Traction Rolling Stock - Indian Railways Institute of Electrical ...

Drying with cold spray: This is the most efficient method of drying oil, and consists
of spraying cold oil into a vacuum. The temperature vacuum, however, must be
lower than the temperature of evaporation of water.
The low pressure and fine dispersion thus obtained considerably aid in the
separation of the water which is drawn off by the vacuum pump.
Oil can only be dried by this method if an airtight vessel is available.
1.3 WORKING PRINCIPLE OF TAP CHANGER
1.3.1 Description
The tap changer is directly built on to the transformer. The tappings of the transformers
are brought out and arranged in circular fashion on an insulated contact plate. There are two rows
of contact segments which are aligned on outer and inner circles of the contact plate. An arm
which is known as selector arm is driven by a shaft at the centre of the contact plate. Two rollers
are situated at the edge of the selector arm. These rollers make connection between the
respective segment in outer or inner rings. These rings are provided in front of the contact plate.
The centre shaft which extends outside the tap changer casing is driven by an air servo motor
known as SMGR. The design of the air servo motor is such that once the selector arm begins its
movement, it can be stopped only at the required tap (not in between two taps). The connection
between the inner or outer ring to the transformer is being established by means of CGR
contactor.
1.3.2 Method of operation (Ref. fig. 1.3.2)
The selector arm is actuated by the driving shaft through an intermittent gearing
comprising of drive wheel, lantern gear pinion and stopping wheel. This driving shaft also operates
the CGR Cam shaft in sequence with the operation of contact rollers. The opening and closing
sequence of CGR contactors are given below :
CGR I CGR II CGR III
Even Notches Open Closed Closed
In bet. Notches Closed Closed Open
Odd Notches Closed Open Open
Before the moving contact roller leaves the zero tap contact segment, it touches the first tap
segment. During this the CGRI contactor closes inserting the diverter resistance RGR in between
the tapped winding. This results in shorting of the section of winding between Tap 'O' and Tap 1
thro. RGR. This resistance RGR restrict the short circuit current when the selector arm further
moves fully on Tap 1, the inner contact roller breaks the contact with zero tap. Meanwhile CGR II
contactor opens and cuts off RGR from circuit. Likewise when the selector arm moves from first
tap to second tap the contact roller (outer) will continue to make connection with Tap 1 segment &
the inner contact roller establishes connection between inner segment and inner ring. (This is due
to over lapping contact segment). At the same time CGRI, CGR II are closed again inserting the
resistor RGR in between the short circuited winding. Now the service current flows from the
contact ring outer in the selector through the resistor RGR, when the selector arm further moves,
the outer roller leaves the Tap l segment & breaks the contact. At the same time CGR I gets
opened and CGR II & CGR III are closed. This again cuts off the resistance RGR.
The opening & closing of the CGR contacts is carried out by a cam shaft which is driven