mains, carries both load current and field producing exciting current.the field produced rotates at synchronous speed in the air gap but rotor speed is always less then synchronous speed in the air gap but rotor speed is always less then the synchronous speed. Two type of 3 phase induction motors are as under:-

 1. SCIM Squirrel cage induction machine.:- Rotor carries copper or aluminium, bars embedded in rotor slots. These bars are short circuited by two end rings, one at each end. SCIM has the most rugged construction and is used where control of starting torque or speed is not needed.

  2. SRIM Wound rotor induction machine.:- Its stator is similar to the stator of SCIM, but rotor carries 3-phase winding with three leads connected to three slip rings mounted on shaft. Brushes pressing on the slip rings allow the insertion of external resistance in the rotor circuit.SRIMs are used where speed, or torque, control is required.

(ii) Single phase induction machines.:-In addition to 3 phase induction machines discussed above, single-phase induction motors are used where single phase low voltage(230 V, 50 Hz) is available, as in homes, offices, classrooms, shops etc.

4. AC Commutator machines.:-As the name suggest these machines are fed from ac source and are fitted with commutators.

D.C.Machines

In DC machines, the field poles are on the stator called yoke. Armature windings and commutator are on rotor.Cross section view of 2 pole DC machine is given.Brushes press on to commutator surface for collecting the power from DC generator or for feeding dc power to the armature of a dc motor. DC machines are of three types: series, shunt ans compound. DC series machine has field winding in series with armature circuit, dc shunt machine has field winding across the armature circuit whereas a dc compound machine has two field windings, one field winding across the armature and second field  winding in series with the armature.

DC machines are also fitted with interpoles. Large dc machines have also compensating windings embedded in pole faces embedded in the pole faces of main poles.

DC machines possess versatile operating characterstics . DC motor is easily adaptable for speed control and electric braking.

Electrical Machines-an overview

Electrical Energy stands on the very first position in energy hierarchy. It is because electrical energy is easily adaptable for all human needs and interests in a economic and efficient manner . At  same time, electrical energy  can be easily controlled and is pollution free at the consumer premises.

For generation and utilization of this energy vast variety  of electrical machinery is used.

 Electrical machinery can be mainly classified as DC machines and AC machines. DC machines can operate as generators and motors: same is true for AC machines.