3 phase sytem 1 phase low/no voltage to ground

[nmark]

A machine has a 3 phase 460 volt feed at the disconnect I measure from phase A-B 460volt, phase A-C 460volt,and phase B-C 460volt.So far so good but when I measure from A phase to ground 70 volts,B phase to ground 450volt and C phase to ground 450 volt.When I measure to ground I am referring to earth.The machine has a problem blowing fuses I don't think it is related to voltage incomeing as I get the same readings at line side of Main disconnect feeding bus duct with machines running happy as a clam. It is a big facility nearby bus fed from different transformers I get a reading of 277v phase to ground and 480 phase to phase.I don't know how to explain to customer that phase A with the 70 volts on it to ground is not the problem or maybe it is ? There maintence department is aware of the different voltages and thinks that is the root of the problem I have different conclusion unrelated to voltage.
Not trying to be mysterious just trying to rule out incomeing voltage.Any help would be appreciated.

 

[GTstartup]

Sounds like 480V delta with phase A inadvertantly not quite solidly grounded (some impedance)

 

[waross]

Can you please answer a couple of questions?
Is the system a delta system or a wye system?
Have you measured the motor windings to ground, either with a multi-meter or prferably with a megger?
It sounds like you have more than one problem.
One problem with the system grounding and another problem with the motor.
yours

 

[nmark]

The electrical system is old I am not sure how to tell if it is a wye or delta.I did not use a megger on the motor I did check motor leads to ground no continuity.The motor is fed with a vfd as soon as power is applied phase A and phase C blow fuses in control panel fuses protect drive only .I do not start/run the motor fuses blow as soon as drive has incoming power.I think the drive is fried but I don't know that it has anything to do with incomeing power.It is about 3 years old it is in a dust tight enclosure but the outside atmosphere is saturated with brass/alloy dust and inside cabinet there is a fine layer.

 

[waross]

With individual transformers;
Delta, there will be a jumper from one bushing on each transformer to the opposite bushing on the next transformer. One transformer may have a connection to center bushing, which may be grounded and will be the neutral if a neutral is used.
Wye, there will be a jumper connecting one terminal of each transformer together. This connection may be grounded and will be the neutral if a neutral is used. The phase connections will be to the opposite secondary bushing on each transformer.
Re the drive. What horse power is the motor and the drive?
Can you move the drive and connect to the good system? If it smokes the fuses on balanced voltages then you have narrowed down one problem.
yours

 

[davidbeach]

Some drives may be designed for grounded wye systems and make reference to ground for surge protection or other purposes. In that case the surge protection might well be fried, other things could be bad. I'm sure the system is ungrounded, may be wye, but most likely delta. If it were a grounded wye, the fault that would pull A phase that far down would also reduce the AB and AC voltages compared to the BC voltage, and that doesn't appear to be happening. It could be a wye system that was at one time grounded but is no longer; there is no measurement that could distinguish between delta and ungrounded wye without disconnecting the transformer.

You need to do one or more of:
1. Repair the neutral-ground bond if this is a formerly grounded wye system.
2. Provide a zig-zag grounding transformer, particularly if a delta system.
3. Use a drive rated for use at 480V line to ground.

 

[nmark]

The drive is 14.9 kw , 20 h.p.The motor is 20 h.p.I like the idea of transferring power to second sytem with balanced voltages it would be easy to do.As far as the voltage rating of drive it says 480v input but makes no reference line to ground.I could get a manual and look 480v input is off nameplate on side of drive enclosure.

 

[itsmoked]

I'm betting the brass dust won... Easily.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[LionelHutz]

A VFD has a converter goes AC-DC-AC. This conversion isolates the motor from the line and means that when you blow the fuses on a VFD the problem is about 99.9% certain to be inside the VFD.

I'm with the others. A VFD "prefers" to operate on a balanced system. The brass has likely damaged the one you're looking at.

If it was being caused by your voltage problem I would expect that the fuses on the 2 lines with high voltage to blow, not one on phase A with only 70V.

As a side note, it is very likely that there is a phase to ground problem on phase A somewhere in that particular system.

 

[ScottyUK]

Lionel,

When the diodes of the input bridge are conducting the pole of the DC link is effectively tied to the line voltage for the phase in conduction. The DC pole is therefore sequentially connected to each of the three phases, and can scarcely be considered as 'isolated'.

VFD electronic overcurrent is very fast acting, and that's the reason why it is rare for a load fault to take out a line fuse on a functional VFD.


----------------------------------

I don't suffer from insanity. I enjoy it...

 

[LionelHutz]

Scotty - Would you accept saying that the conversion process will isolate the line side from a load fault. The combination of DC buss capacity and speed of the IGBTs on the VFD output will keep a motor fault from blowing the VFD input fuses.

At any rate, if a VFD blows input fuses when power is applied it is not a motor fault. A motor fault may have caused the VFD to fail but the blowing fuse problem is inside the VFD.

 

[nmark]

I just replaced the drive yesterday.When I measured voltage at disconnect phase A to ground has 277v.The new drive is running fine.I would like to thank all who replied for there valuable input.It is still a mystery as to the grounded phase condition(it was measured with 2 different meters).But everything is working fine for now.Would this be a problem that could be detected with power logic system to narrow down when ground condition occurs and look at time and loads running.

 

[waross]

Hello nmark
Glad to here that you're up and running.
It is very possible that the faulty drive was grounded internally and pulling the voltage to ground down on one phase.
There are, however some safety related inconsistencies that you may want to check.
It would be well to inquire or investigate whether the system is solidly grounded or resistance or impedance grounded.
It is probable that the drive was faulty and was responsible for pulling the voltage to ground down. The voltage readings may be acceptable with a resistance or an impedance grounded system.
However, it would be well from a safety viewpoint to check the point to which you measured the 70 volts to ground. You may have a piece of equipment with a poor ground connection.
If you have a solidly grounded system you definitely have some faulty grounding connections. This is why I am suggesting a little more investigation even though the equipment is working well.
If you get a chance to look inside the failed drive, let us know if the failure is obvious.

itsmoked (Electrical)
7 Mar 06 23:09
I'm betting the brass dust won... Easily.

This was a good suggestion and some of us are wondering if it was the case.
The poor mans solution to checking ground continuity is to use a dry type transformer for isolation and an electric appliance for a load. An electric kettle works well. You want something that draws about 8 or 10 amps. The isolation transformer should be suitable for the load, i.e. 1000 VA or more. A 2-pole breaker is a nice safety feature. Run 120 volts (or your normal appliance voltage depending where in the world that you are.) from the transformer to the breaker.
From the transformer connect to the appliance and from the other lead of the appliance connect to the point that you used for your ground measurement. Now go back to a point on the system where you are sure that you have a good ground.
CAUTION. You are testing a possibly faulty circuit and there may be a surface voltage on the equipment during this test, (But not nearly as much as when you had the 70 volt reading from the 480 volt system.)
Your appliance current should now be flowing from the transformer through the circuit breaker to the appliance. From the other connection of the appliance the current will flow to the ground connection. The current will follow the grounding path to the other point of connection to the ground system. From the second ground connection the current will return to the breaker and from there to the transformer.
The voltage across the ground path should be close to zero. A voltage of any magnitude indicates a bad connection in the grounding circuit. You can locate a bad connection by measuring voltages "point to point". Use a long jumper to connect one end of your volt-meter to the first ground connection and start measuring voltages along your grounding path. The voltage should be almost zero and a bad connection will be isolated by a jump in voltage from one point to the next. You can follow the current with a clamp ammeter.
yours

 

[mc5w]

nmark,

You should check to see if you can permanently ground this electrical system. My experience with 480 volts ungrounded that does not have ground detectors is that the system is a replication of Benjamin Franklin's kite experiment. This kind of system builds up tremendoud amounts of static electricity during rainstorms resulting in extreme amounts of motor and power electronics damage. There is just no comparison with 277Y480 volts solidly grounded or 480 volt resistance grounded so that ground faults see about 1 to amps of fault current.

A quick and old fashioned patch for these systems is a static electricity drain consisting of 1 megohm 5 watt or more resistors connected phase to ground. In actual practice you would use series strings of 240,000 Ohm 3 watt 750 volt metal film resistors which will coordinate with the clamping voltage of surge arrestors rated for use with 480 volts ungrounded. That is, you would use series strings of 4 of these resistors each phase to ground.

Another perhaps better method would be to connect 600 volt primary 120 volt secondary transformers with the primaries connected phase to ground on the 480 volt system. You would then connect 200 watt or 500 watt 130 volt light bulbs to each secondary. The reason why you want to use 600 volt primaries is so that the light bulbs can withstand a power cross with 120 volt control power. This can happen when somebody is using a jumper wire for diagnostics or if a voltmeter probe bridges 2 wires. The power cross issue is also one reason why surge arrestors for ungrounded systems have a higher clamping voltage than for solidly grounded systems.