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[SOLVED] difference between Relay rated and maximum current

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mamech

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Hello

I want to select a relay to drive an air conditioning unit that is single phase and with power about 4.5 Hp . According to calculations, a 30 A rated relay will be enough, but my question is: will it really be enough?

I know that Ac machines has starting current that reaches up to 5~7 times the rated current. I do not know what is the relation between rated and maximum currents of relay because this is not explicitly mentioned in datasheet.

I am about to choose this one:
https://www.farnell.com/datasheets/1717914.pdf


In fuses, I heard that a rule of thumb is that the maximum current is about 10 times the rated current, does the same rule works with relays?
 

If you look, the motor ratings for 1Hp (at 125v) suggest a contact lifetime of only 1,000 operations.

You might be pushing your luck with 4.5Hp, especially in a refrigeration compressor, which needs to accelerate up to speed under load.

You need a proper mains rated contactor with tungsten contacts, rated for motor starting.

Either that or a solid state relay.
 
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    mamech

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You need a proper mains rated contactor with tungsten contacts, rated for motor starting.

Either that or a solid state relay.

thank you very much. May you suggest a brand or a certain contactor/relay for this application?

actually, solid state relay is good, but its price is high and it is not suitable in my case.
 

Best to go to an electrical supply store, and ask what they have, or can get in for you.
These contactors all have a rating plate, and there will be a stated horsepower rating.
This is just a random picture I quickly found on the internet of a single phase motor starting contactor.
But it shows the type of thing to look for, with suitably heavy construction.
It will definitely not be cheap, but it will do the job and last for a very long time.
CONTACTOR.jpg
 
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    mamech

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I believe that the rated current is the one with which the relay can turn on/off but the maximum current is the one the relay can carry with the contacts on. The rated current depends on the voltage and the load. Single phase compressors use a capacitor start and that needs a high rush current. The inductance causes high voltage at turn off. Most starter relays have W button contacts you will certainly need a heavy duty relay.
 
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    mamech

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An A/C compressor is a special subset of a capacitor-start/capacitor-run induction motor. Relays have to be sized taking into account their particular characteristics.

The task is simple; take a look at your air conditioner's compressor plate, and look for a pair of values: FLA and LRA.

Which means Full Load Amps and Locked Rotor Amps.

Now, on the very first page of the datasheet you sent, look for the heading: contact ratings;
There are very many ratings depending on the nature of the load.

Look up for anything that says LRA/FLA. If your compressor's plate ratings are below the relay's ratings, then you are all set. If they are higher, then you will have to find a higher rated relay.
Otherwise, the operational life will be shortened considerably.
 
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