question

georgev avatar image
georgev asked

Autotransformer output not 120v

Hello. Is my brand new 100a autotransformer defective?! It is putting out 120v and 114v on each leg.

Can I fix this? What do I do?

voltageAutotransformer
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4 Answers
Johannes Boonstra (Victron Energy Staff) avatar image
Johannes Boonstra (Victron Energy Staff) answered ·

Hello, firstly, what is exactly the problem if the balance isnt 100% equal?, due to current unbalance between the legs this is often created. a few % difference will not cause any issues for loads connected Beside this, how are you using the AT ?

As step up/step down or as load balance?

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I am having the same issue - 114V on L1 and 120V on L2

120V current is supplied by a Quattro 48V / 10000 / 140-100/100 to the 100A Autotransformer. I'm using the AT to provide a split-phase 120/240v supply to the 50A main break panel in my RV

I have wired the AT connections according to the diagram #3 shown on the system schematics page from the Victron website: (https://www.victronenergy.com/upload/documents/AT-3-split-phase-120V-to-120-240V-with-Quattro-120V.pdf)

The Ground Relay in the Quattro is set to ON and is correctly grounding the Neutral to earth.

Here are the voltages I read at the terminals:


The problem I have with this is that the voltage on L1 decreases a lot with load, and when occurs equipment will pull more current to compensate for the lower voltage.

Here is a pic of my RV control panel, showing the disparity in voltage between L1 and L2. Even though there are 2 air conditioners on L2 it has only dropped 2 volts - L1 dropped from 114v to 105v. This can't be good.


Question - Is this method of wiring correct, and if so, what is causing the voltage discrepancy?

Thanks!

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blueovalbruin avatar image
blueovalbruin answered ·

George did you ever find resolution on this?

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pigwet avatar image
pigwet answered ·

Any updates?

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echidna avatar image
echidna answered ·

It looks like you are simply experiencing the limitations of the autotransformer in terms of regulation. With transformers, the dominant source of voltage drop under load is often not the resistance of the windings, but the imperfect magnetic coupling between them. This can be modeled as so-called (imaginary) leakage inductance placed in series with a winding of an otherwise perfect transformer.

A consequence of the transformer's source impedance being predominantly inductive is that with resistive loads (load current in phase with voltage), the voltage drop in the leakage inductance is 90 degrees out of phase with the winding voltage. When you add these two together vectorially, a significant voltage across the leakage inductance causes only a small effect in the magnitude of the output voltage.

With an inductive load, the voltage drop across the leakage inductance is in phase with the winding voltage, and subtracts directly from it.

So here is the point: motor loads (like an a/c compressor or fan) which are not purely resistive (another way of specifying this is to say the power factor is less than 1) can cause significantly worse regulation in a transformer than resistive loads. I suspect your system would perform better with a purely resistive load drawing the same current.

This is of interest to me because I'm considering this autotransformer to provide split phase AC from a 240V Quattro. But since Victron don't specify crucial technical data about the autotransformer, the only way to find out how well it will work seems to be to buy one and try it ... or to read of other's experiences on this forum.


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