Motor overload protection

[maurice bracken]

I am an instructor of Electrical Technology in a public technical high school. One of the courses we offer is entitled "Electric Motors and Controls". Now, the situation for discussion:
In the past I have seen three phase squirrel cage induction motors wound with a normally closed thermal disc embedded into the actual stator windings. This disc typically gets wired in series with the control circuit and is designed to open at a pre-set temperature. I personally believe that this method of protecting motors is better that the standard heater or induction type overload device because it protects the motor from any overheating regardless of whether the cause is a mechanical overload or a compromised air flow around the motor.
My specific questions are:
Am I correct in believing that the thermal disc is the best form of motor overload protection?
If I am correct, why do motors not come supplied with this feature as a standard option?
Does this feature have to be installed at a motor repair shop as part of a rewind job?
Can this feature be field installed by an on-site technician?
Any discussion on this matter would be greatly appreciated. Thank you.

{Moderator's Note: Edited to remove personal contact information. If you wish to contact this person directly, then start by sending a Private Message.}

[ January 19, 2005, 12:16 PM: Message edited by: charlie b ]

 

[69boss302]

Re: Motor overload protection

Yes, these give you the best indication of motor winding temperature. Bad things though are if the switch goes bad only one way to fix and that's a rewind. You could always install thermal overloads in the controls like other starters, but theres the added cost you tried to save anyway. There are also some thermals that just go right on the windings, but still they don't give you internal winding temp, actually better off with the control block standard thermals. The control overloads also give you good range selection, where as the ones in the motor windings don't. Some of the electronic overloads give you adjustable overloads which is nice but in many industrial establishment's you get the guy on midnight, that is sick and tired of resetting or the production guy that doesn't want the machine to trip out when he hogs the machine and takes hid little screw driver and tweaks the overload up just to save himself time. When the motor burns up, it's not his problem. Yes they have ways to lock them, but as anything, locks only keep honest people honest.

 

[maurice bracken]

Re: Motor overload protection

Thank you for the insight. I cringed when reminded of the industrial production boss who by-passes safety items. I've seen it done hundreds of times, usually with catastrophic results.
When active as an industrial electrician I always set the MOL lower than spec'ed. I had a few extra calls out to the production floor BUT the savings in motors that did not burn out was tremendous!
Thanks again.... MAB

 

[coulter]

Re: Motor overload protection

Originally posted by maurice bracken:
When active as an industrial electrician I always set the MOL lower than spec'ed. I had a few extra calls out to the production floor BUT the savings in motors that did not burn out was tremendous!
Thanks again.... MAB

MAB -

Your breaking my heart. Say I spent a month designing and installing a production system - motors everywhere. I've got the specs for the production and a budget. The system is designed to run flat out - 100%. Now you come along and decide, "Oh those guys and girls don't know what they are doing. It's too hard on equipment to run wide open. I'll set the OL's down a little and save lot of maintenance costs."

Maybe. Let's look some other aspects.

A few years ago, a maintenance crew was complaining about ops running a particular piece of machinery too hard and it was always breaking down. They were saying what a pain in the a-- it was to have to go out and fix this all the time. My response was this was an accounting decision, not a maintenance decision. One adds up the production costs (including maintenance) and balances that against the production gain. And if run to failure comes out the best income, then that is what you do. As far as I knew, we got paid to keep it running, not decide the production rate.

Here's another. This particular piece of machinery came with a 100hp, 1.0 sf, tin can motor. During normal production, it had a history of tripping out on OL occasionally. I spent a lot of time troubleshooting the starter, circuit, and motor. Nothing wrong with it. I got asked to push up the OL's. I said no, the ones that are in there are the max allowed by code.

One night that dept was being pushed to keep production way up - that night they were the bottleneck for the entire plant. Well the motor tripped out on overload three times within an hour. Fourth time they started it, the smoke came out. I got asked, what happened?. Let's see, tripped three times and had four starts in an hour and it burned up. I don't see anything wrong. I'd say everything worked right as designed.

We replaced it with a 1.15sf motor. They loved it - it got them another 15%.

Supposing you had decided they were running the machine too hard and took it on your self to set the overloads down. Would you have saved the motor? I think it would have burned out sooner - more trips, more starts.

A tech, or engineer, or ops going out, in secret, making changes to calibration, with no documentation, and not telling anyone, that's the stuff troubleshooting nightmares are made of.


carl

 

[69boss302]

Re: Motor overload protection

coulter:
Setting overloads up is not a cost effective method of extending production. Your increase in the Service Factor of the motor with correct verification of properly sized overloads and wiring would be acceptable. OSHA, NEC and others have recognized the fact that when a motor burns up there are many other factors to take into account. One being FIRES! Motor burning up = Heat and lots of it, smoke and usually lots of that. If there is any grease from the associated gear box more than just the motor can burn up. Personnel Safety, again the motors burn up and people are exposed to possible fire's and breathing that smoke. Controllers and Contactors may not be rated for higher amperage. Today's engineering practices do with as minimum as they can get away with and the control panels can burn up easier than the motor (more extensive and expensive damage). I'm sorry but with all the regulations out now and OSHA investigations it is not cost effective by any means to compromise safety for production. Is it better to keep a machine running and produce 100 parts a day and get 500 at the end of the week. Or push it and get 200 parts one day, 50 the next and have it down for a week because nobody remembered to get the last motor fixed when it burned up. Also consideration for the time and frustration of changing said motor. Any time the motor burns up there should also be a check for why it burned up, not just OK change that motor again. Something is wrong, if a motor continually goes bad, it needs to be fixed.

 

[tony_psuee]

Re: Motor overload protection

I agree with 69boss302. We size the motors for our equipment based on the required bhp. If it came with a 1.0sf motor than it should not be drawing more than nameplate FLA under design operating conditions. We have equipment with motors running into the service factor and the motors are designed for that duty. We can provide what the designed bhp and associated FLA are and if it is drawing more than that there is a problem.

However, coulter I do agree with your statement about under sizing the motor overloads. I occasionally get calls from end users about motors tripping only to find out the overloads installed by the contractor are sized at 100-105% of the nameplate FLA on a 1.15-1.25sf motor, or the fuses are sized at less than 110% if FLA.

Tony

 

[coulter]

Re: Motor overload protection

Boss -

Been out in the field a week - just got back

I want to be really clear that in no way do I ever advocate compromising safety nor any installation practice that in any way is not in complete accordance with the NEC and any other known regulation.

I reread my post looking specifically for any thing that could have possibly been construed as not being in line with this philosophy.

Originally posted by 69boss302:
Setting overloads up is not a cost effective method of extending production.

Not sure where your are going with this. If you are saying the overloads can not be set up to the max allowed by the NEC I guess I don't agree. As you recall from my first post, I had said I would not set the overloads above the NEC max.

I saw my comment about the motor failing on the fourth start after the third ol trip as "working per design". The context of that discussion would include the discussions with the production crew/managers that they can't do that sort of stuff to motors (intentionally run the machinery past design spec). They will burn up. So they were aware that if they chose to do that, the motor would likely burn up. With the trouble we had been having, there had been discussions concerning that the motor will not stand repeated restarts after ol trips. They get too hot. I had suggested two starts per hour was all the motors would stand and live. So when they had told me what had happened, my response was, "Well let's see, you overloaded it to where it tripped three times in an hour and on the fourth start it burned up. Sounds to me like everything worked per design." I was pointing out that intentionally overloading a piece of machinery will cause failures.

Originally posted by 69boss302:
Your increase in the Service Factor of the motor with correct verification of properly sized overloads and wiring would be acceptable.

Glad you agree.

Originally posted by 69boss302:
OSHA, NEC and others have recognized the fact that when a motor burns up there are many other factors to take into account. One being FIRES! Motor burning up = Heat and lots of it, smoke and usually lots of that. If there is any grease from the associated gear box more than just the motor can burn up. Personnel Safety, again the motors burn up and people are exposed to possible fire's and breathing that smoke.

I'm not sure where this came from. Your are absolutely correct. FIRES are bad. Certainly nothing I would recommend under any circumstances. I probably wouldn't even need an OSHA code to tell me that. The only thing I could see in my post was the comment about "run to failure". If it was that, well that is an RCM term (reliability centered maintenance). When one does the RCM studies on a system, the group considers all of the known failure modes (some groups also consider unknown modes. ) Safety aspects are considered as well as production. Depending on the safety considerations and reliability requirements, any one of several levels may be applied. Some examples are: An automatic starting installed spare; remote monitoring and shutdown; periodic testing and overhaul to insure satisfactory operating condition; spare parts located on site; and maybe even, run it till it fails and then put in a new one. Just in case you might be thinking these RCM studies are management tools to save money, could be, but all I been involved with were run by the techs. Have to be, they are the ones that really know the failure modes. The example I gave on "run to failure" was a small gear pump that ate up the gears. miserable little bear to change, but cheap. In that context, I not seeing an issue with FIRES. If it was something else, let me know, I'll provide the context or an immediate retraction.

Originally posted by 69boss302:
coulter: Today's engineering practices do with as minimum as they can get away with ...

Couldn't tell you about that. Most of the stuff I deal with has a spec, and the engineers/designers are paid to meet the spec, regulations, and codes in a cost effective manner. I guess I don't see that as trying to "get away" with anything.

Originally posted by 69boss302:
coulter: I'm sorry but with all the regulations out now and OSHA investigations it is not cost effective by any means to compromise safety for production.

No need to be sorry. I absolutely agree with you.

Originally posted by 69boss302:
Is it better to keep a machine running and produce 100 parts a day and get 500 at the end of the week. Or push it and get 200 parts one day, 50 the next...

That I don't know. And with out a cost analysis, how would you know? I likely would tend to agree, but that is different than knowing. Please remember, I am not advocating any safety related failures.

Originally posted by 69boss302:
Any time the motor burns up there should also be a check for why it burned up, not just OK change that motor again. Something is wrong, if a motor continually goes bad, it needs to be fixed.

Yes, generally that would be true. The case I stated above, I think the failure mode was well understood and we did change to one more suited to the use. I think that meet your criteria.

But then again, not always. Several years ago I had pair of 500hp, 3600rpm motors that would take out the bearings twice a year. I tell my boss the motor is under designed, it really should have had sleeve bearings instead of ball bearings. His response is, "Is it going to fly apart to where there is a safety issue." No, when the bearings start to go we see it on the vibration monitor, and take it off line." "Okay what does it cost to fix each time." "Six man-days and $1000 in bearings - say $4000." "So, you want to spend maybe $100,000 to mitigate $8000 in maintenance costs per year." What I did do, was spend about $20000 to balance the pump shaft and impellers and the motor shafts. Got the failures down to about one a year.

My point is that, yes, an analysis of failure modes is needed to insure safety and regulatory considerations are satisfied. Then a cost analysis is required to select a maintenance strategy. Just cause a motor burns up does not mean there is a design flaw that has to be fixed. There may be a design flaw :roll:

carl

[ January 25, 2005, 05:30 AM: Message edited by: coulter ]

 

[69boss302]

Re: Motor overload protection

coulter:
Everything makes sense. I had seen your strategy with the increase in the service factor and you seem to know what you are doing. I just wanted to bring to light that more things need to be looked at when motor's fail. This was a person asking about something to teach people about motor overloads and I didn't want them going out with a simple answer. I have had very similar situations with bearings and cost breakdowns for run to failure analysis. Only thing was that I have seen production people not pay any attention to how things damage other parts of the machinery and mainly electrical distribution and controls.

I had one "engineer" that installed a 250 HP motor on a long wire run and didn't size up the wire for voltage drop and the starter burned up. They replaced the starter and then the motor burned up. They replaced the motor and the starter burned up again. It just get's frustrating to see these things go on because people are trained to look at one thing.

I have had to stop my maintenance guys I don't know how many times from the "just put a bigger horse power motor on" philosophy. That is what production people loved to recommend. They don't understand all that is involved. It's just a motor.

I had a couple of 150 HP DC motors that used to burn up about every 8 months. They wouldn't believe me when I said there was some kind of bind somewhere and they needed to find it. One of my guy's suggested tig welding the com's instead of soldering them like they had been doing. We did that, and the motor's were running like a champ. They started to last up to a year and a half and for some reason or other they started to find all sorts of mechanical problems. Bearings getting literally ripped out and table roll gears that were getting messed up. They tried to blame the motor then, and I said yep it's the motors fault. Makes you find where the real problems are now.

And I have seen a grinder with a 150 HP motor go through it's run to failure because they thought it more cost effective. All they experienced was what I mentioned before. The machine was down more often because it wouldn't last long and the spare motor didn't get repaired, and that was because production wouldn't authorize the req. because they didn't want to spend the money. Usually end up having to by a new one eventually anyway because you can't continually rewind the motor. Damage happens to the core every time you burn out the stator.

Hope you didn't take to much offense, I just get my self rambling some times.

 

[sluton]

Re: Motor overload protection

Embedded overloads are a real pain in most plant applications.

If the equipment can with-stand more horsepower, why not increase the motor size to do the work expected. If the equipment will not with-stand the additional horsepower, then improve the equipment.

Bottom line is to treat the conditions, not the results of the conditions.

 

[izak]

Re: Motor overload protection

itegral overloads are a ***** ....

ive fixed about 3 box type fans by soldering a jumper aross the two leads and "fixing"it...

i mean... how much of a fire can a box fan make, anyway????

heheee