Frequency determination of mains

Thread Starter

Willen

Joined Nov 13, 2015
313
Hi, Few google search did not help me. Problem is to get correct/relevent term to search.

Mains has frequency like 50Hz, 60Hz and I don't know if other frequency exist. Which thing generates the frequency: an oscillator in power house? Or a frequency generator? Or RPM of the turbine?

And how accurate it is?
 

ericgibbs

Joined Jan 29, 2010
13,277
hi Willen.
RPM/Poles of the turbine?
Accuracy, quite good , many clocks use the mains frequency for timing.
E
EDIT:
Video link
 
Last edited:

BobTPH

Joined Jun 5, 2013
3,509
In the U.S. and possible elsewhere, the frequency is adjusted continuously so that it averages to the stated frequency. This way, synchronous motors, used for electric clocks keep the correct time, even if the frequency might be a bit off at any given time.

Bob
 

MaxHeadRoom

Joined Jul 18, 2013
23,389
Mains has frequency like 50Hz, 60Hz and I don't know if other frequency exist. Which thing generates the frequency: an oscillator in power house? Or a frequency generator? Or RPM of the turbine?
In the province I am in, the answer is Both 3 ph generator and electronically.
The many Hydro dams generate initially by way of 3ph alternators with governors, in some of the distant dams, it is then converted to DC for more efficient long distance transmission, at the final distribution end, it is then converted back to AC via electronic switching devices and crystal controlled means.
Max.
 

Reloadron

Joined Jan 15, 2015
6,002
Hi Willen and how goes it?
Hi, Few google search did not help me. Problem is to get correct/relevent term to search.

Mains has frequency like 50Hz, 60Hz and I don't know if other frequency exist. Which thing generates the frequency: an oscillator in power house? Or a frequency generator? Or RPM of the turbine?

And how accurate it is?
Well for the most part in large power generating stations it pretty much comes down to the rotational speed of the turbine. The formula looks a little like this:
• Frequency is dependent on:
• Number of field poles
• Speed of the generator
• f = (N)(P)/120, where
f = frequency (Hz)
N = rotor speed (rpm)
P = total number of poles
120 = Conversion from minutes to seconds and from “poles” to
“pole pairs”
(60 seconds/1 minute) x (2 poles/pole pair)

Even on a small scale AC generator it can get interesting. I have a small 4.0 KW 240 Volt generator out in my shed. Gasoline engine type and the engine has a governor to keep engine speed at about 3600 RPM. The generator is a 2 pole generator which is very common for a gasoline engine driven unit. So for my 60 Hz here is what it looks like:

f = (N)(P)/120 So 60 = 3600 * 2 / 120 and we get our 60 Hz. The same generator running at 3,000 RPM would have a frequency of 50 Hz. This is all well and fine for a RPM of 3600 or 3000. However, if we have a slower RPM from for example a diesel engine we can go to a 4 pole generator where 1800 and 1500 RPM would give us our 60 Hz and 50 Hz respectively.

The Wikipedia has a pretty good description of the evolution of mains power frequency which you can read here. The history and changes to mains frequency over the years is interesting reading.

I have omitted any discussion as to the newer Inverter Generators as I am not familiar with that design being used in any commercial power generating stations. There are several ways to attain mains voltage frequency but I kept it at the popular basics.

Ron
 

Thread Starter

Willen

Joined Nov 13, 2015
313
I understood generally RPM determines the output frequency which has some calculations.

Then can I say 'RPM determines frequency in alternator' means 'frequency determines the RPM in AC motor' generally?
 

Reloadron

Joined Jan 15, 2015
6,002
I understood generally RPM determines the output frequency which has some calculations.

Then can I say 'RPM determines frequency in alternator' means 'frequency determines the RPM in AC motor' generally?
Yes and "generally" is a good way to put it. For example, much like a generator:
  • RPM = (120 * Frequency) / # of poles in the motor
"Since the number of poles of a three phase induction motor is established when it is manufactured, the only way to change the speed of the motor is to change the Frequency". However, we do have a slight caveat in that A four pole three phase induction motor when operated at 60 Hz will be very close to 1800 RPM (synchronous speed).The rated full load speed will be less than synchronous speed by the value of Slip. A four pole three phase induction motor with a rated full load speed of 1750 has a slip rating of 2.7%.By formula": These are known as "pole slip" so as the motor is loaded we have pole slips and often if we read the motor name plate data we will see a 60 Hz AC motor with an RPM rating of 1750 RPM. Additionally this assumes a synchronous AC motor.

VFDS HOW DO I CALCULATE RPM FOR THREE PHASE INDUCTION MOTORS?

The same would apply to most synchronous single phase motors. Just remember the pole slip part of the equation.

Ron
 

RichardO

Joined May 4, 2013
2,271
The frequency of an individual generator is also determined by the grid. If a generator is out of phase with the grid, very bad things happen.
 
Top