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1-Newbie

## Frequency Conversion

Input (in Mathcad) Freq:=60*rpm and the result is 6.283 Hz instead of 1.000 Hz.

I know in terms of angular velocity, 1 revolution per second is = 2*Pi/sec = 6.283/sec, but all of the sources I have found all say that 60 rpm = 1.000 Hz.

2 REPLIES 2
19-Tanzanite
(To:ptc-5646100)

This is an issue that has come up many times. The confusion goes back to when 1 Hz was considered to be the same as 1 cycle per second, and a cycle was usually thought of in terms of a rotating element in some application of mechanical engineering. However, 60*rpm is an angular velocity, not a frequency, and Mathcad therefore converts it to an angular velocity of 6.283 radians per second but unfortunately does not show the radian as a unit. There are some old very long threads on this topic. Here's one of them:

http://communities.ptc.com/message/53939#53939

It's an old thread so some of the thread order got mixed up when the forums were changed, but it's so long I doubt you want to read most of it anyway . The first few posts will give you an idea of where the rest is headed (into a very long argument!). And it's only one of several!

Mathcad can deal with this though. It has an additional unit, Hza, which will give you the answers you want.

23-Emerald I
(To:ptc-5646100)

Mathcad has two versions of Hertz: Hz is literally once per second.

Hza is 2 pi radians per second (angular Hertz?)

Mathcad's built-in rpm converts correctly to Hza, 60 rpm = 1 Hza

If you are working in rotating shafts you need to get used to it. Power is torque times speed; the way it's set up now foot-pounds force (or any torque unit) times rpm will correctly calculate power (hp).

Small price to pay to get rid of 550 (is that the right number?) Try it, you'll like it!

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