120VAC across a 24VDC coil

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Yesterday morning had you asked me what would happen if you put 120VAC across a 24VDC coil I would have told you that the coil would fry.
This apparently isn't the case.

I got a call yesterday afternoon about production down at a manufacturing facility. I was directed to a control panel the facility guys had been trouble shooting for 2 days. They had swapped out 2 contactors with the wrong coils.

What I witnessed was the contactors trying their best to open and close 60 times a second. These were AB (not ABB) coils/contactors.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The exact construction of the coil and magnetic circuit will determine whether the inductance will limit the current from 120V AC to a value that the DC coil can tolerate. I am pretty sure that some coils will melt.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I had that with a solenoid valve....the coil was 24 VDC and we supplied it with 120 AC. It was running very hot, but the manufacturer's guidance said they run hot, and it is not too hot unless it is smoking. The coil worked for a couple of years, but of course, not always energized.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
220817-2205 EDT

You need an understanding of relays.

A DC relay has a coil wound on a magnetic core. The inductance of this coil varies depending upon the mechanical position of the armature. Inductance of the core is lower before energizing the coil, and increases as the armature moves to the fixed part of the core. This change in inductance is generally of no significance in the use of the relay. The coil resistance remains constant independent of the armature position. So in a DC application the coil current will never be greater than defined by the resistance of the coil, and the applied voltage. The relay must be designed so that steady state current is sufficient to pull in the armature from its open position. A partially closed DC relay will not burn out its coil.

Change this DC relay to an AC relay, and two major things need to be done.

At various AC frequencies a DC relay will just sit and mechanically oscillate (buzz). The number of turns in the coil needs to be reduced, and a shading coil added. A convenient relay type to look at is a P&B KUP DC vs AC. Mechanically these are almost identical.

When you drive an AC relay with AC it is necessary to consider the inductance of the coil in both its armature open position and its closed position. In the closed position the impedance of the coil must be low enough to obtained and hold a sealed state. The impedance will be much lower when the armature is in its open position, and if the armature does not move the coil will burn out.

To obtain a more constant magnetic force, and thus avoid oscillation of the armature, a shading coil is added to the fixed core to provide two different magnetic fields that have a phase difference between them. This second field is obtained by a slug of copper around a portion of the fixed magnetic core.

As an approximation a 120 V AC relay can be operated by about a 24 V DC supply. The reverse is not true because DC relays do not have a shading coil.

.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
....

As an approximation a 120 V AC relay can be operated by about a 24 V DC supply. The reverse is not true because DC relays do not have a shading coil.

.
Yet, my real world experience with a solenoid valve says the opposite. I don't see the solenoid valve coil being that much different from the coil in a relay or a contactor. They really do the exact same thing. It appears to me that the mechanical inertia of the armature was enough to hold it in place through the zero crossings of the AC power source.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
220818-2038 EDT

don_resqcapt19:

I opened up an Allen-Bradley 709 Size 2 motor starter. It uses shading coils on the magnetic circuit. This obviously has a much higher inertia mechanism that the P&B KUP relays. But clearly requires shading coils.

I have some AC solenoid valve coils somewhere, and if I can find one I will look to see if any shading mechanism exists.

.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
220818-2038 EDT

don_resqcapt19:

I opened up an Allen-Bradley 709 Size 2 motor starter. It uses shading coils on the magnetic circuit. This obviously has a much higher inertia mechanism that the P&B KUP relays. But clearly requires shading coils.

I have some AC solenoid valve coils somewhere, and if I can find one I will look to see if any shading mechanism exists.

.
One manufacturer of solenoids says their AC coils always have a shading coil.
Differences between AC and DC solenoids
AC service
AC solenoids are always equipped with a shading coil in the plugnut (stationary core) and the top of the core is flat faced and perpendicular.
 
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