Relays with 24VDC coils being turned on by 44VAC

I came into possession of an older robot (mid 90s) and was troubleshooting the existing safety circuits. I set my multimeter to DC Voltage and went poking around, but measuring across the activated relays showed 0VDC. Switched to AC Voltage and it shows 44VAC.

The contactors and relays are all 24VDC coils, but the entire machine is running at about 44VAC, and it works...

The relays that I have had out of the machine and replaced all say 24VDC for the coil voltage. Is there some sort of reason why 44VAC is working, and not damaging the coils, that is explained with math?

Is there some sort of reason why 44VAC is working, and not damaging the coils, that is explained with math?

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to answer your specific question - this might have something to do with it ...

http://www.plctalk.net/qanda/showthread.php?p=177203&postcount=11

Put simply, it takes a certain amount of power to pull in and hold the coil. All other things being equal, a given AC circuit has less average power than a DC circuit at the same voltage. With a higher AC voltage you can develop the same average power. There are other details regarding the construction of the coil that make a coil more suitable for AC. Without knowing more specifics about the relay its hard to give specifics, but chances are its working because the 44VAC circuit is delivering enough average power to hold in the relay.

just curious - but WHERE is the 44VAC being supplied from? ... a transformer? ...

Tharon said:

I came into possession of an older robot (mid 90s) and was troubleshooting the existing safety circuits.

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Obviously this is aside from the question you have, but you should be aware that the safety circuits in pre-1999 robots were not considered "control reliable". Furthermore, if you try to adhere to the ANSI/RIA-R15.06 Safety Standards like my company does, you are not allowed to install such a robot in a new application. You will also probably find that the robot manufacturer will say that making the robot control reliable is not possible.

For this reason, our company no longer buys pre-1999 robots.

I don't really like being a stick-in-the-mud (much), but I think this is one of those things that hasn't been getting around much but really should.

Steve

Yes, the 44VAC is coming from a multi-tap control transformer. I was poking around more today and saw some of the relays/contactors did have coils rated 48V/60Hz. I cannot find the three I replaced last week, but could have sworn they were 24VDC. So I'm pretty sure I saw 24VDC relays in there, but maybe my initial assumptions about the control setup were clouding my memory. One thing I don't like about these AEG and Siemens relays is there is very little information on the relay about what it is. The only part number is a generic series number, which can cover any combination of NC/NO contacts and coil voltages. It makes getting spare/replacement parts a hassle, not as simple as just writing down a part number.

And I do know these are not exactly up to spec as far as the ANSI/ISO safety standards go. But they are part of a pre-existing installation, not being integrated into a new one. Basically a competitor got put out of business and we inherited the production line.

One thing I noticed is the teach pendants for these old robots do not have "deadman" switches for the servo power. Once turned on in teach mode the servo power is on all the time.

Good thing is the safety fence is a pilz safety relay, and the robot uses redundant force guided relays for all it's E-Stop and Safety circuits. So it's not exactly horrible as far as safety goes.

Believe me, I wouldn't be using them if I had the option (and not because of the ANSI/ISO rules). But I don't make those types of decisions. Someone else makes them for me.