Dears,

I just want to ask if when we are talking about line frequency it means 60hz or 3600 CPM. My question is, it can be seen exactly in the spectrum on that frequency or there are instances that it can less or more such as 3550 CPM or 3690 CPM. Then my 2nd question is 2X line frequency is just simply multiply the line frequency by 2? or it is different.

Thank you..
Original Post
In the States line frequency from a public utility is 3600 cpm. That frequency should be dead on with no variation.

I used to work at a paper mill which generated it's own power. They would push the frequency up just a bit, but not by much.

2 times line frequency is 7200 cpm.

Of course 50 hz systems has 3000 cpm line frequency and 6000 cpm 2 times line frequency.
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One needs to be cautious when dealing with power "off grid". On one occasion I was working in a very remote area of an island and the power was supplied on a local basis by a diesel driven generator. Instead of being the usual 50 Hz (for the location) the generator was running slow and the frequency was closer to 48 Hz. One had to be careful in analyzing induction motor vibrations especially.

This is probably much less likely to occur these days; in my instance this was something from the early 1970's, another century!
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I work in one mill where the power plant is required to produce and use a certain amount of power at 25 hz. I didn't know about that until I found a bad bearing in what turned out to be a 2 pole motor powered by 25 hz.
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Alexis,

We measure 2 x line frequency because it is usually alternating current.

Synchronous motor speed (rpm) = 2 x line frequency (cpm)/number of motor poles

In an AC induction motor, there will normally be a small degree of slip so a 4 pole motor on 7200 cpm would have a synchronous speed of 1800 rpm but a running speed of 1750-1800 rpm depending on the construction and load.
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