Switching 170VDC using 120VAC control coil
Switching 170VDC using 120VAC control coil
(OP)
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!
Thanks!





RE: Switching 170VDC using 120VAC control coil
RE: Switching 170VDC using 120VAC control coil
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
RE: Switching 170VDC using 120VAC control coil
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.
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RE: Switching 170VDC using 120VAC control coil
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switching 170VDC using 120VAC control coil
Joslyn Clark has some small DC contactors
h
RE: Switching 170VDC using 120VAC control coil
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
RE: Switching 170VDC using 120VAC control coil
Jeff - that contactor is for switching a 3-phase AC capacitor (power factor capacitor) on-line, not for switching a DC load.
RE: Switching 170VDC using 120VAC control coil
RE: Switching 170VDC using 120VAC control coil
"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: Eng-Tips.com Forum Policies
RE: Switching 170VDC using 120VAC control coil
AC -> rectifier -> capacitor -> relay -> breaker trip coil
RE: Switching 170VDC using 120VAC control coil
"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: Eng-Tips.com Forum Policies
RE: Switching 170VDC using 120VAC control coil
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Switching 170VDC using 120VAC control coil
Thanks, all!
Steve