Motor Speed rpm or min-1

[philr]

Can anyone tell me the difference between a motor speed in RPM and a speed in min-1 and how I convert between the two?

Thanks

 

[Skogsgurra]

philr

There is no difference between RPM and min-1.


This is the explanation:

RPM translates into Revolutions Per Minute - or revolutions/Minute, if you like that better.

Now enters Academia. That is, thin theoretical air and correctness instead of practical engineering.

Academia finds that "Revolution" does not comply with SI (Systeme International). Even worse, it has no dimension at all. The dimension of Revolution is 1.

Division is often expressed with exponent(-1) so RPM translates into R (1) per (division, i.e. exp(-1) minute (min). The whole thing is written min-1 or min**-1.

But, there is more! A minute is not allowed in the SI. The only time unit is the second. So min-1 is not correct either. You shall use s-1.

But that still isn't good, one revolution is not the natural angle measurement - the radian (rad) is. So the correct dimension for rotational speed is rad/sec or rads-1.

1 rad/sec = 6.28 RPS = 377 RPM (or 377 min-1, if that makes you happier)

 

[Skogsgurra]

philr

Please do not read last line in previous posting. It is totally wrong.

This is how it should be:

1 RPS = 6.28 rad/s = 60 RPM (or 60 min-1)

Sorry.

skogsgurra

 

[electricpete]

good comments above. You have to ask the person who wrote min-1 whether they meant rev per minute or radian per minute.

I think there is a typo in the above message. Should be

1 rad/sec = (1/6.28) RPS = 60/6.28 RPM

 

[Skogsgurra]

OK, pete.

You almost got there before me.

I will be more careful next time.

 

[philr]

Thanks Guys

You info has been most helpful

 

[UKpete]

skogsgurra - went I went to school in England I was taught feet & inches, yards, chains, furlongs, miles; 16 ounzes in a pound, 14 pounds in a stone, 8 stones in a hundredweight (abbreviated to cwt for some reason), 20 cwt or 2240 pounds in a ton.

Today my kids look at me and laugh when I describe it, they are taught the metric system. RPM is better than 1/min, but don't blame SI! It's got a lot going for it.

 

[Skogsgurra]

UKpete,

What hardship! My mother had a measure that she called an "aln" it was a stick about two feet (see!) long - or six decimeters in SI lingo.

The "aln" was said to be the length from elbow to fingertip on one of our ancient kings.

Otherwise we do not struggle with feet and inches - except when we are buying wood and nails for building purposes. Then we always use inch for width and thickness as well as length of the nails. But we never have to use these measures in complicated calculations. We almost never use HP either.

 

[UKpete]

skogsgurra - please excuse the slight exasperation!

UKPete