Relays - AC vs DC specs
Relays - AC vs DC specs
(OP)
It seems I learned this a long, long time ago but I'm drawing a blank. A quick Google search didn't help so...
Why are a relay's DC specs so much lower than the AC specs? I'm looking at a relay that can switch 7A at 250VAC (1750W) on a resistive load, but can only handle 3A at 30VDC (90W). Why the difference?
I am selecting a relay to disconnect servo motor phases in the event the E-Stop is activated. The driver is rated for 6.4A peak, 750 VA peak, and a 300mH load. Thus I don't believe the above rated relay will work, because my servo motor could well be stationary and looking like a DC load.
John D
Why are a relay's DC specs so much lower than the AC specs? I'm looking at a relay that can switch 7A at 250VAC (1750W) on a resistive load, but can only handle 3A at 30VDC (90W). Why the difference?
I am selecting a relay to disconnect servo motor phases in the event the E-Stop is activated. The driver is rated for 6.4A peak, 750 VA peak, and a 300mH load. Thus I don't believe the above rated relay will work, because my servo motor could well be stationary and looking like a DC load.
John D





RE: Relays - AC vs DC specs
"When a relay breaks an inductive load, a fairly high counterelectromotive force (counter emf) is generated in the relay's contact circuit. The higher the counter emf, the greater the damage to the contacts. This may result in a significant decrease in the switching capacity of DC-switching relays. This is because, unlike the AC-switching relay, the DC-switching relay does not have a zero-cross point."
So the AC relay has the smarts to switch at the low current point, whereas a DC relay doesn't have that option.
John D
RE: Relays - AC vs DC specs
You can increase the dc breaking capability of relay contacts by putting multiple contacts (from the same relay) in series. This increases the effective arc length.
David Castor
www.cvoes.com
RE: Relays - AC vs DC specs
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If we learn from our mistakes I'm getting a great education!
RE: Relays - AC vs DC specs
I guess you can also use the capacitor or MOV across the coil, rather than the contacts.
There are devices that use this concept, and some electronics to "extent" the contact interrupting capability.
Also, if you don't expect 10,000 operations, you can rerate the contacts for fewer operations (Don't ask, I haven't done this myself).
RE: Relays - AC vs DC specs
E-Stop won't be used that often, so de-rating is possible. Not somewhere I really want to tread with a safety circuit though. It wouldn't be good to have my safety circuit weld itself inactive.
John D
RE: Relays - AC vs DC specs
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If we learn from our mistakes I'm getting a great education!
RE: Relays - AC vs DC specs
The driver manufacturer is recommending disconnection of the phases and disabling of the driver; this seems redundant, so maybe there are other ways to add redundancy without having to disconnect the phases. That gets back to meeting their other safety recommendation - "Never disconnect or connect the product while the power source is energized.". Nice.
John D
RE: Relays - AC vs DC specs
Thanks,
John D
RE: Relays - AC vs DC specs
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If we learn from our mistakes I'm getting a great education!
RE: Relays - AC vs DC specs
I read the requirements again and realized that it calls for removing power from the output connector to the motor; that's a bit difficult unless I'm inside their box. I called their FAE and his response was "don't disconnect the phases, you'll blow the driver on your first E-stop!". He doesn't know why the recommendations got written the way they did, it seems to be some incoherent combination of the European and North American standards.
So I'm back to a simpler, common sense approach. I'll disable the driver so the motors come to a controlled stop. If I have to go one step further I'll shut off power to the driver after a delay.
Thank you all for your responses.
RE: Relays - AC vs DC specs
Here is FANUC's E-stop setup. In the description, the term "Magnetic" is used on Page 3/5 around line 50. My recollection is that "Magnetic" is FANUC's term for Contactor. It's been awhile seen I looked at a FANUC servo chassis, but again my recollection, is that it does have a 3-pole contactor that allows the drive to be powered up.
http://tinyurl.com/y8mb785
And a safety summary from FANUC. Note item 6
http://tinyurl.com/y8nwulb
Ed
RE: Relays - AC vs DC specs
RE: Relays - AC vs DC specs
http://w
RE: Relays - AC vs DC specs
RE: Relays - AC vs DC specs
RE: Relays - AC vs DC specs