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Old 11-20-2003, 09:14 PM
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Default SCR\'s as rectifiers? - building a DC welder

Anyone know if I can use SCR's to build a full wave bridge rectifier for a DC welder? I can get GE 8493622 SCRs for cheap ($10.00 cdn each). They are rated at 110 Amps and 600 volts. Do just need to bias the gate thru a resistor to the one side of the SCR? How do I figure out how much current is needed to bias it? Will the bridge be able to handle more than 110 Amps as the SCR will only see 50% duty cycle as a rectifier? Sorry if this is way ot, but I'm trying to build a cheap rectifier to make a DC supply from my AC welder for my attempt at tig welding.

Thanks.
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Old 11-21-2003, 06:26 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

You just won't give up, will you? EXCELLENT! You're my kind of guy! [img]images/graemlins/RockOn.gif[/img]

I passed your question to an EE buddy who has done a lot of work designing forklift battery chargers. He will give a good answer. Problem is that he's taking a week of vacation for Thanksgiving, and I don't know if he's gone already or leaving tonight.
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Old 11-21-2003, 07:45 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

Thanks Jim. Your reply made me chuckle. [img]images/graemlins/smile.gif[/img]
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Old 11-22-2003, 10:05 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

SCRs are used on solid state variable current DC drives. To change the speed/torque of a DC motor one must change the current passing through the armature. If memory serves SCRs are two thyristors attached together anode to cathode. A thyristor is essentially a diode that requires a gate. A thyristor will only pass current when it is forward biased and a gate pulse is supplied. The amount of current is determined by the area under the curve after the thyristor gate pulse is received and before the thyristor shuts off due to reverse bias. By changing the angle of this gate pulse one changes the current passed through the thyristor.

SCRs are designed for variable current applications, they would make an excellent DC welder. One would have to create some kind of gating circuit. Most are designed to fire through timing when the thyristor is forward biased and the degree of firing will supply the needed current. One could make a circuit that has the thyristors firing all the time and change the current supplied to them. If one used an AC welder for ones power source the incoming current would be the adjustment. I have never seen it done this way but I suppose it would work.

As far as gating voltage/current requirements I don't think they reqire much, maybe a 10 to 30 VDC pulse and very low current. The gate needs to be supplied when the thyristor is well forward biased say 10 to 15 degrees. One has to gate each side (thyristor) of the SCR seperatly and at the right time.

SCRs rated for use in full wave applications so if they are rated at 110 amps that is all one will get out of them without damage.

Hope this helps long post I'm sure I am forgetting tons of stuff.
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Old 11-22-2003, 11:14 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

I see this great circuit where a guy controlled the ac input to his welder using SCR's and an ac light dimmer. Now I know this is different than using them as rectifiers, but the concept looks very interesting and if it could be applied to the rectifier stage, it might make for a way to taper off the power at the end of a weld. For anyone interested:
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File Type: gif 1220707-welder.gif (37.3 KB, 35 views)
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Old 11-22-2003, 11:51 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

lots of good stuff KEG. my memory sucks, I had to grab one of my text books, but an SCR and thristor are the same thing. just different names.

the back to back configuration of SCRs like you mentioned are triacs or a AC staic switch.

It would be nice to have a spec sheet of the part in question.

my answer to the question at large, the SCR could work but I would want more info on the part. connect the gate to the anode and it should act alot like a diode.
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Old 11-22-2003, 12:03 PM
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Default Re: SCR\'s as rectifiers? - building a DC welder

</font><blockquote><font class="small">In reply to:</font><hr />
To change the speed/torque of a DC motor one must change the current passing through the armature.

[/ QUOTE ]
Actually, you vary the FIELD current. The lower the field current the higher the speed. If you disconnected the fields on a running motor, it could go into runaway (speed tries to go to infinity).
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Old 11-23-2003, 09:44 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

Timinb,

The gating circuit looks ok. I do not know much about light dimmer triacs but I assume they adjust the firing angle the full range. Full range of firing angle would be very handy. If you fire at 0 deg as the ac wave is going up you will get full current available. If you fire at 90 deg when the wave is at the peak you will get half the available current. If you fire at 180 deg when the wave is crossing 0 the SCR will reverse bias and you will get 0% of the avail able current. I think you could use that circuit as a good starting point and monkey dink with it and get it to work.

Speed buggy,

That sounds right about Thyristors and SCRs being the same component. I don't remember if triacs make SCRs or SCRs make triacs, but if you looked it up I'm sure you are correct. After a few beers I will burn this out of my brain yet agian and go back to calling them all SCRs.

CJ7Taz,

I do not want to sound mean, but you are incorrect. I know a smidgen about DC drives. The speed regulation of a DC drive is through adjustment of the armature current. The armature firing angle is adjusted by an error signal. Which is generated by the action of the motor speed feedback on the speed refrence to the drive. Although most digital DC drives have SCRs for the field current. These SCRs are used to control the field current to a steady amount, usually the value that is listed on the motor nameplate. The field firing angle will adjust to keep the field current constant at that setpoint. Most changes in field firing angle are due to changing resistance of the field windings because of heat or action of the armature current on the field windings. The only time the field is lowered to adjust speed is if a drive is being run in field weaking mode. The field is lowered a prescribed % resulting in lower CEMF. This makes the armature current higher for any given armature firing angle. Higher armature current results in higher RPM/torque. Field weaking is mainly used when a motor is being run above base motor nameplate speed or extremly hard starting loads. Either way its hard on motors. I have never commisioned a motor/drive combo to run in this manner but it is done. It is true a DC motor can run away if the field is disconnected or dropped, but most modern drives have field detection circuits that fault the drive out in this event.


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Old 11-23-2003, 10:59 AM
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Default Re: SCR\'s as rectifiers? - building a DC welder

[quote}I know a smidgen about DC drives.

[/ QUOTE ]
You better learn a smidgen more. Check the DC motor speed formula. Check into Back EMF too.
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Old 11-23-2003, 02:17 PM
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Default Re: SCR\'s as rectifiers? - building a DC welder

We have hijacked this topic but Iím willing to run with it. When I said smidgen I thought the sarcasm would be apparent, I was wrong. I know these facts because this is what I do. I spec, install, program, commission, repair and maintain DC drives every day at work. I have seen in other posts where you have been in the electrical trade for some years. I gave you a detailed reason why you were wrong because of this fact.

The fact still remains one does not regulate the speed of a DC motor/drive combo by changing the field current. This is a common misconception and I have heard it before. It is true that field strength is directly related to torque produced as by the formula that you quote. This is why modern digital drives have current regulated fields instead of voltage regulated fields. So the field strength is non varying and predictable speed control can be achieved.

If speed regulation is maintained by changing the field current how do you explain the ability to power a permanent magnet motor with a DC drive? The magnets are providing the field and they are not variable. When one does this the field must be disabled and the field loss protection inhibited.

Another good example is a drill motor. Open one up, the trigger varies the rectified current going to the brushes. Which provides current to the ........... armature.

If you still do not believe me check these guys out. My earlier comments where made from memory and they pretty much repeat what I explained.

http://www.joliettech.com/dc_drives_...operatrion.htm

They have some good graphics showing firing angle with respect to armature current
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