Help Sawmill application: Voltage drop.

[lukas86]

Before I go too indepth, I wanted to make sure this was the proper place to post this. I stated in my intro I am doing a co-op work experience. I am working at a sawmill, and I have been asked to look at a few situations and things like that around the mill. I have been here for a couple weeks and the one situation they gave me I am fairly confused about.

This mill is 3 phase, 120/208 VAC supply. My use of terms may be wrong, as I am still getting used to everything. I can draw... or attempt to draw the circuits my supervisor gave me, but basically there is a problem with the voltage.

The 120V supply goes to the one machine, and then right after the machine it is grounded and there are neutral wires as well which the current should flow through. However, instead of that happening, there was 6.7V on the ground end of the load/machine. So to fix this temporarily, there was a jumper put in place from the neutral wire to the machine ground. I am to figure out what is going on and why there is that 6.7V after the load/machine and fix it so the jumper can be taken out of place.

 

[berkeman]

As I mentioned in your intro post, I'm not much help on 3-phase power systems. But there are plenty of other folks here who are, so they should chime in at some point.

For now, here's an intro from wikipedia.org -- you may already know all of this stuff in the basic intro, though.

http://en.wikipedia.org/wiki/Three-phase_power

 

[NoTime]

The 120V supply goes to the one machine, and then right after the machine it is grounded and there are neutral wires as well which the current should flow through. However, instead of that happening, there was 6.7V on the ground end of the load/machine. So to fix this temporarily, there was a jumper put in place from the neutral wire to the machine ground. I am to figure out what is going on and why there is that 6.7V after the load/machine and fix it so the jumper can be taken out of place.

First, the existing "solution" is dangerous.
The thing should be tagged out of service until a proper repair is made.
That is assuming that the issue only affects this one unit.

That said.
The 120v supply indicates, to me anyway, that this particular unit is a single phase motor.
Three wires hot, neutral and ground.
Not that phase count makes a real difference if the problem is with the ground plane.

How was it determined that the ground was at fault rather than the neutral?
Voltage on the ground wire (not a good thing) implies that the connecting cable is damaged or a bad connection at a junction box or panel.

 

[lukas86]

Not sure why the current "solution" to this is being used. This is one of the top priorities at the moment because this 'jumper' is unsafe. Well, right now, not totally sure if it's a problem with the ground wiring, or the neutral wiring. The return (both the ground & neutral I believe) is something like 600 feet of 4/0 wire. Now, if there's bad connections, or burned out busses, that could add resistance, but without that, the resistance for 600 feet of 4/0 wire is only 0.0294 ohms... I think. Very small anyways.

 

[NoTime]

What's the horsepower on the motor?
How exactly is it connected. single, delta or wye?

To find out if the neutral or ground is bad.
Since there should be very little current on the ground (perhaps some leakage) you could try to measure the resistance to a known good ground point (with the motor off).
Or could run a temporary ground to a known good ground point for this circuit.

Particularly if it's Al wire, you could have an oxidized connection.
You might need to remake the connections using a fresh coat of the proper Al grease.

 

[turbo]

In older installations, 3-phase motors could often continue to run with one leg of the 3 phases in ground-fault, and only fail to operate when a second leg faults. You should specify if your mill is wired delta or wye.

 

[dlgoff]

There is the posibility that the motor is causing the problem. Maybe there is leakage from the windings to your ground.

 

[lukas86]

I'll ask and find out about delta or wye, along with the horsepower of the motor/machine. The mill is new and has only been in operation for a year and a half too though.

 

[turbo]

Err, if you are tasked with this responsibility and you are not yet aware of the way that the distribution system and motors are wired, you are in dangerous territory. Get some help, or you could kill yourself or some other employee unlucky enough to contact a couple of pieces of equipment or grounded infrastructure (maybe rub up against a motor housing while standing on concrete). You or a fellow employee may become a conductor across a high enough voltage potential to kill at the amperage levels that this commercial supply can provide. Don't ask for help here. Get expert help, and learn from that expert. It could save lives.

 

[lukas86]

I do have help, and I have electricians taking measurements. I am a student... learning. That's why I am in this environment with this task. I am unfamiliar, therefore I don't touch. My supervisor is helping me with this, but he is trying to figure it out. As for me, since I am going into electrical engineering, and he is a supervisor, I know some theoretical and practical applications he may not, and he knows a lot more than me though.

 

[Averagesupernova]

The 120V supply goes to the one machine, and then right after the machine it is grounded and there are neutral wires as well which the current should flow through. However, instead of that happening, there was 6.7V on the ground end of the load/machine. So to fix this temporarily, there was a jumper put in place from the neutral wire to the machine ground. I am to figure out what is going on and why there is that 6.7V after the load/machine and fix it so the jumper can be taken out of place.

Am I to assume that there is 6.7 volts between the frame of the machine (conduit ground) and the neutral wire? You don't really specifically say, but I would assume. The neutral and ground should NEVER be connected together except at the main service panel. Any panel, disconnect, sub-panel, etc. after the main panel should keep these 2 conductors isolated. If there is a voltage build up between neutral and ground then you need to start at the FAR end of the line (machine) and start disconnecting things. Is the voltage present all the time or just when the machine is running and drawing current? From the little that I've learned so far I'd say there is a poor connection on the neutral wire somewhere upstream.

 

[lukas86]

This is how the circuit was drawn for me by my supervisor,

 |-----------------------------------------------------|
 |                                                   A |
 |                                                     |
Vs                                                     L
 |                                                     |
 |                                                   B |---J---|
 |                                                     |       |
 |-----------------------------------------------------|       |
 |                                                             | 
 |-------------------------------------------------------------|
 |
GND

Vs is the source power, 120 volt supply. "L" is the load which requires this power and the next figure shows the connections a little more clearly. Now apparently between "A" and "B" there is 113.3V. "J" is the jumper and connected directly to the body of the machine, and is temporarily in place to lower that 6.7V.


The setup for the machine/load itself looks like this... which when my supervisor gave to me confused me for a few minutes because I had to understand it as well.

               |----------------------|
               | Optimizer End Column |
|--------------|                      |
|              |                      |
|   A|--N      |                      |
|    |         |                      |
|----O---------|                      |
               |                      |
               |                      |
               |                      |
        |------O                      |
        |      |----------------------|
      B |
        |
        |             16.7A @ 60.0 Hz               To PDC
<-------O----------------------------------------------->

"N" is where the neutral wire is, and the lead coming off from that "A" connects to the machine itself (bolted). "N" also is where the ground and power (from the first diagram) are located. "A" in this diagram is the jumper from the first diagram, and has 5.7A across it, and "B" has 4.2A across it which is the machine ground. This machine ground connects to the main mill ground which has 16.7A @ 60Hz and goes to the PDC.

Now yesterday, I asked my supervisor before he ran off but he said there are also 2 other connections to the machine that are connected to ground for other instrumentation purposes. I believe this may make up for the 1.5A lost from cable A to cable B. An electrician said there will be more data later today if he gets around to it from the cables attached and related to this as well.

I believe there may be poor connections or corrosion, I just wanted some input or advice from others because of their experience.

 

[Averagesupernova]

Your previous post doesn't make a lot of sense to me. It sounds to me like you are mixing up voltage and current since you are referring to something that has current 'across' it. Current is a rate of electrons passing a given point in a given amount of time. It is never 'across' anything. Also, as far as I can tell there are several places where there are currents flowing on a ground wire. Ground wires are NEVER meant to carry current except in a fault condition.

 

[lukas86]

Ya, it's a new mill with some serious problems like this. I meant to say current flowing through... but those are both currents and not voltages. I recently just finished 2nd year of my program, and been working here for 2 weeks, and I recently was asked to help my supervisor with this situation. I understand that in an application like this, the ground shouldn't have any current, only the neutral...

 

[stewartcs]

I presume you actually have 120 VAC being supplied from the source and not 113.3 VAC? If so you'll most likely have an isulation resistance fault. Tell the electrician to "megger" the unit (if possible) and see if there is are any faults.

Hope that helps.

CS

 

[lukas86]

Yes, there is 120 VAC being supplied from the source. My supervisor has asked the electricians to get the numbers from various cables connected to the machine. They are busy, but hopefully I will get some readings soon. Apparently this problem has been there since the mill started up. All the comments are giving me a lot of things to look at which is good. Thanks.

 

[NoTime]

What's the nameplate wattage on your device?
Anything over 2kw on a 120v circuit is rare.

17A is a really large fault current for a 120v unit.
Sounds to me like it's just the canary in the coal mine.
I suspect the actual problem is elsewhere.