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What is a "Voltage Trap"

What is a "Voltage Trap"

What is a "Voltage Trap"

(OP)
Hello!

I have a couple questions for you guys. I'm working with an older Westinghouse generator governor and have come across a component listed as a 'voltage trap'. In the prints it seems to be two diodes with the cathodes facing eachother and a zener symbol inbetween (maybe it's two zener diodes facing eachother?).

The 'voltage traps' in this instance are sitting in parallel with thier respective current transformers and lead to the voltage regulator.

So my questions are:
1) What is this for?
2) How can I bench test it to see if it's functioning correctly?
3) Is there a more common name for it?

Thanks for your time.

Regards,
Paul

  

RE: What is a "Voltage Trap"

The common name is "Surge Protector". The symbol you refer to is usual if there are two zeners as you describe it.

It could also be a varistor, sometimes referred to as a MOV (Metal Oxide Varistor) or a "protisitor" or a "Tranzorb". Many trade names for the overvoltage function.

The overvoltage protection is there in case you open the secondary, which could lead to rather high voltages if there is current in the primary.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: What is a "Voltage Trap"

(OP)
Skogsgurra,

Thank you for your reply.

Any ideas on how to test it?

Also can a selenium rectifier be used as a surge protector?

Thanks!
Paul

RE: What is a "Voltage Trap"

Yes, selenium rectifiers were used back in the sixties/seventies. They can absorb quite a lot of energy and were found on magnetic couplings, excitation windings and other highly inductive loads.

An easy test is to connect to an inductive load with DC fed into it, open the DC and record the voltage. If the device is OK, the peak voltage will stay "within reasonable limits" if not OK, you will have the usual kick back (kilovolts).
And, of course, if the device is shorted, your DC will be short-circuited.

You could also use a high pot tester but you will have to limit the time to a tenth of a second or so. Using an inductive load is easier.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: What is a "Voltage Trap"

"Any ideas on how to test it? "

If you can find a nameplate, it should have a rated voltage and a cutoff voltage.  If not, then you have 'assume' that the device is rated properly for the application voltage.

AT rated voltage, the device will act as an insulator, i.e., low leakage current, high megohms resistance.  If you increase the voltage and read current through the device, you will find that at some point above this, the device will start to conduct.  This should be above the 'cutoff voltage'.

In the absence of information ast to the cutoff voltage, compare the characteristics of like items, remembering that at operating voltage, you should see high resistance.  

You may also find it helpful to measure capacitance of the devices, again, comparing to like devices or previous test reports.

old field guy

RE: What is a "Voltage Trap"

Keep in mind that with some of them, MOVs in particular, the "test" will sometimes result in the demise of the product, i.e. they are a one-time device.

"The operation was a success, but the patient died."

 


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: What is a "Voltage Trap"

(OP)
Thanks for the warning jraef. I'm thinking it's a selenium rectifier (the ship it's on was launched in the mid 70s). Local tests indicate it's not shorted.

I'm working out how to bench test it to see if it's open.

Thanks again for your help all.

RE: What is a "Voltage Trap"

You surely can kill the patient if you test too violently. But if you go by the book, they will do their service any number of times from 100 to infinity.
This link is a start: http://www.taitroncomponents.com/catalog/PDF/Passive/MOV%20Selection%20Guide%20&%20How%20to%20Order.pdf

Siemens does have big blue varistor blocks with bolt connection. They can usually be used instead of the selenium stacks. Search the Siemens site for more data.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: What is a "Voltage Trap"

(OP)
So i've found a test procedure for selenium rectifiers, however i'm having a hell of a time figuring out how to test for the leakage current (see attached).

I put a the meters in the circuit as pictured, however what do I do to calculate leakage? The rectifier I have appears to have 8 cells and is non-polarized.

Thanks again for your help on this subject.

Regards,
Paul

RE: What is a "Voltage Trap"

(OP)
I figured it out. I just had to bring it up to the rated voltage based on the spec sheet of the rectifier. The number of cells was not obvious on the rectifier, which is where I was having my problem. Hope the cal procedure helps someone else in future.

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