paralleling 2 current start relays

[YuriB]

Who think it is possible to effectively parallel current relays ? For instance, if a 1/2 HP one is not available, one can use in necessity two 1/4 HP relays (however, it is better to add by that an ordinary NO relay to bypass the current to the starting winding). Somebody done this ?

 

[ScottyUK]

Doesn't sound like a great idea to be honest. It is unlikely that you will get sufficiently equal current sharing for this to be a workable solution.

 

[waross]

I have a lot of experience doing it right and not much experience doing it wrong.

Who think it is possible

Not me!!!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zeusfaber]

I think that one of them is bound to open after the other one does - and, because by that time it will be carrying all the current, this will not be good for it.

A.

 

[crshears]

Hello YuriB,

Dumb questions on my part, maybe...

Are you asking about paralleling the solenoids or the contacts?

"Current relays" suggest to me that these are interposing relays within a control circuit, in which case if the coils are being paralleled their impedances may differ enough that they won't carry their load current evenly, compromising the accuracy of pick-up and drop-out settings...and I'm guessing that in this case the contacts would be wired in series so if either relay picks up it will initiate whatever needs initiating.

However, you also mention 1/2 HP and 1/4 HP relays, which suggests you're not asking about current relays being supplied from the current transformers of a large motor, but instead about overload relays whose solenoids carry the load current of the motor itself...in which case the impedance sharing characterisic of two coils in parallel would still apply, and the interrupting duty of the contacts is not to open a control circuit but to interrupt the actual load current. If this is what you're asking about, I'd agree with zeusfaber; one will consistently win the race, and the "losing" relay's contacts will end up interrupting the full current...although this wouldn't necessarily be an issue if your design takes into account both the interrupting and continuous current carrying capabilities of the contacts in question.

CR

Carl

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

These may be starting relays for a refrigerator. When the motor current drops enough the contacts open the starting winding. They used to be rated in motor HP.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Note to self: read the title of the post more carefully before responding.

Thanks, Bill.

Carl

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[YuriB]

Yes, one like for "starting relays for a refrigerator. When the motor current drops enough the contacts open the starting winding". They may not be sometimes available of a necessary size. Their paralleled coils would bear running winding current, but their contacts in series switch on and then off an ordinary relay to bear the starting winding current. Unfortunately I do not have an opportunity now to test whether such schematics would work.

 

[jraef]

Sounds more like you are describing a "Potential Relay" used in starting single phase motors. The Potential Relay operates not on current, but on the difference in potential across the start winding. As the motor comes up the speed, the back EMF expressed onto the start winding by virtue of it sharing the same iron as the run winding increases to a point higher than the voltage across the start winding. The difference in potential then energizes the Potential Relay and it's contacts open, removing the Start Capacitor from the circuit. You cannot put two Potential Relays in the circuit in the manner you describe, because you are basing it on the current through them. They are RATED in current because they are in series with the start winding, but they do not OPERATE based on current. If you connect them in parallel to theoretically have them share current, you will not have changed the potential they are designed to operate on, so they would likely operate too soon. To get them to work right, you would have to wire them in series with each other to provide the same impedance. But you would have to determine the exact impedance, then match them to get to the correct value, and i dont think it will be an exact 2:1 relationship. But even then, the current they are rated to handle will be too low, because current rating in series does not incease.

Just go buy the right one...

"Will work for (the memory of) salami"