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PS_100348051-Visitor
1-Visitor
June 15, 2022
Solved

Trying to calculate Power of a motor when using rpm and converting it to rad/sec.

  • June 15, 2022
  • 3 replies
  • 6691 views

I am using Mathcad Prime Release 7.0 and Datecode7.0.0.0

I'm trying to calculate power loss in a motor and rpm will not work as a unit of measure. Also Radians per second also does not work when I convert it from rpm*2PI/60.

Here are the errors that I faced
rpm: not a valid unit.
(Before calculating the output power of the motor, convert speed from units of RPM to rad/sec.)
when trying to calculate: Nm* rad/sec to watts it gives me s*W in the answer.

Best answer by Werner_E

when trying to calculate: Nm* rad/sec to watts it gives me s*W in the answer.

Do you mean you don't see the results shown in the pic below when you type in these expressions?

Werner_E_0-1655377490304.png

Be ware that rpm is defined in Mathcad as   2* pi rad / (60 s)

Werner_E_1-1655377712473.png

and revolution (rev) is defined as 2pi (or 2 pi rad), not as 1.

Werner_E_2-1655377941492.png

 

Is "not a valid unit" the exact error message you get or is it something like "These units are not compatible" or "This variable is undefined" ?

 

You sure should show at least a screen shot including the error message - better of course is attaching the worksheet as well.

 

3 replies

T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
June 16, 2022

Hi,

Make sure you ensure each unit is labelled as a unit (text is blue on default settings)

Go to Math | Style | Labels | Unit to change any that are not units

Capture.JPG

P
PS_100348051-VisitorAuthor
1-Visitor
June 16, 2022
Thank you for the reply. Yes I have done that, but, still the same issue with rpm.
T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
June 16, 2022

Hi,

Can you upload your worksheet?

Cheers

Terry

W
Werner_E25-Diamond IAnswer
25-Diamond I
June 16, 2022

when trying to calculate: Nm* rad/sec to watts it gives me s*W in the answer.

Do you mean you don't see the results shown in the pic below when you type in these expressions?

Werner_E_0-1655377490304.png

Be ware that rpm is defined in Mathcad as   2* pi rad / (60 s)

Werner_E_1-1655377712473.png

and revolution (rev) is defined as 2pi (or 2 pi rad), not as 1.

Werner_E_2-1655377941492.png

 

Is "not a valid unit" the exact error message you get or is it something like "These units are not compatible" or "This variable is undefined" ?

 

You sure should show at least a screen shot including the error message - better of course is attaching the worksheet as well.

 

P
PS_100348051-VisitorAuthor
1-Visitor
June 17, 2022

Ok, I had Base Units selected.  I changed that to Unit System: SI and rpm started being recognized.  It works now.  Thank you.

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
June 16, 2022

Mathcad (even Prime) does an excellent job of managing units.  Including unit conversions.

Fred_Kohlhepp_0-1655380051692.png

The Mathcad unit "rpm" has 2 pi built in so that it will handle angular speed correctly.  Note that the companion unit "Hza" has that too!

 

Prime 4 (Express) file attached, should open and execute in 7.

 

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