Motor or overcurrent relay problem? 3

[dbaec]

Problem
I have had two 2HP single phase dual voltage motors that trip the overcurrent device on startup. The first motor blew the starting capacitor.

History
These are on a hood exhaust fan on a restaurant. The exhaust fan was installed new 12/08. The fan gets turned on at opening runs about fifteen hours and turned off at closing. In two months the OCD tripped three times. The first two times I was called out to check it out. I reset it, checked startup and run currents, both were normal and all else seemed fine. The third time, under pressure to keep it running, the OCD was bypassed. The thinking was that the OCD was defective. Upon starting the motor the starting capacitor blew. A new motor of the same make and type was sent to replace it. One week later the OCD tripped again.

Technical
The motor is 2HP Dual voltage 115/208-230V.
Connected for 230V.
FLA is 12.7 at 230V.
S.F. is 1.15 at 230V.
Motor has two starting caps, both 295uH.
OCD is set for 125% FLA, ~15.8A.
OCD is three phase (ambient compensated bi-metallic)
(class 10 with single phase sensetivity).
Voltage to motor not running is 246, while running is
243.
Start up current is ~57A decreasing to a stable running
current of 12A in four seconds.

Findings
After double checking the wire connections, fan
direction, belt tension and alignment, I cycled the
motor on and off more than twenty times. During that
time the OCD tripped three times. Each time was within
about five seconds of startup. My ammeter would read
in the high 50s and drop to 12A in about four seconds
then the OCD would trip.

Questions
Is there something my ammeter is not telling me?
Even so, that class 10 OCD should handle ~95A for ten
seconds before tripping. (600%x15.8)
If I'm using the wrong or defective OCD what caused
cap in the first motor to blow?

Any suggestions would be appreciated.

 

[jraef]

When I see the term OCD, I think of Obsessive Compulsive Disorder . I prefer to use the term Over Load Relay or OLR.

An OLR is supposed to have a certain amount of retentive thermal memory, meaning it is going to respond to a cumulative amount of thermal stress put on to the motor during start-up. This is as it should be because that heat is cumulative in the motor windings and rotor as well. So I would not be at all surprised to see it trip 3 times out of 20 successive on-off cycles, in fact I'm a bit amazed it wasn't tripping more often. A typical cool-down period after a start up is at least 4-5 minutes and I don't see many motors that can be started more than about 10 times in succession per hour unless they are seriously over sized, and in this case it is not.

The only big mystery I see here then is why the original motor tripped and then the replacement as well. Had it only been the old motor, we could make an assumption that it was just a bad motor; it happens. But the fact that the replacement tripped again after a week adds a dimension of complexity. So here are a couple of possibilities to eliminate up front.

1) You mention that it is a 3 phase ambient compensated OLR. If it was an IEC style OLR, you must pass current through all 3 legs, even on a 1 phase motor circuit. So what you do is loop one leg through 2 poles in series. If you don't, the IEC OL will trip more easily due to what is called a "differential trip bar" that is intended to protect 3 phase motors from running on 1 phase power and is inherent in the IEC design. Older NEMA style OLR didn't have that feature so it was not necessary to do that loop trick. You and whomever wired it up originally may not have been aware of the necessity if you are used to using NEMA OLRs.

2) It could be possible that you have a faulty control deice that s inadvertently cycling the motor on and off, even though nobody is doing that on purpose. So although it may appear that it is running for 15 hours continuously, there is something "bouncing" the control circuit that makes it shut off and immediately turn back on again. It could be happening so fast that nobody notices the slight interruption, like a flickering light, but the effect on the motor starting current would still be the same as I mentioned in the beginning. Could be a loose wire, a defective switch, a bad OLR aux. contact, any number of things. The way to detect it would be a recording meter left on for a week or so. You need to be able to see everything that might be happening electrically.

I would start with those two things and branch out from there if they don't pan out.

A different issue, probably related only to the first complete failure, is that 99% of all OLRs are supposed to be set for the motor FLA; the 125% pick up point is already built-in to their trip curves, so you do NOT add it again. That rule about the 125% pick up point in the NEC is to cover the very few OLRs that do NOT already have that built-in, and those are becoming few and far between. Always read the instruction manuals of OLRs, if they do not tell you to set them at 125%, then you cannot. You can add for the SF if you like, but there is an expectation that you are shortening the motor life if you do so.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Well said jraef. LPS for you.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dbaec]

I think I just found my new favorite forum. I should have caught the OCD as it is often used in reference to me. OLR it is.

Possibility number one could explain the nuisance tripping. I only have current passing through two legs of the OLR. It is an IEC style OLR (Telemecanique LRD21). As for the 125% rule that is also new to me. The box that the OLR came in has a few diagrams and fewer words, but I don't see 125% anywhere on it. So your probably right about setting it at FLA also.

The first motor failure happened with the OLR bypassed. Another company supplied the fan and I supplied the OLR and hookup to thier disconnect switch. After the first two trips I said it seems like a motor issue (maybe wrongly?). Because it was under warranty, the third time it tripped the other company came out to check the motor/fan setup. They said it was an OLR issue. So the OLR was bypassed and when they started it the cap blew. Coincidence?

As to possibility number two, you've given me something else to look at. In the kitchen there is a wall mounted switch wired in parallel with a temperature switch in the hood, both control the fan. If the wall mounted switch is off, the hood sensor could be toggling the fan on and off.
Cause of motor failure, or just a bum motor in combination with a miswired OLR? I'll let you know what I find.

Thanks for the help.

 

[jraef]

Yes, a Tele OLR will have the differential trip feature and that skews the OL trip curve down quite a bit, so that has the highest likelihood of being your issue. And Yes, a Tele OLR must be set to the motor nameplate FLA, NOT the FLA x 1.25. The pick up point is already accounted for in their trip curve. You won't see the 125% mentioned because being an IEC devioce, it is designed to protect IEC motors, which are not as "forgiving" as NEMA motors. There is no such thing as a "Service Factor" in IEC motors, so in essence they are all 1.0 SF and the pick-up point of the OL curve is actually 115%. Not a problem with the NEC of course because the 125% is a maximum, not a minimum value.

But as to the motor, repeated tripping due to the false phase imbalance and having the OLR set to 125% instead of the FLA could very well have caused an early demise.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[dbaec]

By false phase imbalance are you referring to the difference in start and run windings.

 

[waross]

That is my memory of Telemecanique O/Ls also. Use all three poles.
I believe the false phase imbalance is the differential trip when current passes through only two poles of the O/L relay instead of all three.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Yes, Bill is right.

The mechanical trip mechanism inside of the OLR has a spring and level arrangement that counter balances the mechanical force exerted by the bimetal strips in each phase. When there is no current passing through one bimetal strip, the spring force is not countered by the movement of that strip, so it causes the trip bar to inch closer to the trip mechanism. Then when the other two strips heat up, it takes less of their deflection to cause a trip.

The reason why you want this on a 3 phase motor is that in a severe unbalanced current situation (such as the loss of one phase), the motor is getting what are called "Negative Sequence Currents" that make it fight itself, causing it to heat up at a rate disproportionately faster than what the current measurement alone would indicate. So you must bias the trip curve to compensate or risk losing the motor even though you never exceed FLC in either of the other two phases (assuming a lightly loaded motor here). Solid state OLRs do this much better, but the IEC differential trip bar was the state of the art 20 years ago when SSOLs were still impractical.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[skiier]

In the kitchen there is a wall mounted switch wired in parallel with a temperature switch in the hood?
Hood sensor?

What kind of control system do you have?
Sometimes the obvious is the real answer.
Residual heat. Bouncing control switches. Huh?

turn that exhaust fan on manually and leave it on.
Remove the "hood sensor" from the cct. If the hood sensor is a safety then wire it in series with the control but only if it is a "normally closed" approved safety temperature device. Is this not how it should have been wired in the first place?
If you use Halon or some other sort of fire suppression than you must leave the fan on at all costs. In this case no hood sensor can be used or the system may shut down just when it is needed.

My money is on the obvious.
Even if you OLR is miswired it is still working safely. It functions normally under OL conditions . I am not the sharpest knife in the drawer but experience has shown me that big problems are more often than not just little problems.

 

[rockman7892]

Jraef

Would this trick of sending one leg of current through two relay poles work for a solid state relay that may look at current phase angles. I'm not sure that an motor protection relay would look at current phase angles as part of its protection scheme but if it did, it would see that this third relay pole was the same phase as the one it is in series with and would determine an inbalance based upon this. Does this sound possible.

I once had someone in the field try to put the thrid relay pole in parallel with one of the (2) phase legs however I told him that they would need to be in series because if you took one phase leg and jumpered over to the third pole of the relay in parallel and then jumpered back after the relay, the line current would split between these two relay poles and would not be euqal to the line current going throuh the single relay pole.

 

[jraef]

Rockman7892,
Some solid state relays can be "fooled" by putting current through 2 legs in series, others cannot. There is no universal standard between manufacturers on that issue. But many, if not most, solid state OL relays that cannot be used this way also have a switch to be able to turn off the Phase Loss detection so that they can be used on 1 phase motors.

And yes, the current must flow though the 2 legs in series, not parallel. The split would by the way not be equal anyway, and even if it were, it would be significantly lower than the 3rd leg, so again, the OL trip curve would be biased lower.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[rockman7892]

Jraef

Why would the current not be euqal in these two poles if in parallel. Wouldn't this be the same as current splitting in two parallel feeders.

Now I understnad that it will not be equal in the sense that there may be a slight difference due to different impedences, however I dont expect the current to contrast that much. Am I wrong?

 

[waross]

Consider a single phase circuit with 10 amps flowing. One line is a single conductor and the other line is two conductors in parallel. One line will have 10 amps in the conductor and the other line will have about 5 amps in each conductor. Not equal in my school.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Parallel paths would work, to a greater or lesser extent, if you halved the setting of the O/L relay. I can't imagine why anyone would seriously consider doing this as it will not be as effective as the normal series connection and there's a chance the differential trip could operate if the mis-match betwen the two paths is large enough.


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

If we learn from our mistakes I'm getting a great education!

 

[waross]

Scotty, after you split the current through the second and third section of the O/L relay, what about the full load current still passing through the first section?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Well you'd have to use a relay with four poles although where the heck you'd buy one is another question entirely! Good point Bill.


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

If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

Waross

That is what I meant. The one leg would have the full load current passing through while the 2 legs that were split in parallel would each have half of the full load current passing through, just like you stated in your example.

 

[dbaec]

Jraef, in my previous question I had thought you were talking about false phase imbalance in the motor. I realize now you were talking about the OLR. Monday I rewired it (one leg in series) and turned it down to 100%, no problems so far.

Skiier, the thermal switch in the hood is normally open, when the temp hits 85F the switch closes turning the fan on. This insures that if theres a fire the hood will be running. I am concerned about the switch possibly turning the motor on and off all night or bouncing as jraef suggested earlier. This could be the cause of the first motor failure, or it could've just been a faulty motor.

 

[waross]

Is the motor very dirty? You may have a mechanical problem with grease build up in the fan or in the duct work. If the discharge screen is plugged the air movement may be restricted leading to an overheated motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[skiier]

dbaec is this a cooking hood?
Every kitchen exhaust system I have ever worked on has had the gas burners for the heat source interlocked with the exhaust fan. Either by a relay and overload combination or an air proving switch of some sort. The exhaust system is turned on manually when needed and no other control is required. Activating the Halon pull station when there is a fire at the hood has no effect on the exhaust fan operation.The fan keeps running. Just like you want?
If your hood sensor is designed to turn on the exhaust fan upon rising temperature what will you do when this device fails at the end of it's normal operating life? You risk a hood fire if heat builds up too far with a broken hood sensor. Also, you say you want the fan on in case of fire so what will happen when the fires "melts" your hood sensor and shuts down the fan?
Again, turn the fan on manually and leave it on. Turn it off when not needed. Why would you want the possibility of an erractic cycling midnight exhaust fan when no one is there cooking? Get rid of that paralell wiring. Maybe use the hood sensor to activate a light to tell you the hood is unusually hot instead? Before a fire starts. The liability issues connected to your questionable control system could be a heart breaker or worse yet could maybe kill someone. Do you want to risk it? Are you a gambler?
If you don't believe me then you can maybe ask someone in the buisiness in your area.
I have never seen a heat source turn on it's own exhaust fan. At least not in a commercial application. Again, I have to say I don't know everything but I am wondering if your system is safe? Is up to Local code?
I still think the problem lies in the control. The OLR is I feel a secondary problem. It only became a problem because it was pointed out. I know that it's miswired but it still worked when needed. It's important to note that no motor damage occurred while the OLR was in the circuit.
Just putting it out there.