Private tutoring??

[Andy™]

sorry, the question does say rev/sec, not RPM

50 / 3 = 16.6

16.6 x .96 = 15.99. close to C, but id have expected an exact answer? or has anyone else got a different answer?

 

[robinson]

cheers thats better i think where did u get the .96 from ??

 

[Rack Failure]

Andy

The way I was taught many years ago, then rotor speed would be 960RPM or if you like 16RPS.

The formula then was:

Speed (RPM) = 60 x f/pairs of poles, which is equals to

Speed (RPM = 60 x 50/3

Speed (RPM) = 1000 (this 1000 is the synchronous speed of the rotating magneting field)

Slip is the speed the rotor lags behind (and must) the rotating field. 4% =40RPM. Hence 960RPM.

Just in case it is of interest, the value 60 is what converts the frequency from 50 cycles per second to cycles per minute, which in turn calculates speed in revs per minute.

It seems the modern way to calculate RPM is to use the value 120 x f / number of poles ie 120 x 50/6 = 1000 same answer.

The 120 is called a constant.

Hope this is of interest.

Steve

 

[Andy™]

cheers thats better i think where did u get the .96 from ??

if you need a % off something, its always 1 - (number / 100). so if you need to take off 20%, time value by 0.8, if you need to remove 4%, times value by 0.96. just a shorter way

 

[Andy™]

AndyThe way I was taught many years ago, then rotor speed would be 960RPM or if you like 16RPS.

The formula then was:

Speed (RPM) = 60 x f/pairs of poles, which is equals to

Speed (RPM = 60 x 50/3

Speed (RPM) = 1000 (this 1000 is the synchronous speed of the rotating magneting field)

Slip is the speed the rotor lags behind (and must) the rotating field. 4% =40RPM. Hence 960RPM.

Just in case it is of interest, the value 60 is what converts the frequency from 50 cycles per second to cycles per minute, which in turn calculates speed in revs per minute.

It seems the modern way to calculate RPM is to use the value 120 x f / number of poles ie 120 x 50/6 = 1000 same answer.

The 120 is called a constant.

Hope this is of interest.

Steve

it still gets the correct answer, so just go with what you find easier

 

[robinson]

thanks everyone i dont think anyway u can get a exzact answer for it so i am just going to go with the closest i have the answer paper here too so i will check it when i have done them all cheers everyone

 

[robinson]

hey everyone thanks for yesterday you all made it seem easier, alot better than other people who have tried and actually get paid to teach me it lol sorry but i have another question i just cant get my head around....

What is the RMS value of a AC supply that has a apeak value of 325.22 volts?

a.) 324.583

b.) 460

c.) 229.93

d.) 459.36

i know the answer is C because this is a practice paper that i have already done but i cant remeber how i done it, i know that RMS means root means squared !

cheers

 

[Andy™]

hey everyone thanks for yesterday you all made it seem easier, alot better than other people who have tried and actually get paid to teach me it lol sorry but i have another question i just cant get my head around....What is the RMS value of a AC supply that has a apeak value of 325.22 volts?

a.) 324.583

b.) 460

c.) 229.93

d.) 459.36

i know the answer is C because this is a practice paper that i have already done but i cant remeber how i done it, i know that RMS means root means squared !

cheers

325.22 x 0.7071 (RMS value) = 229.93

 

[robinson]

ahhh thanks !! may sound stupiid where did the 0.7071 come from isnt that just standard on everyone i cant remeber i think it is??

 

[Andy™]

ahhh thanks !! may sound stupiid where did the 0.7071 come from isnt that just standard on everyone i cant remeber i think it is??

0.7071 is the RMS value. times peak by this to get RMS, divide RMS by this to get peak

 

[robinson]

cheers mate brillient!!!! been revising two days straight if i dont pass think i will break down lol ! headbang <<<< me

 

[robinson]

A transformer has a primary supply of 240 volts and a primary winding of 950 turns and secondary winding of 380 turns. The secondary voltage will be?

a.) 600v

b.)330V

c.) 96 V

d.) 25V

i have looked in my notes and books and seems to have no straight answer on how to do this ?? no-one from college seems to know either :S ???

 

[Andy™]

A transformer has a primary supply of 240 volts and a primary winding of 950 turns and secondary winding of 380 turns. The secondary voltage will be?a.) 600v

b.)330V

c.) 96 V

d.) 25V

i have looked in my notes and books and seems to have no straight answer on how to do this ?? no-one from college seems to know either :S ???

240 / 950 x 380 = ....... v

 

[robinson]

haha thank you that makes sense u should see my notes for that its not as simple as how u have put it !

 

[robinson]

sorry everyone about the questions lol

a single phase circuit desipates 2.5kw of power and operates at a power factor of 0.95 what is the current flowing in the circuit??

is it something to do with cosine???

 

[Andy™]

W / (V x PF)

 

[robinson]

but i am not given the volts??

 

[RussellR]

If it is single phase and not stated then use the nominal voltage (230V for single phase, 400V for Three Phase).

 

[Andy™]

but i am not given the volts??

then you cannot work it out. it doesn mention voltage, so would usually be 230v, but the question may state 200v or other

 

[robinson]

haha oh yeah sorry tht was a stupid question i just been doin revison non stop all weekend gettin stupid now sorry