AC vs. DC solenoids: inrush, hum, and instant burnout
On DC the current is set by resistance alone, so the force is predictable. On AC the plunger position is part of the magnetic circuit, so pull-in current runs three to ten times holding current, a shading ring is mandatory to stop the force collapsing at every zero crossing, and a coil energised with its stroke blocked sits at the inrush value until it fails.
Why this happens
The two types are not the same device on two supplies. They behave differently, fail differently, and need different magnetic circuits, and treating them as interchangeable is where most of the confusion starts.
On DC the current is set by resistance alone. A DC coil of 12 Ω on 12 V draws 1 A. That value does not depend on where the plunger is, does not depend on frequency, and changes predictably with temperature. Because force goes with the square of current, and current goes with voltage, force varies with the square of the applied voltage: a 10 % dip in supply costs about 19 % of the force. Heating changes it too, in the same direction. A winding that stabilises 70 K above ambient has risen 27.5 % in resistance on the 0.393 %/K copper coefficient, so current falls to about 78 % and force to about 61 %. On DC, heat and low voltage both reduce force.
On AC the plunger is part of the magnetic circuit. Current is set by impedance, and impedance is resistance in series with reactance, and reactance depends on inductance, and inductance depends on the air gap. With the plunger out, the gap is large, reluctance is high, inductance is low, so impedance is low and current is high. Settle the plunger and the gap closes, inductance rises, impedance rises, and current falls. The ratio between the two states is typically three to ten to one, and it is geometry, not supply.
That single fact produces the AC failure mode. If the plunger cannot seat, the current never falls to the holding value. The winding then dissipates several times its rated power continuously, and the coil fails on a timescale of seconds to minutes rather than weeks. Class B insulation permits an 80 K rise over ambient, class F 105 K and class H 125 K, so a 40 °C panel already consumes half of a class B allowance before the winding produces any heat of its own. A jammed damper, a binding linkage or an obstruction all produce exactly this, which is why an AC coil can die on a supply that measures perfectly correct. It is a mechanical fault presenting as an electrical failure, and it is covered from the thermal side in what actually kills a coil.
AC also needs a shading ring, and DC does not. The force from an AC winding passes through zero twice per cycle, so at 50 Hz the armature would be attracted and released 100 times a second. A short-circuited ring set into part of the pole face delays the flux in that section by roughly a quarter cycle, so while one part of the face is at zero the other is still producing force. With a 20 ms mains period at 50 Hz, the force collapses twice inside every 20 ms without the ring, and the armature never settles. Remove or crack the ring and the unit buzzes and will not hold cleanly, which is a fault that looks like a weak coil and is not one. Splitting magnetic noise from mechanical noise is covered in diagnosing solenoid noise.
The magnetic circuit itself differs. At 50 Hz the flux penetration depth into ordinary steel is under a millimetre, so an AC magnetic circuit has to be laminated or the core behaves as a surface rather than a volume. A DC circuit can be solid, because there is no alternating flux to drive eddy currents. This is why an AC and a DC solenoid of the same force rating are not dimensionally interchangeable.
Speed is where DC takes the advantage back. On DC the current rises with an L/R time constant. A coil of 30 mH and 12 Ω has a time constant of 2.5 ms, so it takes about 7.5 ms to reach 95 % of its final current, and the armature moves after that. Shortening it means driving with a higher voltage for the pull-in interval, not adding turns, which would raise the inductance and slow it further. Pull-in intervals of 20 to 50 ms are typical, and holding phases measured in hours are just as typical, which is why the two states deserve separate treatment.
Check these in order
1. Confirm which type the application actually needs before comparing specifications. Mains-powered, short duty, simple function points at AC. Battery, precise control, PWM holding, low noise or frequent operation points at DC.
2. For AC, size the wire and the switching device from inrush, not holding. The inrush is what sags the supply and what welds contacts.
3. Convert watts to volt-amps before sizing anything electrical. A solenoid has a power factor commonly between 0.4 and 0.7, so apparent power runs roughly twice the real power, and every device in the chain is limited by current. A 24 V coil rated 5 W at a power factor of 0.5 draws about 10 VA, which is roughly 0.42 A rather than the 0.21 A the watt figure suggests.
4. For AC, prove the stroke can complete. The whole failure mode depends on whether the plunger seats. Check the linkage, the stop and the mechanism for anything that could hold the armature off.
5. For DC, check force at the low-voltage limit and at the hot resistance. Both reduce current, and force follows the square.
6. For DC on a rectified supply, check the smoothing. Without a reservoir capacitor the force pulses at twice line frequency and the unit behaves like an unsmoothed AC one.
7. For AC, confirm the shading ring is present, intact and correctly fitted. It is a small part carrying a lot of responsibility.
8. Check the magnetic circuit construction. A laminated core for AC, a solid core for DC. A substitution in the other direction will not behave as expected.
What actually to change
| Finding | What to change | Why not the other thing |
|---|---|---|
| AC coil failed on a correct supply | Find why the stroke never completed | Replacing the coil restores the fault, not the function |
| AC unit buzzes and will not hold | Inspect and replace the shading ring | A stronger coil cannot fix a zero-crossing force collapse |
| Transformer or contacts overheating | Size from VA and inrush, not watts | The watt figure understates the current by roughly half |
| DC force short at low supply | Check force at the voltage minimum, not nominal | Adding turns raises resistance and slows the response |
| DC unit on rectified AC is noisy | Add a reservoir capacitor | The ripple is in the supply, not in the coil |
| Response too slow on DC | Overdrive the pull-in, then drop to a holding level | More turns increase inductance and slow it further |
| Frequent operation on an AC unit | Move to DC or to a rectified supply with smoothing | AC contacts and shading rings both suffer at high rate |
| Force fine cold, short hot | Check the resistance rise on the copper coefficient | Core material is rarely the limiting factor |
When it IS the harder problem
The application needs AC simplicity with DC behaviour. Then the answer is usually a rectified supply with smoothing, or a shaded-pole arrangement, and the design work moves into the power supply rather than the solenoid. The mechanical package often stays as it was.
Several coils share one AC supply and can pull in together. Then the inrushes coincide and the supply sees a demand no individual datasheet describes. Either stagger the pull-ins in the control logic or size the supply on the summed inrush.
The mechanism occasionally fails to complete its stroke. On AC this is not a degraded performance issue, it is a destruction issue. Where the stroke cannot be guaranteed, the AC coil needs a thermal protector or the design needs to move to DC. The same blocked-stroke logic applies to AC actuators generally, as set out in the 24 V AC sizing trap.
The unit has to hold for long periods. Then AC is the wrong starting point, because continuous holding is where its current and its heat are least favourable. PWM on DC, or a mechanical or permanent-magnet hold, is almost always the better answer. Compare the two on the failure modes as well as the force, because the failure modes are what will actually be experienced.
AC vs DC solenoidAC solenoid inrush currentshading ring solenoidDC solenoid force voltagesolenoid AC hum
Frequently asked
- Why does an AC solenoid draw so much more current at pull-in?
- Because the air gap is part of the magnetic circuit and the plunger position sets the inductance. With the plunger out the gap is large, reluctance is high, inductance is low, and since impedance is dominated by reactance the current is high. As the plunger seats, the gap closes, inductance rises and the current falls to the holding value. The ratio between the two states is commonly three to ten to one, and it is a property of the geometry rather than of the supply.
- What is the shading ring for?
- On AC the force passes through zero twice per cycle, so at 50 Hz the armature would be pulled and released 100 times a second and would chatter instead of holding. A short-circuited ring embedded in part of the pole face delays the flux in that section by roughly a quarter cycle, so part of the pole face is still producing force while the rest is at zero. The result is a non-zero net force throughout the cycle. A cracked or missing ring is the classic cause of a unit that buzzes and will not hold.
- The coil failed on the correct supply voltage. How?
- Because it never reached its holding state. An AC solenoid energised with the stroke blocked, by a jammed mechanism, a binding linkage or an obstruction, stays at the pull-in current indefinitely. That current is several times the value the continuous rating assumes, so the winding dissipates several times its rated power. This is the most common way an AC coil dies with a healthy supply, and it is a mechanical fault presenting as an electrical failure.
- Can I run a DC solenoid on rectified AC?
- Only with smoothing. Full-wave rectified AC still delivers a pulsating supply, so the current follows the ripple, the force pulses at twice line frequency and the armature buzzes in exactly the way a shading ring exists to prevent. Add a reservoir capacitor sized so the voltage does not fall appreciably between peaks, or accept that the unit will be noisy and will fretting-wear its pole face. An unsmoothed rectified supply is the most common reason a DC solenoid sounds like an AC one.
- Which type is more tolerant of low supply voltage?
- DC, in the sense that the behaviour is more predictable. On DC at constant gap the force varies with the square of current, and current varies with voltage, so a 10 % voltage drop costs about 19 % of the force. On AC the current near pull-in is largely set by reactance rather than resistance, so it is less sensitive to supply voltage but more sensitive to whether the plunger has seated. Predictability is the DC advantage.