HP Rated Relays

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fifty60

Senior Member
Location
USA
What is the difference between inrush ratings of a relay and HP rating. Would the HP rating have to be used to confirm that you can break the inductive load, while the in rush is just to cover the LRA on startup? If I have a relay that has the correct inrush ratings, but does not have any HP ratings, is it ok to use on an inductive load or would I still need a HP rating?
 

fifty60

Senior Member
Location
USA
The relays actually have a 2500VA interrupt rating. For an inductive load this would still be the measured current multiplied by the voltage. My load is 2.9A at 230V gives me 667VA...
 

fifty60

Senior Member
Location
USA
The relays are definitely rated to make and break the load, yet they are still turning black after less than a year of use. Is it normal for the relays to turn black after only a years use?

I will need to find an arc suppressor to use across the contacts. I would prefer to use a contactor, but for such a small compressor there is not much room and the contactors are 3 times as expensive.

I see there are several arc suppression options. I've never sized one before. My load is 3 amps at 230V volts. My inrush is approximately 18A. I am not sure what the voltage peaks at and for what duration when the load is broken. Any recommendation on a quencharc or diode that will not cost too much relative to the relay?
 
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Besoeker

Senior Member
Location
UK
The relays are definitely rated to make and break the load, yet they are still turning black after less than a year of use. Is it normal for the relays to turn black after only a years use?
Might it be related to atmosphere in the environment?
I've seen this happen in anodising plants.
 

mgookin

Senior Member
Location
Fort Myers, FL
I have a customer who has 750,000 relays in service continuously and he replaces 65,000 of them per year. He does lots of testing and he has an incredible lab to do it in.

He tells me that it's the break (as in make - break) which causes a relay to fail and if you can break it when sine=0 you will get greatest longevity out of your relay.

That's as much help as I can be.

Curious - where are you seeing black? Are you disecting the relay and finding black on the contacts of a failed relay?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The difference between an Inrush rating and a HP rating on a set of contacts has to do with the PF when switching. Inrush in an AC load, TRUE inrush that can be as high as 20x the FLA, is only the current that has to establish the magnetic flux, so it lasts only a cycle or two at most, then drops off once inductance, and therefore impedance, is established. But for a motor load, the STARTING current will be 500-800% after that initial inrush, for as long as it takes for the motor to accelerate to about 80% speed, and because the PF during that time is very low, the contacts must supply all of that current. Worse yet is the possibility that the contacts should OPEN when the PF is very low like that, because the current is high, but because the load is inductive, the arc formed by opening the contacts is sustained longer and does more damage to the contact surface. So that high switching current at a low PF is then used to essentially reverse engineer the HP rating of the contacts.

Why the "inrush" rating on a relay contact without a HP rating? Transformers and anything else with a coil have inrush too, but only the extremely short duration magnetic inrush. The real root of the problem is the widespread misuse of the term "inrush" to mean "starting current" on AC motor loads. They are not the same thing.

Per the NEC and I believe UL508A by the way, you must have a HP rating shown on anything that is switching an AC motor load, you cannot "roll your own". There are specific tests done for attaining those HP ratings. That's why often times people are confused as to why the HP rating of a relay or contactor is seemingly so much lower than the "thermal" current rating, it's all about the switching capability, not just the current carrying capability.
 

fifty60

Senior Member
Location
USA
Thanks for the information everyone, as always extremely helpful. If anyone can recommend a 230V single pole ice cube relay that has a HP rating o 1/3HP, I would greatly appreciate it. My usual suspect Omron does not look very impressive at 230V, with a 1/3HP rating for only 1000 operations.
 

mgookin

Senior Member
Location
Fort Myers, FL
most manufacturers and the biggest suppliers of electronic components (digikey, avnet, mouser) have pretty good parametric search on their websites.
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
I will need to find an arc suppressor to use across the contacts. I would prefer to use a contactor, but for such a small compressor there is not much room and the contactors are 3 times as expensive.

How much are the repeated service calls costing?

I do understand the size issue, and needing to shoehorn a contactor into a too-small enclosure. I just went through this on a much larger compressor, at the shop where I work. In your case, it might be worth it to install a larger enclosure and a proper contactor. Many small compressors don't have an unloading valve and start loaded. This is very hard on relays. Oversize the contactor and you should be alright; contacts rated at 20A or greater will definitely give good service life.

http://www.automationdirect.com/adc...ctors_-z-_Overloads/9_to_25_Amp/SC-E05-110VAC

A suitable enclosure (assuming NEMA 1) would be about $25. For $60 in parts and an hour of your time, this problem becomes not-a-problem. Keeps the client happy with increased up-time too.


ScenreryDriver
 
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