Inrush Ratings for Relays

[fifty60]

I am looking for a SPDT relay to operate small compressors. I generally use HP rated contactors for larger compressors, but would prefer to use the less expensive and smaller blade relays for these compressors.

The LRA of the compressor is 33A. I can find some relays that have inrush ratings, so I would try to match the LRA of the compressor with inrush rating of the relay.

Some relays only have inductive ratings, PF .04 = 7.5A for example. I have heard that it is common practice to multiply the inductive ratings by 4 in order to derive the momentary inrush ratings. Can anyone verify/offer any insight into this?

 

[petersonra]

You can buy a real motor contactor with a 9A FLA rating for $15. Why would you screw around with a relay of dubious specs?

 

[templdl]

I am looking for a SPDT relay to operate small compressors. I generally use HP rated contactors for larger compressors, but would prefer to use the less expensive and smaller blade relays for these compressors.

The LRA of the compressor is 33A. I can find some relays that have inrush ratings, so I would try to match the LRA of the compressor with inrush rating of the relay.

Some relays only have inductive ratings, PF .04 = 7.5A for example. I have heard that it is common practice to multiply the inductive ratings by 4 in order to derive the momentary inrush ratings. Can anyone verify/offer any insight into this?

I would go with an IEC contactor. Have you considered one of those?

 

[Jraef]

You cannot do this on your own by calculation. You CAN use a "blade type plug-in relay" for a motor load, but ONLY if the nameplate of the relay SAYS it is rated for motor loads, and of course only at the load they say it is rated for. That HP rating is what is giving it the starting current capacity.

For the most part, you are not going to find one rated for more than 1/2HP at 120 or 230V (even though the current is lower at 230V, the HP rating remains the same). Those will be the ones rated for 15A or 16A. Those rated 10A are usually 1/3HP max, the ones rated less than that usually have no HP rating at all.