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
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
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Motor calculations
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
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor calculations
Math... skills... severely... compromised... this... week...
"Will work for (the memory of) salami"
RE: Motor calculations