AC Relay/Contactor Powered by DC

[Fish4Fun]

AFAIK the only physical difference between an AC relay//Contactor is that a DC relay relies entirely upon the DC resistance in the coil to limit current where as an AC relay/contactor uses the impedance.

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If this is false, then I need someone to set me straight.


So, if I have a Relay designed for 24Vac @ 50/60hz, and I measure its DC resistance @ 10.8 ohms and it's inductance @ 23.78mH, then I can assume it is designed for 240-280mA via the following:

50hz = 3.141 x 10^3 Rad/Sec
60hz = 3.769 x 10^3 Rad/Sec

(3.141 x 10^3) * 23.78mH ==> 74.70 
(3.769 x 10^3) * 23.78mH ==> 89.64

24 / (74.70 + 10.8) = 280mA
24 / (89.64 + 10.8) = 240mA

Soooooo, if I wanted to power the above relay with DC, I should use ~ 10.8 * 0.25 = 2.7V OR I should use a series resistor of ~ V/(.25 * (R + 10.8)) so for a nominal 12Vdc supply, I would need roughly a 30 ohm resistor with a power rating of at least 3W. For a 5V system, I would need roughly a 10 ohm resistor rated @ 1W.

The problem with all this theory is that it doesn't work, LOL. The relay will not work at all @ 5V, and @ 12Vdc the maximum series resistance that will still reliably operate the relay is 1.25 ohms, and it "works more like 24Vac" with no added series resistance at all @ 12Vdc.

Soooo, would somebody please '"splain to Lucy" what the physical difference between an AC relay and a DC relay really is? 'Cause obviously I am confused.

Thanks

Fish

 

[duke37]

A DC relay run on AC will buzz a lot since the magnetic field reverses and passes through zero.

An AC relay has a shaded pole. Part of the pole is surrounded with a copper ring. This produces a phase shift so that the total magnetic force never drops to zero. The ring carries current so dissipates power. A similar constuction is used in shaded pole motors to give phase shift and so motor direction. These motors are inefficient and are often used to drive fans which helps to keep the temperature down.

I have done some measurements on Norslo 240V AC relays. 2 pole changeover, 7500ohm.

50 Hz AC
Pull in 130V Drop out 130V

50 Hz bridge recitifier
Pull in 65V Drop out 10V

50 Hz single diode feed
Pull in 110v Drop out 75V

50 Hz diode feed with diode across coil
Pull in 90V Drop out 20V

DC
Pull in 60V Drop out 7V

When running on DC the inductance will not restrict the coil current so lower voltage should be used.

 

[Fish4Fun]

Part of the pole is surrounded with a copper ring.

Thanks Duke! I knew I wasn't completely crazy FWIW, I need to control some heavy AC current (~25A @ 220Vac) and it appears the best "cheap" relays (technically "contactors") are built for the HVAC industry which uses 24Vac for almost everything. I could certainly add a 24Vac HVAC transformer, but my project already has 12Vdc & 5Vdc, so I thought I would play a bit, but my calculations were way off my measurements, so I thought I would ask why, lol. A double pole 24Vac"contactor" can be bought in 30A or 40A for under $10 delivered where a similar capacity "relay" is just silly expensive, lol.

Again, THANKS!

Fish

 

[duke37]

Contactors are slower than relays. They use a bar which is thumped up to a pair of contacts so when the contactor is opened, there are two contacts in series. There are guides to separate the arcs of the different phases.

Have you considered a solid state relay?

 

[Fish4Fun]

Have you considered a solid state relay?

I have, but I don't need speed, and I really like a completely "broken" contact. There will actually be 4 of these "contactors" that control various heating elements in my home brewing setup. It is a wet environment and I want both hot legs completely "broken" when off. These just seem cheap, easy and safe; how often do those three go together? I have also considered using scr/triac "dimming" (x number of full AC cycles on/ y number of full AC cycles off) to control power input, but in reality, the volume of water/wort I typically heat only rises ~1C per minute per 5.5kW heater, so even in maintaining the mash temps the shortest possible cycle time is ~2 minutes, with more typical cycle times >15minutes. So even a remarkably slow switch is more than adequate.

too many hobbies, not enough time, lol.

Fish