why the Transformer Rating may be expressed in kVA, Not in kW.???
why the Transformer Rating may be expressed in kVA, Not in kW.??? a crispy answer...
Definition of Transformer
A transformer is a static machine used for transforming power from one
circuit to another without changing frequency. This is a very basic definition of transformer.
History of Transformer
The history of transformer was commenced in the year 1880. In the year 1950, 400KV electrical power transformer was
introduced in high voltage electrical power system. In the early 1970s,
unit rating as large as 1100MVA was produced and 800KV and even higher
KV class transformers were manufactured in year of 1980.
Losses in Transformer
As the electrical transformer is a static device, mechanical loss in transformer normally does not come into picture. We generally consider only electrical losses in transformer. Loss in any machine is broadly defined as difference between input power and output power.
When input power is supplied to the primary of transformer, some portion of that power is used to compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy currentloss in transformer core and some portion of the input power is lost as I2R loss and dissipated as heat in the primary and secondary windings, because these windings have some internal resistance in them. The first one is called core loss oriron loss in transformer and the later is known as ohmic loss or copper loss in transformer. Another loss occurs in transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and winding conductors.
When input power is supplied to the primary of transformer, some portion of that power is used to compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy currentloss in transformer core and some portion of the input power is lost as I2R loss and dissipated as heat in the primary and secondary windings, because these windings have some internal resistance in them. The first one is called core loss oriron loss in transformer and the later is known as ohmic loss or copper loss in transformer. Another loss occurs in transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and winding conductors.
Copper Loss in Transformer
Copper loss is I2R loss, in primary side it is I12R1 and in secondary side it is I22R2 loss, where I1 & I2 are primary & secondary current of transformer and R1 & R2 are resistances
of primary & secondary winding. As the both primary & secondary
currents depend upon load of transformer, copper loss in transformer vary with load.
Core Losses in Transformer
Hysteresis loss and eddy current loss, both depend upon magnetic
properties of the materials used to construct the core of
transformer and its design. So these losses in transformer are fixed and do not depend upon the load current. So core losses in transformer which is alternatively known as iron loss in transformer can be considered as constant for all range of load.
There are two type of losses in a transformer;
- Copper Losses
- Iron Losses or Core Losses or Insulation Losses
Copper losses ( I²R) are variable losses which depends on Current passing through transformer windings while Iron Losses or Core Losses or Insulation Losses depends on Voltage.
So the transformer is designed for rated voltage (iron loss) and rated current (copper loss). We can't predict the power factor while designing the machine, because power factor depends upon the load which varies time to time.
When a manufacturer makes a transformer, UPS etc, they have no idea of the type of load that will be used & consequently they can only rate the device according to its maximum current output that the conductors can safely carry (at unity Power Factor) & the insulation rating of the conductors (voltage & temperature).
That’s why the Transformer Rating may be expressed in kVA, Not in kW.
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