ac or dc contactor coils

MNJ96 · Apr 21, 2006

hello everyone

i am preparing to build a control panel for an in house machine upgrade . i have 24 vdc inputs estop . pushbuttons , etc . i want to know which contactors would be best to use i have a power source for either type the 24 volt are a deeper contactor . can someone give me the pros and cons of each

elevmike · Apr 21, 2006

What size is the contactor?

In most cases for large contactors I would use two piolet relays. The contacts on the poilet relays are wired in series with the contactor coil. Additionally, mount the piolet relays upside down so if the return spring breaks, the armature might fall way by gravity.



 n/c E-Stop

--*_|_*-------------------+--(P1)

                          |   

                          +--(P2)

 

       P1           P2

-------| |----------| |-----(Contactor)

keithkyll · Apr 21, 2006

24 AC control transformer seems cheaper at first, until you add primary/secondary fusing and holders. DC Switchers have a primary fuse and Overcurrent Shutdown for the secondary. No extra fusing needed, unless required by code.

Large DC Contactors (60 Amp-3 pole and up) need a pull in coil and hold coil. DC will cost more. I think DC runs cooler. AC just needs 1 coil.

Some items are in stock in 24 DC, special order for 24 AC.
Noise suppression is just a diode with DC. Snubber (resistor-cap) with AC.

MNJ96 · Apr 21, 2006

i have a choice of controlling these from 24 volts dc and 120 volts ac not 24 volts ac . the dc models we have in stock are the allen bradley 100-c series . 100-c30d*00 . and we have ac 120 volt models the same size these will control some small motors and such.

keithkyll · Apr 21, 2006

Relay outputs with 120AC need snubbers to keep from burning the contacts.
120 AC will draw less current - a motor that takes 1 amp at 120AC will be about 4 amps in the 24DC version.
120vac input cards cost more.
120vac outputs require safety grounds.
120V indicator lamps are expensive and get hot.
24DC is the standard today. 120 AC appears to save money at first, until you get into the safety issues.
Safety is the #1 reason I have a very strong objection to 120AC.

MNJ96 · Apr 21, 2006

there will be 120 volt ac components no matter which contactor i choose . with that said . other than safety issues which would be best

elevmike · Apr 21, 2006

I'm not sure I get the question. At 1st you seemed to be asking if you should use a 24vdc or 120vac coil in the contactor. Now you seemed to be saying that the contactor coil will be 120vac.

Maybe you can re-phrase the question so you get better answers..

keithkyll · Apr 21, 2006

phil buchanan said:
there will be 120 volt ac components no matter which contactor i choose . with that said . other than safety issues which would be best

Yes, but not on the PLC. Minimize the amount of lethal voltages by using 24DC wherever possible.

The PLC outputs should be 24DC driving 24DC contactors. AC means triacs or relays. Either one needs snubbers to supress the back EMF from the coils. With DC, all you need is a diode. In many cases, the diode is built in.
If you have 100 Amp contactors, then this might be a different story. A DC version would be expensive and the PLC output wouldn't have enough current to drive it.

Safety verses cost.

Tark · Apr 21, 2006

There are slight advantages to using both, but the biggest advantage is using the one you are most comfortable with.

I personally don’t consider using a 24V system any safer than 120VAC systems, especially if you have mixed voltages. I would go as far as saying there’s a bigger safety issue using 24V in a mixed voltage system. The last thing you want people thinking in a mixed voltage system is that something isn’t going to hurt them.

Thinking 24V is safer than 120VAC is a false sense of security.

jthomes · Apr 21, 2006

Tark said:
Thinking 24V is safer than 120VAC is a false sense of security.

I have to agree WIth Tark on this one. We once had a guy wire a 240 V source into the positive side of 24 VDC power supply.

I usually found that the 120vac coils ended up saving me money in the long run. I didn't need a different supply and they seemed to last longer. Our cost on them was about the same and on the small budget I had, every penny I could save was something cooler to build next time.

MarkTTU · Apr 21, 2006

In my applications I'm moving all the PLC inputs to 24vdc, but I've left the outputs 120vac. All of my discrete output cards are relay outputs which saves on both external relays for light loads and makes wiring things much easier.

Just my 2c.

Gerry · Apr 21, 2006

typoglycaemia strikes again

...or something similar

elevmike said:
In most cases for large contactors I would use two piolet relays. The contacts on the poilet relays are wired in series with the contactor coil. Additionally, mount the piolet relays upside down so if the return spring breaks, the armature might fall way by gravity.

Like the guy in the ****pit of your avatar - PILOT

phil buchanan said:
i have a choice of controlling these from 24 volts dc and 120 volts ac not 24 volts ac . the dc models we have in stock are the allen bradley 100-c series . 100-c30d*00 . and we have ac 120 volt models the same size these will control some small motors and such.

AB sell a little pilot relay for the 100-series contactors that attaches to the coil terminals and operates on 24VDC. Can't remember the part no. but I think it had 'BZ' in it.

elevmike · Apr 21, 2006

Gerry said:
...or something similar

Like the guy in the ****pit of your avatar - PILOT.

No No... a Pilot flys an airplane, but a poilet drives a contactor....

Gerry said:
AB sell a little pilot relay for the 100-series contactors that attaches to the coil terminals and operates on 24VDC. Can't remember the part no. but I think it had 'BZ' in it.

That would be the 100-JE DC interface. We've had some trouble with the interfaces so we use the relays instead.

Jody M · Apr 25, 2006

I agree w/ marksji, over the last 24 months everything I do seems to be 24 VDC input 120 VAC relay output. To me the maintenance techs like it more. breaks up the troubleshooting. (they can identify input or output faster) uhhhhhhhhhhh they can identify an input or output problem faster, thus getting the production line back up.

JDM

Gerry M · Apr 26, 2006

In my applications I'm moving all the PLC inputs to 24vdc, but I've left the outputs 120vac. All of my discrete output cards are relay outputs which saves on both external relays for light loads and makes wiring things much easier.

Just my 2c.

I'm in this boat too. I even use the same brand. Except when I have a machine with high cycle rates(1 Million per year or so) I use AC Outputs, not relays due to the expected lifetime calculations.

ac or dc contactor coils

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