AC Relay excitation using PWM, benefits ?

Hello! Everyone,

Yesterday i had discussion with one guy who works on Home Appliance Products.
He told me something, and i just want to confirm and also the benefit of using such technique for exciting relay.

They use AC Relays to turn on the AC load, when they have to turn on the load, they just provide PWM pulse to the coil of the relay. They provide PWM in such a way, that relay doesn't get unexcited, and load remains on during the PWM cycle, and will only be turned off when they remove PWM cycle.

Guys, please explain, is it possible, because whenever i had to turn on the relay, i just provide a constant DC signal not any PWM pulse.
What is the advantage of such technique ?? ( One thing i am thinking is that by using this technique we are not energizing the relay coil always, which in turn may increase the relay life )
This is only my assumption.
Need some expert comments.
Thanks

It depends on the design of the relay. When the relay is at rest the magnetic path is broken so the coill looks like the DC resistance with a small inductor in series with it. When the relay is energised the magnetic path is completed and the value of the series inductances rises. In high power contactors with 50 HZ AC coils this gives a high pull current and force and the current falls a lot when the contactor is "in" but enough to hold the contacts in but it saves on power. If the contactor cannot be pullled in because it is sticking, the high initial current will burn out the coil.
How much of this applies to relays is doubtful as the closed magnetic path is not as efficient as a contactors one.
Frank

Recent DC actuated contactors have built-in energy saving circuits, either using PWM or a dual coil drive.

To decide if the method can be applied for your relays, check the data sheet specifications about pull-in and hold current. For standard relays that are designed for 100 percent drive duty cycle, its more a matter of energy saving and reduced power supply than relays lifetime.

Are these relays designed for AC loads with DC coils, or with coils designed for AC, e.g.110Vac?

PWM drive of DC coils is commonplace to reduce power consumption of a control board and Trise of the coil wires.

Easy peasy said:

Are these relays designed for AC loads with DC coils, or with coils designed for AC, e.g.110Vac?

PWM drive of DC coils is commonplace to reduce power consumption of a control board and Trise of the coil wires.

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These relays are designed for 110V AC loads with DC Coils, which is drive by PWM.

Suppose we have to use more than six such relays, so how we will drive the coils using PWM, because microcontroller doesn't have that much PWM pins.

That guy told me, they just use one PWM signal and then mix it with DC signal, such that DC signal becomes PWM signal and hence they just require one PWM signal and only 1 Output pin of micro-controller per relay to control the state of a relay.

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FvM said:

Recent DC actuated contactors have built-in energy saving circuits, either using PWM or a dual coil drive.

To decide if the method can be applied for your relays, check the data sheet specifications about pull-in and hold current. For standard relays that are designed for 100 percent drive duty cycle, its more a matter of energy saving and reduced power supply than relays lifetime.

Click to expand...

Thanks for your comment.
I will try to find the relay and then search relay datasheet to figure out more information on this topic.

Relay reliability is not a mechanical or a coil driver issue but a load surge current issue. Thus all contacts are derated for motors and incandescent strings of ceiling lights due to surge currents.

Read details from OMRON's website.

You can easily AND the uP output with e PWM signal to drive the coil...

Relay reliability is not a mechanical or a coil driver issue but a load surge current issue.

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yes, it is the inrush current that flys through the contacts at the point of closure....or indeed the gap sparking when you break an inductive current (all current is inductive due to strays).
So if you want relay to last long time, then PWM is not necessarily really the way, you should arrange for zero current to be flowing through the relay at the point of making, and make the current through the relay = zero just before you break the contacts.

There are relays with PWM inputs, and this is because your coil driving voltage may not be the required one, so they have circuitry inside them so you can pwm it to get the relay to work without you having to put in another voltage rail just for the purpose of driving the relay coil.

you should arrange for zero current to be flowing through the relay at the point of making, and make the current through the relay = zero just before you break the contacts.

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Of course the above is usually if not always practically impossible, relays are rugged devices - hence their use, they are intended to switch on heavy and high inrush loads and to break inductive and other loads, simply choose one that is slightly over-rated.

Of course the above is usually if not always practically impossible

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In eg a mains powered SMPS, switch on the relay that shorts out the inrush resistor after the inrush event has happened, and before the SMPS's have been started up.....that way you switch the inrush relay on with zero current flowing through it.
There are other situations.

Same with battery circuits…..you want to switch the battery out when its charged up so as to minimise its discharge. You want to use a relay for this , not just a fet, because a fet will suffer leakage current. When you switch the battery back in, there would be often inrush through the relay contacts into eg input capacitors….so you have also an inline fet switch…….then you first switch the relay on with no current flowing through it, then you switch the fet switch on to connect the battery to the circuit...no inrush through the relay contacts as they are contacting..

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relays are rugged devices - hence their use, they are intended to switch on heavy and high inrush loads and to break inductive and other loads

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True, though as you know, contactors replace relays for the higher loads, and above that, they use "switchgear"

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As you well know, not all relays are rugged devices, reed relays are often not.

No one is talking about reed relays, rather relays with 110V coils...