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How to get synchronous speed of an ac motor in RPM?

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powersys

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We know that:

n = 120f/p ...... (1)

... where n is the synchronous speed of ac motor in RPM, f is power supply frequency, and p is the number of pole.

If we look at the right hand side (RHS) of equation (1), the unit is Hz or 1/s. Also, we know that the unit for n is RPM. If my opinion, the unit at RHS, i.e. Hz or 1/s, does not match RPM. Anyone could explain about this? Thanks.
 

1RPM => 1 / 1min = 1/60s

1Hz => 1 / 1s => 60RPM

In both cases the unit is 1 / time ..

Regards,
IanP
 

IanP said:
1RPM => 1 / 1min = 1/60s

1Hz => 1 / 1s => 60RPM

In both cases the unit is 1 / time ..
Click to expand...
One of the useful features of Mathcad is its ability to track standard units during calculations and to convert the units of calculated quantities automatically. If the unit is not available in Mathcad's built-in unit library, then we can create one. As rpm is not available in the library, I tried to create one as shown in the figure below:

unit.jpg


Pls note that I define rpm ≡ 1/s. I should have defined rpm ≡ 1/(60s). But if I define rpm ≡ 1/(60s), then all the results were wrong... Therefore I asked the question here... any idea to modify the Mathcad file? Thanks.
 

powersys,
let f = power frequency (Hz)
. p = # of poles
. fsyncs(Revolutions/second) = 2f/(p)
. fsync(RPM) = 60fsyncs.
Examples;
. A 2 pole motor with 60 Hz power has a sysnchronous speed of 3600 RPM.
. A 4 pole motor with 60 Hz power has a synchrounous speed of 1800 RPM.
Regards,
Kral
 

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