AC to DC solenoids

[PhaseShift]

I recently came across some solenoids in the field that used 120V AC primary input and then rectified voltage to DC for use with a DC coil in the solenoid.

What is the point of rectifying voltage to DC? Why not just have an AC rated coil in the solenoid?

Would DC coil have a larger impedance and therefore require less current when reflected on AC?

 

[iwire]

I was under the impression DC coils are less noisy

I could be wrong.

 

[hillbilly1]

I was under the impression DC coils are less noisy

I could be wrong.

This is true, I ran across a 200 amp electrically held contactor in NJ a couple of months ago, what my guy was telling me didn't make sense until I got up there and seen it was DC! It was very quiet once I moved the 4/0 wire out from behind the guide on the side. (It was blowing control fuses)

 

[jeremysterling]

I'm guessing the DC solenoid is stronger. When I serviced Budgit chain hoists, the solenoid that released the holding brake was energized through a 4 way bridge rectifier. These rectifiers would fail from time to time, and I replaced about as many rectifiers as I did brake discs.

 

[SG-1]

The DC solenoids produce a stronger magnetic field than their AC counterparts. They have greater pull-in power. We use them for closing & opening Med. Voltage circuit breakers, so they are only energized for a few cycles. Even the clapper type are DC coils. The designers tried AC, but they did not have the strength to operate the latches especially at minimum voltage.

 

[gar]

100311-0807 EST

Given essentially the same space and magnetic circuit a typical AC solenoid will have greater pull-in capability than a DC solenoid for the same steady-state closed power dissipation.

This is because magnetic force is a function of current. The current in a DC solenoid is the same no matter where the core is positioned. This also makes the DC solenoid burn out proof for a stuck solenoid plunger. A common problem in hydraulic or pneumatic valves. It is DC burn out proof if designed for continuous on operation.

In a typical AC solenoid the impedance is much higher when closed than when open. So initially when open the current is higher and when closed the current is lower.

An AB 709 Size 2 709COD has a coil with 40 ohms DC resistance. This just pulls in at 45 VDC in a horizontal position. Power dissipation at 107 VDC would be about 286 W. At this point this is not a fair comparison, but it is the power that would be dissipated if fed from a full wave bridge rectifier from 120 V AC. But note that even at the pull-in voltage the power dissipation is 50 W.

On AC this same contactor pulls-in at about 60 V, and has a actual power input of about 8 W at 120 V AC. This ratio of nominal to pull-in voltage is about 2 to 1.

In the DC case if we used the same nominal to pull-in ratio the DC voltage would be 90 V and the power dissipation at nominal would be 202 W.

In the vertical position the AC pull-in voltage is about 68 V. DC pull-in is about 55 V.

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