Principles of electrical engineering E.X

 

ii) Answer the Problems:

1. The number of turns on the primary and secondary windings of a single-phase- transformer are 350 and 35 respectively. If the primary is connected to a 2.2 kV, 50-Hz supply, determine the secondary voltage on no-load

- the secondary voltage on no-load is: ……..

 

2. A 3000/200-V, 50-Hz, 1-phase transformer is built on a core having an effective cross-sectional area of 150 cm2 and has 80 turns in the low-voltage winding. Calculate; (a) the value of the maximum flux density in the core. (…….…)

(b) the number of turns in the high-voltage winding. (……….)

 

3. A 3,300/230-V, 50-Hz, 1-phase transformer is to be worked at a maximum flux density of 1.2 Wb/m2 in the core. The effective cross-sectional area of the transformer core is 150 cm2. Calculate suitable values of primary and secondary turns. (NP = …….. & NS = ………. )

 

4. A 40-kVA, 3,300/240-V, 50 Hz, 1-phase transformer has 660 turns on the primary, internal drops in the windings are to be ignored, Determine;

(a) the number of turns on the secondary. ( ………… )

(b) the maximum value of flux in the core. ( …….…… )

(c) the approximate value of primary and secondary full-load currents.

 

5. A double-wound, 1-phase transformer is required to step down from 1900 V to 240 V, 50-Hz. It is to have 1.5 V per turn. Calculate the required number of turns on the primary and secondary windings respectively. The peak value of flux density is required to be not more than 1.2 Wb/m2. Calculate the required cross-sectional area of the steel core. If the output is 10 kVA, calculate the secondary current.

( the secondary current = ……….. )

 

6. The no-load voltage ratio in a 1-phase, 50-Hz, core-type transformer is 1,200/440. Find the number of turns in each winding if the maximum flux is to be 0.075 Wb.

(NP = ………… & NS = …………)

 

7. A 1-phase transformer has 500 primary and 1200 secondary turns. The net cross-sectional area of the core is 75 cm2. If the primary winding be connected to a 400-V, 50 Hz supply, calculate;

(a) the peak value of flux density in the core. ( ………… )

(b) voltage induced in the secondary winding. ( ………… )

 

8. A 10-kVA, 1-phase transformer has a turn ratio of 300/23. The primary is connected to a 1500-V, 60 Hz supply. Find the secondary volts on open-circuit and the approximate values of the currents in the two windings on full-load. Find also the maximum value of the flux.

( ES = ……….., IP= …….. , IS= ……… & ϕm= …………..)

 

9. A 100-kVA, 3300/400-V, 50 Hz, 1 phase transformer has 110 turns on the secondary. Calculate the approximate values of the primary and secondary full-

load currents, the maximum value of flux in the core and the number of primary turns. How does the core flux vary with load?

( IP= …….. , IS= ……… , ϕm= ………….. & NP=………….)

 

10. The no-load current of a transformer is 5.0 A at 0.3 power factor when supplied at 230-V, 50-Hz. The number of turns on the primary winding is 200. Calculate; (a) the maximum value of flux in the core. ( ϕm= …………..)

(b) the core loss. (Pi = ……….……..)

(c) the magnetising current. (IM = …………….)

 

11. The no-load current of a transformer is 15 at a power factor of 0.2 when connected to a 460-V, 50-Hz supply. If the primary winding has 550 turns, calculate;

(a) the magnetising component of no-load current. (……..……)

(b) the iron loss. (…………...)

(c) the maximum value of the flux in the core. (……………)

 

12. The no-load current of a transformer is 4.0 A at 0.25 p.f. when supplied at 250-V, 50 Hz. The number of turns on the primary winding is 200. Calculate;

(i) the r.m.s. value of the flux in the core, (…………………)

(ii) the core loss. (…………………)

(iii) the magnetising current. (…………………)