Shunting a Thermal Overload Relay

It would certainly be a simple thing to stick some contacts around the O/L ... but... beware of Murphy's law.

I would think it prudent to do some thorough troubleshooting first. Use a clamp-on and see what actual current you are running during this period and verify the elements are sized correctly.

I personally wouldn't risk simply closing around the overload but if I absolutely had no other recourse I might, but I would use some fuses that gave me locked rotor and instantaneous protection.

Thanks Switchman--Fuses, how simple! I could calculate the neccesary current/size the wires and contacts and fuse them! Brilliant!

I may try it with a timing relay and put it into the PLC later--Thanks and a GA from me.

if all the electrical component is in good condition, the first start o/l could be due to fluid accumulating in the compressor chamber , after the first crank probably the fluid had been pump out and the second start should be a little diffcult for the motor but eventually it will just run fine. i could be wrong , but this is similar to my twin run compressor. One will fail to strt-up if rest for more than 6hrs--- sometime.

Powermass--You have two compressors and one overloads? This is the same as that, I believe--I still haven't looked into it any further yet, but I will.

Dear friend test the relay for its thermal characterstics and short circuit characterstics, record the motor starting current and verify the coordination. I feel the characterstics of the relay may be at slightly under curve for higher currents ( motor Starting current ), which causes tripping. Alternatively also findout if there is enough dust in the relay which reduces the gap of the thermal element releasing the trip faster.

M.I.KHAN

Kigore,

Check the oil temperature in the morning. This sounds like it could be caused by cold thick oil causing extra starting load. Also check the oil viscosity.

LG_DAVE

Sure thing--

This only happens once and it is always in the morning at start up. The compressor is healthy and you are probably correct on temp and all that--But it is PLC controlled and the overload is sized correctly and I am not inclined to walk away from a ongoing problem.

Either keep heat on the compressor or shunt the O/L relay one time at start up with the PLC and never have to reset the motor protection.

One thing I do want to find out is: There may be two compressors in tandem on the system and they may be starting at the same time. I will try to sneak away and look at it.

Kilgore,

One further thought - if the motor was starting against pressure in the cylinder it would see a starting overload. Is the dump valve functioning properly? Is pressure somehow bleeding into the cylinder overnight and not being dumped during the initial startup?

LG_DAVE

I haven't been to it yet, but, per the information I have is, that once it gets out of the "hole" in the morning, it runs fine all day.

I believe that, in the morning, the pressure in the system is has bled off to zero.

Now I'm going to have to get over there today and find out what is there. Check you later today.

If pressure has bled to zero, it may be a time-related issue, as the compressor will need to run longer to reach its pressure setting and shutting off. Re-starts will take place with a significant percentage of the pressure still in the tank, and the time needed to raise that to the upper limit will be far less. Cold vs. warm oil and similar aspects may contribute to the overall result. Simply watching the first morning start, and observing the sequence of events, will tell you what is actually happening, I believe. Take notes: start time, initial pressure reading, whether trip occurs almost immediately or after some delay (note the time!), pressure at time of trip, pressure at re-start after relay is reset, pressure at automatic re-start during the day, and time to raise pressure to upper limit [during that rest / restart] are all worth noting. If you can read oil temperature, even better! You may be able to observe additional aspects, and possibly find out what other loads are being started at the same time, or during the period while this compressor is building pressure after its first start of the day. Cure may be as simple as a slight reprogramming of timing of events.

One other thing you may want to check. Many motor starters have the option of manual or automatic reset on the trip. If yours has that option, put it in automatic and let it resatrt itself in the morning.

Yes and when there is a real overload problem then this auto start will barbacue your motor.

Hi,

I have seen this done, once, and it did not appear to cause issues. But if it was my choice, no, I would not go this way. Check out the compressor a little more, is there an isssue with the unload? What type of starter is it, and is there a problem here? Excessive start time?

If it does look like the problem is the overload then look into changing the overload to one that has a modified curve to cover the starting. Maybe look into using motor thermistors and raise the overload setting. Some of the electronic overloads can be quite smart with their thermal modelling and are ways to get around this.

Regards,

Trevor.

Maybe look into using motor thermistors and raise the overload setting.

Raise them one thermistor range...cheapest/easiest thing to do...better than fusing.

MR. GUY

Is the problem only noticed during the starting in morning.

Further , Ifeel that the tripping may be due to surge current during starting. The surge current can be measured using a CRO and it is better to analyze the starting current waveform and proceed further.

Regards

BINNY, Kerala, INDIA

I would be cautions, as you are, before bypassing a protection feature. Compressors usually have a pneumatic circuit that bypasses the compressor load until it is up to speed. Something in this circuit may be faulty.

Rather than bypassing the overloads with a time delay relay or PLC, you might consider adding a solenoid valves to unload the compressor head until it is up to speed.

I worked long time ago (22 years) as maintenance responsible. I remember that all the motors compressors started with a reduced tension starter. All that thoseof the type two step (with an auto-transformer) square D starters.

You do not mention if your starter is full load type.

best regards.

Francisco.

Hello and welcome to all!

BOOOO!

Kids are getting ready to go get treats and I have to go unburry the duck eggs from this spring, so---I don't have a lot of time.

I actually ran in and opened the MCC bucket and took a gander at the starter(2 seconds only)--

It is across the line,

With adjustable overloads(Square D and I am not familiar with Square D),

33.2 A

I didn't see the compressor/motor, but I am sure it is fine. They have troubleshot it properly and have abandoned it for one nusiance trip on start-up.---I will look at the compressor Monday--I don't know if it is a screw type or piston(with unloaders) or what.

I think that it falls into the crack between good protection and no protection. Up size overloads--Not a good idea. Go with resetting overloads every morning, I don't like that either. Shunt them for a definite time, once in the morning---Maybe.

I think you are on the right track; investigate thoroughly, then only solve the problem that exists, not the ones you ASSume exist.

And just so that you know in case that remains your only viable option, it is an accepted (albeit not too common) practice to shunt the OL on startup with hard to start motors, regardless of size or voltage. The National Electric Code has a specific section dealing with this in Article 430; find it, read it understand it and follow it.

Here is a link to selection and use of Square D thermal overloads.

Perhaps you should use a slow-trip version.

Check out the instructions for furthur details at this link:

https://www.schneider-electric.us/en/download/document/9065CT9701/

Hope this helps

HTRN

HTRN-You should register(if you haven't already)--Your input is and will be valuable and it will be nice to bump into you in different threads on a social level. If you were registered, there would be an e-mail alert that I responded to you and it will have a link to this thread.

I will try to gather all the information about the compressor/motor this week and sit down here and come up with the best solution, with all of your help. Until then--thanks.

When referencing the thermal overload element (heater) size, be sure to include the service factor of the motor.Starter manufacturers have a chart for proper sizing, but the chart is brand-specific.Since this has been going on for years, the thermal overload may have been improperly sized from the start.You may also have a loose connection on a single phase at the motor or starter that heats up,trips out,and due to expansion from heat, becomes solid.Once reset,the integrity is maintained by the constant cyling of the motor.The overnight cooling may open the circuit again on 1 phase,causing the cycle to repeat.

Remember, fusing is to protect the wiring, overloads are for motor protection,and fusing cannot be substituted for properly sized overloads.

Another good point brought up by others is viscosity of the oil. A crankcase heater might be required, and various types are available, including some that screw right into a pipe thread, or bands that strap around the compressor.

Keep us posted on your findings.

HTRN

The system as is--Is sized, fused and overload protected properly. That isn't in question. Single phasing can be ruled out out--It starts and stops all day long. The overloads work properly and aren't suspect.

The fuses that were mentioned are for the (possible) shunt wiring and timing relay only. The combination starter is already protected.

The compressor has one hard start after sitting overnight. I want to tap dance around this one start without sacrificing any motor protection.

Thermal overloads can't help themselves here--Electronic ones I have used with automatic resets create bad habits. If a motor generates enough heat when starting and has the overload drop it out--the heat will dissipate out of the overload faster than the motor. Restarting it right away multiple times can be very bad.

My theory is that if there is a condition where this motor loads up just enough on the first start to trip the O/L--If it had a chance to run without the inrush current any longer, that it would cool and heal up faster while running than if it were when it stops dead in it's tracks. How can this be done?

Try putting a heat lamp on the compressor overnight.This will keep the oil warm,and tell you if it is due to thick cold oil. If heating the crankcase does not help,check for the bleed-off valve sticking and causing the compressor to start under load.The amps have to be relatively high for more than a few seconds to trip out the OL's.

Try to determine what is different in the conditons of the system in the morning:Temperature of the oil? What other factors are different? Motor bearing grease temperature?Are the motor bearings greaseable?If so, is the drain plug for the bearings left open for 5 minutes with motor running after adding grease?The reason I ask, is if the bearings are overlubed, the grease will churn in the bearing until it is forced out of the way by the balls.This will create extra load for a short period.,eventually, the grease will overheat if the overfilling is severe, and the bearings will fail.

If the amps are increasing,that is a sure sign of increased load from somewhere,if the wiring is good,it has to be mechanical.

I would consult the manufacture's manual for type and viscosity of the oil,and perform an oil change if it hasn't been done recently.Likewise with the motor bearings, if grease fittings are present.

I do not advise shunting around the OverLoads.If you do, the contacts will have to have the same rating as the motor starter to handle the load.This is not good practice, and shade tree at best.

You say you have not visited the MC panel, yet you say the overloads are working properly, and sized correctly.Are you sure?I would recommend double checking the OL's against the heater chart and motor amps, including the service factor mutiplier on the motor data plate.

If you will give me the motor data, and the brand of starter, I can do this for you, if you like.

There must be hundreds or thousands of compressors like yours,and I am sure they are working propely, or the factory would be swamped with customer complaints.

Good point on the contactor sizing. However, if your motor is mid sized and you want to avoid using a shunt contactor sized for the motor load, you can use CTs to get the motor current down to 5A and use a 5A contactor to shunt the CT circuit, just like you said you have done on larger motors. Perfectly acceptable.

Another alternative is that there are a few SS OL relays that provide either automatic shunting of the OL curve during starting, or provide "dual curves" that allow you to, for instance, program a Class 30 curve for starting, then drop down to a Class 20 curve for running. The problem with either of those options is that they will be in place for every start, not just the first one in the morning.

But still, as I said earlier, shunting the OLR with a contactor is a perfectly acceptable practice, done all the time. In your case, I woud say to use a small programmable relay like a Siemens LOGO that can be set up to shunt the OL only on the first startup in the morning, then ignore that routine the rest of the working day.

My friend,

I don't see you have a problem. The problem with your compressor motor is a "maintenance". Do you have a monthly- bi-annual or yearly shutdown maintenance? Do your system has only one compressor? I hope, there is stand-by or back up compressor? If yes, please do this.

* Fill up and Maintenance Request Form, indicate all problem you have seen and observed.

** Change-over the problematic compressor. Turn it off, while the stand by or back up compressor run. Isolate the system.

** Remove the compressor or the motor alone (If, this is assembly, remove all)

** Disassemble motor from compressor and check and record the following:

1. Lube oil or grease. (Maybe, it is solidified already) Clean and removed, change lube)

2. Bearings both end. (Change, if necessary) (probable cause)

3. Insulation test. (1Mohm= 1kV) (probable cause)

** While doing on compressor, somebody must check the MCC.

1. Safety tag in and tag out.

2. Clean the motor cubicle and retighten all screws and bolts. Check and recoird all carbon burns.

3. Insulation test of feeder line from MCC to terminal box of the motor.

All of the above must in record.

** PLC

1. Using the "shunt" from motor starter for "fail to start" or "Overload" indications in the PLC is okay, but this will not solve the problem. You may add this system to your OPEX and you must consider this to revise (drawing) or update (rewire) the system.

Eren

Service voltage fluctuates 24 hours. Your urtility voltage may be lower in the morning when your trying to start the motor than during normal hours of the day. Lower voltage mean higher starting current, and may be enough to trip your protective devices.

That is right--But, what we have witnessed is that late night/early morning the voltage jumps from 480v to just over 500v.---Amps go down and we have had suspected problems with burning our radiant heaters up because of the voltage. We changed from 480v to 600v heating elements and the problem subsided--Another story, though.

Our current problem--through the overload is amps.

Forgive me--I still haven't made it out there yet.

Ever try to judge a "pig in a poke"? Cannot be done properly with incomplete information.

You are making a lot of presumptions that have not been substantiated by facts.

Look at the motor,compressor, data plate on the motor,motor amps,service factor,listen to the motor running.Then determine the proper size overload for the application.Slow-trip OL's are available.These provide a safe amount of delay without endangering the motor windings.

Then let us all know what you find.

HTRN

Read my posts--I haven't been able to get on it yet.

The overloads are protecting the motor and have been for over two years, since it was installed.--What's your hurry--This is one of a thousand things I am going to have to deal with--IN TIME. I don't own the place, but I will--believe me--I will, get to it when time permits.

Ahhh, the plot thickens...

You say you have high voltage in the morning when you start? That could also be a problem, especially if the ability for your motor to start the compressor is normally on the ragged edge. The high voltage over-excites the motor and actually increases the current draw at startup.

Not here(that I know of). It was at the last place I worked --A more rural community--I won't rule it out and I will add it to the list when I check.

Well, since there is no rush on this problem,and real information is scant,only assumptions,when you get time to do a proper evaluation of the problem, and truly know the ACTUAL conditions,someone may be better able to help you.You are presenting only a sketchy outline filled with assumptions,based on the fact that it has been running for a long time.

I have done electrical/industrial maintenenae for over 30 years, and am a licensed unlimited electrical contractor, and I would not even think about bothering anyone without thoroughly checking everything PERSONALLY FIRST. NO assumptions.

I'll check back in about 2 years or so,when you have more experience,and when you own the place.

Are you going to offer an IPO?

good luck

On a thermal overload relay--Can a relay(controlled by time from the PLC) close in parallel with the resistive heaters, to defeat them for a definitive time? Is there an accepted practice for doing this? I can imagine that there would be the risk of overloading the shunting circuit--any ideas?--Copied and pasted from the top.

That was the question and it has been answered--And I thank all of you for the input.

The rest is conversational. You don't have to participate, but if you do, I will not interact with you as a guest from here on out. Register or login then:

Check back in a month, and I may have the latitude to cut my own work schedule. I am getting a hand-off from a 30 year veteran and I can wait and so will you and everyone else. It is a combination motor starter anomaly they have lived with long before I came along, NO BIG DEAL.