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| AC single Phase Inductace motor welds relay contacts | |||
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| Posted by: bill12780 ® 05/10/2007, 01:40:23 Author Profile eMail author Edit |
Hello all,
I need a lesson on this subject because I am obviously missing something. We have a small .4 hp 110v single phase AC 3-wire induction motor that is driven by two electromagnetic relays that upon breaking the contacts archs and welds the NC contacts together. The circuit is has follows: trac to control the AC on and off to the common contact on the relays. Then a Micro-controller drives a standard NPN transistor that in turn drives the 28V coil of the relay(this is all standard with snubber diode arcoss the coil etc...) The Triac and relays are fired at the same time. Why you ask? Cause we all know that if I turn the triac off first and then turn off the relays the spark would not happen. We do this because the motor also has a spring loaded brake that is also controlled by the triac. If we don't turn them off at the same time we will get some coasting in the motor. Not much but its unacceptable. When the the relay is engaged there is no spark. But when the the relay is dis-engaged there is a pretty large spark and the contacts are stuck. I should also mention here that the contacts that are being stuck are the NC contact and we are not using them (there just floating...no connection) Now I know that I need to install either a MOV or a Snubber circuit arcross the windings of the motor so the engery stored in the winding can disapate. Now I thought at first I needed a 130V MOV (adding a little from line voltage to account for differences) but that reduced the spark but did not eliminate it. So I tried my standard 100 Ohm/.047uf RC network. Also, reduced but not eliminated. I tried both ideas together. Still no go. I should also mention that I am pretty much stuck to this design. We are trying to "mod" an exsisting board to work with this new motor so we don't have to turn a "new" board. I have tried every possible "guess" I have. So what I need is some one to explain exactly how to "calculate" what I need rather then going on my gut and experience. I have "googled a oodle" and there is all but NO information on this. Lots on DC motors stuff but nothing I could find helped. Any takers to teach me a lesson? hahahaha! This will teach me to try and do something I don't know anything about! thanks to all in advance! Bill12780
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| Re: AC single Phase Inductace motor welds relay contacts | |||
| Re: AC single Phase Inductace motor welds relay contacts -- bill12780 | Post Reply | Top of thread | Forum |
| Posted by: Bruiser ® 05/10/2007, 11:33:27 Author Profile eMail author Edit |
Have a look at Randykimball's post on relays - should be right next to yours, and is a good a-z on the subject. Apart from the following queries, it sounds like your relays are too small. Arcing across contacts that are not even connected suggests collateral damage from an overloaded main contact. If so, you can stick with the board you have by using these to control larger relays. Queries:
Does the motor have run or start capacitors? Check your centrifugal contact if so. I don't understand why you'd have a triac AND a relay. You could use your control signal to turn the triac on and avoid this, although it doesn't help you staying with an existing board. (As a point of interest, hindsight has shown me that trying to stay with an existing board costs more than it saves - errors are costly, but at least you gain from fixing them). Using a triac to control a brake could be a problem if the triac is doing speed control (you haven't said this). eg if you're running the motor at reduced voltage, the brake may not be fully disengaging, = overcurrent. Why two relays?? Double pole switching?? Schematic? I'm intrigued. |
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| Re: Re: AC single Phase Inductace motor welds relay contacts | |||
| Re: Re: AC single Phase Inductace motor welds relay contacts -- Bruiser | Post Reply | Top of thread | Forum |
| Posted by: bill12780 ® 05/10/2007, 15:21:55 Author Profile eMail author Edit |
Thank you all for your responses! I have figured out what was going on. Later tonight when I get home I will draw a quick schematic and explain what was happening. Thanks again!
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| Re: Re: Re: AC single Phase Inductace motor welds relay contacts | |||
| Re: Re: Re: AC single Phase Inductace motor welds relay contacts -- bill12780 | Post Reply | Top of thread | Forum |
| Posted by: bill12780 ® 05/14/2007, 12:05:18 Author Profile eMail author Edit |
Hello all, I have been trying to find sometime to draw up a print and a full explination of what was happening. But I have not had time to do so (work is whipping me pretty hard as of late) Anyway to make a long, long, long story short. The Motor has two windings right...Well it was the winding NOT being used that was collecting the back emf. After watching closely I realized that the arc was comming from the un-used NC contact to the common. NOT common to NC. SO......All I had to do was cut a trace to seperate the two relays NC pins. Thanks to everyone who responded I do appreciate all your input. One day maybe I will publish all the details for everyone. Thanks again! Bill12780 |
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