Electrical Motor Calculations

[iken]

Hi, I have a spreadsheet relating to pulley changes on fans etc, and would like to know if what I have used for Power and running amps are correct. I have assumed the fan effeciency as 87%, motors are 400V, 3 phase.

Take a look and let me know if I am off the beaten path.

Thanks,

 

[waross]

I'm a little confused. Why are you changing pulleys if you have a VFD?
If you don't have a VFD, why are you confusing the issue.
But a point that may be important:
The motor speed is given as the synchronous speed. The motor actual speed is synchronous speed minus the slip. The actual speed of the motor will be from 1440 to 1460 RPM.
Running at 48 Hz from a VFD, the actual motor speed will be around 1392 RPM.
That slip frequency error may throw your fan discharge of by 6% to 10% or more. (The amount depends on the actual slip frequency. To make it more fun, as the load decreases, the slip frequency also decreases.
You can ignore the slip error for many applications, but if you are concerned with a 4% speed change, the slip[ may throw your calculations off.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iken]

Thank waross for your feedback.

put the VSD in there for an option (if there was a VSD, what setting would be required to acheive design flow).

Are my calculations for amperage draw and actual power consumption correct? This is what I need confirmation on.

Thanks again.

 

[waross]

I'll leave the kW calculations to someone more conversant with fan applications. Your amperage calculations are probably way off. You have neglected magnetizing current (Which is responsible for the less than unity power factor of most motors.)
At your light loadings, the magnetizing current is a significant fraction of the total current and may predominate. The efficiency also varies with the loading.
My suggestion, use a variable pitch pulley and adjust the pitch until you have the discharge that you need. The rest "It is what it is".
Hopefully someone conversant with fans will check your fan load calculations for you.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[automatic2]

your spreadsheet appears to have a linear speed to flow relationship. You'll want to visit that further

 

[iken]

Automatic2

I thought that was what the fan laws stated?

P2 = P1 * (N2/N1)

 

[krsh]

So you have a 1.8 kW motor installed? If yes then your running current should be between 1 and 4 amps(depending on the gear and fan propeller size) and the motor protection set around 4 amps(depending on motor specifications).

 

[bibe2011]

How will you know the capacity of the motor if the motor doesn't have its nameplate?