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rglory avatar image
rglory asked

100A autotransformer hot without load

Hi,

I have just installed Quaddro 15kW 230V and I have 100A autotransformer on the output to convert 230V to split phase 120V for my house. I connected inverter and tuning batteries for a while and connected autotransformer input to inverter output, but I haven't connected autotransformer output yet (there is a bypass switch) so there is no load. I left it overnight and today morning I found that temperature LED is on and transformer body quite warm (not hot though). Is there a problem that there is no load connected or I have faulty autotransformer?

Autotransformer
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rglory avatar image rglory commented ·
I did some research today and found that power drain happens when neutral is connected to the output of the transformer. If I disconnect neutral then there is no leak. In disconnected state there is 3V difference btw actual neutral and middle point of autotransformer output. What could be the reason of this?
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Bruce Fergusson avatar image Bruce Fergusson rglory commented ·

Short version: I think the A/T neutral is “competing” with the house power to define neutral, hence current flow when neutral is connected

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Kevin Windrem avatar image
Kevin Windrem answered ·

An autotransformer is not an acceptable solution in this type of system. The proper device in this scenario is an ISOLATION transformer. Then the output winding can be centertapped and form a new neutral which can then safely be connected to safety ground.

Careful consideration to joining two power systems and their neutral and safety grounds. An input from 230 volt will have the neutral connected to one leg which is also connected to safety ground. The hot leg will be 230 volts above neutral/safety ground.

When this passes through an autotransformer with the intent to create a 120/240 split phase system, the center tap of the transformer becomes the neutral for the split phase system. This places the neutral at 115 volts above safety ground. Attempting to connect this split-phase "neutral" to safety ground will cause significant fault current! Not connecting the neutral to safety ground is the only option but will cause it's own set of problems. Mainly, the neutral typically has no overcurrent (circuit breakers) so a neutral fault could cause overheating, fire or shock. Presumably the input to the Quattro will have a circuit breaker and that should trip under such fault conditions. But I suspect a neutral at 115 volts above safety ground is against electrical codes.


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