Electrical Machine Design

E-mail

WEEK 07: TRANSFOMERS: CALCULATIONS & TEST DATA

Sections: Volt-Turns | Density | Insulation | Stampings | Window

Volt-Turns

Volt-Amperes, V-A = 4.44f x F x TI x 10^-8

But the Volts per turn, Vt = E/T = EI/TI

Thus Vt² = 4.44 f F x 10^-8 x EI/TI = k x (fF/TI) x EI
where: k = 4.44 x 10^-8

Thus:

Core (DT)

c = 40 - 70

Core (PT)

c = 30 - 55

Sheel (DT)

c = 25 - 40

Shell (PT)

c = 20 - 30

where: , thus

Sections: Volt-Turns | Density | Insulation | Stampings | Window

Flux and Current Density

Flux Density	f = 25	f = 50 or 60
Distribution T	75,000 to 90,000	70,000 to 85,000
Power Transf.	85,000 to 95,000	80,000 to 95,000

Average Values of Current Density, amp/sq.in.

Standard Distribution

900 to 1,600

Transfromer in Central Generating Station

1,500 to 2,100

Large, oil-, forced-, water-ventilated

1,800 to 2,600

Usually, permissible current densities are limited by heating.

The resistivity of copper is usually at 1.0W per CM in 60°C and 1.05W per CM in 75°C. Note that 1 cu. in. of copper is equal to 4/p x 10⁶ CM, thus, the watts lost per cu. in. would depend on the current density.

Then, Watts lost per cu. in. = D² x 10⁵ / [(4/p) x 10⁶] = 0.825D² x 10^-6 but 1 cu. in. of copper = 0.32 lb., thus Watts lost per lb. of copper = 2.57D² x 10^-6.

Sections: Volt-Turns | Density | Insulation | Stampings | Window

Insulation

Voltage

Thickness,
mills

220

40

440

45

1000

55

2400

80

Characteristics.
Insulations should be mechanically strong, high dielectric strength and not soluble in oil. Usual types include: cotton tape, empire cloth, micanite, press board and horn paper.

When used in dry oil, up to 50,000 volts alternating between metal disks 0.5 in in diameter, there should be 0.2 in. separation.

Note: 25 parts of water per million by volume will lower dielectric strength of approximately 50%. The oil ducts should be at least 3/16 to 5/16 of an inch. The creepage distance (under oil) must be approximately 0.25 + 0.07kV.

Generally, the insualtion thickness should be about 0.2 + 0.35 kV, where kV is the rms value of kilovolts of the transformer. Increase this up by 25% for coils that are rectangular.

For 69kV or higher, end turns should be insulated from breakdown due to surge. 2.5% of the end windings should be with special insulations up to 10,000 volts. Add ¼% for each increase of 10 volts, and add 15% for 220,000 V transformers.

Stampings