Well Pump transformer advice

[DanB]

I don't have a lot of experience with this.  I run my 3/4hp 240AC well pump off a very large old variac and an old Trace SW5548.  I am trying to help a friend run his well pump off the same sort of inverter... on a budget, so searching for appropriate transformers.  His pump is also 3/4hp.  I wonder if a 3KVA transformer is big enough for this.  I don't want to gamble with his money (much).  I'm looking at this:
http://www.ebay.com/itm/Jefferson-211-101-Dry-Type-3-kVA-Single-Phase-Transformer-240-480V-to-120-240V/171671171173?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D28772%26meid%3D0d00cbde6cf0422da383a8e65f18743e%26pid%3D100005%26rk%3D3%26rkt%3D6%26mehot%3Dlo%26sd%3D151618798960&rt=nc

[mab]

well 3/4 hp is a tad under 600W IIRC, so I'd have thought you could get away with a 1KVA transformer (the startup power will be greater, but transformers are quite overload tolerant); 3KVA should be plenty.

[joestue]

some others:

http://www.ebay.com/itm/Acme-Transformer-T279741S-NOS-1-5-KVA-120-208-240-277-/291397132478
http://www.ebay.com/itm/GE-9T51B0012-2-KVA-1-PH-240X480-HV-120-240-LV-TRANSFORMER-NEW-/31131177058
http://www.ebay.com/itm/JEFFERSON-1-5KVA-1PH-TRANSFORMER-211-081-HV-240-480-LV-120-240-/131458300481
If you notice.. all those transformers specify 100, 115, or 135 C temperature rise.. that is in addition to ambient.

I would take the gamble and buy this one or something similar.
http://www.ebay.com/itm/1KVA-1000VA-120V-240V-Isolation-Toroidal-Power-Transformer-p-n-AN-104115-/251881333270
I would take a toroid and overload it 50% before I bought a traditional isolation transformer for the same price.... especially considering its only a 3/4 hp motor.

--now keep in mind that a 120:120 transformer configured as an auto transformer effectively doubles its "KVA" rating.
so if you don't need isolation but just a voltage doubler, you can buy a 600 watt isolation transformer and it should safely run your 3/4th hp provided it is configured as an auto transformer.

You may want to help that toroid out with a case designed to maximize ambient flow..

Toroid dot com for instance has a 1 KVA isolation transformer for 168$.. its temperature rise under full load is only 40C.. and that's mounted flat on a table with the rubber mounting pads on top and bottom.. I once called them up and asked them for the winding resistance, and determined it would be reasonable to run it at 1.5KW (doubling copper losses) provided that some kind of better cooling system were provided.. such as allowing air to flow through the center hole.
I am not familiar with Antek.

I think you would also prefer 5 watts no load losses over 50 or 100 for those 3KW dry type transformers.

also, cancel out the reactive component at the pump with a correctly sized capacitor, if you haven't done so already.
if that is hard on the inverter, you can connect those capacitors to the motor through a NTC resistor.. they will come online slowly in a few seconds rather than a dead short.

[kitestrings]

A 3/4 hp motor that is fully loaded would be (746w * .75 = ) 560 watts; but it won't be.  You could check yours with an amp-probe, but I'd be surprised if it were over 70-80% of full-load rated amps.  Having said that, if you don't have the pf correction that joestue describes you have to provide the reactive component, so you could still be looking at .55-.6 kVA.

[joestue]

I would expect power factors in the 50% range unless I measured it as otherwise.
And efficiency is probably less than 75% below 1 hp.. unless measured otherwise.