Contactors

[RMS]

Hi all,

this may seem like a silly question to some but i was at my local gym today and an electrician was working on the panel for the steam room. I got chatting to him and he said that the a contactor had flash marks on it and this caused the contactor to generate a higher current and a new one was required.

My question is if it is possible for the contactor to generate a higher current?

Thanks for any replies.

 

[ricicle]

As contactors wear their contacts develop high resistance this in turn makes heat due to the I²R until they probably go infinite resistance (open circuit - even when the coil is energised)
So no they will not cause an increase in current, the opposite in fact!

 

[stevesey]

Higher arc current perhaps?

 

[breezer]

same current, smaller point to arc on

 

[Adam_151]

Another effect thats probably relevant here which came up in college today (though its more relevant to the older more open contactors), is if dust builds up on the faces of the magnetic parts, etc and stops it pulling in quite as far as it should, this will make a difference to the reactance of the coil, and the current drawn will be higher and can burn out the coil.

So its not just the contacts that can cause a worn/dusty contactor to overheat

 

[ricicle]

Another effect thats probably relevant here which came up in college today (though its more relevant to the older more open contactors), is if dust builds up on the faces of the magnetic parts, etc and stops it pulling in quite as far as it should, this will make a difference to the reactance of the coil, and the current drawn will be higher and can burn out the coil.

So its not just the contacts that can cause a worn/dusty contactor to overheat

Yes, if the iron isn't fully 'home' inside the coil then there is not enough magnetic flux to create the current to oppose the energising current (Lenz Law)

 

[IJWS15]

He probably did mean a higher cut off (or arc) current.

Got used to these beasts working for British Rail years ago. The older locomotives used 750-1000 V DC for the traction motors and they were turned on/off/reversed by big air operated contactors complete with arc chutes. sometimes the contacts deteriorated so much they would draw an arc on opening that was bigger than the arc chute could cope with - i.e. they did not break the current flow!

On the Class 37s one of the contactors was behind the drivers seat at one end and many driver would not drive the locomotive from that end.

All solid state in the new ones!

 

[ricicle]

Yeah - there is no zero crossover on DC is there.

 

[plugwash]

On the Class 37s one of the contactors was behind the drivers seat at one end and many driver would not drive the locomotive from that end.

how did they get out of doing it? surely they have to drive from the end that lets them see where they are going and there aren't exactly many places to turn trains round on the railways now steam is gone.....

 

[bernardgreen]

Many years ago a young lecky replaced the contact pads on contactors controlling DC motors on a copying lathe. As he had to fit pads much larger then the one fitted ( obsolete contactors ) he removed the "cooling coils" which he thought only served to disapate heat.

When the machine was next used the tool carrier wouldn't stop when commanded to and went into the rotating chuck. The contactors were arcing brightly with the motors running on the arcs.

The lecky learnt the hard way that the "cooling" coils created a magnetic field to stretch and snap the arcs.

 

[IJWS15]

On the Class 37s one of the contactors was behind the drivers seat at one end and many driver would not drive the locomotive from that end.

how did they get out of doing it? surely they have to drive from the end that lets them see where they are going and there aren't exactly many places to turn trains round on the railways now steam is gone.....

They would either

Put the loco on the turntable
Report it as low on power
Report it as drafty
Report it as cooker not working
Report it as heater not working
Report it as cab too hot
.....


You get the picture?

 

[RMS]

thanks for all the replies.

So the conclusion is that contactors may pull more current due to a change in XL of the coil.

That explains why he said that the MCB was tripping.

Thanks all.

 

[ricicle]

Could be - but in that case it would be whichever is the control circuit breaker