Slight voltage rise after passing through dry type transformer.

[11bgrunt]

Worked a complaint that has single phase 240/480 from the POCO to a meter pole. There is a service assembly that includes a lighting contactor for area lighting. That panel is tapped to serve a dry type transformer 240-480 to 120-240. Only one 120 volt leg is used past the 3kVA transformer. Measured voltage under the meter at 227.6 - 227.1 / 456. 10 amps on each leg. Measured voltage after the transformer and measured 115.7. Same DMM used on all checks, Fluke 87V. With the impedance of the dry type of 3.7%, I expected a voltage drop not a voltage rise. A thermal shot of the meter socket and of the 120-240 disconnect that I had access to was done with no hot spots found. I found nothing today that caused me concern. I am curious about the voltage readings.
I did not measure load on the 120 volt leg and I did not check to see how the little transformer was connected. The complaint was a low voltage alarm on the 120 on the customer's side.

 

[iwire]

I do not understand.

You have 456 going into the primary of 480 volt transformer so of course the 120 volt secondary is running low at 115 which I imagine is why the low voltage alarm is going sounding.

 

[templdl]

My first thought was that this is a characteristic of a machine tool or a control power transformer where the turns ratio is not proportion such a 4:2, 2:1 like a power transformer. The voltage on the secondary of a machine tool or control power transformer will be slightly higher with little or small load to compensate for a voltage being less nder load.
But yours is obviously a power transformer. You weren't cear on your secondary connections but I will assume that it is from X2X3 which iyou should have grounded and either X1 or X4 to get your 120v. In doing so you are only using 1/2 of the transformer windings. Not that this is the reason for you slighter higher than 120v but try reconfiguring the secondary connections in order to parallel the 2 secondary weingings. Simply disconnect the X2-X3 and connect the X2 to X4 and X1 to X3. This will double your 120v capacity by using all of the secondary windings grounding the X2X4 with an OCPD on the X1X3.
This may or may no solve your slight over voltage condition, if not the only other thought is the pri-sec turns ratio is off, the the primary may have fewer turns than it should or the secondary having more which does happen. But, since both the primary and secondary both consist of double windings maybe one of the primary or secondary windings could be off.
But I definitely would parallel your secondary windings though as it may reduce your over voltage condition.

 

[11bgrunt]

I do not understand.

You have 456 going into the primary of 480 volt transformer so of course the 120 volt secondary is running low at 115 which I imagine is why the low voltage alarm is going sounding.

My brain says the transformer is a two to one turns ratio. 256/2=228 on the LV side. The 120 is one half of the LV 240 coil so 1/2 of the 228 measured voltage suggest to me that this transformer output should not be greater than 114 volts on one leg when high side is 456. Customer is reporting alarm sounds at 106 volts. He and POCO would be happy if 115 was delivered at load. Something is out of wack, just haven't figured it out yet.

 

[iwire]

115 volts is not an overvoltage condition.

What the heck am I missing here? :huh::huh:

 

[iwire]

My brain says the transformer is a two to one turns ratio. 256/2=228 on the LV side. The 120 is one half of the LV 240 coil so 1/2 of the 228 measured voltage suggest to me that this transformer output should not be greater than 114 volts on one leg when high side is 456. Customer is reporting alarm sounds at 106 volts. He and POCO would be happy if 115 was delivered at load. Something is out of wack, just haven't figured it out yet.

I think you are overthinking this, and you kind of left some info out about the 106 volts.

Typically small unloaded transformers run a bit high but I seriously doubt the transformer is the issue here.

Do you have access to a recording meter to monitor the 480 volt service?

 

[11bgrunt]

I think you are overthinking this, and you kind of left some info out about the 106 volts.

Typically small unloaded transformers run a bit high but I seriously doubt the transformer is the issue here.

Do you have access to a recording meter to monitor the 480 volt service?

I suspect their alarm settings or metering. I will be adding some type of recorder, probably on the 480 side. If the neutral on the LV side of the dry type was the problem, I expect the loaded 120 leg voltage to fall and the unloaded leg to rise relative to current and resistance of the neutral connection. A recorder on the 480 side would never reveal that. Finding a place to hide a recorder on the 120 side for a week will be the problem.

 

[kwired]

I suspect their alarm settings or metering. I will be adding some type of recorder, probably on the 480 side. If the neutral on the LV side of the dry type was the problem, I expect the loaded 120 leg voltage to fall and the unloaded leg to rise relative to current and resistance of the neutral connection. A recorder on the 480 side would never reveal that. Finding a place to hide a recorder on the 120 side for a week will be the problem.

Have you measured voltage while normal loads are operating?

If you have a poor connection you may find it faster to find it if you apply a temporary heavy load and monitor voltage, 1000-1500 watt heater would be a perfect test load.

 

[K8MHZ]

115 volts is not an overvoltage condition.

What the heck am I missing here? :huh::huh:

I am puzzled about this as well.

 

[kwired]

115 volts is not an overvoltage condition.

What the heck am I missing here? :huh::huh:

I he is just questioning why he reads 115.7 volts instead of 114 if the primary voltage is 456.

The alarm issue is under voltage not over voltage from what I can tell, and apparently it is dropping to/below 106 volts at some point in time, but this very well could be a problem beyond the transformer and up to the monitor as well.

 

[11bgrunt]

I he is just questioning why he reads 115.7 volts instead of 114 if the primary voltage is 456.

The alarm issue is under voltage not over voltage from what I can tell, and apparently it is dropping to/below 106 volts at some point in time, but this very well could be a problem beyond the transformer and up to the monitor as well.

Yes, you are right. The question is why does it read 115.7 volts instead of 114 if the primary voltage is 456.
The 106 low voltage complaint is a red herring. I control what I can control. The dry type is not mine and I didn't have input on whether the secondary would be cut over or not to get the 3 kVA at 120. I expect the 120 is control voltage for something and never sees much load. My responsibility is power delivery and part of that is to control voltage drop.
A recorder will be added and if a low voltage condition is not measured, we will advise the customer to dig in on his side and offer as much assistance as practical short of design/build ourselves.
I preach turns ratio as a starting point for what we expect the voltage to be after it passes through an unloaded transformer.