Transformer/apparatus

Which of the following is not a basic element of transformer?

• Core

• Primary winding

• Secondary winding

• Mutual flux

Transformers are used to change the value of the following EXCEPT:

• alternating current

• alternating voltage

• power

• resistance

A transformer having 1000 primary tums is connected to a 250-V a.c supply. For a secondary voltage of 400 V, the number of secondary tums should be

• 1600

• 250

• 400

• 1250

4. A transformer has a primary winding of 2,000 turns and voltage of 2,400 v and current of 8.66-j5 A with an impedance Z2 connected across the secondary winding. The secondary winding has 500 turns. What is the value of the secondary impedance?

• 8.66 + j5 ohm

• 26+ j15 ohm

• 15.32 + j10 ohm

• 13+ j7.5 ohm

When a welding transformer is used in a resistance welding, it will

• step up voltage

• step down voltage

• step up current

• step down current

The primary of transformer has 200 turns and is excited by a 60 Hz, 220 V source. What is the maximum value of the core flux?

• 6.12 mWb

• 4.13 mWb

• 3.41 mWb

• 5.31 mWb

A transformer is rated 2-kVA, 230/115 v, 60 Hz. Because of an emergency, this transformer has to be used on a 50-Hz system. If the flux density in the transformer core is to be kept the same as at 60 Hz and 230 v, what voltage should be applied at 50 Hz?

• 195.83 v

• 200 v

• 183.33 v

• 191.67 v

The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer. Esc=72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the percent regulation at a power factor of 0.75 lagging.

• 2.54%

• 2.86%

• 2.62%

• 2.71%

An 8 MVA, 69/13.2 kV transformer has a 6.75% impedance. What is the actual impedance?

• 0.147015 ohms

• 147.015

• 14.7015 ohms

• 1.47015 ohms

A 7.5 MVA, 69/13.8 kV, 3-phase transformer has 8% impedance. What is the per unit

impedance at 10 MVA base?

• 0.24

• 0.106

• 0.06

• 0.096