a strange question

[lofty84]

an a.c load has an output power of 12 kw and an input of 15 kva. determine the reactive power

can anyone work this out it looks simple but i cant remember what to do again

 

[IQ Electrical]

KVAr^2 = KVA^2 - KW^2

Or use KVAr = sqare root of KVA^2 - KW^2

KVAr = sqare root of 225 - 144

= 9 KVAr

 

[lofty84]

you know your stuff mate, nice one

 

[IQ Electrical]

No problem, it keeps my AC theory fresh too!

 

[lofty84]

Heres a weird one

what are the copper losses in the primary winding of a step up tansformer, where the resistance of the winding is 0.3 ohm and the transformer is drawing 8 amps from the supply ?

any Ideas IQ ?

 

[IQ Electrical]

Copper loss = I^2 R

= 64 X 0.3

= 19.2 Watts

 

[lofty84]

Can I just ask, is this all of the top of your head ?

 

[IQ Electrical]

Yes but I've been running revision classes for 2 years so it sticks!

 

[lofty84]

ive got more

what is the current that can be drawn from a 100va transformer if it has an out put voltage of 20 v


another is

if a transformer has a maximum output voltage of 200v and supplies a current of 12a what is the KVA rateing of the transformer

 

[johnboy6083]

first one assuming 100% efficiency is 5A

 

[johnboy6083]

2nd question is 2.4KVA

 

[lofty84]

thanks mate how did you work out the second one please

 

[johnboy6083]

P=VI

KVA translates into Kilo volt amperes
KVA is apparent power, and Kw is real power.

So to work out apparent power, you multiply the real power by thepower factor. As you haven't given me a power factor, i asuume unity ( a power factor of 1).

So in other words, KW and KVA are one and the same, with a power factor of one.

 

[lofty84]

some more if thats ok and youve got time

a single phase transformer with 2000 primary turns and 700 secondary turns is fed from a 230v ac supply determine the secondary voltage


a transformer haveing a turns ratio of 2.8 (primary to secondary) is connected to a 230 v supply. calculate the output voltage

 

[lofty84]

some more if thats ok and youve got time

a single phase transformer with 2000 primary turns and 700 secondary turns is fed from a 230v ac supply determine the secondary voltage

= 700/2000 x 230 = 0.85 v is that right

a transformer haveing a turns ratio of 2.8 (primary to secondary) is connected to a 230 v supply. calculate the output voltage

= = 8/2 x 230 = 920 v is this right

When a current of 200mA flows into a capacitor for 40mS, the charge developed in the capacitor would be ???

I dont suppose anyone knows the answer for this