AC/DC Coils

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shawn73

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I am an industrial electrician at an automotive factory. The other day I was having issues with fuses blowing on a machine. I found that someone had added a solenoid to the machine's circuit. A 24VAC coil was incorrectly installed, the controls of the system are all 24VDC. How does the design of a DC coil differ from that of an AC coil and can I get a detailed explaination of the way each reacts when supplied the incorrect types of voltages. Thank you for your responces.
 
shawn73 said:
I am an industrial electrician at an automotive factory. The other day I was having issues with fuses blowing on a machine. I found that someone had added a solenoid to the machine's circuit. A 24VAC coil was incorrectly installed, the controls of the system are all 24VDC. How does the design of a DC coil differ from that of an AC coil and can I get a detailed explaination of the way each reacts when supplied the incorrect types of voltages. Thank you for your responces.
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I'll touch on the basics. AC coils have much lower resistance (DC) than DC coils for similar circuits. You'll also USUALLY find that the magnetic components are different. The "reactance" (AC resistance) is much higher than the DC resistance. Think about inrush on AC ... that is more DC resistance determined, and if maintained, would either burn up most coils, blow a fuse or the PLC output, depending on degree of (in)coordination.

There is a big exception; many hudraulic and pneumatic valves use only DC. The coil, if marked for AC, has builtin rectifiers. Hard to test ...

Think about your motor starters; if it cannot close and is AC, it continues to draw inrush. If the same problem happens on DC, it just doesn't close.

In my world, electrohydraulics, MOST manufacturers use differing sizes of "pole tubes" or "core tubes" to prevent the incorrect ones being used. Soem designs don't offer that capability.
 
The AC solenoid will have a shading coil to keep the plunger in place at the zero crossings of the AC wave form.
Don
 
to add to GeorgeB's reply-

the DC coil, operated on AC, will be really "weak" and may not seal in, which may cause it to vibrate, get hot, or draw too much.

Resqcap'n- shading coil? please explain?? seen shaded pole motors, but this is new to me...
 
This is the way I have always looked at it:
AC current produced hysteresis where DC current does not. The hysteresis caused be the AC current not only provides impedance that the DC current does not but the hysteresis also causes a current to flow in the iron core of the coil. Because conducting current is not the best attribute if iron the iron gets hot. One of the things that is done to reduce the heating effect within the iron core is to build that core with laminations of steel each of which is insulated from one another. As such current flow within each lamination if limited to that lamination only which reduces heating.
When it gets down to it AC coils are more suited to be used with DC than DC coils with AC. But one also must remember that the hysteresis caused by an AC current also produces counter EMF that basically results in a resistance stated as impedance. When DC is applied to that same AC rated coil you no longer have a counter EMF and the DC voltage sees that coil and a simple resistive load. So expect to get a different response from a 120vac coil if one were to apply 120vdc to it.
 
templdl,

There is hysteresis alright, but it is un unwanted characteristic of the iron core. An ideal solenoid would have no hysteresis, and it would work just fine. Hysteresis is related to the core loss in the iron. Laminations reduce the eddy currents but do not reduce hysteresis.

The solenoid works because the current causes a magnetic flux to flow in the iron. Magnetic poles are created which attract each other. The solenoid is a linear electric motor with limited motion.
 
Translation:

Translation:

LarryFine said:
Generally, a DC voltage equal to a coil's AC voltage rating will overheat the coil.
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What Larry is saying is that that a relatively high resistance limits the current in a DC coil. An AC coil would have a relatively low resistance with the current being limited mainly by the inductance. This lower resistance allows too much current when DC is applied to an AC coil.

BTW, you can smoke an AC solenoid by leaving the plunger out. The missing plunger reduces the inductance which leads to an overcurrent.
 
rattus said:
What Larry is saying is that that a relatively high resistance limits the current in a DC coil. An AC coil would have a relatively low resistance with the current being limited mainly by the inductance. This lower resistance allows too much current when DC is applied to an AC coil.
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Yeah, that's what I was saying. ;)

Another example is a loudspeaker; the DC resistance is much lower than the impedance; often less than half.

Keep in mind that an inductor exhibits resistance to a change in current through it, while a capacitor exhibits resistance to the current through it remaining steady.

That's why a series inductance filters (blocks) AC while a series capacitance blocks DC.
 
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