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UNIT 3Single Phase Transformers Q1) Explain the necessity of transformers?A1)

It is used for transmitting power from one voltage to another.

It steps up or steps down the voltage power accordingly. It is made of coils, cores, windings and other materials that help the device to transmit power at an adequate voltage level and light up the entire area.

Transformers are devices used in electrical circuits to change the voltage of electricity flowing in the circuit. Transformers can be used either to increase the voltage (called "stepping up") or decrease the voltage ("step down").

Energy is lost in the process of transmitting electricity long distances, such as during the journey from a power plant to your home. Less energy is lost if the voltage is too high, so electrical utilities use high voltage in long-distance transmission wires. However, this high voltage is too dangerous for home use.

Electrical utilities use transformers to change the voltage of electricity as it travels from the power plant to you. First, the voltage of electricity coming from the power plant is "stepped up" using transformers to the right level for long-distance transmission.

In order for the electrical power distribution network to function, voltages must be stepped up before power is transmitted great distances over power lines.

One major problem is that power is lost between the power plant and the consumers because currents use some of the power to heat the transmission lines. The power transmitted along the line is equal to the voltage times the current.

The higher the voltage the lower the current that must flow within the transmission lines to deliver the same power. Lower currents produce much less heating and much less power loss.

Of course, the high voltages must be stepped back down before power is supplied to our homes. Transformers are the critical elements that step up and down the voltages at each end of the line. Q2) Explain the principle of operation of transformer?A2)

A transformer consists of two electrically isolated coils and operates on Faraday’s principal of “mutual induction”, in which an EMF is induced in the transformers secondary coil by the magnetic flux generated by the voltages and currents flowing in the primary coil winding.

Both the primary and secondary coil windings are wrapped around a common soft iron core made of individual laminations to reduce eddy current and power losses.

The primary winding of the transformer is connected to the AC power source which must be sinusoidal in nature, while the secondary winding supplies electrical power to the load.

A transformer could be used in reverse with the supply connected to the secondary winding provided the voltage and current ratings are observed.

The ratio of the transformers primary and secondary windings with respect to each other produces either a step-up voltage transformer or a step-down voltage transformer with the ratio between the number of primary turns to the number of secondary turns being called the “turns ratio” or “transformer ratio”.

If this ratio is less than unity, n < 1 then NS is greater than NP and the transformer is classed as a step-up transformer.

If this ratio is greater than unity, n > 1, that is NP is greater than NS, the transformer is classed as a step-down transformer.

If the turns ratio is equal to unity, that is n = 1, then both the primary and secondary have the same number of coil turns so therefore the voltages and currents will be the same for both the primary and secondary windings.

This type of 1:1 transformer is classed as an isolation transformer as both the primary and secondary windings of the transformer have the same number of volts per turn.

The efficiency of a transformer is the ratio of the power it delivers to the load to the power it absorbs from the supply. In an ideal transformer there are no losses so no loss of power then PIN = POUT.

Q3) Explain the types of transformers?A3) Acc to input supply : I phase and

phaseAcc to construction: core and shell typeAcc to 0/P : step up and step down Q4) Explain the emf equation of transformer?A4)A transformer basically consists of two coils wound around a common soft iron core. When an alternating voltage ( VP ) is applied to the primary coil, current flows through the coil which in turn sets up a magnetic field around itself, called mutual inductance, by this current flow according to Faraday’s Law of electromagnetic induction.The strength of the magnetic field builds up as the current flow rises from zero to its maximum value which is given as dΦ/dt.

As the magnetic lines of force setup by this electromagnet expand outward from the coil the soft iron core forms a path for and concentrates the magnetic flux. This magnetic flux links the turns of both windings as it increases and decreases in opposite directions under the influence of the AC supply.However, the strength of the magnetic field induced into the soft iron core depends upon the amount of current and the number of turns in the winding. When current is reduced, the magnetic field strength reduces. As the magnetic flux varies sinusoidally, Φ = Φmax sinωt, then the basic relationship between induced emf, ( E ) in a coil winding of N turns is given by:

emf = turns x rate of change

E = N dɸ/dt

E = N x w x ɸ max x cos (wt)

Emax = N w ɸ max

Erms = Nw/ ɸ max

= 2 π / x f x N x ɸ max

Erms = 4.44 fN x ɸ max

Where:

ƒ – is the flux frequency in Hertz, = ω/2π

Ν – is the number of coil windings.

Φ – is the amount of flux in webers

This is known as the Transformer EMF Equation. Q5) Explain the losses in transformer?A5)There are 2 types of losses occurring in a transformerA) 1. Core loss or Iron lossB) 2. Copper loss

Core losses:

This loss is due to the reversal of flux

The flux set up in the core is dependent on the i/p supply

as the i/p supply is constant in magnitude

the flux set up will be constant and

core losses are also constant.

Core losses are voltage dependent loss they can be subdivided in 2

1 Hysteresis loss2 reedy current loss

Hysteresis loss: The iron loss occurring in the core of T/F due to the Hysteresis curve of the magnetic material used for core is called as Hysteresis loss.

Hysteresis curve is the curve as loop which shows the properly of magnetic material to lag the flux density B behind the field Intensely H

Above is the 3 different loops (Hysteresis of 3 diff. Materials)

the selection of magnetic material for the construction of core depends upon Hysteresis loop of that material having tall and narrow Hysteresis loop is selected for the T/F core

silicon SteelHysteresis loss depends on fold factor PH = KH. Bm1.67 F

V – watts Where KH = constant (Hyst)Bm = max Flux densityF = Frequency

= Volume of core.2. Reedy current loss :This loss is due to the flow of reedy (circular) current in the core caused by induced emf in corePE = Ke Bm2 f2 t2 v – watts Where Ke = reedy current const.t = thickness of coreIt can be reduced by using stacks of laminations instead of solid coreB] Copper loss : PCU The Copper loss is due to resistance of the primary and secondary winding It is load dependent / current dependent loss

As load on a transformer is variable (changing) current changes copper loss is a variable loss

Primary secondary

Total C is loss = I12R12 + I22 R22

Copper loss depends upon load on T/F and is proportional to square of load current or KVA rating of transformer

PCU 2 (KVA)2

F.L = full load

PCU (at half load) = 2 PCu F.L

= (0.5)2 PCU F.L.

Or PCu ( load) = ()2 PCu F.L

Q6) Explain the condition for maximum frequency?A6)

Find no load vtg E2

remove the load and measure the reading of V2 meter ew will get n load vtg E2

E2 = V2 when load is absent

Now connect load and measure V2 this is now the load voltage

For each reading E2 will be same but V2 will change acc. To load

Form Results Plot graph for efficiency and regulation against I2 and O/P power W2

Efficiency: it is the ratio output power to input power of transformer = Output power = input power – total loss Input power = O/P + losses
		O/P power = KVA Cos Ø2 1000 Or = V2 I2 Cos Ø2 Losses = Pi + PCU (F.L) = iron + copper loss Full load = Half load(H.L) or 50% or 0.5 = =
	Maximum efficiency – for numerical: The efficiency of T/F is maximum when copper loss equates iron loss this is the condition for max efficiency ie Pi = PCU = PCU = Pi at max n Where KVA at max n given = Full load KVA

Q7) What is domestic wiring?A7) Wiring done in domestic premises (houses) for providing electrical power for lighting fans and domestic appliances with all safety precautions followed is called domestic wiring. Durability: Type of wiring selected should confirm to the standard specifications so that it is durable that is without being affected by weather conditions, fumes etc Electrical safety: The wiring must provide safety against leakage shock and the operating personnel. Appearance: Electrical wiring should give an aesthetic appeal to the interiors. Cost: It should not be prohibitively expensive that is the system chosen should depend upon the type of building and the purpose for which it is used keeping economy in view. Accessibility : The sitches and plug points provided should be acessible and there must be provision for further extension of the wiring system if necessary. Maintenance cost: The maintenance cost should be minimum. Mechanical safety: The wiring must be protected against any mechanical damage. Q8) Explain concealed conduct wiring?A8)There are two additional types of conduit wiring according to pipe installation

Surface Conduit Wiring

Concealed Conduit Wiring

Surface Conduit WiringIf conduits installed on roof or wall, It is known as surface conduit wiring. in this wiring method, they make holes on the surface of wall on equal distances and conduit is installed then with the help of rowel plugs. Concealed Conduit wiringIf the conduits is hidden inside the wall slots with the help of plastering, it is called concealed conduit wiring. In other words, the electrical wiring system inside wall, roof or floor with the help of plastic or metallic piping is called concealed conduit wiring. It is the most popular, beautiful, stronger, and common electrical wiring system nowadays. Q9) Explain two-way or three-way control?A9) A two-way switch (2 connections to the switch, not including ground) turns lights on or off from 1 location only.A three-way switch (3 connections to the switch, not including ground) can turn lights on or off from 2 locations. A four-way switch (yup, you guessed it, 4 connections to the switch, not including ground) enables you to turn lights on or off from 3 or more locations (a 3-way switch is at each “end” of the circuit, and 1 or more 4- way switches in between them).A two-way switch just connects or disconnects the 2 connections. A 3-way switch will connect/disconnect the common connection to 1 of the other connections.So If you needed to turn a light on or off from 1 location, use a two way switch; from 2 locations a 2 three-way switches; from 3 locations use 2 3-way switches and 1 4-way switch; from 4 locations use 2 3-way switches and 2 4-way switches; from 5 locations 2 3-way switches and 3 4-way switches…. Q10) Explain MCB?A10)

Find no load vtg E2

remove the load and measure the reading of V2 meter ew will get n load vtg E2

E2 = V2 when load is absent

Now connect load and measure V2 this is now the load voltage

For each reading E2 will be same but V2 will change acc. To load

Form Results Plot graph for efficiency and regulation against I2 and O/P power W2

Efficiency : it is the ratio output power to input power of transformer

Output power = input power – total loss

Input power = O/P + losses

O/P power = KVA Cos Ø2 1000

Or = V2 I2 Cos Ø2

Losses = Pi + PCU (F.L)

= iron + copper loss

Full load =

Half load(H.L) or 50% or 0.5 =

Maximum efficiency – for numerical:

The efficiency of T/F is maximum when copper loss equates iron loss this is the condition for max efficiency

ie Pi = PCU

PCU = Pi at max n

Where KVA at max n given = Full load KVA

Q11) What is electronic shock? Explain its precautions?A11) Electric shock can be avoided. Faulty installation, improper grounding, and incorrect operation and maintenance of electrical equipment are always sources of danger.Some of the precautions include:1.Ground all electrical equipment and the work piece.

Prevent accidental electrical shocks. Connect power source, control cabinets, and work piece to an approved electrical ground.

The work lead is not aground lead. It is used to complete the welding circuit.

A separate connection is required to ground the work or the work lead terminal on the power source may be connected to ground.

2.Use the correct cable size.

Sustained overloading will cause cable failure and result in possible electrical shock or fire hazard.

Work cable should be the same rating as the torch cable.

3.Make sure all electrical connections are tight, clean, and dry.

Poor electrical connections can heat up, and even melt.

They can also cause bad welds and produce dangerous arcs and sparks.

Do not allow water, grease, or dirt to accumulate on plugs, sockets, or electrical units.

4.Keep dry.

Moisture and water can conduct electricity.

To prevent shock, it is advisable to keep work areas, equipment, and clothing dry at all times.

Fix water leaks immediately.

Make sure that you are well insulated.

Wear dry gloves, rubber-soled shoes, or stand on a dry board or platform.

5.Keep cables and connectors in good condition.

Improper or worn electrical connections can cause short circuits and can increase the chance of an electrical shock.

Do not use worn, damaged, or bare cables.

6. Avoid open-circuit voltage.

Open-circuit voltage can cause electric shock.

When several welders are working with arcs of different polarities, or when using multiple alternating-current machines, the open-circuit voltages can be additive.

The added voltages increase the severity of the shock hazard.

Q12) What is earthing? Explain its types?A12)The process of transferring the immediate discharge of the electrical energy directly to the earth by the help of the low resistance wire is known as the electrical earthing. The electrical earthing is done by connecting the non-current carrying part of the equipment or neutral of supply system to the ground.Electrical EarthingThe process of transferring the immediate discharge of the electrical energy directly to the earth by the help of the low resistance wire is known as the electrical earthing. The electrical earthing is done by connecting the non-current carrying part of the equipment or neutral of supply system to the ground.Mostly, the galvanised iron is used for the earthing. The earthing provides the simple path to the leakage current. The short-circuit current of the equipment passes to the earth which has zero potential. Thus, protects the system and equipment from damage.

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