QUE-ANS: 10-30 words
[1] What is the voltage in the secondary winding of a voltage transformer?
The voltage in the secondary winding of a voltage transformer is 220V (this may vary from ship to ship).
[2] Does the transformer draw any current when its secondary is open?
Yes, no-load primary current. For supplying no-load iron and copper losses.
[3] Is cu loss affected by power factor?
Yes, cu loss varies inversely with power factor. Cu loss depends on currents in the primary and secondary winding. It is well known that current required is higher when power factor is lower.
60-100 words
[1] What effects are produced by change in voltage?
Iron loss – varies approximately as V2
Cu loss – it also varies as V2 but decreases with an increase in voltage if constant kva output is assumed.
Efficiency – for distribution transformers, efficiency at fractional loads decreases with increase in voltage while at full load or overload it increases with increase in voltage and Vice versa.
Regulation – it varies as V2 but decreases with increase in voltage if constant kva output is assumed.
Heating – for constant kva output, iron temperatures increase whereas cu temperatures decrease with increase in voltages and vice versa.
[2] Distinguish between an induction coil and a transformer.
Induction coil such a mutual induction in its simplest form, a transformer consist of two inductive coils which are electrically separate by magnetically linked thru a path of low reluctance (a path that offers low resistance to the flow of magnetic flux).
A transformer of AC system is the ease with which an alternating voltage can be increased or reduced. The transformer may be an air cooled type
[3] State the normal care and attention needed for a transformer in operation.
Normal care and attention needed for a transformer in operation:
At regular specific interval, transformer must be disconnected cover removed and all accumulated dust and deposits remove by a vacuum cleaner and suitable brushes.
Windings must be inspected for any sign of damage or overheating.
Windings continuity resistances are measured.
Insulation resistance of al winding must be measured with respect to earth and to other phase windings.
Cable connections must be checked for tightness and covers must be secured.
Transformer generally air cooled, thus ensure no obstruction at that area.
100-150 words
[1] How does change in frequency affect the operation of a given transformer?
Iron loss – increases with a decrease in frequency. A 60-hz transformer will have nearly 11% higher losses when worked on 50 hz instead of 60 hz. However, when t 25 hz transformer is worked on 60 hz, iron losses are reduced by 25%.
Cu loss – in distribution transformers, it is independent of frequency.
Efficiency – since cu loss is unaffected by change in frequency, a given transformer efficiency is less at a lower frequency than at a higher one.
Regulation – regulation at unity power factor is not affected because IR drop is independent of frequency. Since reactive drop is affected, regulation at low power factor decreases with a decrease in frequency and vice versa. For example, the regulation of a 25Hz transformer when operated at 50-hz and low power factor is much poorer.
Heating – since total loss is greater at a lower frequency, the temperature is increased with decrease in frequency.
[2] Where on board ship transformer may be used with advantage?
Onboard ship transformer used:
Electric generation is typically 3 phase 440V, 60Hz, while fixed lighting and other low power loads are applied with 220V or 110V single phase from transformer units.
Auto transformer starter consists of a three coil which normally use 65% large motor drives.
Advantages:
3 phase lighting transformer, if transformer fault develops, the faulty unit can be disconnected. Power supply will still be available at a reduced rating from other transformers.
By auto transformer starter, it will reduce starting voltage thus limit the starting surge current by step it down such as electric cargo pumps, ballast pumps, FD fan etc.