Current Through a Relay

fifty60

Senior Member
Location
USA
A 4PDT relay has an amp rating of 5A. Would this be 5A through each of its poles, or 5A total through all of its poles combined. I have checked on some other relays and was told that the amp ratings was per pole. So a 10A DPDT relay is really rated for 20A total. Is this common across the board, that the amp rating pertains to each pole?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
A 4PDT relay has an amp rating of 5A. Would this be 5A through each of its poles, or 5A total through all of its poles combined. I have checked on some other relays and was told that the amp ratings was per pole. So a 10A DPDT relay is really rated for 20A total. Is this common across the board, that the amp rating pertains to each pole?
Why are you totalling the currents per pole? What is your goal? Are you attempting to parallel them?
 

mgookin

Senior Member
Location
Fort Myers, FL
As a general principle, you can run the rated load through any contact. And when there are multiple poles, you can run that rated load through every one of those poles. If you have a specific application you can call the relay manufacturers who from my experience have very good engineering support. We deal with several relay manufacturers who all give us fabulous service and support.
 

fifty60

Senior Member
Location
USA
I have 1 pole turning on solenoids, and another pole activating a small motor. They are energizing separate circuits.
 

kwired

Electron manager
Location
NE Nebraska
A 4PDT relay has an amp rating of 5A. Would this be 5A through each of its poles, or 5A total through all of its poles combined. I have checked on some other relays and was told that the amp ratings was per pole. So a 10A DPDT relay is really rated for 20A total. Is this common across the board, that the amp rating pertains to each pole?
Rating is per pole. A 10 amp double pole is not rated for 20 amps, it is rated for two circuits @ 10 amps each.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I have 1 pole turning on solenoids, and another pole activating a small motor. They are energizing separate circuits.
Then you have two separate circuits. If each one is under 5A then that's fine, maybe.

There is a difference between amp ratings and motor ratings, because switching induction motors is hard on contacts. You must look at the HP rating of your relay and compare that to the HP on your motor nameplate. If the relay does not list a HP rating, it doesn't have one and you cannot use it. If the motor only says watts, there are 746 W/HP.

Some small relays may also give you a maximum watt rating on AC solenoids as well, so be careful there. All of that info may not show on the relay itself, you may have to look up the specs.
 
Last edited:

a.bisnath

Senior Member
type of load

type of load

please look at the type of load -inductive loads should be rated by horsepower or kilowatt not current ,inductive loads stores a charge in series with the circuit being released when the contact is open .Also think about cross pole arcing are the loads locally or remotedly located the longer the distance is more arcing on contact opening should not different relay be used for different loads?check this out ,it really happened- a relay controls motor and solenoid plunger for brakes of an elevator winch,brakes is energised to release ,motor contact burns arcing ionises air crosses over to contact pole with brakes fusing contact ,elevator starts dropping.I will stop here.
 

ATSman

Senior Member
Location
San Francisco
Occupation
Self Employed
As others have stated, high inductive loads are hard on relay contacts when they break the circuit.
We usually series 2 contacts to share the arc between contacts. In your case you have 4 poles:
series 2 poles to the solenoid & 2 poles to the motor for longer contact life.
If you want to get serious then use a contactor Nema size 00 or 0, depending on the HP of the motor.
 

fifty60

Senior Member
Location
USA
Could someone please go into a little more depth on why there is arcing across the contacts. Why is it worse when breaking inductive loads? I know that the energy stored in the magnetic field of the motor coils is released when power is removed from the coils, but how does this cause arcing? Is it basically a small amount of energy stored in the magnetic field discharged rapidly as high power?
 

GoldDigger

Moderator
Staff member
Yes to that last statement.
With a resistive load, current will stop flowing as soon as the contact separation gets wide enough that the breakdown voltage of the air gap is higher than the applied voltage.
With an inductive load, the emf of the inductor itself will add whatever voltage it takes either to keep the current flowing across the contact gap or to drive that same current into whatever stray voltage capacitance exists.
As the stored energy in the inductor decreases, the voltage and current in the arc decrease too



Tapatalk...
 

fifty60

Senior Member
Location
USA
So when the contact opens there is a potential difference between the contacts and with the air dialectric (weak dialectric) the contacts momentarily become a capacitor. The energy released from the magnetic field of the motor windings is stored in the electric field between the contacts...is it the same amount of energy stored between the contact, I would think it would have to be besides what is lost to heat.

Where does the energy stored in the air gap between the contact go when the potential difference between the contacts is zero?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Could someone please go into a little more depth on why there is arcing across the contacts. Why is it worse when breaking inductive loads? I know that the energy stored in the magnetic field of the motor coils is released when power is removed from the coils, but how does this cause arcing? Is it basically a small amount of energy stored in the magnetic field discharged rapidly as high power?
A common misconception is that "AC is crossing zero 120 times per second, so it stops flowing faster". While it's true that it stops flowing FASTER than DC in an equivalent air gap distance, that doesn't mean it is stopping fast, at least not at the molecular level where the contact material is being vaporized. Yes, the current crosses zero, but by then, the arc started and the air is ionized, so when the sine wave rises again it takes LESS current to re-ignite the arc, which keeps happening until there is sufficient air dielectric resistance to extinguish it. At voltages above about 12V there is ALWAYS arcing of some sort whenever you open a contact under load. With an inductive load, the arc is just sustained longer, so it does more damage. It's based on the "Inductive Time Constant" concept, that you cannot instantaneously change current flow in an inductor, it has a specific rate of change. In addition as you open a set of contacts in an AC circuit, the separation has a capacitive effect as current tries to continue flowing across the gap, which in effect increases the voltage across it. That, combined with the increased time it takes to make the inductive current stop flowing, increases the energy flow across the contacts, and as long as you have sufficient energy flow, the arc will be maintained.
 

GoldDigger

Moderator
Staff member
So when the contact opens there is a potential difference between the contacts and with the air dialectric (weak dialectric) the contacts momentarily become a capacitor. The energy released from the magnetic field of the motor windings is stored in the electric field between the contacts...is it the same amount of energy stored between the contact, I would think it would have to be besides what is lost to heat.

Where does the energy stored in the air gap between the contact go when the potential difference between the contacts is zero?
Nope. Not at all right. Sorry. :)
The factor of the air being a poor dielectric (low dielectric constant) just means that for any given contact configuration the capacitance will be low. The capacitance across the opening contacts will be small compared to that in other places in the circuit in any case.
What is a factor is the dielectric breakdown voltage, where electrons and ions actually move across the gap. That is an arc. The arc has a resistance, and will dissipate energy as heat and light. For as long as it lasts. The greater distance the arc covers, the greater the energy dissipation and the more difficult it will be to re-establish the arc if it stops momentarily.
 

kwired

Electron manager
Location
NE Nebraska
The old "points and condenser" ignition systems in older internal combustion engines take advantage of some of what has been brought up here to perform their function.
 

mgookin

Senior Member
Location
Fort Myers, FL
A relay cycle is one make and one break. When it fails, it's going to be on the break. As the contact opens, there is arcing. Eventually it welds itself and it sticks cuasing failure.
 

GoldDigger

Moderator
Staff member
I find it hard to figure out how contacts could weld together on break.
More likely the last break does sufficient damage (roughening) to the surfaces that the concentrated inrush current welds them the next time they close.


Tapatalk...
 

kwired

Electron manager
Location
NE Nebraska
I find it hard to figure out how contacts could weld together on break.
More likely the last break does sufficient damage (roughening) to the surfaces that the concentrated inrush current welds them the next time they close.


Tapatalk...
correct, for them to be welded together means they had to fail to break, which means the weld had to happen either during closing or during the period when they were in an intended closed state.
 
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