The power supply alone takes around 78A at rated load.
The AC is likely around 30A with locked rotor which is what is relevant when you're comparing line sag during start up.
Transformers look plentiful if you can source from Canada. There's a 112.5 kVA 2% z available now. If you can sort out freight and getting it through customs.
https://www.kijiji.ca/b-ontario/600-...former/k0l9004
Here is the response i got from Ametek, and the calculations i did are below that.
If they do not look accurate, please advise!
Manufacturer's response:
almost there with the calculation but not quite. The formula is correct. The numbers are depended on worse case scenario which is full load at low line condition. So instead of nominal 480V we will would be looking at 396V as low line condition. Applying the formula gets us to 56A which is exactly what we already have stated on page 29 of the manual under the PFC section for Maximum AC current rating for this 30KW unit with the 480V nominal input. Please see Table 2-1 for reference in the manual.
Sizing the breaker at 125% of 56A gets us to around 70A breaker to use.
my calculations:
Efficiency = 0.87
Power factor = 0.9
AC amps = (30,000W)/(Vac x 1.732 x PF x Efficiency)
AC amps = (330VDC x 91Amp)/(Vac x 1.732 x PF x Efficiency)
Amps = 30,000/(1.732 x 480 x 0.9 x 0.87) = 46.086 Amp (at 480VAC supply)
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this unit is then being powered thru a transformer.
Power source is 600VAC (Canada)
75KVA transformer is a 600V to 480V delta-WYE
this big power supply should never be maxed out either (customer couldn't provide accurate data for the VFD that it powers direct to the DC bus)
the customer also wouldn't accept the calculations using 396VAC input voltage to that power supply since it will actually be powered by 480VAC
Last edited by emiller233; 09-11-17 at 04:39 PM.
Power factor:
>0.69 typical for 440/480 VAC input (40V, 60V - 800V models, 0.9 available with modification “PF”)
I used 0.69. The value you use depends on if your rig was ordered with the modification they mentioned
For the AC, 30A is around the appropriate value to use for voltage drop. The voltage available with 30A added is what's available for motor while starting which isn't a big deal.
Initially you didn't give much details so my comments didn't exclude the possibility about something like a 40 hp motor that must start across the line under full load where the transformer impedance becomes a big deal.
it was ordered with the PF mod.
do we need to be concerned with voltage drop, the transformer is like 10 feet away from this power supply
sorry about that, i didn't know it was relevant or i would have! I'm still getting my feet wet with details such as these involving transformers.
is there a difference between a autotransformer, and a wye-wye autotransformer? or is it the same thing (which i believe it is)
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