Motor Contactors

[tshea]

I'm looking for information on what would cause the points on a contactor to burn/melt.
One possibility that I have been told is the circuit breaker feeding the motor tripped and burned the contacts. Is this true?
Another is mis-alignment (I knew that).
Dirty contacts from the environment is another possibility.

Opinions?

 

[69boss302]

Many different things. First the thing that burns them up is excessive current, which can come from several sources. I've seen burned up contacts many times from
-people setting overloads to high to keep the motor from tripping out.
-no proper maintenance on contactor (contacts wear and will burn up when they are to worn to make proper contact)
-some contactors even need to have adjustments made for contact surface wipe.

The contacts can burn when the ckt bkr trips because of the inductive kick back on them. Many contactors have arc suppressors across the contacts to protect them from this.

What size contactor by the way?
Just one contact burned? Or all three? If this is a three phase motor?

 

[coulter]

tshea -

When you say points, I'm translating to "main power contacts" as opposed to control contacts.

You have two good ones - minalignment and dirty. But, If the the CB trips, then those contacts take the fireball from the inductive load - contactor should be okay.

Others might be:

Chattering - 100 years normal service in a day - bum control ckt
Opening under heavy inductive load - when the mag field collapses the energy comes out in a fire ball at the contacts - jogging, ovld trip while stalled are examples
Severe overload - even if resistive, worse if inductive
Resistive rated and used on tungsten or inductive loads

 

[petersonra]

Re: Motor Contactors

I'm looking for information on what would cause the points on a contactor to burn/melt.
One possibility that I have been told is the circuit breaker feeding the motor tripped and burned the contacts. Is this true?
Another is mis-alignment (I knew that).
Dirty contacts from the environment is another possibility.

Opinions?

The idea that a CB tripping once would cause the contacts to fail is ludicrous. Forget that suggestion.

The things are either seeing WAY too much current (unlikely), operation frequency, or it is seeing severe environmental conditions.

As another poster suggested, it is possible the thing is chattering, either due to a bad control circuit, or because the coil is failing or might be the wrong voltage. Excessive jogging or especially plugging could do it, but if it is properly sized the motor would probably go long before the contactor.

Environmental factors such as corrosive or dusty environments could play a role as well.

Are these reversing? IEC? How old?

 

[tshea]

Are these reversing? IEC? How old?

Not reversing. IEC (AB 100-D180) New.
Tripped the breaker the first time the start button was pressed. Then twice more.

I will be able to see the contacts Friday after 2 and try to post a picture.

 

[jim dungar]

If the breaker tripped on the first attempt to start, then I would blame the power circuit wiring and the motor, not the breaker.

Make sure the IEC contacts are rated for the motor. A good practice is to size IEC contactors by their current rating not their HP rating. For example most 10HP 480V IEC devices are rated for about 18A versus a NEMA starter which is rated at about 27A. You need to compare apples to apples. Use the life curves of the starter to verify number of expected operations at anticipated loading.

As it was pointed out if the contacts hang up for any reason (dirt, jamming, low coil voltage) they may burn up very fast.

 

[petersonra]

Are these reversing? IEC? How old?

Not reversing. IEC (AB 100-D180) New.
Tripped the breaker the first time the start button was pressed. Then twice more.

I will be able to see the contacts Friday after 2 and try to post a picture.

sounds like there is a problem with the motor or wiring. maybe a ground fault of some time.

try meggering the wirring and the windings.

 

[augie47]

sounds like there is a problem with the motor or wiring. maybe a ground fault of some time.

try meggering the wirring and the windings.

sounds like your best advice.
also,
you might take a look at OEM specs on your starter. They often recommend a faster acting fuse in lieu of a breaker to help prevent contact damage in the event of a short, etc. especially on IEC units.

 

[69boss302]

[quote="petersonra
sounds like there is a problem with the motor or wiring. maybe a ground fault of some time.

try meggering the wirring and the windings.[/quote]

I agree.

 

[shocker3218]

what about the possibility of a high inrush "design E" motor?

 

[petersonra]

what about the possibility of a high inrush "design E" motor?

could be an undersized OCPD too. not really a lot of information to go on here.

its like when I get a call for help from someone and basically all they can tell me is that "it don't work". not a lot I can do with that.

 

[oliver100]

If the breaker/contactor have worked before properly something is wrong down the road (the motor or the driven machinery). In similar cases, the user is pushing many times the Start Push Button and somehow the system works with cumulative contact damage. At some point the contactor melts with very prominent bangs and fireworks.

If it were new, the selected devices are undersized or defective.

 

[tshea]

The plot thickens...

170 Amp (total load) machine with 50HP motor, 208 V, 3 ph. Data tag says 160 Amp total package.
200Amp fusible disconnect w/ 200 Amp dual element time delay fuses
wired to a 3p200 Amp I-line breaker
wire size is 3/0

200 Amp breaker tripped 3 times, then a 400 Amp was installed and machine ran fine.

Client claims main power contacts were melted/ burned. (I still have not seen them). Machine is running now.

My calculations:
160A * 115% = 184A fuse (200A) & 200A disconnect
160A * 125% = 200A conductor = 3/0
160A * 250% = 400A breaker

Client claims because original 200A breaker tripped, that was cause for burning the main power contacts.

I disagree. Opinions?

 

[petersonra]

the cb opening a few times will not cause the contacts any damage. maybe if it happened 10,000 times, but not once or twice.

I don't have any major objection to your calcs off the top of my head, not having any reference material handy.

i am not overjoyed with the idea of just replacing the 200A cb with a 400A cb. what protects your 3/0 wiring?

The plot thickens...

170 Amp (total load) machine with 50HP motor, 208 V, 3 ph. Data tag says 160 Amp total package.
200Amp fusible disconnect w/ 200 Amp dual element time delay fuses
wired to a 3p200 Amp I-line breaker
wire size is 3/0

200 Amp breaker tripped 3 times, then a 400 Amp was installed and machine ran fine.

Client claims main power contacts were melted/ burned. (I still have not seen them). Machine is running now.

My calculations:
160A * 115% = 184A fuse (200A) & 200A disconnect
160A * 125% = 200A conductor = 3/0
160A * 250% = 400A breaker

Client claims because original 200A breaker tripped, that was cause for burning the main power contacts.

I disagree. Opinions?

 

[oliver100]

I disagree. Opinions?

Me too.

Most likely the the melted contactor was tripping the 200 A breaker.

 

[jim dungar]

tshea,

I believe, you can only use 250% when the breaker feeds a single motor (I am away from my code book).

There is no way the breaker could have caused damaged to the contactor, otherwise we would be destroying lots of devices every time we turned off their circuits when they are loaded.

What was the condition of the overload relay on the failed contactor?

 

[69boss302]

I disagree. Opinions?

Most likely the the melted contactor was tripping the 200 A breaker.

I'd go along with that.

 

[benaround]

I disagree, 160a motor starting current about 960a, then the 200a fuses

open,or even worse only one opens at first single phasing the still starting

motor. I would say opening the contacts under those conditions could

easily arc into a fireball.

The 400a breaker is the answer imo.

 

[jim dungar]

To be UL listed with a HP rating the device must be capable of making and breaking the motor locked rotor current (for most devices this can be estimated as 10x the current from the NEC).

UL listed motor overloads are sized to allow 6x full load for either 10secs (IEC standard) or 20secs (NEMA standard). before opening.

Taken together this means a UL listed continuous rated 160A starter is expected to make and break 1600A and to carry 960A for at least 10 seconds before opening without damage.

 

[tshea]

Thanks all! I will finally get to see the contacts on Friday. Bear in mind this machine is running and Friday is the first time it can be shutdown. Will post picture and any other infor I am able to discern.