heavy duty power relay
I am looking for suggestions on a heavy duty spdt relay, 24 volt coil. It is for an oil furnace that has a 3/4 hp motor. The standard relay that says it is rated up to 1 hp, and 13 amps only seems to last about two years. I know RIB makes so heavy duty ones for baseboard heat, but I was not sure if they could handle the inrush current of a motor. Thanks to all
Comments
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Before you slap an expensive contactor in there — and there are lots of them — check one thing: are you really getting 24 volts AC to the coil? Or is something dropping it to maybe 21 or even less? Is the transformer powering the system big enough?
Reason I ask is that the quickest way to destroy any relay or contactor is to not give it enough voltage. It will pull in slowly, and that will give you contact bounce and arcing, and you're done.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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or it is chattering or short cycling for some other reason. a 3/4 hp motor controlled by a quality 1hp relay should last for decades. it is rated 1hp at 120vac, right? just upsizing it to a 1.5 or 2 hp 120v relay of a similar type would be more durable, but it should last a long time as it is. Bad terminations allowing the contacts to be heated from the terminals will destroy a relay too.
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So many relays, so little time. Can you share the part number of the original (or a picture) ? So we can see what you are basically looking for. There is so many variables with picking a relay, giving us a clue may help find something that matches the application with a higher load rating.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Mars #92340 about $16 at supply house.com
Its only rated 3/4 hp.
What I would do is switch both the hot and neutral through the relay to cut down on the load on 1 set of contacts.
The relay I mentioned is DPDT 24 volt coil.
And make sure your 24 volt trans is up to the job of pulling in the relay.
If you need more amps you will need a contactor, but finding those DT will be $$$$$
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" switch both the hot and neutral through the relay to cut down on the load on 1 set of contacts. "
Well that won't help much since the contacts are effectively in series, probably increasing the chance of failure. Each contact is switching the same series current. Still either contact fails your broken again.
Since it is a DPDT in this example put the contacts in parallel, redundancy and/or in a perfect world the load current would divide equally between the contacts. This would switch only one wire.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
I was taught — and maybe the sparkies on the Wall will correct me (please!) if I'm wrong — that one should never switch the neutral. The reason being that if you switch the neutral, the device may appear to be off — but the hot or hots are still energized, and worse the neutral wiring in the device and anywhere between the device and the switch is now also energized at full voltage. This can ruin your whole day…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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You can switch the neutral if you also switch the hots with the same control.
I meant switching the hots in parallel as you described. Brain Fart.
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I'm thinking that this oil burner is on some commercial boiler of some sort since most residential oil burners for the last 80 years have have 1/6 and 1/7 HP burner/blower motors that operate the combustion fan and fuel pump. So the relay that is operating that burner motor is not the Primary Control like a R8184G, or Carlin 70200. The controls that provide flame safety function will power up a simple contactor that you might find on a Air Conditioning Condensing unit. Those are available in a variety of ratings based on the size of the compressor or other motor that are being powered.
As you can see, the bigger the rating the higher the price. But if you have frequent failures then the cost of the relay that does not fail is meaningless. As long as the more expensive relay solves the problem.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Usually never (almost) switch the neutral for the reasons you stated.
However, if the controlling device is such that it mechanically disconnects both the hot and neutral simultaneously then it is approved.
But a DP relay can have one contact hang up and defeat the safety feature.
Anyone with their fingers in the wiring would have the disconnect switch available and operate it accordingly as needed.
A greater danger is in most central AC units that have only a single pole contactor.
Being a 240 VAC unit, when not running, leaves all wiring in the unit at 120 Volts to ground……….again the service disconnect switch is available.
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The voltage of the motor is less than or equal to the voltage of the relay contact rating right? 240v will arc more so the contacts for 240v need to be beefier even if the current is less.
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I think in general the higher the voltage rating the greater the open circuit air gap or arc squelching means. Contact size is more about the current ratings.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
how often does it cycle on off? That will destroy contacts.
1-HP230 volt 6-8
115 volt 12-16
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If the ampdraw on the motor is too high that may cause arcing which can pit the contacts..
This can cause a premature failure of the relay which is failing frequently !
I would set my Ampprobe to "max hold" and check the startup and the running ampdraw.
It may be time for a new Motor/Burner. 😑
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even that might miss the peak inrush, it is only sampling a couple times a second at most.
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The way I read this the OP mentioned a 3/4hp blower motor on a furnace.
Not the oil burner.
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I read "Oil Furnace"…In my book that would be a furnace that blows warm air in ducts and has a oil burner as a heat source..
Soooo is it a Oil burning furnace that blows warm air
OR is it a "Boiler" that has 140F to 180F water and baseboard with a oil burner on it
OR is it a "Boiler" that supplies Steam for Radiators using a Oil burner.
Either way there is a 3/4 Hp motor that uses a contactor and is in need of replacement frequently. That is why i recommend checking the Maximum Ampdraw instead of replacing the Relay over and over again.
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how much higher can the? If it goes above FLA it shuts off.
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Yeah…..As in a Locked Rotor.
I personally have seen this over and over again.
Blower is drawing high amps….Technician changes the failed part (Relay).
Furnace Blower works fine for a little while until….Again Bad relay.
Not sure why checking the Amp draw on the equipment is "Frawn upon". 😫
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drawing LRA is totally different!
That wouldn’t let it cycle.
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I know that it is not a LRA situation….
Can we agree that high ampdraws reduce the life expectancy of relays 😀
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motors draw 80 - 95% of FLA. Any more and the safety opens!
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i think we learned enough about selling the same motor under different ratings and such that we can only get a general idea of where the overload will trip. somewhere in the overloaded range but where might vary.
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Here's an interesting book about relays, or at least the contacts thereof:
(At 51 MB, it's just a touch too large to attach, sorry.)
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When the Safety Opens does the Enduser know ?…Or will it reset it self after it cooled off and the Homeowner/Enduser is oblivious about this situation !
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I'd still start by checking the voltage at the coil when it tries to pull in… it's a sim[le check. Multimeter. Two probes. Get someone to turn the device on…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
I was looking at the oil burner blower (not the ductwork blower) for a commercial oil burner. And since the OP has not replied we can't really know for sure, But you could be correct because a large residential furnace could have a 3/4HP blower motor. 5 ton of cooling and 2o0,000 BTU furnace might have that big of a motor. And on older furnaces that use Fan Center relays for the cooling side of the system could have the standard R8222 relay that keeps failing. Interesting observation Ed. I went the the contactor relay because of my assumption of the oil burner being commercial. Not the fan relay on the older furnaces.
As old as I am, I should have thought of that.
EDIT: Looking at the R8222B 1076, that relay has contacts that are rated for 12 running amps and 60 LRA for 3/4 HP motors, but there are other R8228 relays that are rates as high as 18 running amps and 96 LRA for up to 1 HP motors. My guess is that no one really looks at that rating.
If it fits it must be the right one. Ain't that how it works?
EDIT:
You need the R8228B relay for the higher HP rating If you are talking about the fan center type relay.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I used one of those fan center relays to control a window air conditioner for a while then to control an electric baseboard heater for quite a while after that.
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thanks so much to all that responded
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