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from the Electrical department...
24 VDC
Continuous process industries, DCS questions. topic
Posted by Chip on 30 July, 2008 - 1:35 am
Hello Everyone,

My nick is chip and I'm glad to find an informative place to get electrical control answers that's fun! Anyway here goes.

I found a 24VAC 3 pole relay in a piece of our equipment at work and the coil was fried. I suspect someone replaced what was originally a 24VDC relay with the AC relay. And I think I know who did it, not that it matters.

Until he said you can use either a 24V AC or DC relay. Well I'm no engineer... but this is not right and I would like someone to let me know if this is really possible and is this a common practice. What kind of damage can this do?

This equipment is a PLC controlled autoclave and I checked out another unit that was total 24VDC.

Come on guys! Let me know what you think.

Chip


Posted by Roy Matson on 30 July, 2008 - 11:34 pm
No, you shouldn't use an AC relay on DC it will draw too much current and burn out as you can see.


Posted by Russ on 30 July, 2008 - 11:54 pm
I would guess that the AC coil looked to be a low resistance at DC (a few ohms at most vs. the normal 1000+ impedance at 60 Hz.) and the 24 VDC power supply was more than happy to provide enough current to burn up the coil.

It might be possible for a 24 VDC coil to be used on AC by adding a diode of adequate current capability (the coil might not like the pulsating DC being generated but it may work for a while) but going the other way just doesn't work in my experience.

There is a reason to specify a particular coil in an application, you saw that first hand.

Russ


Posted by Zerokewl on 31 July, 2008 - 12:03 am
24VDC and 120VAC relays are 2 different items. A 24VDC relay needs a DC signal to turn ON, where as a 120VAC relay needs an AC signal.

Now, saying that, it does not matter what voltage you are switching on the contacts (Normally Open/Closed), you could have DC or AC Voltage on the contacts, but ONLY the required voltage to PULL it IN.

So whoever said to you that you could use BOTH is full of S***!


Posted by Michael Griffin on 31 July, 2008 - 12:08 am
You are correct, and your friend is wrong. If you use a 24VAC relay in a 24VDC application, the relay coil will usually burn up (although it may take some time). I have seen this happen.

A 24VAC coil has a lower resistance than a 24VDC coil. This is because in an AC circuit, the total impedance is the sum of the resistance and the inductive reactance, while in a DC circuit, you only have resistance. This means that in a DC circuit the coil must have a higher resistance to keep the current to the same level. Since the AC coil has a lower resistance, the resulting current is too high and the coil over heats.

You cannot use a DC coil in an AC circuit because the current will then be too low (the relay might not pull in), and the DC coil does not have a shading coil (which keeps the relay from chattering during AC zero crossings).

The strength of the solenoid in a relay is proportional to the amount of current and number of turns in the coil. If the current is too high, the coil will overheat and fail. If the current is too low, the coil will be too weak. The relay designer has to balance these two considerations when they design the relay. It is hypothetically possible to design a relay which would work in either type of circuit, but I can't recall ever having seen one.


Posted by Curt Wuollet on 31 July, 2008 - 12:11 am
They are wound differently. The AC coil has fewer turns because the current is limited by the inductive reactance. The DC coil has to have enough resistance to limit the current. That's why the AC coil fried. You can use an AC coil on DC in an emergency if you lower the voltage to limit the current or use a resistor for this purpose. A DC coil on AC may or may not work but will often buzz and may break contact because it does not have the shading ring to maintain the magnetism through the zero crossing. They are not interchangeable, but sometimes you might luck out and it appears to work.

Regards

cww

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