Motor calculations

[guilio2010]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[guilio2010]

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?

 

[Skogsgurra]

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

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



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

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

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

 

[jraef]

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

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



"Will work for (the memory of) salami"

 

[7anoter4]

The connection type [Star or Delta] is not important. In any way the line current will be the same: 9.355 A.