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sfrounds
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Join Date: Mar 2014
Posts: 3

460/3 Line Power to Multiple Frequency Speed Drives

03/17/2014 8:15 PM

Hello, first time I have ever ran into this problem. 460/3 line power to multiple frequency speed drives for fan and pump motors. Line power shows good between phases and to ground. However, the lowest voltage leg has the lowest current.

For example one leg will show a 2 to 3% less voltage to ground or between the other phases. However that low voltage leg is only drawing less than 1/2 the current of the other legs.

This condition is happening form multiple Mcc's throughout the building, some emergency power and some not.

Lost? Thanks Steve
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SolarEagle
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Join Date: Mar 2007
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#1

Re: Please explain this problem

03/17/2014 8:58 PM

Sounds like a bad connection somewhere...I would start scanning with an infrared thermometer, look for a hotspot.....How far back have you traced? I would start at the source to make sure it's right first...

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Fredski
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#2

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/17/2014 9:44 PM

with all loads offline ( no current draw) perform all the same voltage checks verify your Line supply is good from your provider, assuming ITs good add a load, repeat, record your findings until your red flag revealed itself
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sfrounds
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Join Date: Mar 2014
Posts: 3
#3

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/17/2014 10:02 PM

It exists in both a 2500 amp emergency power MCC and another 5000 amp non emergency MCC. There are multiples of these throughout the building but since it exists in both of these, i'm thinking more toward utility source.

I Just can't comprehend the occurrence of a low voltage leg along with a low current association when connected to an inductive load. Yes, I made tests with all loads off and multiples of loads on. It does not seems to affect motors operating across the line as much as when they are on a vfd/vsd?

Typically one would associate high current with low voltage. Steve
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JRaef
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United States - Member - New Member Engineering Fields - Power Engineering - New Member

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#4
In reply to #3

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/17/2014 10:09 PM

It's not connected to an inductive load any more, you said it is feeding VFDs. All the VFD uses the line power for is as a resource for converting the AC to DC. The VFD front end is a rectifier, and the diodes in the rectifier only conduct at the peak voltages. On a low incoming line, the peaks on that phase are lower, so the forward conduction voltage is relatively lower. That means current is being drawn from the other two phase before it is drawn from the low voltage one, so more current goes though those two.

This by the way is a very bad situation for your VFDs and needs to be corrected sooner than later. You are stressing the diodes on the other two phases by making them do most of the work. They were not likely designed for that and will over heat and fail sooner. There are other negative consequences as well, especially with regard to ripple on the DC bus stressing the capacitors and creating more common mode noise that is likely leaking out all over the place.

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sfrounds
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#5
In reply to #4

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/17/2014 10:31 PM

Thank you for that explanation, now it makes perfect sense too me. It does happen on straight inductive loads as well but not nearly as dramatic. As mentioned in another post, I think one leg is seeing a high resistance due to connection/hot spot. I need to track that down tomorrow. I have already lost a 75HP ABB drive, and the 2 others are faulting occasionally on bus related issues. Thanks everyone..
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The.Tinkerer
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#7
In reply to #4

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/18/2014 3:45 PM

Does this guy work in the same facility where the chillers were chewing up Bacnet cards due to induced or harmonic currents?

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Johny451
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#8
In reply to #4

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/18/2014 11:13 PM

Right on the money JRaef. Absolutely correct. GA from me.

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PWSlack
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#6

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/18/2014 7:17 AM

Think in terms of single phase loads and brown-outs at neutral connections.

Concentrate on the neutral. Voltage phase to ground is a bit esoteric as it is a function of the neutral current.

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Geoffrey36
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#9

Re: 460/3 Line Power to Multiple Frequency Speed Drives

03/19/2014 6:05 AM

Two questions:

1. Are other users, with other loads, connected in common with your supply?

2. Are you measuring against the true neutral - the star point of the supply; are you sure the neutral conductor is not loaded?
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Fredski (1); Geoffrey36 (1); Johny451 (1); JRaef (1); PWSlack (1); sfrounds (2); SolarEagle (1); The.Tinkerer (1)

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