Switching 170VDC using 120VAC control coil 3

[SBMerrick]

I have an application where I need to switch a 1200uF capacitor charged by a half-wave rectified 120VAC supply, or 170VDC across the contacts. The relay coil will need to be supplied from the 120VAC source. I have spent all day to no avail looking for a suitable relay with a 120V coil and contacts rated at or above 170 volts DC. Even relays with contacts rated for 240VAC are typically derated down to 28 or 30 volts in DC applications. Can any of you gurus please help guide me in the right direction?

Thanks!

 

[davidbeach]

Probably need to go to solid state, or look at something like what an Electro Switch LOR would provide. It certainly isn't an ice cube relay.

 

[dpc]

Google up "dc contactors" instead of relays.

http://www.hubbell-icd.com/icd/contactors/T720.asp

Hope you have a big space to put it in.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[jraef]

No need to go so far as the Hubbell contactors, there are plenty of DC rated contactors out there if the voltage is below 240VDC. It's 600VDC that's a bugaboo.

But more to your point, there are specific "Capacitor Contactors" available from most of the IEC device mfrs, i.e. Siemens, ABB, Telemecanique etc. They are the standard AC contactors derated but are also fitted with surge suppression across the contacts. That's more important than the DC rating.

The one shown here is a 9A contactor, fairly inexpensive, about 45mm square footprint.






"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[itsmoked]

You can also greatly increase your typical relay's abilities to that end, by putting all the contacts of a multipole relay in series.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Can you elaborate on the operation. Is this a capacitor trip unit? What current would these contacts have to break? Depending on the application, I would consider installing a small 600VAC rated motor contactor with the poles wired in series. If there is little for the breaking requirement then I'd even consider a 3 or 4 pole control relay with the contacts in series. The difficult part is breaking DC.

Joslyn Clark has some small DC contactors

http://joslynclark.com/pdfs/joslyn_clark/JC_DC_REHO_Sec_J.pdf

 

[SBMerrick]

Thanks all for your excellent responses. I have some good leads now, thanks to your input. It looks like there are several DC contactors that will meet the need.

Lionel - yes, the device I need to switch is a CTD-2 capacitor trip unit made by Electromagnetic Industries, LLP. It is installed on some Cutler Hammer Medium Voltage switchgear. The switchgear control power is supplied by a UPS system which supplies the CTD with 120VAC. The CTD provides brief backup power to a SEL-351A multifunction protective relay to enable it to ride through brief bumps in the UPS power.

I need to install a relay with a normally closed contact in the 170VDC CTD output circuit such that if UPS power is lost for more than a second or two, the new relay will drop out and close its normally-closed contact to discharge the CTD through the main breaker breaker trip coil. This would have the effect of providing a safe shutdown of the switchgear on loss of control power. I think I now have enough information to go by, thanks to all of your help.

Thanks again,
Steve

 

[LionelHutz]

That is a fairly easy load to switch. Once you apply the DC to the trip in the breaker the breaker will open and a contact in the breaker will disconnect the trip unit. So, the relay you use will not be required to break the DC current. You could use a decent quality control relay. Just put a couple of contacts in series.

Jeff - that contactor is for switching a 3-phase AC capacitor (power factor capacitor) on-line, not for switching a DC load.

 

[davidbeach]

As Lionel indicates, closing into a dc circuit is easy and the breaker clearing contacts will be what interrupts the dc trip coil current.

 

[jraef]

Is half wave rectified AC really DC? It will still go to zero 60 times per second.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

I think you're confused where the relay contact is. The relay is switching the DC after the capacitor and not the rectified AC before the capacitor. The circuit goes;

AC -> rectifier -> capacitor -> relay -> breaker trip coil

 

[jraef]

Ahh, missed that it was a capacitor trip unit. Thanks.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

Nothing critical at all with that switching. The capacitor is charged to 170 V DC. A voltage that most relays can handle easily. Even Ice cube relays.

There is no switching off DC. Only switching on. And the switching on is not short circuiting of the capacitor - only connecting it to an inductive coil that doesn't take much current and with a very moderate di/dt.

No need to series connect or parallel contacts. But do, by all means, if you feel better doing that.

In short: Just because the voltage is DC, it is not a DC specification. Only a mild switch-on, discharge spec.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[SBMerrick]

Excellent! You've given me what I need.

Thanks, all!
Steve