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Tuesday 27 September 2011

Transformers Short Interview Questions 2

What are the advantages of oil over air as a cooling medium in transformers ?
Ans. For large transformers, oil cooling is needed, especially when high voltages are in use. Oil-cooled transformers must be enclosed in steel tanks. Oil has the following advantages over air as a cooling medium :
(i) It has greater specific heat than air so that it will absorb larger quantities of heat for the same temperature rise.
(ii) It has greater conductivity than air and enables the heat to be transferred to the oil more quickly.
(iii) The breakdown strength of oil used for cooling is about six times that of air. Thus it provides very good insulation and ensures increased reliability at high voltages.
Illustrate the impedance matching property of a transformer with a resistive load.
Ans. The maximum power transfer theorem states that in order to transfer maximum power from a source to a load, the load resistance (or impedance) should be equal to internal resistance (or impedance) of the source. Let us illustrate it with a practical example. Suppose we have a source of 50 V (= VT) with internal resistance of 5 kQ. We want this source to transfer maxi­mum power to a load of 200 Q. For this purpose, we shall connect the source through a transformer of suitable turn ratio (NI/N2) to the load of 200 Q.
Illustrate impedance matching property of a transformer with a capacitive load.
Ans. When the source has actual internal impedance (inductive or capacitive) rather than internal resistance, the situation is slightly different. For maximum power transfer, the load imped­ance must be conjugate of the internal impedance of source. Conjugate means that phase angle of the load impedance must be equal in magnitude but of the opposite sign to the inter­nal impedance of the source and that the magnitude of the two impedances must be equal
 
What is a current transformer
Ans. A current transformer is a device that is used to measure high alternating current in a conductor, Fig illustrates the principle of a current transformer. The conductor carrying-large current passes through a cir­cular laminated iron core. The conductor constitutes a one-turn primary winding.. The secondary winding consists of a large number of turns of much fine wire wrapped around the core as shown.
Due to transformer action, the secondary current is transformed to a low value which can be measured by ordinary meters.
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 A current transformer must never be operated with its secondary winding open-circuited. Explain.
Ans. It is because when there is no secondary current to oppose the core flux generated by the primary, a serious overheating of the core can occur. Secondly, it is also possible that the secondary open-circuit voltage may reach a dangerously high level.

What is a potential transformer ?
Ans. It is a device that is used to measure high alternating voltage. It is essentially a step down transformer having small number of secondary turns The high alternating Supply voltage to be measured is connected directly across the pri­mary. The low voltage winding (secondary winding) is con­nected to the voltmeter. The power rating of a potential transformer is small (seldom exceeds 300 W) since voltmeter is the only load on the transformer.
The.primary of potential transformer is connected directly across the high-voltage mains. The indication of the voltmeter multiplied by the ratio of primary to secondary turns of potential transformer is the high voltage value, The ratio of primary to secondary turns is usually marked on the nameplate as a multiplier. The primary of the current transformer is connected in series with the load. The indication of ammeter multiplied by the ratio of secondary to primary turns of current transformer gives the value of load current.