Page 253 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

No - load input power_W0 = V1 l cos f
0 0
I has two components ;
o
i One in phase with V1.
active or working component Iw

supplies small quantity of primary Cu loss.
I = l0cos f0
w
ii other component is in quadrature with V1 and is known as magnetising component Iμ
its function is to sustain the alternating flux in the core.

It is wattless. Iµ = l0 sin f0
• I = √(OΙμ² + Iw²).
O
Cooling of transformers
Types of transformer cooling systems
• AN – Air Natural

• ONAN – Oil Natural Air Natural
• ONAF – Oil Natural Air Forced
• OFAF – Oil Forced Air Forced
• OFWF – Oil Forced Water Forced

• OD Applications - Oil Directed -are always forced oil flow that is directed through predetermined paths in the
transformer winding.
ONAN - Transformer Cooling Systems
• ONAN – Formerly known as OA.

• Natural convection flow of hot oil is utilized to dissipate heat.
• Radiator or tube applications are predominately used.
• Least expensive form of cooling if application and cooling levels are met.
ONAF - Transformer Cooling Systems
• ONAF – Formerly known as FA.

• Natural convection flow of hot oil is utilized in conjunction with cooling fans.
• Heat dissipation is increased across cooling surfaces by air movement.
• Most common application in the field.
• Offers dual ratings as an ONAN/ONAF system.

• ONAF – Formerly known as FA.
• Natural convection flow of hot oil is utilized in conjunction with cooling fans.
• Heat dissipation is increased across cooling surfaces by air movement.
• Most common application in the field.
• Offers dual ratings as an ONAN/ONAF system.
Auto transformer: An auto transformer works on the principle of self-induction. It has onlyone winding, part of
this being common to both primary and secondary. Primary and secondary are not electrically isolated. Theory
and operation is similar to two-winding transformer. This type of T/F uses less copper and hence is cheaper. They
areused where transformation ratio differs little from unity. Their voltage regulation is superior

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CITS : Power - Electrician & Wireman - Lesson 41-49