A320sRcool
7th Oct 2005, 05:31
How many poles needed for 400hz AC gen to provide 6000 rpm
How do I get the correct ans of 4 ?
Grass strip basher
7th Oct 2005, 06:54
I may be shot down here... see my earlier thread about how much I hate electrics but I'll give it a go!
6000rpm = 100 revolutions per second (6000 rpm/60seconds)
So to get 400 Htz you need 4 poles as each pole will revlove 100 times a second so 100*4=400 Htz
Now in all likelyhood I have just displayed my complete ignorance in terms of all things electric in nature...
Alex Whittingham
7th Oct 2005, 07:31
The formula is the same one for a synchronous motor, RPM = (Frequency (Hz) x 60) ÷ pole pairs
This means pole pairs = (Frequency x 60) ÷ RPM = 400 x 60 ÷ 6000 = 4 pole pairs = 8 poles
You've picked up some feedback from an early edition of the CQB. The question didn't match the given answer. The CAA say they have got the JAA subject experts to correct the error, we believe by modifying the 'given' answer from 4 to 8. It would make life much simpler if the JAA examiners referred to 'pole pairs' like everyone else rather than 'poles', one is always left wondering whether they really mean it.