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I am trying to choose a relay to control a compressor. The compressor alimentation is 230V (50Hz), the max intensity is 0.9A and the power needed is 130W.

Then I found two relays: the ALD112W and the GNDB-1A-E12VDC with Max switching current 5A and max switching voltage 250VAC.

So that seems ok but when I looked at the max switching power it is 1250VA, 90W.

Since W is the real power needed and VA is the complete power I don't understand why there is a difference for a relay which should just let the current pass or not.

Which one is important? 1250VA or 90W? Since the compressor is I think like a motor.

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  • \$\begingroup\$ Give us some links to the relay data sheets. \$\endgroup\$ – Dave Tweed Oct 11 '12 at 12:02
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The difference between VA and W rating is where the current is with respsect to the voltage, or vice versa. However, the difference between 1250 VA and 90 W is surprising. I don't recall seeing that wide a difference before.

Now that you've posted links to the datasheets, we can see what was really said. Here is the relevant section from the first datasheet:

I know this is the political season (at least here in the US), but it's still not nice to misrepresent someone else's statements. Notice the all-important AC after the VA rating, and DC after the W rating. That makes all the difference.

Think of the problem a relay or any mechanical switch has when it is turning off. There are two issues it has to deal with, interrupting whatever current is flowing, and eventually providing enough of a gap so that the open circuit voltage can be withstood without arcing.

Immediately interrupting the current is the hard part. The contacts can't instantly go from touching to having the full gap between them. The gap starts at 0 and then over time grows to the full size. When it just starts to open and there is any current flowing, a arc will form between the contacts. A arc is air turned into a plasma by the flowing current. Plasma has lots of free electrons, so is conductive. Once the arc forms it creates its own low resistance path and thereby perpetuates itself. It therefore takes a much larger gap to break the current once the arc has started. Also consider that if the load has a significant inductive component, like a motor, how that will tend to sustain the arc even more.

Now think about how AC helps. At 50 Hz, the voltage and current will each be zero twice per cycle, or every 10 ms. This means any arc will be extinguished automatically in at least 10 ms. By the time the voltage builds up enough on the next cycle to possibly re-establish the arc, the contacts have moved farther apart and the ionized air has returned to normal state. The result is that much higher voltage is required to re-start the arc. That's a good thing for the relay.

So the net result is that AC is a lot easier for relays to switch, hence the large difference in the AC and DC power handling spec. The AC part is specified as VA instead of W because the problem is related to the peak volts and amps, even if they are out of phase.

Since you are switching AC, you apply the AC VA rating, not the DC W rating.

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  • 3
    \$\begingroup\$ +1, I've seen some of your replies, and I must report that reading them is a pleasure - even if I'm (well, more or less) in possession of the knowledge being detailed. Thank you for conveying such a level of professionality wrapped in a so enjoyable form! \$\endgroup\$ – ppeterka Oct 11 '12 at 13:05
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The data sheet for the Panasonic relay is clear on this: it states 831VA (AC) and 90W (DC). So for AC applications, such as this, it can habdle 831VA power, while for DC loads, it would only tolerate 90W of load.

I'd also take a look if the compressor features any kind of soft-starting mechanism, that would make the relay last longer.

Also, I'd try to see if there are any other, non-mechanical solutions, such as using a solid state relay, or a triac zero-crossing appliance, as I'm against using any kinds of moving parts when considerable amount of current is involved, as relays' contacts wear our from sparking over time...

Also recommended reading: AC Power factor

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