Burned 3 phase motor

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livewire99

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Location
Garland,TX USA
I have a 7200 volt motor with a 97 amp FLA, that burned up due to single phasing and what I need help on is interpreting the events recordings. This motor is loaded down to about 45% load and I can tell that the customer did not properly troubleshoot the faults leading up to the motor catching on fire, but my question is if my Multilin 369 is seeing incoming voltage on C phase or is the voltage coming from A and B phase? Voltage is present on all three phases but current is not present on C phase.My fuses did not open and now my contactor has A and B phase welded closed and C phase is open, fuses never opened since they are sized for short circuit protection and the current at the time of the motor fire was at 605 amps for 4 minutes. Before the motor was stopped the current was about 125 amps. Normally the motor runs at about 45 amps when everything is normal.
Let me know if more information is needed, thanks and look forward to your comments.
 

Jraef

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I have a 7200 volt motor with a 97 amp FLA, that burned up due to single phasing and what I need help on is interpreting the events recordings. This motor is loaded down to about 45% load and I can tell that the customer did not properly troubleshoot the faults leading up to the motor catching on fire, but my question is if my Multilin 369 is seeing incoming voltage on C phase or is the voltage coming from A and B phase? Voltage is present on all three phases but current is not present on C phase.My fuses did not open and now my contactor has A and B phase welded closed and C phase is open, fuses never opened since they are sized for short circuit protection and the current at the time of the motor fire was at 605 amps for 4 minutes. Before the motor was stopped the current was about 125 amps. Normally the motor runs at about 45 amps when everything is normal.
Let me know if more information is needed, thanks and look forward to your comments.
You don't ever have voltage on only a phase, you have current on a phase, but voltage BETWEEN phases, i.e. A-B, B-C, C-A (referred to a Vab, Vbc, Vca).

If you have a 7200V system, the 369 measures voltage through Potential Transformers that step it down to a safer level, typically 120V. Those transformers are usually connected using two PTs in an Open Delta configuration, typically connected to Vab and Vbc, then Vca is derived from those other two. There is nothing wrong with this connection by the way, this is how most protective relaying equipment is connected. But the PTs are fused, so if you blow one of those fuses, the relay will not show one of the voltages. Default settings in the 369 would have it trip off on Phase Loss since it would not know it was just a PT fuse. What I have seen happen is that someone unfamiliar with how this all works will see the relay trip on phase loss, they check the incoming phases and see there is no phase loss, so they ASSume the relay is defective and "fix" the problem by disabling the phase loss trip function! So that's one thing to check first, what is the 369 SET to do?

The danger of single phasing a motor that is very lightly loaded is that the severe imbalance of current in the stator creates negative sequence current flow in the rotor, which in turn causes counter-rotating torque to be produced. So the motor "fights itself' and the heating effect in the motor is greater than it should be for the given current flow. So if the motor is very lightly loaded, the increase in current under single phasing may still be below the Thermal Overload Trip setting, but the EFFECT that current has on motor heating is OVER the thermal damage curve of the motor. That is why, in something like a 369 relay, you want to program a current imbalance protection scheme into it. Again, check to see what it is set at. If it is disabled (as I would suspect), that is the root cause of your failure in the motor.
 
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livewire99

Member
Location
Garland,TX USA
Burned 3 phase motor

Jraef,
My Current Unbalance was Latched:
Block Unbalance From Start : 15 s
* Current Unbalance Alarm : Off
* Current Unbalance Trip : Latched
* Assign Trip Relays : Trip
* Unbalance Trip Level : 20%
* Unbalance Trip Delay : 10 s
 

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junkhound

Senior Member
Location
Renton, WA
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EE, power electronics specialty
Let me know if more information is needed

Understatement of the year?

Some paragraph breaks would also be helpful, and also application, power sources, etc...

Have seen drive motors aboard ships fail because of generator field current decay being slower that engine stop, results in loss of PT as it saturates and PT fuse blows. Next time gen starts, no PT connected.
 

livewire99

Member
Location
Garland,TX USA
Burned 3 phase motor

This is for a blast hole rig so mining would be the application. Utility power is already crappy as you can see in the events recorder.
1400 HP motor by the way.
 

Jraef

Moderator, OTD
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Location
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Occupation
Electrical Engineer
Well, in my opinion, the REASON why your motor fried is because someone chose to ignore the single phase condition and attempted to continuously restart this motor, at first by overriding the Starts/Hour block, then when one phase opened (likely as a result of a fuse opening because of the successive restarts), with one phase missing, over and over and over again. The poor relay was TRYING to protect the motor, but someone was purposefully unblocking it and forcing it to restart. Your facility needs some SERIOUS education of operators here.
 

Ragin Cajun

Senior Member
Location
Upstate S.C.
Well, in my opinion, the REASON why your motor fried is because someone chose to ignore the single phase condition and attempted to continuously restart this motor, at first by overriding the Starts/Hour block, then when one phase opened (likely as a result of a fuse opening because of the successive restarts), with one phase missing, over and over and over again. The poor relay was TRYING to protect the motor, but someone was purposefully unblocking it and forcing it to restart. Your facility needs some SERIOUS education of operators here.

Sounds like it's overdue in enabling password protection on programming changes - for starters. I would be surprised if an "operator" would be making such changes, but you never really know. Perhaps a technician needs to have his knuckles wacked?

RC
 

livewire99

Member
Location
Garland,TX USA
Burned 3 phase motor

Thanks guys,
Jraef, help me understand, in your original reply you said if one of the fuses on my PT's blew, "the relay will not show one of the voltage. " Since I do show all of my voltages then my fuses did not open. Since we know the motor burned due to single phasing and C phase is not showing any current then the voltage recorded is coming back from the other 2 phases that are still active? And the current was not recorded because the CT is now orientated the wrong way?
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Thanks guys,
Jraef, help me understand, in your original reply you said if one of the fuses on my PT's blew, "the relay will not show one of the voltage. " Since I do show all of my voltages then my fuses did not open. Since we know the motor burned due to single phasing and C phase is not showing any current then the voltage recorded is coming back from the other 2 phases that are still active? And the current was not recorded because the CT is now orientated the wrong way?
More likely because there was no current. There can be voltage measured at a location (like a terminal at the midpoint of a resistive voltage divider, for example) without any current flowing through that terminal.
 

Jraef

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More likely because there was no current. There can be voltage measured at a location (like a terminal at the midpoint of a resistive voltage divider, for example) without any current flowing through that terminal.
Right. The flown PT fuse issue was just an example of how something could happen like that. In this case looking at that report now, it's clear that someone was looking at some sort of incorrect data and ASSUMING that something was wrong with the relay, instead of looking for the REAL problem. So they kept overriding the relay's protection lockout which eventually killed the motor. What we don't know is how your meter is connected, what kind of starter you have, why your operator was allowed to override the Blocked Start etc. etc. etc. That's something that someone who is there needs to investigate.
 

livewire99

Member
Location
Garland,TX USA
Burned 3 phase motor

Jraef,
The stop circuit goes to the multilin trip contact but the start circuit does not so the multilin interprets a start when it sees a current rise of 5% of FLA. I am attaching the schematics so you can see how the multilin is tied into the system. Originally I was looking at the contactor being jammed with only two poles closed and the C phase open but that is unlikely??
Bottom line here is that the mine electricians did not trouble shoot properly the trip since power was only off for about four minutes unless they bypassed the trip contacts on the multilin. The motor was doing ok while running on single phase based on the RTD temps, but when they turned the medium voltage off then restarted that is when the motor burned up!

 

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GoldDigger

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Retired PV System Designer
Jraef,
The motor was doing ok while running on single phase based on the RTD temps, but when they turned the medium voltage off then restarted that is when the motor burned up!
That fits perfectly, since by far the greatest stress on the motor will be when trying to start single-phased, and particularly if it is also under load.
As long as it was not running anywhere near full load power, it could keep on going when single phased and just run hotter than it normally would for that same load.
 

Jraef

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So the voltage on C phase actually the returning voltage of A and B phase?
Maybe not. If it is TRULY wired as shown on that diagram (never ASSume, always check), then if a line fuse had blown, or your dropped a phase from the source, then the Multilin would have shown no voltage on that line and dropped out. There is nothing in the record indicating that.

So because the CTs are ahead of the PTs, you know that the loss of current flow took place AT OR BEHIND the PTs somewhere. So either they lost a motor lead connection inside of the peckerhead, or, as I suspect, they smoked a contact in that main line contactor. Your first fault was from exceeding the Starts-per-Hour limitation, so someone did an override of that and restarted anyway. That may have damaged the contactor pole, so from that point on the motor was single phasing. But since the PTs were happily telling them that voltage was present, they kept trying to restart it, probably thinking the relay was stupid or something (I can just hear them swearing at it). It kept trying to tell someone that there was a phase loss when it didn't see current flowing in one phase, but they kept ignoring it because they were only thinking voltage, not current, so they kept restarting it until the windings smoked.
 

GoldDigger

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Does anyone know of a formula to calculate what the induced voltage would be on an open leg of a three phase motor?

As long as the motor is running and is not heavily loaded, a rough approximation will be exactly the expected phase voltage for that line. The motor is simultaneously acting as a motor and as a generator. This is how a rotary phase converter works (once you get it started.) In practice, the voltage will be lower than that, but it is hard to predict just how much lower. That will depend on the motor design, slip and load. That phase will also be roughly the normal phase for that line.
 
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livewire99

Member
Location
Garland,TX USA
As long as the motor is running and is not heavily loaded, a rough approximation will be exactly the expected phase voltage for that line. The motor is simultaneously acting as a motor and as a generator. This is how a rotary phase converter works (once you get it started.) In practice, the voltage will be lower than that, but it is hard to predict just how much lower. That will depend on the motor design, slip and load. That phase will also be roughly the normal phase for that line.
Based on C phase having no current then I can say that the leg is open because if no current then my "circuit" is incomplete?
 

GoldDigger

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Based on C phase having no current then I can say that the leg is open because if no current then my "circuit" is incomplete?
Yes, it is much more important to know that there is no current than to know what the voltage is when trying to detect a phase loss. If instead of being open the defective phase line is connected to a load on the motor side of the break, then there will be current but it will be flowing at all times in the opposite direction to what it would be if the motor were absorbing power from that lead.
 

livewire99

Member
Location
Garland,TX USA
Yes, it is much more important to know that there is no current than to know what the voltage is when trying to detect a phase loss. If instead of being open the defective phase line is connected to a load on the motor side of the break, then there will be current but it will be flowing at all times in the opposite direction to what it would be if the motor were absorbing power from that lead.
Thanks Golddigger,
i think I have confirmed my gut feeling but glad I found this forum to throw the question out there. Thanks to all that posted, I work in the mining industry and have not found a forum dedicated to the industry.
 
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