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Topic: How to test a relay for exagerated specs (Read 554 times) previous topic - next topic

PeterPan321

Apologies if this is not the best place to post this, but there is this China based company "Songle" that is marketing these relays, and several vendors on ebay, ali-express and others are selling them on convenient PC boards, often touting the usual "for use with arduino, Raspberri PI", etc. Here's a photo of one I've bought a few of to test...



So like a "good little designer", I like to pick components that well exceed my requirements, and these relays say they are good for 30 amps at up to 250VAC (or 240 depending on what nameplate data you read. The relays also have some rather conflicting specs printed righ on them...

30A 250VAC/32VDC

But they are also marked...

1HP 120VAC
1/2HP 240V
30A 240V

Well I need to control a 1HP pool pump motor that draws about 15 amps peak at startup, settling to about 12. But that's a lot of conflicting information on the label! I have downloaded the English data sheet but I am still confused as to how much AC current this relay can REALLY switch. The company has a product link here, where you can also download an English version of its data sheet (PDF). It doesn't offer much help.

But my REAL concern comes not from looking at the numerical specs, but life experience with relays. Looking at physical relay contacts rated for different AC loads, the single contact in the SONGLE simply does not "LOOK" up to the task. The photo of the internal contact made me skeptical,so I finally cut one open to really look at the actual contact. I'm still uneasy.

I realize there have been advances in everything, and I guess if the relay contact is made of some amazingly conductive and electrically durable substance I'm not aware of, maybe it can do the job. I've seen some other industrial load control relays/contractors that also had what I'd consider "amazingly small" contacts, so I don't want to write it off immediately. But currently, I've been using an OMRON (LY4-12VDC) 4 position (10 amp per contact) relay, with all contacts ganged together, and even one of the OMRON 10A contacts looks bigger than the single so called "30 Amp" contact in the Songle!


So I'm writing to ask  how I can test the relay  to see whether they are vastly exaggerating the specs, without having to field test it for years. It is sealed, so cutting one open surely makes it less likely to live up to its spec. But is there a temperature I should look for (with a sensor) mounted at the contact to determine the likelihood of longevity? The company claims its good for 100,000 operations. With that little contact, and at 30A AC, I'm VERY skeptical!!! It would be MUCH more convenient to use that what I'm doing now. But convenience is not as important as making a reliable product!

Of course if any of you have actual experience (positive or negative) with this relay, I'd liike to hear that too!

CurtCarpenter

I think it boils down to whether you trust the data sheet or not.

The 1hp/16A/120VAC spec. seems definitive to me, but I've no experience with the device and don't have any ideas about how to verify it short of using it for a while. 

Your motor's 15A peak gives you a 6% safety margin (assuming it's 120VAC).  That may or may not be enough to make you comfortable.  If not, I'd go with a different relay. 

Another alternative though, to buy a little more peace of mind, might be to put a snubber across the relay contacts.  See here for some helpful information on that approach.

DVDdoug

The main problem I see is that at 1HP you are already running it at it's limits so I don't know what you could do for stress test or accelerated life test.

I'm not sure the specs are "conflicting".   If I'm remembering correctly, 1HP is about 750W so that's around 7A at 120V, plus some more for inefficiency, and it will depend on the actual mechanical load on the motor.     But, the start-up current could be even higher than what you're measuring...  If you are using a multi-meter it may be fast-enough.

It is common to "de-rate" components....  I'd "feel better" if it was rated for 2A or 5A.  ;)

Relays are surprisingly reliable (or a mechanical "thing").   I had a horn relay in a car fail once and that's the only time I remember seeing a failure.   Where I work we make a board with 16 reed relays and I've seen DOA relays (or relays that fail the resistance test) during production test but we don't get returns from the field.    These are low current relays and it's a "technical" product so our customers usually know what they are doing, but we can't control what they connect.

hammy

Some of the low voltage connections are a bit  close to
the high voltage side on a lot of these .
Rather than mess about trying to test it , just buy a decent relay spec'd way above what you need.

Grumpy_Mike

Quote
. The relays also have some rather conflicting specs printed righ on them...

30A 250VAC/32VDC
What is wrong with that?
It can switch 30A when the voltage is 250V AC but if you have DC then you can only switch 30A if the voltage is 32V or less.

Because AC is constantly changing and reversing polarity the current "sticks the contact together" less than if it was DC.

larryd

#5
Sep 14, 2018, 11:28 pm Last Edit: Sep 14, 2018, 11:28 pm by larryd
My gut says 7.5kW for that relay is high.
But what does my gut know?

Let's cut one open.
.
No technical PMs.
The last thing you did is where you should start looking.

CurtCarpenter

Let's cut one open.
.
Strictly out of curiosity, what do you expect to do after you've cut one open?

larryd

#7
Sep 15, 2018, 12:17 am Last Edit: Sep 15, 2018, 12:18 am by larryd
Just to look at the size of the contacts and the quality of workmanship.
Also, it would be interesting to see what happens to the plastic if it was heated to 150°F, i.e. if it becomes soft.


No technical PMs.
The last thing you did is where you should start looking.

Paul_KD7HB

My gut says 7.5kW for that relay is high.
But what does my gut know?

Let's cut one open.
.
I did that a couple of years ago when they failed with my Peltier project. 12 volts dc at 3-4 amps. One failed shorted and failed open.

The relay blades have very tiny balls for contacts. In my case the shorted unit had migrated materiel between the contacts. That kept the armature from moving. The open relay also had a build up of material between the contacts but, that was not conductive and kept the armature from closing.

My experience shows these are cheap hobby relays not suitable for real world use. They are usable for signal switching and perhaps low level AC.

Paul

larryd

I did that a couple of years ago when they failed with my Peltier project. 12 volts dc at 3-4 amps. One failed shorted and failed open.

The relay blades have very tiny balls for contacts. In my case the shorted unit had migrated materiel between the contacts. That kept the armature from moving. The open relay also had a build up of material between the contacts but, that was not conductive and kept the armature from closing.

My experience shows these are cheap hobby relays not suitable for real world use. They are usable for signal switching and perhaps low level AC.

Paul
That's what my gut was thinking too.


No technical PMs.
The last thing you did is where you should start looking.

PeterPan321

What is wrong with that?
It can switch 30A when the voltage is 250V AC but if you have DC then you can only switch 30A if the voltage is 32V or less.

Because AC is constantly changing and reversing polarity the current "sticks the contact together" less than if it was DC.
As I tried to explain, what is "wrong" with it is that visually, its "so called" 30A contact is smaller that a single 10A contact in a USA made (OMRON) relay. I'd love it if the 30A label were true, but if someone handed you a motor that fit in the palm of your hand and said it was 20HP motor, I'm sure you'd want to test that somehow before believing it enough to use in a possible product, right?

PeterPan321

I'm not sure the specs are "conflicting".   If I'm remembering correctly, 1HP is about 750W so that's around 7A at 120V, plus some more for inefficiency, and it will depend on the actual mechanical load on the motor.     But, the start-up current could be even higher than what you're measuring...  If you are using a multi-meter it may be fast-enough.
So between the power factor and the motor efficiency, its not uncommon for the run current to be (like the motor I've been testing with here) about 13 amps at 120.  My problem with the "conflicting" specs is that is says 30A, a long way from any reasonable calculation (or real world measured) 1HP motor. So if the company prints 30A on the relay, but then says its good for 1HP at 120V, I don't know how to reconcile the two except to assume one is a "lie" or exaggeration.

allanhurst

#12
Sep 15, 2018, 12:47 am Last Edit: Sep 15, 2018, 12:57 am by allanhurst
An ac motor can take up to 10 times it's rated full load current at start up. That's one reason why special relays called contactors are generally used .

Allan

larryd

#13
Sep 15, 2018, 12:55 am Last Edit: Sep 15, 2018, 01:12 am by larryd
The image says 'Contactor', that's what made me suspicious.






Now this is a 7.5kW contactor ;)








No technical PMs.
The last thing you did is where you should start looking.

allanhurst

Another reason for the contactor is that they have 2 contacts in series. This is because breaking a highly inductive load such an ac motor would otherwise cause the resulting arc to quickly burn out the contacts. 2 in series reduces this effect .

It's also why a dual rated  ac/dc relay has a much lower voltage rating at dc than ac - the arc will be extinguished at the next zero crossing with ac, but not with dc.

Allan

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