Autotransformer

[jes25]

I installed my first auto transformer this week. I bucked 277 to 230 to operate an HVAC unit. On one of the secondary leads of the Xfrmr the voltage read around 40V to ground. If this conductor faults will it still open the OCPD?

My second question involves the KVA rating of these units. The unit I was given to install was labeled as 2 kva, yet it was rated at 50amps (or close to it) at 230V. Obviously way more than 2 kva. How is kva calculated for these? And what would be the line current of the the primary related to the secondary? I imagine the the line kva would equal the load kva plus any losses and my final question is how efficient are these units; are the losses significant?

 

[templdl]

Remember that an auto transformer does not isolate and only needs enough KVA to make up the differance in voltage. The entire load does not actually go through the transformer itself,

 

[__dan]

autotransformers

autotransformers

The autotransformer secondary is in series with the line and the voltages add because the sinewaves are synchronized, the delta phase angle between the voltages = 0. The autotransformer only carries load for the fraction of the voltage it raises, secondary voltage = 16 or 32 typical. Calculating load kva for the auto xfmr, use the secondary voltage at 16v or 32v x load amps. Line amps are the sum of the load amps + the primary amps of the autotransformer. Line amps of only the primary of the xfrm x line voltage = load amps x xfmr secondary voltage (16 or 32) = autotransformer kva. This is for single phase. For 3 phase or open delta I would need to look at the wiring diagram.

The auto transformer voltage is added in series to line voltage , so that portion of line voltage is untouched and efficiency = 100%. The portion of kVA carried by the transformer = secondary voltage (16 or 32) x load amps, for this efficiency is in the range of 97%. The unit probably runs nearly cool to warm to the touch, quiet also.

Faulting the secondary to ground, the line amps could be in the range of 1/7 of load amps and could have trouble making fault clearing current at the primary, line side. Voltage will drop through the unit (secondary voltage) as the iron core saturates and impedance through the unit manifests. The output secondary is a welder, it will have no problem making secondary current but maybe a problem making secondary voltage.

 

[Volta]

I installed my first auto transformer this week. I bucked 277 to 230 to operate an HVAC unit. On one of the secondary leads of the Xfrmr the voltage read around 40V to ground. If this conductor faults will it still open the OCPD?...

I think that you are using a transformer with a 240 volt primary and a 32 volt secondary, and are applying 277 volts, which would buck or boost around 40 volts.

The primary is supplied by one grounded and one ungrounded conductor, and by a 50/50 chance you did not carry the grounded and derived conductor through, but rather the ungrounded and derived conductor. That meets code, IMO 210.9 or 215.11, but if you had done it the other way you could have had two wires that were 240ish and 0 volts to ground, rather than what I think you have as 277 and 40ish.

Check out the nameplate, I bet it is designed for a lower voltage (240) than you are using. A 277 to 40 volt transformer with the secondary identified for the elevated voltage would be a rare creature.

 

[gar]

100528-0757 EST

jes25:

Something is strange about your description.

Your source is 277 V. One of the source wires should be a neutral and the grounded wire. The other wire of the source is a hot, ungrounded, wire.

How many wires on this thing you are calling an autotransformer? Are there 3 or 4 or more?

See
http://en.wikipedia.org/wiki/Autotransformer
for a description of an autotransformer.

What does the transformer nameplate read?

If you refer to the Wikipedia drawing. then the bottom end of the winding should go to the grounded source wire. The very top end should go to the 277 hot wire. Your load would be connected between the very bottom terminal and the tap shown as the secondary side. If your transformer is three terminals, then there should be no 40 V to ground or neutral.

If you have connected your transformer incorrectly, then your 277-40 = 237 output will be floating off of ground by 40 V.

.

 

[steve66]

On one of the secondary leads of the Xfrmr the voltage read around 40V to ground. If this conductor faults will it still open the OCPD?

That sounds like it is installed wrong. The neutral wire should be common going in and out of the transformer. Sounds you have the 277V line in run to the common point, and you are bucking the neutral voltage up 40 volts.

That should be fixed pronto.

Steve

 

[steve66]

I assume the second schematic would be a code violation. I guess it might be allowed if both lines on the secondary had a fuse or a circuit breaker installed. Does anyone know a code reference that would allow or disallow this?

View attachment 4688

Steve

 

[jes25]

Thanks guys lots of useful responses. Here is pictures of the paperwork of what I installed. The way I connected it (fig B) was the ungrounded (277v) to A and the grounded (neutral) to B. I had 40 something (can't remember exactly) on the secondary B.

 

[gar]

100528-2234 EST

I really can't read the photos very well. However, if the phasing is correct, then A should go to neutral, B to 277 hot, neutral, point A, to your load as a grounded neutral, and your tap point that I can not identify (possibly D) is the reduced voltage to your load. 277 minus something gives you about 277*240/288 = 230.

.

 

[Smart $]

Thanks guys lots of useful responses. Here is pictures of the paperwork of what I installed. The way I connected it (fig B) was the ungrounded (277v) to A and the grounded (neutral) to B. I had 40 something (can't remember exactly) on the secondary B.

Flop your inputs. Hot to B. Neutral to A. "a" and "b" are your outs.