# How is an A.C. Relay works ?!!

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#### aljamri

##### Member
I know how DC relay works using a DC to create a temporary magnet that will attract and hold the contact.

But what is the concept used in AC relay ? and why do we need the free-wheeling-diode in AC? !

#### dknguyen

##### Well-Known Member
All relays work the same way. Current flows through the relay coils to produce an electromagnet which pulls the spring-loaded contact (which can either be open or closed in the unpowered state).or closed. The only difference between AC and DC is the type of current used to magnetize the coil.

aljamri said:
and why do we need the free-wheeling-diode in AC? !
Where are you getting your information? Are you saying you need a diode across the AC relay coils but not DC relay coils? You need them for DC relays because the coils are inductive and when you de-energize them (stop the current from flowing through them) it creates damaging voltage spikes. The diode is used to supress this voltage spike.

I don't think using a diode to supress the voltage spike works for AC relays because the current flow changes every half cycle causing the diode to become a short-circuit every half cycle. I don't know how it's done for AC relays. It's probably some other type of snubbing.

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#### Roff

##### Well-Known Member
An AC-powered relay has what is known as a "shading coil" (Google), which is a shorted turn on the relay's solenoid. A current is induced in this shorted turn which is 90 degrees out of phase with the main coil's current, and provides just enough magnetic field to keep the armature engaged during the zero crossing of the current in the main coil.
I don't understand how the current is shifted 90 degrees. Perhaps someone else can explain this.

#### dknguyen

##### Well-Known Member
What is a shorted turn? You mean just a loop of wire? ?? ??

An inductor's transfer function is jwc, so wouldn't that account for the 90 degree phase shift?

V=IZ, Z = jwL

So that would mean that the voltage across an inductor leads the current in the inductor by 90 degrees in the complex plane.

#### Roff

##### Well-Known Member
dknguyen said:
What is a shorted turn? You mean just a loop of wire? ?? ??

An inductor's transfer function is jwc, so wouldn't that account for the 90 degree phase shift?

V=IZ, Z = jwL

So that would mean that the voltage across an inductor leads the current in the inductor by 90 degrees in the complex plane.
Yes, it's a single heavy loop of wire.
What you say is true, but, since the main coil is also an inductor, the current in it also lags the voltage by about 90 degrees. Remember that the magnetic flux is in phase with the current, not the voltage.
It would seem that the shorted turn would be the secondary of a transformer (the main coil being the primary), and the current in both primary and secondary would be in phase.

#### dknguyen

##### Well-Known Member
I found this:
While relays are usually taught and more easily understood as DC devices, in
practice AC relays are commonly used. These relays typically employ shaded
poles, similar to the shaded pole induction motor. A shaded pole is a coil loop
which is not separately excited by the relay source; it is excited by the flux on
the main relay coil. This coil then produces an opposing current, flux and
voltage (Faraday’s and Lenz’s law) which holds the contact during zero voltage
intervals on the main coil. Failure of the shaded pole leads to “relay
chatter”— a 120 Hz clicking that occurs every time the main relay voltage
crosses the zero point.

I see...so the shaded coil is ACTUALLY a loop of wire (normally a loop of wire does not meet end to end to close itself because it has + an - terminals instead). I was getting confused about the previous description of a shaded loop being a short-circuit (I was imagining the loop as completing a short-circuit whicih didn't make sense).

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#### Hero999

##### Banned
Suppression of an AC relay is normally done using a snubber network consistin of a capacitor in series with a resistor in parallel with the relay coil.

#### eblc1388

##### Active Member
dknguyen said:
I was getting confused about the previous description of a shaded loop being a short-circuit (I was imagining the loop as completing a short-circuit whicih didn't make sense).

Now you are confused.

You've imaged correctly. It is physically a completed loop joined end to end to form a perfect short-circuited arrangement. It make perfect sense.

#### hawk2eye

##### New Member
Ron H said:
....
I don't understand how the current is shifted 90 degrees. Perhaps someone else can explain this.
................................................................................
Ron H.
relay coil AC current 90_degrees, wrt Line Voltage which produces the short circuit current

-AC Line Voltage generates current I1 through the relay inductance, L
I1 = Vac/wL at 90_degrees relative to Voltage

-AC Line Voltage causes magnetic field to operate relay

-AC Line Voltage through ' transformer", coil inductance as primary
sc_turn as secondary, causes current I2 through sc_turn
0_degrees relative to Voltage

hawk2eye

#### Roff

##### Well-Known Member
hawk2eye said:
................................................................................
Ron H.
relay coil AC current 90_degrees, wrt Line Voltage which produces the short circuit current

-AC Line Voltage generates current I1 through the relay inductance, L
I1 = Vac/wL at 90_degrees relative to Voltage

-AC Line Voltage causes magnetic field to operate relay

-AC Line Voltage through ' transformer", coil inductance as primary
sc_turn as secondary, causes current I2 through sc_turn
0_degrees relative to Voltage

hawk2eye
But the magnetic field in the relay's solenoid is in phase with the current in the "primary". Since the shorted turn is in that same field , won't the current in the shorted turn be in phase with the primary?

#### dknguyen

##### Well-Known Member
eblc1388 said:
Now you are confused.

You've imaged correctly. It is physically a completed loop joined end to end to form a perfect short-circuited arrangement. It make perfect sense.

What I meant as I imagined two coils in parallel inside the relay, with the shaded coil being so short as to basically form a short-circuit across the primary coil.

#### aljamri

##### Member
Thanks for all.

My next question is when to call it Relay and when to call it Contactor.

Thanks

#### zevon8

##### New Member
Generally, and this is not a perfect rule, relays are "low power" and contactors are "high power" devices. Mostly contactors are what you expect in industrial settings for operating motors, large power power loads, etc. Relays are generally everything else lower power. Often contactors have more robust mechanical designs that ensure solid and vigorus closing of the contacts, to help prevent arcing or welding of the contacts.

It can just be a choice of words. I have used mercury wetted 3 phase devices capable of several hundred amps that were called a "relays" by the manufacturer.

#### Brian Park

##### New Member
But the magnetic field in the relay's solenoid is in phase with the current in the "primary". Since the shorted turn is in that same field , won't the current in the shorted turn be in phase with the primary?

The magnetic layout is slightly more complicated than a "simple" transformer (with a single magnetic path linking 2 coils). The magnetic iron from the main coil is divided into 2 parts, one goes straight to the armature, the other has to pass through the short-circuited single-turn shading coil. Because of this, the shading coil has "leakage inductance" (formed by the magnetic flux path linking the shading coil, but not the main coil.) this inductance causes the current in the shading coil to lag the current in the main coil.

(If this lag didn't happen, the effect would be to just cancel a part of the flux linking the shading coil, causing a simple weakening or a "shading" of the flux, as described in your question.)

The shading current, being 180 degrees out of phase, retarded more by the leakage inductance, leads to the "imperfect cancellation". The flux in the shaded part of the core is weaker and delayed from the main flux. The cross-section of the shading coil has to "be right"; fat coil, too little resistance, lots of delay, but the delayed flux then would be weak. Thin coil, lots of delayed flux, but not much delay.

Think of an RC circuit excited by an AC source. If C and R are both large (so the time constant is much greater than the AC half-period) then nearly 90 deg delay, but almost no voltage on the cap. If C and R are small (time constant much less than AC half-period) then big voltage on C, almost in phase with applied voltage.

So in effect we have 2 separate relay coils and magnetic-path iron, each excited by AC of different phases, acting on a common armature, both trying to attract it with 120Hz "vibratory forces" (for a 60Hz relay). The armature "sees" the sum of these forces, which always add, never subtract (due to the square-law rule of magnetic attraction of iron to magnetic flux). The phase shift guarantees that when one is zero, the other is positive. So the holding force, while being still vibratory, never goes below some value. (In an ideal world, the phase shift would be 90 degrees, and the 2 fluxes would be equal in magnitude, giving a perfect vibration-free attraction). But such perfection is rarely achieved, and is not needed, since once the armature contacts the core face, only a small fraction of the pull-in current ("holding current") is needed to prevent the relay from releasing during the zero-crossings of the current in the main coil. This is why the cross-section of the shading coil can be a small fraction of that of the main coil.

#### kinarfi

##### Well-Known Member
The purpose of the shaded pole is to create a hysteresis loop so the magnetic field switches so fast that the magnetic armature does not have a chance to open before it's pulled back in. If the shorted ring opens up, the relay or contactor will buzz like hair clippers that are adjusted too tight. If you look at the face of the pole piece, you'll see that only part of the armature has the shorted turn on.
Kinarfi

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#### Brian Park

##### New Member
Hysteresis is not necessary and is undesirable in a relay

The purpose of the shaded pole is to create a hysteresis loop so the magnetic field switches so fast that the magnetic armature does not have a chance to open before it's pulled back in. If the shorted ring opens up, the relay or contactor will buzz like hair clippers that are adjusted too tight. If you look at the face of the pole piece, you'll see that only part of the armature has the shorted turn on.
Kinarfi

The shaded pole does not create, nor require, a hysteresis loop to function. The phenomenon involves linear circuit theory only. It is an RL circuit. In fact, the iron is doped with silicon, and carefully heat treated to minimize the amount of hysteresis (too much could cause the relay to magnetize and "stick"). If the magnetic field switched "suddenly", a huge voltage spike would be produced in the coils. Spikes can not occur while the relay is energized.

If the shading coil opens up, you have no shading coil anymore, and as you indicate, the relay will buzz.

#### aljamri

##### Member
the relay will buzz.

That is exactly what I'm asking about. The concept of DC coil, as a magnet is very clear to me, it is simply fixed North and South poles , but AC magnet ?

Now the idea is a bit clearer to me.

Thanks for all

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