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Motor calculations

Motor calculations

Motor calculations

(OP)
I'm going through old notes to remember 3 phase motors and it's not making sense and didn't know if I could get this cleared up.

If I have a 7.5hp motor, 5.6kW, 3 phase. We'll assume .8PF and .9eff
Voltage 480V, 277V LN

In single phase:

I = 5.6 / (480 * .8 * .9) = 16.2 A
This is 7776 VA (480 * 16.2) which multiple by .8 PDF and .9 eff, you get 5.6kW.

In 3 phase

I = 5.6 / (480 * .8 * .9 * 1.73) = 9.4 A/phase

From here I'm trying to do the check. To make sense of this, is the total power divided by 3 for each phase? Then from calculated above, 9.4 x 277 = 2602 VA and to check 5.6/3/.8/.9 = 2592VA?

Also, if the motor is operating at full power, is the 3 phase motor operating at 16.2A but because 3 phase power, one leg is at 9.4 A and the other two legs at 3.4A? Will using 480V give you the full draw of the motor instead of using Vln?

Thanks,
Guilio

RE: Motor calculations

Remember there is a 120 degree shift between the three phases. So you need to get back to your notes and study a little more. You cannot add currents or calculate power if you do not take the phase relationships into account.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor calculations

(OP)
I'll dive back into it. It's been years.

I'm just trying to grab onto the visual pictures of how the current is flowing. If the motor is 5.6kW, and I calculated 9.4 A for a leg, if I look at the 3 phase voltage graph, when Va is at 90 degrees, Vb and Vc is 2/3rds of Va peak voltage. If the motor is 5.6kW, then each leg should be producing power to contribute to that?

RE: Motor calculations

You can look at the motor as three single-phase systems. Then your input power to each system will be Ul-n*I*PF or, since Ul-n is Ul-l/sqrt(3), equal to Ul-l*I*PF/sqrt(3).

Adding the three systems together (they all contribute with one third of the total power) results in 3*Ul-l*I*PF/sqrt(3), or Ul-l*I*PF*sqrt(3).

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Motor calculations

480V x 9.4A x 1.73 = 277V x 9.4A x 3

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

RE: Motor calculations

Must.... resist... commenting...

Math... skills... severely... compromised... this... week...

"Will work for (the memory of) salami"

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