Browsing: Transformers

Questions and Answers on Single Phase Transformers

  1. What is a transformer?

Ans: – A transformer is a static device which transfers electric power from one circuit to another circuit at same frequency but voltage level is usually changed.

  1. What are step-up and step-down transformers?

Ans: – When the voltage level is raised on the output side, the transformer is called step-up transformer, whereas, the transformer in which the voltages is lowered on the output side, is called a step-down transformer.

  1. Which are the two winding are present in a transformer?

Ans: – Primary and secondary windings

  1. Transformer is called static electrical machine. Why?

Ans: – There is no rotating part in the transformer, therefore, it is called static electrical machine.

  1. What are the functions of transformer oil?

Ans: – In provides additional insulation and protects the insulation from dirt and moisture and it carries away the heat generated in the cores and coils.

  1. Why an iron or steel core is provided in a transformer?

Ans: – To ensure a high permeability of the magnetic circuit. Because of high permeability, the magnitude of exciting current necessary to create the required flux in the core is small. The presence of steel core also causes hundred per cent of magnetic flux.

  1. What is the application of a isolation transformer?

Ans: – An isolation transformer has equal turns in primary and secondary windings. These transformers are employed for isolating the load from supply.

  1. What is the order of magnitude of no-load current?

Ans: – No-load current is transformers ranges from 2 to 5 per cent of full-load primary current.

  1. How is magnetic leakage reduced to a minimum in commercial transformers?

Ans: – By interleaving the primary and secondary windings.

  1. Define voltage transformation ratio.

Ans: –The ratio of secondary voltage to primary voltage is called a transformation ratio. It is denoted by capital letter K.

  1. From the construction point of view, name different types of transformers?

Ans: – According construction point of view, the transformers are divided into two types: (i) Core type and  (ii) Shell type.

  1. In practice, what determines the thickness of the stampings?

Ans: – Frequency.

  1. Why the core of a transformer is laminated?

Ans : – To reduce eddy current loss, the core of the transformer is laminated.

  1. What are the functions of transformer oil?

Ans: – Transformer oil’s primary functions are to insulate and cool a transformer.

  1. What is the cause of noise in a transformer?

Ans: – In a transformer, noise occurs mainly due to loosening of stampings and mechanical forces developed during operation.

  1. Which test gives the copper loss of a transformer ?

Ans: – Short circuit test.

  1. Tap changers are usually employed on H.V. side of a transformer, why?

Ans: – Tap changers are usually employed on H.V. side because, the high voltage side has a large number of turns which allows smoother variation of voltage. It is easily accessible physically and above all, it has to handle low currents.

  1. What would happen, if a power transformer designed for operation on 50Hz, were connected to 5Hz source of the same voltage?

Ans: – The power transformers are designed to operate at 50 Hz. On the other hand, if the primary is connected to a source of 5 Hz frequency, the primary winding will have an insufficient inductive reactance (XL = 2πfL). The result will be that, the primary will have excessive current producing considerable copper losses. There is every possibility that the transformer may start to smoke.

  1. What would happen, if a power transformer designed for operation on 50Hz, were connected to 500 Hz source of the same voltage?

Ans: – The power transformers are designed to operate at a particular frequency, generally at 50 Hz. If the frequency of the supply will be high, it will result greater inductive reactance. This high inductive reactance of the winding will prevent the primary from drawing sufficient power. Moreover, the iron losses i.e. hysteresis and eddy current losses will be excessive.

  1. Silica gel placed in the transformer breather. Why?

Ans: – Silica gel is placed in the transformer breather. The function of the silica gel is to absorb the moisture so that the life of the transformer can be increased.

  1. What is the thickness of laminations?

Ans: – 0.35mm to 0.5mm

  1. Why silicon content in electrical sheet steel is limited to 4.5% to 5%?

Ans: – Silicon content exceeding 5%   makes the sheet steel brittle and so causes in punching.

  1. Why LV winding is placed first on the core and then HV winding in the case of a core type transformer.

Ans: – Placing of LV winding near the core and HV winding around the LV winding in a core type transformer reduces the amount of insulation material required.

  1. Why circular coils are always preferred over rectangular coils for winding in a transformer?

Ans: – Circular coils are preferred for winding a transformer as they can easily wound on machines, conductors can easily be bent and winding does not bulge out due to radial forces developing during operation.

  1. Does the transformer draw any current when its secondary is open?

Ans: – Yes, no load primary current.

  1. Why are iron losses constant at all loads in a transformer?

Ans: – Since the induced primary ampere-turns and secondary ampere-turns always neutralize one another, the flux in the core on load is the same as the flux on no-load. Hence, the iron losses are constant and are independent of load.

  1. Where is shell-type construction suitable for a transformer?

Ans: –In shell type core, both the windings are wound on the central limb. This type of core is used for those transformers which work on poor power factor.

  1. What are the advantages of open and short-circuit tests on a transformer?

Ans: – The efficiency of a transformer is always determined by open-circuit and short-circuit tests due to the following reasons: (i) The power required to carry out these tests is very small. (ii) The tests give the core loss and copper loss separately.

  1. What are the types of bushing used in transformer terminals?

Ans: – (i) Porcelein insulator bushing (ii) Oil filled bushing (iii)  Capacitor bushing

  1. Why is it advantageous to make short circuit test on the high voltage side rather than on low voltage side?

Ans: –  It is due to the fact that the ranges of the meter at the high voltage (HV) side at the short circuit condition of the low voltage (LV) side is much more suitable.s

  1. What is tapped transformer?

Ans: – A tapped transformer is one whose windings are fitted with special taps for changing in voltage or current ratio.

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Difference Between Power Transformers and Distribution Transformers

Applications

Power Transformer

These transformers are used to step-up voltage at the generating station for transmission purposes and then step-down the voltage at the receiving sub-station.

Distribution Transformer

These transformers are installed at the distribution sub-stations (in localities) to step-down the voltage value.

Capacity

 Power Transformer

The rating of such transformer is more than 500 KVA.

Distribution Transformer

The rating of such transformer is upto 500 KVA.

Cooling

 Power Transformer

These transformers may be self oil cooled, forced air cooled or forced water cooled.

Distribution Transformer

These transformers may be Air natural cooled or oil natural cooled. They are of self cooling type and are almost invariably oil immersed.

Losses

 Power Transformer

These transformers are usually designed to operate at full load, which would cause continuous capacity copper losses, thus affecting their efficiency. To have minimum losses during 24 hours, such transformers are designed with low copper losses.

Distribution Transformer

In such transformers, iron loss occurs for all the time where the copper loss occurs only when they load. Therefore, distribution transformers should be designed with iron loss in comparison to the full load copper loss.

Connections

 Power Transformer

These transformers may be single phase or three-phase; three-wire delta/delta or delta/star connected.

Distribution Transformer

Such transformers are usually three-phase; four wire, delta/star connected.

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EMF Equation for a Transformer

When a primary of a transformer connected to the ac supply, an alternating flux is set up in the core which links with primary and secondary winding.

Let,

Φm = maximum value of flux in Weber.

F = supply frequency in Hz

N1 = number of turns in primary

N2 = number of turns in secondary

E1 = e.m.f. induced in primary

E2 = e.m.f. induced in secondary

Flux changes from +Φm to 0 in one forth a cycle i.e. 1/4f seconds, as shown in figure

Average rate of change of flux

= 4fΦm Wb/sec

Now, the rate of change of flux per turn is the average induced e.m.f. per turn in volts.

∴ Aveage e.m.f. induced per turn =  4fΦm volts

∴ R.M.S. value of em.f. induced per turn E = 1.11 × 4fΦm = 4.44 fΦm volts

Since primary and secondary have N1 and N2 turns respectively.

Therefore, R.M.S. value of e.m.f. in primary,

E1 = 4.44 N1 m volts

Therefore, R.M.S. value of e.m.f. in secondary,

E2 = 4.44 N2 m volts

From above equation, we concluded that induced emf primary and secondary winding is directly proportional to the number of turns, supply frequency and value of flux.

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Losses in a Transformer

Transformer losses are broadly classified as no-load (core or Iron losses) and load losses (copper losses). No-load losses occur when the transformer is energized with its rated voltage at the primary winding, but the secondary winding is open circuited so that no through current or load current flows. In this case, full flux is present in the core, and only the necessary exciting current flows in the primary and secondary windings. The losses are predominately core losses due to hysteresis and eddy currents produced by the time-varying flux in the core steel. Copper losses occur when the output is connected to a load so that current flows through the transformer from input to output terminals. So, from the above explanations transformer losses are :

  1. Core losses
  2. Copper losses
  3. Interlaminer losses
  4. Stray losses
  5. Tieplate losses
  1. Core losses

These consist of hysteresis and eddy current losses and occur in the transformer core due to the alternating flux. These can be determined by open-circuit test.

Hysteresis loss, = kh f B1.6 watts/m3

Eddy current loss, = ke f2 B2 t2 watts/m3

Both hysteresis and eddy current losses depend upon :

  • maximum flux density Bm in the core
  • supply frequency f.

Since transformers are connected to constant-frequency, constant voltage supply, both f and Bm are constant. Hence, core or iron losses are practically the same at all loads.

Iron or Core losses, Pi = Hysteresis loss + Eddy current loss = Constant losses

The hysteresis loss can be minimized by using steel of high silicon content whereas eddy current loss can be reduced by using core of thin laminations.

  1. Copper Losses

These losses occur in both the primary and secondary windings due to their ohmic resistance. These can be determined by short-circuit test.

  1. Interlaminar losses

The core laminations are coated with a glass-like insulating material. This is usually very thin to keep the space factor reasonably high (>96%). Like any other material, the coating is not a perfect insulator. Thus, eddy currents driven by the bulk flux in the core can flow across the stacked laminations that comprise the core, that is, normal to their surfaces. Of course, the eddy current paths are completed within the laminations where the resistance is much lower. The coating must be a good insulator to keep these losses low relative to the normal interlaminar losses. The insulating value of the coating is determined by measuring the resistance across a stack of laminations.

The interlaminar loss should be compared with the normal loss at the same peak induction. For typical values of the parameters, this loss is generally much smaller than the normal loss and can be ignored. However, a high enough interlaminar resistance must be maintained to achieve these low losses.

  1. Stray losses

Stray losses are losses caused by stray or leakage flux.

  1. Tieplate Losses

The tieplate (also called the flitch plate) is located just outside the core in the space between the core and the innermost winding.

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An Ideal Transformer

An ideal transformer is one which has no ohmic resistance and no magnetic leakage flux i.e. all the flux produced in the core links with primary as well as secondary winding. In other words, we are postulating the following:

  • The core of the transformer is highly permeable in a sense that it requires vanishingly small magnetomotive force (mmf ) to set up the flux.
  • The core does not exhibit any eddy-current or hysteresis loss.
  • All the flux is confined to circulate within the core.
  • The resistance of each winding is negligible.

An ideal transformer

Bahaviour and Phasor diagram

The primary winding is connected to an alternating voltage source V1, while the secondary winding is left open. A current  Im  flows through the primary winding. Since the primary coil is pure inductive, the current  Im  lags behind the applied voltage V1  by 900. This current sets up alternating flux in the core and magnetises it. Hence it is called magnetising current. Flux is in phase with Im. The alternating flux links with both primary and secondary windings. When it links with primary, it produces self induced e.m.f. E1 in opposite direction to that of applied voltage V1 according to Lenz’s law. When this flux Φ links with secondary winding, it produces mutually induced e.m.f. E2 in opposite direction to that of applied voltage.

phasor diagram of an ideal transformer

Transformation Ratio

In an ideal transformer there is no power loss, so that, output must be equal to input.

E2I2cos Φ = E1I1cos Φ

E2I2 = E1I1

The induced emf in primary and secondary winding is directly proportional to number of primary and secondary winding turns.

E1 α N1

E2 α N2

The ratio of primary to secondary induced emfs is equal to the ratio of primary to secondary turns is called transformation ratio. It is denoted by letter K.

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Why Transformer Rated in kVA or MVA?

An important factor in the design and operation of transformer is the relation between the life of the insulation and operating temperature of the transformer. Therefore, temperature rise resulting from the losses is a determining factor in the rating of a machine. We know that copper loss in a transformer depends on current and iron loss depends on voltage. Therefore, the total loss in a transformer depends on the volt-ampere product only and not on the phase angle between voltage and current i.e., it is independent of load power factor. For this reason, the rating of a transformer is in kVA and not kW.

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Transformer Construction

The main  elements of a transformer are ; two coils and a laminated steel core. The two coils are insulated from each other as well as from the steel core. In order to keep the core loss to a minimum, the core of a transformer is built up of thin laminations of highly permeable ferromagnetic material such as silicon sheet steel. Silicon steel is used to reduce hysteresis losses. The lamination’s thickness varies from 0.014 inch to 0.024 inch.  A thin coating of varnish is applied to both sides of the lamination in order to provide high inter lamination resistance.

Basically two types of construction are in common use for the transformers: core type and shell type.

Core Type Transformers

In a simple core type transformer the magnetic core is built up of laminations to form a rectangular frame. The laminations are cut in the form of L-shape strips. In a core-type transformer,  each winding may be evenly split and wound on both legs of the rectangular core. While placing these windings, an insulation layer is provided between core and lower winding and between the two windings. To reduce the insulation, low voltage winding is always placed nearer the core.

Core type transformers - transformer construction

Core type transformers are used for High voltage applications like distribution transformer, power transformer etc.

Shell Type Transformers

In case of shell type transformer, each laminations is cut in the form of long strips of E’s and I’s. In order to avoid high reluctance at the joints where the laminations are butted against each other, alternate layers are stacked differently to eliminate continuous joints.

shell type transformer - transformer construction

In such type of transformer, the core has three legs or limbs. The winding is placed on the central limb so that, central limb carries whole of the flux, whereas the central limbs carry half of the flux. The width of central limb is about double of the outer limbs.  Similarly, core type transformer, low voltage winding also placed nearer the core and high voltage winding is placed outside the low voltage.

Shell type transformers are used in Low voltage applications like transformers used in electronic circuits and power electronic converters etc.

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Working Principle of Transformer

The basic principle of a transformer is electromagnetic induction.

When an alternating voltage V1 is applied to the primary, an alternating current starts flowing through the primary winding. This result, an alternating current set up in the core. This alternating flux when links own turn (primary winding), an emf E1 is induced in the primary winding. The direction of this emf is opposite to the applied voltage, according to the Lenz’s law.

working principle of transformer

The alternating flux also links with the secondary winding, an emf is induced secondary winding is called mutually induced emf.

The induced emf in the primary and secondary winding depends upon the rate of changed of flux linkages. The rate of change of flux is the same for both primary and secondary winding. Therefore the induced emf is directly proportional to the number of turns of the primary winding and in secondary is proportional to the number of turns of the secondary winding.

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Transformer on DC

A transformer cannot work on d.c. supply. If a rated d.c. voltage is applied across the primary, a flux of constant magnitude will be set up in the core. Due to constant flux, there will not be any self induced emf in the primary winding to oppose the applied voltage. According to the Faraday’s law of electromagnetic induction, whenever a conductor cuts by the varying magnetic flux an emf is induced in the conductor. But when transformer is connected with dc supply, a flux of constant magnitude is set up. As the resistance of the primary winding is very low, the primary current will be very high as given by the relation;

This current is much more than the rated full load current of primary winding. Thus it will produce large heat in core (due to copper losses) and burns the insulation of the primary winding, this results the transformer will be damaged. That is why, transformer is not operated with dc supply.

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What is Transformer?

The Transformer is a static device that transfers electrical energy from one electrical circuit to another electrical circuit with same in the frequency but voltage level is usually changed.

transformer

The transformer has two winding one is called primary winding and other is called secondary winding.

  • The source is connected to the primary side.
  • The load is connected to the secondary side.
  • The transformer is an electromagnetic energy conversion device, since the energy received by the primary is first converted to magnetic energy and then it is reconverted into electrical energy in the secondary circuit. Thus the primary and secondary winding of a transformer is not connected electrically, but are coupled magnetically.
  • The transformers may be stepped up or stepped down. If the secondary voltage has higher than the primary winding, then such transformer is called a step up transformer. If the secondary voltage is less than the primary voltage, then the transformer is called a step down transformer.

Necessity of Transformer

The electrical power is generated at the voltage level of 11kv. For economical reasons, ac power is transmitted at very higher voltages over a long distance. Therefore, a step up transformer is used at generating station to raise the voltage level. Then voltages are stepped down to suitable level by a transformer at various substations. Ultimately for utilization of electrical power, the voltage is stepped down to 400/230 for safety reasons.

Efficiency

Transformer is a static device, owing to the lack of rotating parts. There are no friction or windage losses, so that the efficiency of a transformer is high. Typical transformer efficiencies at full load lie between 95% and 98%.

Transformer on DC

A transformer cannot operate on dc supply and never be connected to a dc source. If the primary winding of a transformer is connected to a dc supply mains the flux produced will not vary but remain constant in magnitude and, therefore no emf will be induced in the secondary winding except at the moment of switching ON. There will be no induced emf in the primary winding and therefore, a heavy current will be drawn from the supply mains which may result in the burning out of the transformer winding.

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