Now then party ppl, In need of help to get me ready for my 202 exam in a few days

[Chrisuk]

Been working all day and now trying to revise be having some problems working some stuff out
we had a test on wednesday and i got a crappy 20 out of 30 so i have 10 questions to get my head around!!!

if someone could help it would help me

question 1)

A distribution cable ABC is fed at A at 230V and supplies loads at B and C
if the volt drop from A to C is 4V, and from B to C is 1V, then the voltage at B is

a) 229V
b) 227V
c) 226V
d) 225V <----correct answer

obviously adding them together gets the 5V
but how is it 5V at B???

Q2)

An induction motor rotates at 16/s and has a slip of 4%.
if the frequency of the supply is 50Hz the number of poles on the motor is

a) 8
b) 6
c) 4
d) 2

All we have done is questions when we know the amount of poles, we have not done any questions to work out how many poles

i know the answer to the above (well if college gave the correct ones) can someone please tell me how to work it out

cheers

 

[Simonslimline]

Same here with the motor, I would be interested to see the answer for this .

I may have some notes to work this out but cant get them as am away from home for a while due to a broken leg.

 

[Chrisuk]

Hi Si

its getting close, work needs to be done and i am working nights from monday for the next 2 week so i think its highly likely that today and tomorrow and a bit wednesday will be my only days to get the stuff straight in my head

its easy to add the 4v and 1V, ok thats 5V im not that thick
A to C is 4V
so how can its also be 5V at B, baffles my brain

explaination is greatly needed

 

[telectrix]

a 2 pole motor rotates @50 c/sec, less losses. so 1/3 of 50 = 16.667 . 16.667 x 0.96 = 16c/sec. so the answer is 2 x 3 = 6 pole.QED.

 

[Chrisuk]

if you say 4 pole a.c three phase machine is fed with a supply at a fequency of 50Hz, calc the synchronous speed i can do that

i can work out per unit slip

but we just havent done it so we can work out how many poles!!

in all questions we have been given the number of poles!!!! now i need to work out how many poles i'm totally foooked!!!!

didnt see your post Tel until i clicked send!!!

will have a look at what u said

 

[Chrisuk]

a 2 pole motor rotates @50 c/sec, less losses. so 1/3 of 50 = 16.667 . 16.667 x 0.96 = 16c/sec. so the answer is 2 x 3 = 6 pole.QED.

ok i get that a 2 pole motor rotates @ 50c/sec
why did you do a 1/3 of 50??
the question said a 4% slip , isnt that 0.64?
and where did the 0.96 come from??

thanks for helping me though

and Yes 6 poles is what college said it is

 

[telectrix]

OK. 50c/s is the sych. speed for 2 poles, the 16c/s motor is approx. 1/3 of that, so it's 2 poles x 3 =6 poles.

 

[Chrisuk]

OK. 50c/s is the sych. speed for 2 poles, the 16c/s motor is approx. 1/3 of that, so it's 2 poles x 3 =6 poles.

good i think i get this, so for 50/16 is approx 3, thats the 3rd
if the question was 10c/s it would be 5 so the answer would be 10 poles?? 5N 5S

i hope so

what through me was the 4% slip

and then the 0.96

but i may have it now...maybe

many thanks

 

[Sintra]

ok i get that a 2 pole motor rotates @ 50c/sec
why did you do a 1/3 of 50??
the question said a 4% slip , isnt that 0.64?
and where did the 0.96 come from??

thanks for helping me though

and Yes 6 poles is what college said it is

The 0.96 is the 96% left after the 4% slip has been taken off.

So 2 pole at 50/sec then take off the 4% slip. 4% of 50 = 2 leaving 48 or alternatively 50 x 0.96 = 48

2 pole @ 48/sec
48 / 16 = 3
2 x 3 = 6 poles

Surely the answer to the first question is b or am I drunk.

 

[telectrix]

the 0.96 is 1.00- 4%.

and for the VD one, i get 227V @B and 226V @C.

 

[Chrisuk]

The 0.96 is the 96% left after the 4% slip has been taken off.

So 2 pole at 50/sec then take off the 4% slip. 4% of 50 = 2 leaving 48 or alternatively 50 x 0.96 = 48

2 pole @ 48/sec
48 / 16 = 3
2 x 3 = 6 poles

Surely the answer to the first question is b or am I drunk.

Huge thanks for that sintra

I also said the answer to the first question was b but college said it was wrong, but the tutor said many times C&G makes plenty of mistakes