Direct start 10 HP motor - Faulty contactor 1

[lukin1977]

Dear Experts

We have in operation a machine that has a 10 HP electric motor – 3x380 V – 50 Hz
The motor has direct start with a contactor. The problem is this:

Every 2 month (aprox) there is a fault in the contactor. Most of the time the contacts melt and weld together. This is a happening for a long time and what we do is have a spare contactor and change it and then wait for the next 2 month.

The nature of the work made by the machine is so that the operator makes many starts and stops throught out the day on this motor

How can I do to finally solve this problem? Should I used an over rated concator, for example 20 HP?

Thanks

 

[jraef]

Contactors, especially IEC contactors, must be sized by current and duty cycle, with an eye for expected life span. Unfortunately many OEMs, in an effort to keep cost down, ignore that last part except as it relates to THEIR warranty.

So yes, you most likely have to increase the size of that contactor. Every contactor manufacturer has a chart available that is used to plot the current, duty cycle and expected lifespan of their products. Elicit the help of your manufacturer in selecting the proper size for your application.

That said, another cause of rapid failure is sometimes attributable to faulty control circuit issues causing what is called "chatter" in the contactor. For example a low control voltage can cause the contactor to not hold in properly, or a bad connection on a contact block in a control element such as a push button or relay can cause intermittent operation. Check those things fully before jumping into a new contactor, because if a problem like that exists you will smoke the new contactor too, it just may take a little longer.


"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]

I agree with jraef. Another issue that may arise with rapid duty cycles is short off times. If the contactor is re-closed before the residual voltage has decayed in the motor, You may encounter an out of phase closing event. This may subject the contactor to currents in excess of any ratings. The solution is to avoid fast re-starting of the motor. Allow 2 or 3 seconds minimum before restarting.

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

 

[itsmoked]

You could consider using a soft starter which removes contacts from the starting procedure entirely:

http://www.factorymation.com/s.nl/it.A/id.59035/.f?sc=2&category=83875

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lukin1977]

Yes. The problem is the restarting. Thats what I meant when I said "many starts". Sorry for my mistake.

Should I install some kind of timer to prevent restarting?

 

[ScottyUK]

What brand of contactor is it? At least one big manufacturer has taken so much copper and iron out of their current range of small contactors that they run blistering hot and 'might' meet spec under perfect conditions but are a reliability problem under real-world conditions. So if it happens to have 'TeSys' written on it then dump it in a skip. If it says 'Made in China" then also dump it in a skip. The Siemens Sirius range, ABB's A-series and A-B's range who's name escapes me are all pretty good.

You might consider choosing your contactor based on its AC-4 rating rather than the normally quoted AC-3 rating. AC-4 is a severe duty rating for plugging and reversing applications: normally it effectively results in using a contactor one frame size larger.


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

 

[lukin1977]

Brand: WEG
Model: CMW 25

 

[itsmoked]

lukin1977; it is not just the starting more than likely it is the stopping while the motor is still actually in the process of starting. That is when the motor is trying to draw its 8-10x running current. Interruptions during that period are very hard on contactors, causing welding etc.

Yes timers would be a good solution as they would also remove switch chatter or hesitation.

You would want a timer to let the motor reach speed before turn off and ideally some sort of time to prevent turning the motor on more frequently than absolutely necessary.

Perhaps the process doesn't need to be turned on and off more often than every few minutes, most motors that size would not like to be turned on/off more than ~ 20 times an hour, so then set the on relay to a few minutes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Rated 11kW, that's about 15HP so it is already a little bit over sized. You may need more.

Maximum ops/hr = 1200 at AC-3 but drops to 360 at AC-4.

AC-3
Utilization Category AC-3 is typical for most common applications utilizing a standard squirrel cage induction motor. The Contactor or Starter is closed to line start the motor, and then opened while the motor is running to stop it. Opening the Contactor while the motor is running reduces the current the Contactor has to break to the motor’s full load value. Under this condition only a low voltage is developed across the contacts as a result of the small difference between the motor’s back EMF voltage and the line voltage.

AC-4
Utilization Category AC-4 typically also applies to standard induction motors, but in this case the start and stop actions are more severe. The Contactor or Starter makes into the locked rotor current of the motor, but can be opened while the locked rotor current is still flowing, with a large voltage present across the contacts, because the motor’s back EMF voltage is still low while starting. The combination of high current and voltage significantly reduces the life of the contacts. Typical applications require jogging or plugging. Plugging duty can be more severe because the motor is rotating in the reverse direction when one Contactor is opened and a second closed, potentially causing a much larger current to flow for a longer time period. If a Contactor or Starter carries both an AC-3 and AC-4 rating, the AC-4 operating life is usually a fraction of the AC-3 value.

"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]

I am not concerned with the starter opening under starting current so much as I am concerned with the contactor re-closing out of phase before the motor residual current has decayed. The resulting currents may be much greater than locked rotor current.

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

 

[ccjersey]

Some kind of re-think of the process operational requirements seems to be in order.

Constant jogging with the starter has to be avoided if possible. It could involve both opening under starting surge amperage as well as restarting out of phase with the residual magnetic field in the motor. Or not.....the difference between basically operating with the contacts chattering and just concern about the number of starts per hour is only a matter of seconds.

On the other hand, a new contactor every 2 months might be the cheapest solution.

 

[lukin1977]

My first and common sense option was to try to work with the machine operator and his boss to stop using the machine that way. Unfortunatly that did not not work. I wont start explaining why but I am sure that you all know what I mean (people)

So, that is why I want to electricaly protect the contactor.
I understand that the reasons the contactor fails are 2:

1. opening contactor before it finishes starting the motor.
2. Re-closing with residual current

Can anyone recommend an instalation scheme? drawings? to avoid these

Thanks

 

[lukin1977]

almost forget

the cost of one contactor is: 32 USD

 

[wayne440]

Consider that you have already talked to both the machine operator and his/her boss, and that they are unwilling to modify their operating methods. Given this, what are the chances that any electrical protection of the motor that forces a change in operation methods will be accepted?

Changing the contactor every 60 days may be a reasonable price to avoid the complaining that will almost certainly result from making a change to the machines operational characteristics.

 

[waross]

I used to service a number of overhead cranes with push-button control pendants. When a poor operator was positioning the hook there would be multiple rapid reclosings.
One out of four had multiple contactor issues. We finally replaced every contactor in the control box with Allen Bradley contactors and never had another contactor failure. We lost a lot of work. grin.
You may want to consider an Allen Bradley, size 2, 25 HP@380V, Part number 509-CO* (*=Coil voltage).
The choice between Eutectic melting alloy O/Ls and electronic O/L protection is another discussion. Either will work. Eutectic has no differential protection but has very good over current protection.


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

 

[waross]

PS At $32 USD it is a wonder that that piece of crap lasts a week. Quality costs. Be sitting down when you see the price of the AB starter.
Don't be frightened by list prices in a fairly new catalog. Catalog list prices must be used with a discount schedule. There may be a 4:1 ratio between list prices and actual cost.
I have seen a $20,000 quote at replacement prices drop to $5000 at competitive bid prices.

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

 

[dpc]

In the NEMA world, this would take a Size #2 NEMA contactor I'm guessing it would have a much longer life.



David Castor

http://www.cvoes.com

 

[waross]

One out of four had multiple contactor issues."
By this I meant that three cranes worked well (and most of the other cranes that I saw only once or twice.) One crane had undersized, poor quality contactors and in damp weather we had a failure and trouble call once a week or more. That's a lot of down time for a working truck loading yard. Good quality contactors, properly sized solved the issues.

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

 

[lukin1977]

ok. So a good oversized contactor will eliminate the problem for many years

I have 3 options here:

Telemecanique
Siemens
Moeller

Allen Bradley: I Did not find anyone who sells this brand here
I am writing from Paraguay

 

[ScottyUK]

Of those three, Siemens without a doubt. Moeller in second place. Telemecanique's current TeSys range was blacklisted at our site because of the number of failures we suffered - a shame because the earlier LC1 series was pretty good.

Size the contactor from the AC-4 ratings, not AC-3. That might well push you up one frame size compared with AC-3, and you'll end up with a contactor roughly the same size as the NEMA equivalent.

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