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Relay specifications

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alphacat

New Member
Hey,

I wanted to ask some things about these specifications.

1. the 1A version is rated 16A/250Vac.
Meaning that the contacts and armature can handle 16Arms and can be connected to 250Vac, right?

2. Why do they mention the Power factor in the rating?
if the Power factor is less than 1, it means that more than 16Arms current can go through the contacts for a given 250Vac?

3. What does it mean Max. switching power?
shouldn't it be 16A*250Vac*(cosPHI=1) instead of 2200VA?

4. What is the difference between electrical life and mechanical life?

Thanks a lot!

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crutschow

Well-Known Member
Most Helpful Member
1. No. See 3.

2. The rate it for a PF of 1. If the PF is less than one, power handling would decrease (The VA stays constant).

3. You can not switch 16A at 250VAC. The switching power is as shown. For example, form 1/A/B/C can switch 2200VA, which is 8.8A @ 250VAC or 16A @137.5VAC.

The switching power is given as apparent power (VA) which includes the effect of power factor.

4. Mechanical life is how many times the relay will operate before it's likely to mechanically fail (something breaks).

Electrical life is how many times the relay will operate with rated load before the contacts fail due to contact pitting or erosion.
 

alphacat

New Member
Thank you!
Here are my responses.

2. The rate it for a PF of 1. If the PF is less than one, power handling would decrease (The VA stays constant).
But if i understand correctly, even thought I_MAX = 16A, and V_MAX = 250V, it doesnt mean that they can "come together", meaning that the relay can switch a 16A@250V load.
So how mentioning the power factor makes a difference?


3. You can not switch 16A at 250VAC. The switching power is as shown. For example, form 1/A/B/C can switch 2200VA, which is 8.8A @ 250VAC or 16A @137.5VAC.
Wow, i didnt know that.
There are many times that I use the relay in order to switch a 16A @ 230Vac load.
Does it mean that i'm damaging the relay?


4. Mechanical life is how many times the relay will operate before it's likely to mechanically fail (something breaks).

Electrical life is how many times the relay will operate with rated load before the contacts fail due to contact pitting or erosion.

Got it, thanks.
 
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crutschow

Well-Known Member
Most Helpful Member
But if i understand correctly, even thought I_MAX = 16A, and V_MAX = 250V, it doesnt mean that they can "come together", meaning that the relay can switch a 16A@250V load.
So how mentioning the power factor makes a difference?
Not sure why they mentioned the power factor. The VA value should tell you all you need.
Wow, i didnt know that.
There are many times that I use the relay in order to switch a 16A @ 230Vac load.
Does it mean that i'm damaging the relay?
That can seriously shorten its electrical life.
 

alphacat

New Member
Thanks a lot crutschow! really.

Not sure why they mentioned the power factor.
The VA value should tell you all you need.
So when they say
Rating: PF=1, 16A/250Vac
They mean that
- the max. current than can flow through the contacts/armature is 16Aac.
- the max. potential difference between each fixed contact and the armature (moveable contact) is 250Vac
?

If its correct, does it mean that there could be a situation where
V_Contact1 = 500Vac
V_armature = 250Vac
?

That can seriously shorten its electrical life.
Got you.
I need to find a relay with max switching power equal to 4000VA then.
 
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alphacat

New Member
Are you saying that :
Max switching power = V_contact_armature(right before they make contact) * I_contact_armature(right after they make contact) ?
 

Sceadwian

Banned
There's no exact calculation for it, once you get past a certain volt/amp rating which isn't exact, when the relay contacts break you'll get arcing welding and generally all around relay life decreasing events. What blueroom is saying is that you don't have to worry about this is if you switch the relay during a zero cross event when the relay current is exactly 0. However the exact timing of when the relay is switched has to be exactly synchronized with the zero crossing.
 

crutschow

Well-Known Member
Most Helpful Member
Zero crossing switching is not practical with a mechanical relay, only a solid-state relay.
 

alphacat

New Member
I think i just dont understand the definition of switching power.
Is it the power dissipated on the load at the moment when the contacts make or break connenction?
 

Sceadwian

Banned
Zero switching is practical with a mechanical relay, with the right feedback, and even trying is better than not.

No, switching power is the TESTED power the device is rated to switch. In the real world under high power loads this often simply speaking can't be done, the arc and current to follow will bypass the physical linkages themselves.
 

alphacat

New Member
the TESTED power the device is rated to switch

I really don't understand that exactly do you mean.
What power is the relay switching?

In the following example, the relay switches the appliance at t0.

What is the switching power in this case?

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Sceadwian

Banned
Alphacat that's pretty self explanatory, if the relay ACTUALLY closes at T0 then the switching power is zero, thing is relays don't switch instantly and there's contact bounce, so realizing this in practice isn't always easy.
 

alphacat

New Member
I'm not sure i got you.
If the relay closes at t0, and say keeps bouncing for 10ms (this is the average bouncing time i measured in my relay), what would be the switching power in this case?

It would be very helpful if you could refer to terms like v(t0), i(t0), Rapplinace as appeared in the shceme.

movs-jpg.35732


Thank you again.
 
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Sceadwian

Banned
Depends on the voltage and frequency of the live wire alpha, and what the effective contact resistance was when the contacts were bouncing. 10ms is almost a complete cycle of 60hz AC so it's not going to be good if the resistance of the load itself is small.
 
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alphacat

New Member
Oh, i think i got your point!


Is this the switching power?
relay-jpg.35797


Is Reff(contacts) equal:
[Duty_cycle * 10ms] * R(contacts)

Where the duty cycle is the fraction of switching time which the contacts are connected to each other.

If its true, then why do they mention P(switching) in VA, while the R(contacts) is pure ohmic?
 

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Sceadwian

Banned
By the way, if I'm not mistaken electrical life is the number of hours the device can operate for before failure, and mechanical life is the number of switching cycles it can go through before failure, they are approximations.

I have no idea how they actually calculate switching power, there's no standard that I know of. Suffice to say whatever is written on the datasheet is what that company thinks that device is capable of switching safely and reliably. If you violate the rated switching power of the relay you'll end up with carbon scoring and/or welded contacts. Also the power factor is mentioned if the relay is driving an inductive motor load, as a low power factor will force you to derate the relays switching current. As you can tell by the datasheet the switching current of the relay is abysmally small compared to the maximum the contacts/conductors are rated for. This is typical of a relay and why they tend not to be used in high power applications anymore or as frequently as they used to.
 
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