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Nema 6 square starter coil problem

Nema 6 square starter coil problem

Nema 6 square starter coil problem

I have a square D nema 6 starter for a 400 Hp motor.  Recently the coil burned up, and I was called out to replace it.  I didn't have a size 6 coil, but the plant did have a size 5 starter as a spare.  I took the coil out of that one and put it into the nema 6.  The coils looked identical.  The new coil was 480V, and the old was 120V, so I had to put an interposing contactor in place.  I was told by the operators that prior to the burn up, the coil had been chattering badly.  When the operator opened the door, the coil was on fire.  I don't know what caused the original chattering, but I am pretty sure that this chattering was the coil not being pulled in all the way.  

So after installing the 480V coil, I disconnected the motor and tried starting the system.  Same results.  Lots of chatter, contactor won't pull in.  I pulled the thing apart and put it back together 4 times, looking for mis-alignments.  Nothing.  

One other worthy note on this mystery.  The starter has on board a "solid state motor control".  This device is wired into the controls, the aux switch, and the 120V side of the control transformer.  It appears that this device is used to deliver current to the starting coil.  Because the original starting coil was 120, this device was not performing it's proper role once the 480 coil was installed.

So, now to some questions....
1. Is it possible that a coil from a 5 size starter is not strong enough to pull in the contacts for a size 6 starter?

2. Does anyone know what this solid state motor control device does, other than logic functions?  Is it possible that this device provides an initial extra surge current for pulling the contactor in?

Any help on this would be appreciated.

RE: Nema 6 square starter coil problem

Can you push in the moveable contacts by hand? I like to check contactors by hand to make sure that nothings binding up. Is it possible that the 120V coil is DC. A lot of bigger contactors use DC coils, and have rectifier and economizing module built in. The economizing module allows for full current to pull in the starter and then reduces the current to hold it in.

RE: Nema 6 square starter coil problem

Everything moves easily, except for the last 1/4".  It takes quite a bit of force to push until the shading coil makes contact.

RE: Nema 6 square starter coil problem

In that size it is common to use a DC coil. This will have an economizer winding. If the connections are not changed properly to take the pull in coil out of the circuit, the coil will fail.
But that won't explain the chatter.
If this is a DC coil check the diodes. It may be running on half wave instead of full wave DC.
If this is an AC coil, check to see if a shading coil is broken. The shading coil is one shorted turn of a conductor in a groove in the magnetic pole face, enclosing part of the pole face.
Heavy resistance in the last 1/4 inch or so is normal, but check the contact springs in all three poles to make sure that one is not binding and producing extra force. Dirt or debris that prevents the magnetic circuit from closing all the way may also cause this issue. Any thing which prevents full travel of the magnet may cause chattering. Severe chattering may cause burnout.

"Why not the best?"
Jimmy Carter

RE: Nema 6 square starter coil problem

Look at the part number of the coils in their catalog. If they are the same, the part numbers will be the same. Not likely between a Size 5 and a Size 6 in my opinion.

What exactly is the "solid state motor control"? Have a part number? It may be a Solid State Overload Relay and if that's the case, you may have killed it by applying 480V to it (if you did, not sure of what the circuit looks like from your description).

Causes of coil chatter (others will likely add to the list):
1) Low control circuit voltage. This can be the result of a voltage drop on the entire system from starting a 400HP motor X-Line. it could also be a bad or under sized control power transformer or a high resistance somewhere in the control circuit causing a voltage drop (distance, bad contact etc.) Being that you used a 480V coil and it didn't fix the problem, that would eliminate the bad CPT, but still could be something else, especially overall system voltage drop from X-Line starting. Coils on NEMA contactors typically have a drop-out voltage of 70% but as you approach it, they will sometimes chatter before dropping out completely.

2) Rapid cycling of a control circuit element. For instance, if you have something like a float switch or pressure switch in the circuit, usually you must have either a hysteresis in the device (difference between on and off state) or you need a "de-bounce" timer in there. Without something to prevent fluttering of the control element, you will toast the coil rapidly.

3) A mechanical jam or separation of the pole faces of the armature in the contactor. one that prevents the yoke from making full clean contact with the magnet face. Rust, debris, drill filings, insect carcasses, any little thing that prevents full mating of the faces can cause that.

4) Previous damage. It could be that you had a low voltage problem, that caused the initial coil fire, but that has now cause a slight alignment problem in those pole faces. The Sq. D 8536 starters use what is called a "Bell Crank" mechanism in the mechanical movement, that is why you feel it being difficult to push in only at the last part of the stroke. But any damage to any of those levers and fulcrums in that system may leave you with an incomplete close. In that case, toss the entire contactor and start over.

If it were me, I would junk the thing and replace it with a solid state soft starter. 400HP is too big a motor to be starting X-Line in my opinion.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Nema 6 square starter coil problem

I found some Square D documentation which states that sizes 00 - 5 use AC coils.  Size 6 uses DC.  This would explain why the 480V coil wouldn't work.  The "solid state motor controller" is a rectifier/logic unit.  I will put a scope on the output to see what the current waveform to the coil looks like.  I agree that it may be sending a 1/2 waveform.  This might cause chatter.  I also agree to junk the thing.  We're heading back up this morning with a new coil and a complete starter, if the coil doesn't work.

RE: Nema 6 square starter coil problem

FYI, it was the coil.  The rectifier unit was in good shape.  Once we put the DC coil in, the starter pulled right in.  I still don't know what the original chatter was caused by, but I have two theories.  1. The cabinet was very dirty.  There may have been dirt in the coil.  2. The starter was installed in the early 80s, and the coil might just have died from old age.  

Thanks for your help.

RE: Nema 6 square starter coil problem

Thanks for the update eeprom.

"Why not the best?"
Jimmy Carter

RE: Nema 6 square starter coil problem

For future reference I stock those coils and any other parts you would need for that starter.  

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