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Autotransformer Rating

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ScotchTapeLord

Tue Jun 15 2010, 12:45PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

So I was rummaging through the family business's junk room, and I found a 208V to 230V autotransformer. It's about the same size as an MOT, though the windings are fully potted.
The strange thing is that the rating listed is 3.2KVA, whereas I thought most MOTs get very hot over 1KVA.
Why this autotransformer would be rated so much higher than a similarly sized MOT?

Steve Conner

Tue Jun 15 2010, 02:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

An autotransformer can always be physically smaller than an ordinary ("double-wound") transformer of the same VA rating. The autotransformer's advantage gets bigger, the smaller the transformation ratio. And 208 to 240 is quite a small ratio.

1: The autotransformer's VA rating is really just the difference between the input and output.
208V @ 3200VA = 15.4 amps
Autotransformer's actual rating: (240-208) * 15.4 = 492VA, which agrees with it being about the size of a MOT.

2: An autotransformer doesn't provide isolation between input and output. Therefore a 1:1 autotransformer is no transformer at all: if you like, at 1:1 the advantage over a double-wound transformer is infinite. Lack of isolation is the price paid for the advantage.

3: An exposition in the form of a question. Why can't MOTs be constructed as autotransformers? It would make them a bit smaller.

radiotech

Tue Jun 15 2010, 03:45PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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" An exposition in the form of a question. Why can't MOTs be constructed as autotransformers? It would make them a bit smaller."

If the primary and secondary were wound as an autotransformer, it would require metalic isolation of the magnetron anode from exposed metal in the oven.

Ther would still be a rerquirement of about 35 VA for the magnetron heater and the only savings would be the difference
between either 220 or 110 squared / the difference between the ohmic difference between the number of primary turns wound with the heavy guage copper and the smaller secondary.

Now the only thing to consider is the core gap/shunt design to limit current to the magnetron facing wide SWR swings between warming a cup of tea and a whole chicken.

Now it makes sense to use smps ovens but short of mandating them by law like compact fluorescents, the coppercore pigs will be around for a while yet.

BTW 208/240 autotransformers are for appartment dwellers whose voltage is 120/208. Many jurisdictions wont let you use stoves labeled 240 volts in a 208 volt volt building. A motor rated at 208 volts would actually be happier on 240 volts whereas a 240 volt motor running on 208 would overheat.

ScotchTapeLord

Tue Jun 15 2010, 04:35PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Thanks! All great information. So this is one of those cases where the VA and wattage cases are quite different. I believe the autotransformer was meant to be used in a refrigeration system, perhaps for a motor or pump.

I wonder what I can do with it...

klugesmith

Tue Jun 15 2010, 04:50PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I will take Scotch's 230-volt figure as a nameplate value, not a typographical error.
Tho' 208:240 is much more common in USA today, as radiotech points out. At work we use 208:32 boost transformers for 240V AC fans in equipment running from 120/208 3-phase. [edit] they investigated that, but concluded that maximum RPM at 208V moved enough air.

So the autotransformer of OP is like a 208:22 volt, 306 VA boost transformer with primary & secondary tied internally.
The 22 volt section carries the 230-volt output current (13.9 A), and the 208 volt section carries only the difference current (1.5 A) and will use much thinner wire. Can't tell by inspection, 'cause it's potted. But I expect the 208 volt section to have roughly 90 times as much DC resistance as the 22 volt section. That based on wire gauges chosen to evenly divide the winding area, for minimum copper loss.

[edit] You asked what you could do with it. How 'bout experimental measurement to resolve the voltage question for us. Connect 120V (through fuse or protected power strip) across the main input or main output, then directly measure & report the boost or buck voltage.

Co

radiotech

Tue Jun 15 2010, 09:43PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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120/240 is a "legal" voltage even if you dont get the exact value.
The sizing of wiring is based on watts/m^2 of building area and demands of specific load wattages divided by 240 volts to get amps.

ScotchTapeLord

Wed Jun 16 2010, 01:17AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I'll double check the writing on it. This thing was salvaged from piles of junk that have been around for a few decades, so anything goes.
I'll also get back with what it does with 120 on it.

Thanks for all the input.

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