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Dc Thyrister Drive


AB2005

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Dear All,

 

We have 17 DC Thyrister drives in one of our machines. Each drive has a 3 phase reactor connected in series with input power. I want to know that what is the purpose of this reactor?

Thanks in advance.

 

"Don't assume any thing, always check/ask and clear yourself".

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Hello AB2005

 

The AC reactors are often an optional extra.

 

If you do not use the reactor, the current drawn from the supply is very peaky and has very high harmonics. The line reactor reduces the level of harmonic current in the supply as well as reducing the effects of line transients on the controlled bridge SCRs.

 

I always recommend the use of the line reactors with DC drives.

 

Best regards,

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The AC reactors are often an optional extra.

 

If you do not use the reactor, the current drawn from the supply is very peaky and has very high harmonics. The line reactor reduces the level of harmonic current in the supply as well as reducing the effects of line transients on the controlled bridge SCRs.

 

I always recommend the use of the line reactors with DC drives.

 

Dear Sir,

 

Thanks a lot for your kind reply.

"Don't assume any thing, always check/ask and clear yourself".

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Long before there was a concern about power quality and harmonics on the mains, reactors were used at the input of the DC Drives ( DC Converters) that employed SCRs. In fact, before reactors were used, DC drives were usually supplied with a 1:1 Isolation Transformer.

 

Reason: to provide some impedance between the converter and the mains. You see, when the AC line commutates the SCRs, there is a momentary short-circut of the AC lines which produced nothes in the AC supply.

 

The Isolation transformer added impedance between the SCRs and the mains such that the rest of the plant AC distribution network was immune from the notching.

 

Well, Isolation transformers were expensive and added considerable cost to a project. Imagine if Allen B. were to have an isolation transformer ahead of each DC drive— 17 transformers !

 

Back at that point in time ..... circa late 1960s - early 1970s ...... AC line reactors were not in common supply; that is, they were not a stock item as they are today.

 

But looking to reduce the cost of DC drive projects, the customers pressured the DC drive manufactures for a more cost effective solution. Ergo, the advent of the AC line reactor of about 1.5 - 3% impedance.

 

The folks at Rockwell Automation have written an excellent paper on the subject. (Click on the underlined link below.)

 

 

NB! The article points out an important consideration; that is, while some impedance is a good thing, too much impedance is NOT ! Read the article to find out why.

 

For another consideration, one having to do with the CEMF of the motor rising above the peak of the AC line voltage, resulting in SCR commutation failure with consequent fuse blowing, loud noise, and smoke, see (click on) the below link from Magnetek.

 

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Dear jOmega,

 

I have already appreciated you and Mr. Marke for a very kind consideration for replying against the questions asked by juniors like me.

A bundle of thanks for a very lengthy and correct answer. Now I have completely understood about reactors.

 

"Don't assume any thing, always check/ask and clear yourself".

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