contactor

[kwame]

Hi folks
A general purpose plug in miniature relay with 12 VDC ,10mA coil can switch 120/240VAC load. Can a 24O VAC 20A /120 20A contactor be used for switching an inductive load of 12V DC 6A?
I am not very familiar with contactors ;tomorrow i want try one out.Has anyone ever tried this out before?

 

[crutschow]

It would probably work if you use a diode across the inductor to suppress the inductive transient, but it's best if you can get the data sheet for the contactor to see what it says about a DC load.

 

[carbonzit]

The difference 'twixt a relay and a contactor is what they say is "a difference without a distinction". They're the same thing, really: contactors are just bigger relays. (Check this page, for instance. Hey, found an interesting site while searching: "differencebetween.com".)

I really don't see why DC and AC ratings would be all the much different. While we know there can be inductively-generated transients when switching DC, they can also occur with AC.

The ratings for the previous relay we were discussing with Kwame (an Omron) made no distinction at all between AC and DC.

In any case, if the "contactor" is rated at 20 amps, I'd say it's safe to switch a 12V, 6A load with it.

 

[Reloadron]

Would this be that motor load? The 12 volt 6 amp motor (actuator)? A "contactor" is a relay on steroids. What do you plan on using a contactor for?

Ron

 

[crutschow]

...........
I really don't see why DC and AC ratings would be all the much different. While we know there can be inductively-generated transients when switching DC, they can also occur with AC.

The ratings for the previous relay we were discussing with Kwame (an Omron) made no distinction at all between AC and DC.

The reason for a DC rating being less for a mechanical relay is that the AC waveform periodically goes through zero which extinguishes the arc as the contacts open. This doesn't happen for DC thus a given relay has a lower current rating for DC then for AC.

This difference does not apply to solid-state relays since obviously they do not generate an arc when they open a circuit.

 

[kwame]

Ron
The Omron spec sheet states that the contaction is for N.O is 10AMP.The N.C contact rating is 5A resistive ;although the contact rating for inductive loads is not given,it will not be any different.My actuator consumes close to 6A .I am finding difficult building the 'EMF protection;so obviously a contactor can withstand the EMF better since it has stronger and bigger NC/NO .
I want to forget about reverse polarity protection using diodes,capacitors and resistor by simply using a contactor rated at may 20A .

 

[kwame]

Ron
The Omron MK3P-I spec sheet states that the contact rating is for N.O is 10AMP.The N.C contact rating is 5A resistive ;although the contact rating for inductive loads is not given,it will not be any different.My actuator consumes close to 6A .I am finding difficult building the 'EMF protection;so obviously a contactor can withstand the EMF better since it has stronger and bigger NC/NO .
I want to forget about reverse polarity protection using diodes,capacitors and resistor by simply using a contactor rated at may 20A .

 

[kwame]

It is not just about the current ratings.I want to use the contactor because they are much robust and should be able to cope with the EMF transients without protection diode,capacitors,resistors ets .I am finding it difficult determining the ideal rating the diodes and resistors should have;that is why i want substitute the relay with the contactor.
Different webistes keep giving different values and designs.I doubt if i can a non polarized for the EMF protection.The common diode on the market is electrolytic diodes.

 

[kwame]

One thought

It is not just about the current ratings.I want to use the contactor because they are much robust and should be able to cope with the EMF transients without protection diode,capacitors,resistors ets .I am finding it difficult determining the ideal rating the diodes and resistors should have;that is why i want substitute the relay with the contactor.
Different webistes keep giving different values and designs.I doubt if i can a non polarized capacitor for the EMF protection.The commonest capacitor on the market is electrolytic capacitor.

 

[carbonzit]

The Omron MK3P-I spec sheet states that the contact rating is for N.O is 10AMP.The N.C contact rating is 5A resistive

I think you're misinterpreting the datasheet. I found what you're referring to, but those aren't Omron's ratings: they're regulatory agency ratings (SEV, DEMKO). Notice that the UL and CSA ratings make no distinction between NO and NC contacts.

Is SEV or DEMKO used in your country? If not, I'd just ignore them and use Omron's ratings, which are plenty high enough, if you use a snubber diode across the motor to absorb reverse EMF spikes.

And why are you going on about using capacitors for this purpose? A diode is what you want to use.

 

[Reloadron]

OK, then use a contactor if it makes you feel more comfortable, the relay would have worked fine especially with minor filtering. You will need a DPDT contactor if you plan to reverse direction. However, when starting a new thread try to make sure you include all the details. Carl's first reply was based on a purely inductive load and not reversing polarity. You may want to go back and read this thread as to a motor being an inductive load also. Note Carl's replies as well as the others.

Ron

 

[kwame]

Carbonzit,What must be the rating of the diodes?

 

[kwame]

Carbonzit, you are right,the UL,CSA ratings made no distinction between NO/NC contacts.The Omron MK3P-I actually has an inbuilt diode supression plus varistor surge suppression mechanism.
I think i have to make do with the omron relay,contactor is not necessary.



Breaks relatively large load currents despite small size.

Long life (minimum 100,000 electrical operations) assured by silver contacts.

Built-in operation indicator (Mechanical, LED), push button, diode surge suppression, varistor surge suppression.

Standard models are UL, CSA, SEV, DEMKO, NEMKO, SEMKO, TÜV (IEC), and VDE.

Conforming to CENELEC standards. **broken link removed**

 

[ronv]

A picture is worth a thousand words. If you can post your schematic showing the actuator and relay and power supply we can show you how to connect the diodes. If you can't let me know and I will make one.

 

[Reloadron]

Carbonzit Said in part....

If not, I'd just ignore them and use Omron's ratings, which are plenty high enough, if you use a snubber diode across the motor to absorb reverse EMF spikes.

And why are you going on about using capacitors for this purpose? A diode is what you want to use.

Carl mentioned the same thing. This is exactly what happens when you split threads all over hell's half acre despite your love of doing so. You failed to mention to those who are trying to help in this thread anything about the relay thread and failed to mention this motor will be getting reversed and that precludes the use of a diode. You can't place a diode across the terminals of a motor you plan to reverse using reversed polarity. You didn't give these guys any prior information, you never mentioned motor at the start of this thread. I suggest that you provide a link or links to the previous threads that deal with what you are doing. It would make for much better help.

Ron