need help on a D.C. motor contactor?

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charlie tuna

Senior Member
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Florida
i'm in the middle of a problem that i need some advice figuring out how this d.c. contactor works.
this is a big 4160 volt - 2500hp - starter - that pulls the contact vertically up thru a right angle hinged bracket by the function of a coil the size of a football! the contactor voltage is 110 volts D.C.! i just had the coil rewound because it failed (blew up). upon installing the new coil it pulls in and then chaters like it doesn't have the strength to hold the contact up!
i can't have this since it controls 4160 volts to a big chiller unit. connected to the coil is a siries of capacitors and two ceramic coil adjustable wire wound resistors. it also has an auxillary contact. can anybody tell me whats wrong or how this circuit works?? capacitors in a d.c. circuit are probibly to help hold the voltage up for the initial action of the coil???
thanks, charlie tuna
 
Re: need help on a D.C. motor contactor?

I know I do not have to tell you this but the relay is dropping out because the DC voltage is not holding up. Check the voltage at the coil when it is energized and see how low it gets.
 
Re: need help on a D.C. motor contactor?

bob,
what could be causing this voltage drop - there must be a simple circuit for d.c. contactors using capacitors and resistors in their control circuit? i guess i will have to pull the controls apart and make a drawing to determine which device can be suspect of cauing this chatering!
 
Re: need help on a D.C. motor contactor?

i just had the coil rewound because it failed (blew up).
Click to expand...
It takes more current to pull the contactor in than it does to hold it in.

Large DC contactors often have resistors that are switched into the coil circuit, by the auxilliary contact, when the contactor is closed, to reduce the voltage to the coil, to prevent overheating.

In other words, the resistor is bypassed during pull-in to increase the voltage to the coil.
The capacitor reduces the severe arcing at the auxilliary contacts.

Some possibilities-

Coil rewound properly-wire size/no of turns?
Resistors/capacitors connected correctly?

Ed
 
Re: need help on a D.C. motor contactor?

Interesting. Are you sure that the coil assembly doesn't already have a build in diode/ rectifier system already built in as an integral part? Also, it is common for a DC operated coil to have an economizing circuit, that is after it pulls in an electrical interlock automatically switches the rectifier assembly such that the coil receives less power. I know that this is a common design on the Westinghouse/C-H Ampgard medium voltage starters as well as some size 5 and larger starters, such as the vacuum cotactors and starters,that incorporate DC coils where 120vac is actually applied. If I recall they they used an L56 electrical interlock.
If the interlock isn't operating correctly the coil may be getting full voltage continually and may overheat and burn out.
 
Re: need help on a D.C. motor contactor?

thanks for the information guys! the rectifier is a heavy duty unit located in another section of the switchgear. i'm beginning to suspect the auxillary contacts may not be functioning? we isolated the coil and applied d.c. directly to it and it closed the main contacts but with little force and drew about 3 amps. in the original setup with the normal circuit intact this system drew 9.6 amps? there is a small resistor across the terminal block that really gets hot - too hot i think? it has an amperage rating on it that says 6.2 amps -- resistors don't usually have amperage labelled on them? remember this contactor is 33 years old - made by cutler-hammmer and of course they are of no help! one of the wire wound resistors has been replaced and is a little shorter than the original - but it has been working up until the coil blew up. the guy i had rewind the coil is good and they actually counted and replaced the wire and turns as was originally in the coil. it's something simple - and the problem is accessible - and like mentioned above -"whatever blew the coil is still there!" thanks for your ideas - i will take them and run..... charlie tuna
 
Re: need help on a D.C. motor contactor?

C-H? I'm not sure that C-H was into med. voltage starters. They may have name branded them at one time. Are you sure it isn't a Westinghouse Ampgard starter? (w) manufactured thier LF air contacor from the early 60s until the mid 1990s.
They used a pick-up and hold designh. If this is true 120vac power is placed A and B. a N/C elec. interlock, M, is wired to C and D which opens after the contactor pulls in. However, I don't see a resistor in the wiring diagram,
Also, with regards to your resistor, they are normally rated in watts. If you can determine the wattage measure the voltage across the resistor and multiply that with the cuurent that you have determined.
 
Re: need help on a D.C. motor contactor?

this is definently cutler-hammer and the nameplate even gives me drawing publication numbers and i have contacted them and they refer me to "ex" employees that do referal type work but i keep running into a wall on this one? with the knowledge you have bestowed upon me i should be able to troubleshoot this problem. i'll first make an "as built" drawing of the circuit and then check the auxillary contact's operation. from the "as built" i should be able to figure out what it's supposed to be doing... thanks everyone for your help and thanks to mike holt for providing us this tool. thanks, charlie
 
Re: need help on a D.C. motor contactor?

bob-ed-temp,
thanks for the help on this contactor and as a result the chiller went on line today! going back to "basic electricity" and your insight we figured out the problems with this high voltage starter.
first step was to draw out the way it was wired and investigate it's method of operation. as temp stated "auxillary contacts had to be there to reduce the voltage to the dc coil after it closed". the way it was set up - on initial activation normally closed aux. contacts directed the voltage to only a short section of a 10 ohm resistor. upon closure the aux contacts would open and place in series with this 10 ohm resistor a 100 ohm resistor which would reduce the voltage to the coil. original current was 9 amps - then it dropped to 1.2 amps. the capacitors are to reduce the ark on the auxillary contacts with the help of a shunt resistor.

the problem occured during the weekend but we could re-inact the sequence.
during normal operation the 100 ohm resistor opened circuited. this dropped the contact out which brought the low resistance resistor into the circuit to close the coil and as soon as it closed it dropped out since the 100 ohm resistor was open - result chatter - chatter - chatter until the main coil failed from the near constant high current (9 amps) and when it failed it blew out the full bridge rectifier which feeds this circuit. and the rectifier failure caused the control circuit fuse blew! four problem areas in a circuit while troubleshooting is a challenge!
thanks again guys - thats what is good about this webb page!!!!!!!
 
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