What would cause this Electrical anomaly?

Status
Not open for further replies.
R

rodneyl

Member
We design and build Industrial Automation Machinery, we have a customer that has one of our pieces of equipment. They called us experiencing an issue with a variable frequency drive in our machine (over-voltage fault). We talked them through voltage measurements on the machine. They are reading 210vac to ground on L1, 390-410vac to ground on L2, & 210vac to ground on L3. What would potentially cause this condition? Our machine expects 480vac 3ph wye + ground. One of our techs mentioned the possibility of a transformer not being tapped correctly in their building that is feeding the distribution panel which in turn is feeding our machinery. One other thought that was mentioned is that there may be some form of a dual element fuse that only one element has blown which they claimed can lead to strange readings. I am not enough of an Electrical expert to come up with any sound conclusion and would appreciate any insight that someone might have.

Thanks,
 
I think the first thing we have to clear up is what the customer supply is supposed to be. Is it wye, or an ungrounded delta or a corner grounded delta?

If it is a wye or a corner grounded delta it seems to be missing a system bonding jumper.

The next thing to clear up is what your machine expects, you said

3ph wye + ground.
Click to expand...

Just to be clear, 3 ungrounded, a grounded and a grounding conductor?
 
cowboyjwc said:
Sounds like a stinger leg, but something else must be wrong.
Click to expand...

It does and the voltage readings are close enough that it could be. That is why I asked about the L-L readings.

If the L-L readings are around 430 that would be about what they would be if it was a high leg delta system, give or take a few volts. no idea why anyone would do that on a 480V system though.
 
Is this a brand new install or has the machine been running for some time and these are new faults/problems? and I third the need for line to line readings, as well as line to neutral, not just line to ground.

L-L would be 480V on high leg delta. B-N would be ~416V. A or C to N, 240V. These are all out of spec for a machine that needs a 480Y/277V supply.
 
petersonra said:
what are the line to line readings? If it i a delta power system you can get all kinds of goofy readings to ground.
Click to expand...
ESPECIALLY if it is an UNGROUNDED Delta system, which in this case, it must be. Either that or it was SUPPOSED to be a "Corner Grounded Delta" and someone removed the GEC, as iwire suggested. I've been seeing that happen a lot lately because copper thieves mistakenly believe that cutting an exposed bare copper grounding conductor is "safe", so it happens in the middle of the night and nobody notices until something like this comes up.

Background: A lot of facilities used to use an ungrounded Delta system years ago, because the plant will keep running after the first ground fault, i.e. you don't shut down production, kind of important on a continuous production line like a steel mill or a refinery where a plantwide shut down means major revenue loss and clean-up expenses. but it was also a "Damn the torpedoes, full speed ahead!" approach to facility management, with inherent safety issues that lead to the NEC requiring continuous ground fault monitoring if you have that system, or changing to a "Corner Grounded Delta" system by grounding one corner of the service transformer secondary. In a Corner Grounded Delta, all three phases will read 480V line-to-line, but line to ground, one "phase" will read 0V and the other two will read 480V. Since that option only involved the cost of a piece of ground wire for the service transformer, a lot of people opted for that. Enter the newer phenomenon of copper thievery and you wind up with weird things like this happening because once that connection is lost, all three phases end up "floating" with reference to ground, so the voltage you read is affected by the capacitance at the moment you connect your meter.
 
Sounds to me like the transformer isn't bonded. At all. The neutral and ground must be bonded to keep your phases at line to zero earth potential.I'm not getying that from your voltage reading.
 
Have the POCO test the transformer for full primary voltages on all three phases. I've seen a single phase primary, usually due to a blown primary fuse, produce very weird secondary voltages. Most padmount POCO transformers are "common core", meaning the core has three vertical legs joined top and bottom, with the windings around each vertical core element. The flux from any two energized windings can induce a smaller but signifigant voltage on the "blown" secondary winding, allowing the motor or other loads (especially motors) to continue to run. Starting current will be very abnormal and (usually) eventually cause overload trips. Happens mostly on 277/480 Wye transformers. Primary can be either Wye or Delta and it'll still happen.
 
petersonra said:
It does and the voltage readings are close enough that it could be. That is why I asked about the L-L readings.

If the L-L readings are around 430 that would be about what they would be if it was a high leg delta system, give or take a few volts. no idea why anyone would do that on a 480V system though.
Click to expand...
No idea why they would use a 480 system with a high leg?

We have many of them around here, primarily on irrigation systems with fairly limited load, yet three phase is still needed. POCO can just build an open delta bank that has a cost of two single phase transformers instead of three. Also works great in remote areas where they can run one less conductor out to that area, or even run single phase from one direction and single (but a different) phase from the other direction and put an open delta on a pole that looks like it only has single phase supply to it, but if you look close the ungrounded primary lines never connect to one another because they are not the same phase.
 
Status
Not open for further replies.
Top