Arc quenching in a switch with an inductive load (Reducing Commutator Arcing)

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ezekielizuogu@yahoo.com

Joined Apr 13, 2012
8
i have a circuit containing an inductor,a coil of insulated copper wire of 664 turns.The source voltage is 50 vdc and the coil has a resistance of 0.7 ohms.The coil is energised and de energised by a split ring,carbon brush swithc arrangement.I find there is heavy arcing at the switch especially when the switch is opening.One expected this from an inductive circuit.However,the puzzle is that i have applied all the techniques to quench the arcing,or at least reduce it considerably but to no avail.

I have used the quencharc,the metal oxide varistor,the capacitor-resistor arrangement etc.
Nothing seems to be working.I will be extremely grateful if any member can assist me to quench this arc which is ruining the switch all the time.
 
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CDRIVE

Joined Jul 1, 2008
2,219
Neglecting the effects of inductive kickback, your coil is drawing 71 Amps after settling time. Have you considered power FETS and back EMF diodes? An alternative might be an automotive starter solenoid/relay. They're rated > 200ADC.
 

crutschow

Joined Mar 14, 2008
25,108
Have you tried a diode connected directly across the coil (cathode to positive side)?

Of course that will slow the collapse of the magnetic field and if that's a problem you can add a resistor or zener in series with the diode.
 

Kermit2

Joined Feb 5, 2010
4,162
magnets are sometimes used to draw out an arc and extinguish it much more quickly.

Placing a magnet near the switch might have some observable effect. Worth trying along with the other suggestions.
 

Thread Starter

ezekielizuogu@yahoo.com

Joined Apr 13, 2012
8
May i sincerely thank our members who responded to my request for assistance in quenching the arcing in my device.I am particularly thankful to our senior members CDRIVE,KJ6EAD,and Crutshow.However,i have tried all the suggestions they made but the arcing remains unsolved leading me to think that perhaps i am not doing somethings well.(though an electronics engineer,i am not particularly good at practical circuit constructions).I think i need some help from some person experienced in practical circuit construction to assist quench this arc and i wonder if i can be directed to some person/workshop around the New York area when i can get some assistance? (i dont mind paying for expert assistance).May i request email addresses of our above senior members? Kind regards.ezekiel
 

CDRIVE

Joined Jul 1, 2008
2,219
You're surrendering far too early in the thread. We've just begun here. Like Bertus said post schematic and pix too if you have them.

BTW, don't be embarrassed because you're an engineer. What you described about yourself is more common than you think. Early in my career I worked as an engineering tech for the company's the chief engineer. Joe Lamag--- was the most brilliant engineer I've ever known. He was a walking, talking, living calculator but when it came to hands on work he could turn a beautiful prototype into an ugly lump of yuck. The damage he could inflict with a soldering iron was legendary within the company. :D Might as well have been an arc welder that got in there! He was equally skilled with the use of the specialized needle point test probes that we used. :eek: Nothing would depress the wiring/soldering (Thick Film IC) dept more than seeing Joe walk through the door saying.. "Girls, I made some mods in here. See if you can clean it up. We've got military brass visiting this week and I want this to look really good." ;)
 

crutschow

Joined Mar 14, 2008
25,108
...................
However,i have tried all the suggestions they made but the arcing remains unsolved leading me to think that perhaps i am not doing somethings well.
So you have connected a large diode (at least 25A) directly across the inductor and you still get the arc? That does seem strange. With a diode across the inductor you shouldn't get more than about a 1V transient to generate the arc. :confused:

How long is the lead between the inductor and the contact? Perhaps you need to connect the diode at the contact.
 

Thread Starter

ezekielizuogu@yahoo.com

Joined Apr 13, 2012
8
Thank you so much,CDRIVE,for your words of encouragement.I have always been very intimidated by electronics people who have a great hands on the job when it comes to dexterity to put components together and get the device working.THe fascinate me.
I will send in a sketch soon.
Crutshow,i connected a big diode across the contacts,not across the load.The diode i have is rated a whopping 500A,and carries heat sink fins which wont let it connect directly across the inductor.But the idea to connect across the inductor is beautiful and i will look for anther big diode that can go in there.I hope it works.
The length of cabling between the inductor and my contact points is like 100mm.
You have all been marvellous and am so grateful for your ideas.I look forward to receive more ideas on this until the arc is quenched.
ezekiel
 

CDRIVE

Joined Jul 1, 2008
2,219
If I'm not mistaken, the diode will carry the full 71A of current, even for a brief time. should it not be a higher capacity that 25A?
No, you're not mistaken and I'm not at all surprised about the arc, even with the diode. 71A is a lot of juice to interrupt. and I'm not even referring to the back EMF.
 

Thread Starter

ezekielizuogu@yahoo.com

Joined Apr 13, 2012
8
May i once again thank members who have given valuable ideas on how i can quench the arcing in a switch in an inductive circuit.I have taken a few more days to try out the suggestions made to me.I have used a big diode rated 800v,500A connected across the inductor but the arc remains unchanged.I then added varying values of resistance in series with the diode.These were 100 ohms,22,4.7,3.9,0.47,0.33 and none made a difference in the arcing.
I know that some members suggested i attach a sketch of the circuit am using to enable them make more valid suggestions on how to quench the arc.
I have attached a rough sketch of the circuit.The rotor is being rotated by a small electric motor(not shown)and it rotates at approximately 540 rpm to work the switch.
May i send special greetings to CDRIVE and CRUTSCHOW.
ezekiel
 

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