![]() |
Fig. 1 Drysdale phase shifting transformer |
The rotor winding is housed in the slots provided on laminated
structure of the rotor. A small air gap is present between stator and
rotor. The rotor can be moved from its initial zero position through a
desired angle such that the equal phase displacement in rotor e.m.f. can
be observed. With the help of pointer and scale provided at the top of
the instrument, the angle through which the rotor has been moved can be
indicated. The scale contains angles in degrees of phase displacement as
well as cosines of these angles.
When stator winding is excited by voltage then rotating magnetic
field is produced. This field gets linked with the rotor winding to
induce e.m.f. in it. The windings on the rotor are arranged in such a
way that change in magnitude of induced emf is not followed after the
change in phase of this emf.
When the stator is excited from single phase supply as shown in
the Fig. 1, then phase splitting device is to be used as rotating
magnetic field is not produced by single phase supply. Thus two separate
stator windings displaced from each other by an angle of 90o
in space are used. The phase splitting device consists of a variable
resistor and variable capacitor connected in series with one of the
stator winding. The other winding is directly connected to the supply
lines. The values of resistor and capacitor are successively adjusted
till the currents in the two windings are displaced from each other by
an angle of 90o for producing uniform rotating magnetic field. This is called quadrature adjustment.
1.1 Operation
The action of phase splitting device can be understood by considering the Fig. 2(a) and (b).
![]() |
Fig. 2 |
The corresponding phasor diagram is shown in the Fig. 3.
![]() |
Fig. 3 |
The circuit shown in the Fig. 2(b) is the equivalent circuit. When the currents are equal and quadrature ( 90o apart from each other) then we have,
I2 = j I1
Equating real and imaginary parts,
Note : Hence by adjusting the value of R and C, phase splitting is possible.
If we considering a particular position of the rotor then the
induced emf in each of the two stator windings is proportional to the
cosine of the angular displacement from the position of maximum emf.
Emf induced in rotor winding due to first stator winding is given by,
e1 = KI sinωt cosθ
Similarly emf induced in rotor winding due to second stator winding is given by,
e2 = KI sin (ωt + 90o) cos(θ + 90o)
= - KI cos ωt sinθ
The resultant emf in the rotor is given by,
e = e1 + e2 = KI sinωt cosθ - KI cosωt sinθ
= KI (sinωt cosθ - cosωt sinθ)
We have, sin (A-B) = sin A cos B - cos A sin B
... e = KI sin (ωt - θ)
From the above relation, it can be seen that the emf induced in
the rotor has a constant amplitude but its phase depends on the position
of rotor i.e. an angle θ.
With the availability of three phase supply, three phase shifting
transformer can be used. It is less sensitive to frequency changes than
the phase splitting circuit which uses single phase supply.
In case of three phase shift transformer the phase angle between
incoming and outgoing lines is shifted without change in the voltage
ratio. It is shown in the Fig. 4.
![]() |
Fig. 4 Phase shift transformer |
The three phase lines is connected to incoming terminals of phase
shifting transformer. The voltage at output terminals is displaced from
incoming voltage by some angle without change in magnitude. Thus
voltages at output terminals are displaced incoming voltages which is
shown in the phasor diagram in the Fig. 5.
![]() |
Fig. 5 Phasor diagram |
then why not we use it for power factor correction?
ReplyDeleteNice post. Thanks for sharing this blog. We also Manufacturers of Control Transformers Control Transformers Manufacturers
ReplyDelete