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Low leg on 3 Phase 480V

886 Views 20 Replies 10 Participants Last post by  gpop
I was called out to a residential midrise building adjacent to Pacific Coast Highway to investigate fluctuating voltage that feeds an LG Multi V series 4 roof mounted A/C . The A/C units keep frying their circuit boards up on the roof and the manufacturer is claiming it is due to voltage fluctuations.
LG Multi V-IV: Variable Refrigerant Flow Cooling System | HVAC | LG India Business
I started in the basement on the 480V 400 amp dedicated feeders and found two legs at 290 and the center leg at 260. This circuit is feed from it's own 500 KvA Delta-Y transformer. The voltages were constant but the current was constantly fluctuating between 25 and 80 amps and my meter would flash 62.5HZ every so often. Inspecting the distribution panel and the roof disconnects it became obvious why the current was fluctuating, when these units cycle on and off they ramp up and down slowly. I've never worked on a VFD type system before other than mounting one of those Big Ass fans with the 12' blades. These units were amazingly quiet and smooth as the current ramps up and down. There was no voltage variance as the units cycled though. All the readings from basement and roof were consistent so there was nothing abnormal other than the center leg being low.
I've read through all the posts on this site I can find about voltage fluctuations and there was some great input about if one of the phase legs is 5% low it can cause a 20% increase in current. I can't remember the exact numbers that were posted but the gist of it was that you need to derate three phase motors by 50% because of one low phase voltage.
So my question is wouldn't the motors burn out and not the control boards due to a low phase voltage? Or are VFD drives very finicky? My hunch is that it is the salt air from being so close to the ocean.
The Utility primary has a voltage monitor and it shows it as supplying 4888 on two legs and 4964 on the 3rd leg on a 4800V system. So the Utility feed has one high leg. There are 3 other transformers connected to the primary feeding adjacent buildings. If those transformers are out of balance what is the chance it will cause a low leg on the transformer feeding the A/C units? It seems I will need to go back and measure for a low leg on the output of the other 3 transformers to isolate whether it is a utility problem or not.
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@gpop does your Poco take power quality trouble calls? I ask as the one I deal with up here has told us to call in a power quality trouble call when we suspect it may be a bad supply issue. We've done it a few times and it gets them to drive the line and check their gear. Maybe it's the same thing, just different terminology?

Depending on the size of the customer.

Last place i worked owned the sub station and would contract poco to do maintenance. As we were there largest customer we had the directors personal phone number and could get any assistance we needed 24hrs a day.

Now i work for a smaller utility im lucky if i can get a person to answer the phone and will probably end up having to file a ticket online. I also get the standard "if the fault is found on your side there will be a call out charge".
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