# 3 phase Main contactor chatter problem



## 460 Delta

If you replaced the contactor with a good one, then the problem is voltage drop, check with a 260 Simpson, a DVOM isn’t fast enough to test for this.


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## Wirenuting

Welcome to ET,,,, Please finish filling out your profile.


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## MikeFL

Did you have a spare new contactor?
Or did you have a spare contactor which was replaced because it was chattering?


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## varmit

If possible, disconnect hoist motor(s). Have someone hold hoist push button depressed (or keyed on on radio control). Measure "fall of potential" between the control power "hot" in the hoist panel, and the coil terminal. If this reads more than 50 to 70 mv, Check for worn contacts or loose control wiring. If there are motor overload relays, the contacts in these can fail also.


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## Helmut

You replaced the contactor and coil?

Sounds like a shorted coil if voltage is good.


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## frenchelectrican

What kind of overhead crane are we taking about the track type or jib type ? 

If that is the track type crane any chance that did you check the concants on the slider fingers ? 

How big those crane hoist is rated ? 


as other mention about the methold of testing you will need a fast acting test meter to catch the dip and spike on voltage.


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## Bsmithjf

Hey. So using an analog meter, I checked for voltage drop. It stays constant at about 119V. I rechecked the resistor bank just for verification and it checks good. I checked the brake contactor and it's good. I checked the hoist brake to see if it was causing current spike due to possibly being locked up and it was good. I unhooked the hoist motor wires on hoist 2 and checked resistance and they all read the same resistance. Once I unhooked the motor wires I checked the main contactor and it stopped chattering. All points to a bad motor but it checks good. What else could I check on this motor to see if it is bad? It spins freely when the brake is open. Thanks


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## telsa

Nothing stops a contactor from having bad internals -- pitting -- the works.

Without a load, it will read fine as your meters draw zilch in amps or watts.

I'd remove a suspect contactor, force it to close manually and then megger across it, phase by phase to see if I get weird ( differing) resistances// fall of potentials. 

You've already checked the stuff that you can see with the Mark I eyeball.

I'd also take a look at the Momentarily Closed circuit to see that it's not compromised. It's gotten plenty of use. IT might be passing marginal voltage -- under load -- on to the coil.


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## gpop

Is the contact sized big enough to handle the load. Ive seen them buzz when they are overloaded (good clue a contractor is overloaded when you hear the buzz).

If you still have the old one field strip it and look at the internals as that may hold a clue. Ive seen loose screws, fireants and a bunch of other reasons for a contractor to rattle. With out hearing it everything is a guess.


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## CMP

One of the most common causes of buzzing and chattering, can be dirty pole pieces of the electro-magnet. Its not uncommon to have surface rust or metallic dust on the pole piece faces. The pole pieces contain residual magnetism when turned off, and any metallic dust or rust will not fall away, but just moves around the pole face when re-energized.


The fix can be to open the unit and remove the pole pieces for cleaning. If it's rust, burnish the pole faces on some 400 grit wet/dry sandpaper, backed up with a hard flat surface behind it. Blow clean the entire assembly before putting it back together.


IEC type contactors, may not be so easy to disassemble for cleaning, depending on the size and type.


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## jaacrane

As some one else mentioned, disconnect the motor and check function again. If not chattering, then yes motor has an issue. Does this hoist have single speed or is it a double speed?


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## jaacrane

I’m sorry I just noticed you mentioned it had resistors.


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## Rakesh kumar soharu

Bsmithjf said:


> Hello,
> 
> I Have a Street Brand 40 Ton overhead crane(2 hoists @ 20T each). The main contactor just starting chattering this morning but only when i use both hoists at the same time going up and down. Input voltage to the coil remains at 120V while chattering and if you manually hold in the contactor, the chattering goes away. I had a spare contactor so i replaced it and the problem continued. I checked connections throughout the crane and they all are tight. I checked resistance on the 3 phase motors and all three are giving me the same resistance, which usually means the motors are good. Checked resistor banks and they test good too. This is unusual to me. Does anybody have any other ideas on what to test?
> 
> Thanks,
> Brandon Smith


I also found contactors chattering while starting the crane. Found control voltage going to the contactor coil was going to the limit switch. 
Limit switch was earthing so grounding the control voltage. 
Removed earthing and was ok.


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## paulengr

Are you measuring voltage across the contactor itself as in measuring on the two terminals directly, not somewhere else?

Contactors chatter for 4 reasons:
1. Dirt or foreign material blocks the armature or it’s so old, springs worn, or the body is so warped it’s shot.
2. A pole face coil is broken. This is a tiny wire that looks like a retainer crossing the pole face that helps shape the magnetic field that often gets broken. Very common on large ones.
3. Low voltage to coil usually because something else has worn contacts or corroded or broken wiring.
4. Coil is failing. Check resistance but this isn’t always reliable. Check visually for a swollen coil. Also a common problem is larger modern contactors are DC with an “economize” circuit…it’s a rectifier with an SCR and a circuit that cuts the output voltage to about 10% once the contactor closes. I have seen external versions on old ABB starters and certain brands (Advantage) are notorious for failures. Often just one diode/SCR fails and so the contactor chatters because it’s running at half power or shorted so it outputs AC.

Contactors are so cheap these days just replace with new.


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## wiz1997

CMP said:


> One of the most common causes of buzzing and chattering, can be dirty pole pieces of the electro-magnet. Its not uncommon to have surface rust or metallic dust on the pole piece faces. The pole pieces contain residual magnetism when turned off, and any metallic dust or rust will not fall away, but just moves around the pole face when re-energized.
> 
> 
> The fix can be to open the unit and remove the pole pieces for cleaning. If it's rust, burnish the pole faces on some 400 grit wet/dry sandpaper, backed up with a hard flat surface behind it. Blow clean the entire assembly before putting it back together.
> 
> 
> IEC type contactors, may not be so easy to disassemble for cleaning, depending on the size and type.


This is probably the most common fix to chattering contactors I've used over the years.
Something else to look for is to see if the shading coils are still on the pole faces.
The shading coil is a metal ring embedded in the surface of the coil.
If cracked or missing, the contactor will chatter.

Here's an explaination of what the shading coil is and does.









Shading coil - Wikipedia







en.m.wikipedia.org


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