When a load is connected to a transformer
current (say I2) flows through secondary coil thus an M.M.F (N2I2)
is produced, this produces the secondary flux. This flux reduces the main flux
induced in the primary & also reduces E.M.F E1 in the primary. As a result more current is
drawn from the supply. This additional current drawn is due to the load
component I2’ .This I2‘ is anti-phase with I2.This I2‘
sets a flux which opposes the secondary flux & helps the main flux. The
load component flux neutralises the secondary flux produced by I2.The
M.M.F N1I2 balances N2I2.Thus
the net flux is always at constant level
Give an example where
transformer is used in DC applications
Transformer never work on DC .but it is
Why an induction motor
is called a rotating transformer?
The rotor of an induction motor receive
electric power by induction in exactly the same way as the secondary of a
2-winding transformer receives its power from the primary. Thus an induction
motor can be treated as a rotating transformer i.e. one in which primary
winding is stationary but the secondary is free to rotate.
Why is transformer
rating in KVA?
Copper loss of a transformer depends on
current and iron loss on voltage. Hence, total transformer loss depends on
volt-ampere (VA) and not on phase angle between voltage and current i.e.
independent of load power factor. That is why rating of transformers is in Kva
and not in Kw.
How do we specify the
rating of a transformer?
Transformer rating is specified in
Kilovolt-Ampere (KVA)
What are the cooling
methods to cool a transformer?
The cooling methods are:
- Oil filled self cooled
- Oil filled water cooled
- Air blast type
What is Scott-
connected transformer?
A Scott-T transformer (also called a Scott
connection) is a type of circuit used to derive two-phase electric power (2-φ, 90-degree phase rotation) from a three-phase (3-φ, 120-degree phase rotation) source, or
vice-versa. The Scott connection evenly distributes a balanced load between the
phases of the source.
Does the transformer
draw any current when its secondary is open?
Yes, no-load primary current
Is Cu loss affected by
power factor? Why?
Yes, Cu loss varies inversely with power
factor
Cu loss depends on current in the primary and
secondary windings. It is well-known that current required is higher when power
factor is lower.
What are Instrument
Transformers? Current Transformer
and Potential Transformer are Instrument transformers. Current Transformer
(CT) is a step-up
transformer which steps down the current to a known ratio. The primary of this
transformer consists of one or more turns of thick wire connected in series
with the line. The secondary consists of a large number of turns of fine wire
and provides for the measuring instruments and relays a current which is a
constant fraction of current in the line
Potential Transformer
(PT) is a step down
transformer and steps down the voltage to a known ratio. The primary of this
transformer consists of a large number of fine wire connected across the line.
The secondary winding consists of a few turns and provides for measuring
instruments and relays a voltage which is a known fraction of the line voltage.
