Page 272 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Transformer rating

• Transformer is rated in kVA and not KW
• iron loss of a transformer depends on voltage.
• Copper loss of a transformer depends on current
• Total transformer loss depends on volt-ampere (VA) and not on phase angle between voltage and current

• Transformer loss is independent of load power factor.
• Hence rating of transformers is in kVA and not in kW.
Voltage regulation
No load voltage - Full load voltage
• % regulation "down" =
Full load voltage
• unless stated otherwise, regulation is to be taken as regulation "down"
No load voltage - Full load voltage
• % regulation "up" =
No load voltage
Sumpner back to back test (Fig 18)

Fig 18

• Find efficiency, regulation and heating under load conditions
• Can be done if 2 similar tx. Are available
• Power drawn is only for meeting losses

• Primaries of tx. Are paralleled
• If switch s1 open, W1 indicates core loss of 2 transformers
• Secondary potentials are in opposition
• V = V
AB CD
• A is connected to C and B to D

• No secondary current due to potentials are in opposition
• T is auxiliary transformer
• Through T, rated I2 is passed in secondary’s by closing switch S1
• I2 flows from D to C and then from A to B.
• Flow of I1 is through the loop FEJLGHMF

• I2 does not pass through W1

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