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1. Originally Posted by jumper
I thought I did.

240v(rms 60Hz) 3ph, 20A per leg (measured)
240 x 20 x 3 = 14.4kW

even under 240 x 20 x 0.9PF x sqrt3, = 7.7kW, more than the motor rating?
Last edited by FionaZuppa; 08-10-18 at 11:43 AM.

2. Originally Posted by FionaZuppa
I thought I did.

240v(rms 60Hz) 3ph, 20A per leg (measured)
240 x 20 x 3 = 14.4kW
7.5HP x 746= 5.5 kW

Motor chart FLC for a 7.5 240V 3P motor per NEC 430.250=22A

Seems normal.

3. Originally Posted by jumper
7.5HP x 746= 5.5 kW

Motor chart FLC for a 7.5 240V 3P motor per NEC 430.250=22A

Seems normal.
yes, but the RMS measurements were 20A per leg. 240v per leg, so its a 4 pole delta?

4. Originally Posted by FionaZuppa
yes, but the RMS measurements were 20A per leg. 240v per leg, so its a 4 pole delta?
You are going way too given the OPs info.

We know only what he posted, we cannot hypothesize beyond that with any accuracy. No?

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I thought I did.

240v(rms 60Hz) 3ph, 20A per leg (measured)
240 x 20 x 3 = 14.4kW
No, use square root of three.

Then there is efficiency, so input KW is greater than output KW always! Or you invented a perpetual motion machine....

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Originally Posted by FionaZuppa
I thought I did.

240v(rms 60Hz) 3ph, 20A per leg (measured)
240 x 20 x 3 = 14.4kW

even under 240 x 20 x 0.9PF x sqrt3, = 7.7kW, more than the motor rating?
We are assuming that this is a 240V delta, so the voltage to neutral is 139V. And 139V x 20A x 3 = 240 x 20A x sqrt(3).

That is another simple way of deriving the sqrt(3) term in the formula.

You cannot mix line current and phase voltage in calculating watts (or VA).

7. Originally Posted by GoldDigger
We are assuming that this is a 240V delta, so the voltage to neutral is 139V. And 139V x 20A x 3 = 240 x 20A x sqrt(3).

That is another simple way of deriving the sqrt(3) term in the formula.

You cannot mix line current and phase voltage in calculating watts (or VA).
??
240 3ph wye (3P4W), where does 139v come from? Ph-N is 240v
sorry, i meant Y. but even with "3P4W 240v Open Delta" or the "3P3W 240 Delta", the ph-ph is 240v, and wild leg to N (B-N) is 208 while AC to N (A-N and C-N) is 120v.
Last edited by FionaZuppa; 08-10-18 at 06:50 PM.

8. but even in 240 delta, 20A per leg is still ~2.5kW over rated HP, if PF=0.9 under load assumably.

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Originally Posted by FionaZuppa
??
240 3ph wye (3P4W), where does 139v come from? Ph-N is 240v
sorry, i meant Y. but even with "3P4W 240v Open Delta" or the "3P3W 240 Delta", the ph-ph is 240v, and wild leg to N (B-N) is 208 while AC to N (A-N and C-N) is 120v.
IF the supply is 240 wye you need to specify that clearly, as it is relatively uncommon. For 240 wye you would NOT generally use a motor nominally rated at 240V since the motor rating almost always specifies the line to line (across winding) voltage. But, yes there can be motors wound for 240 wye.

You are correct that when 240 is the wye voltage the power calculation uses a full factor of 3.

If you are using the line to line voltage, you need to match it with the line to line current, which is NOT equal to the individual line current. That is where the factor of sqrt(3) comes in then.

10. Fair enough. Can we see a pic of the motor spec plate.

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