50 hz motor... 1500 rpm! :(

[Fahad Bhatti]

hI ce's,

I have never been able to get a very basic idea about the frequency the generators generate.

LET for example the case of a generator who is run by mechanical source at 1500 r.p.m, the frequency we get from it is 50 hz, if frequency depends on revolutions, shouldn't it be 1500 hz?

I know the formula
speed=120*f/p or f= speed*p/120

but this does not satisfy me,

Is this not the case in motors that 1 hz is 1 mechanical revolution?

[sauravgoswami]

buddy frequency is no of cycles per sec,if u rotate the generator one revolution per sec u will get 1 hz of frequency provided it generate 1 complete cycle of AC voltage

[Fahad Bhatti]

but saurav according to this rule, our motors should run at 50 revolution per seconds (as the frequency of supply is 50 hz), and that should be the only available speed, but they run at various speeds depending upon their number of poles, why is that so? why don't the always follow the same 50 rev/sec ?

[kashish0711]

Mate that formula does not explain you directly

Look the frequency of the generator depends upon the no. of cycles of emf induced at a given speed.

Just imagine a generator having "P" no. of poles on the rotor revolving at speed at "N" r.p.m.. When a conductor passes through a pair of poles, one cycle of emf is induced in it.

Therefore, No. of cycles made per revolution = P/2

No. of revolutions made per second = N/60

Therefore, No. of cycles made per second =
No. of cycles/revolution X No. of revolutions per second

Frequency, F= P/2 X N/60

or, F= PN/120 cycles/second or Hz


This is basically the derivation of the formula you mentioned

This explains the point

But If any confusions, let me know, Ill draw a diagram for you and explain you in detail. : )

[Fahad Bhatti]

Thanx kashi, I get the concept of r.p.m and hz but m still having some confusion that HOW is No. of cycles made per revolution = p/2 ?

Please post a diagram if you have one, that would definitely clear the idea.

[kashish0711]

Firstly I am really sorry for this so late reply, I totally forgot, I am really sorry
OK here you go, made the diagram for you







Firstly understand that frequency is not generated by no. of revolutions but how many times the cycles of emf are produced per second.

Now in the above figure a motor is shown having "P" no. of poles on a rotor revolving at a speed of "Ns" r.p.m.

When a conductor passes through a pair of poles, one cycle of emf is induced.

Now the frequency of the motor should be the no. of these emf cycles produced per second.

Now we know that one cycle of emf is produced when the conductor passes through a pair of poles. Therefore we know,


No. of e.m.f. cycles made per revolution = P/2

Now we know that the speed of the motor was "Ns" r.p.m., therefore we know,

No. of revolutions per second = Ns / 60

Now we know that no. of revolutions made per second, and also we know that those revolutions per second will have certain no. of emf cycles produced. Therefore we will find the no. of emf cycles in the no. of revolutions per second.

Therefore, we know,

No. of e.m.f. cycles per second = No. of cycles / Revolution X No. of revolutions per second

Now we know that no. of emf cycles per second forms the frequency of the motor, therefore the last equation can also be written as

Frequency, f = P / 2 X Ns / 60

simplifying the above equation we get

Frequency, f = P Ns / 120 cycles/second or Hz


This is how the frequency of a motor is formed.


I tried to eloborate it a lot, I hope its clear now.

Good Day