Autoformer issue

That implies a 12A draw. What is the actual amp draw?

A DC resistance of 10 ohms sounds rather believable. Offhand I would expect a lower resistance, based on a rather simple understanding of how it may be set up, but I don't have any specific values or very detailed design info to be be very certain. The DC resistance of transformer coils is by design quite low since any resistance there simply results in inefficiency and heating. The primary (and also the secondary, or at least a portion of it, since it's the same winding in an autotransformer) would presumably be connected more or less directly to the plug.

Signs of excessive heat at the plug prong is due to a poor connection right in that area, either where the wire attached to it, or between the prong and the spring fingers in the socket it was plugged into, or between the wire and the socket. Replacing the plug is a very good idea if you find it in that condition.

How long has it been since the plug you had heating problems was disconnected.

If a plug is left for a long time corrosion will build up and cause a high resistance connection.

I have a Hughes Autoformer that was permanently mounted in my trailer. I had used the plug connectors and they were never disconnected as I always used the Autoformer. After about 5 years I discovered the same heating and melting problem you described.

I removed the plugs and made a permanent connection with wire crimps which I soldered after crimping. This required opening the case of the Autoformer which I did not mind as the Autoformer was way out of warranty.

If you do not want to wire direct I would recommend disconnecting and reconnecting the plugs after each trip. This will break the corrosion that forms.

Autoformer is not an isolation transformer but rather one big coil with several tap points to get the different voltages. How would it work with an open circuit? There is no power switch.

If this is about the plug melting, this is fairly common during high usage as many pedestal connectors are worn, pitted, dirty and corroded causing a poor connection and some heat.

I would guess it is normal without breaking out the meter.

A transformer tends to be right close to a short circuit at DC levels. But at AC it is a much higher Impedeance This is true of all inductive devices.

Sounds like my idea of what to expect was wrong.

Let me back up for a minute and explain why I was even checking the resistance in the first place.

This is a used Autoformer purchased from CraigsList. I plugged it in at a campground here in FL where we were camping for a week and it resolved the low voltage issues that we were having during peek AC usage (2-3pm). However, at some point my daughter got a little bit of a shock on the metal door handle and I thought it might be the Autoformer so I unplugged it and it went away. That is when I noticed the corrosion/melting on the plug.

I decided to check if on of the plugs was shorting to ground, which they weren't, but then I noticed that there was measurable resistance between hot and neutral and wasn't sure if that was expected.

I have thought about wiring the unit directly, but I want to be sure that it does not have any other issues before doing that. For now I have changed the plug, and hopefully that fixes it.

rfaulkner99 wrote:
However, at some point my daughter got a little bit of a shock on the metal door handle and I thought it might be the Autoformer so I unplugged it and it went away. That is when I noticed the corrosion/melting on the plug.

I suspect either reverse polarity at the pedestal or more likely a poor ground connection and your RV has a small current leak to ground.

Assuming you are back at home... consider plugging the RV into a GFCI outlet and see if it trips.

time2roll wrote:
I suspect either reverse polarity at the pedestal or more likely a poor ground connection and your RV has a small current leak to ground.

Assuming you are back at home... consider plugging the RV into a GFCI outlet and see if it trips.

I can try that, however if that were the case, wouldn't the problem (shocking) continue after removing the Autoformer?

Bad ground could have been intermittent. When you removed the AF your RV plug may have made ground contact. OK to plug into the AF and check ground continuity from the AF plug ground pin to the ground bus. Hard to check the exact situation after the fact.

An EMS surge protector would continuously monitor power for voltage, ground and polarity issues and cut power if anything was wrong.

The Hughes Autoformer I have has triacs that determine which transformer tap is active. As such measuring dc resistance between hot input and the hot output may be something much higher than expected as you are going through a triac.

There will be some resistance between the neutral and hot due to the voltage sensing circuit in the Autoformer.

The neutral lead goes direct from the input to the output so it should measure very low in dc resistance.

When I first received my Autoformer Hughes had reversed the output leads which resulted in the hot lead being on the neutral of the output. Also my trailer was wired wrong in that the ground and neutral had been bonded. This resulted in the trailer frame connected to the hot lead. Thus you will receive a shock if you touch the trailer while standing on the ground outside the trailer.

This can be checked on the Autoformer. As stated above the neutral goes direct through the Autoformer. Thus input neutral to output neutral should be a very low dc resistance. The hot goes through the transformer and because of the triacs there will be some resistance measured between input and output hot leads.

There should be infinate resistance between ground on the Autoformer and either the hot or neutral lead.

I discovered my problem when my Hughes was new and Hughes replaced my Autoformer under warranty.