What is the difference between AC coils and DC coils?

Solenoid valves are available with different coils, which enable the valve to be connected to the available mains voltage. The most common voltages are:

• 230Vac
• 24Vdc
• 110Vac
• 24Vac

AC and DC coils differ fundamentally in terms of operation and the range of switchable pressure differences. At this point we take a small mathematical excursion into the world of electromagnetism and distinguish between two cases: direct current and alternating current.

The electromagnetic force of a coil

The electromagnetic force of a coil arises due to a change in magnetic resistance in the iron circuit of the solenoid valve. This change in magnetic resistance is due to the air gap between the magnetic core and the pole of the guide tube, since air is a much poorer magnetic conductor than the used stainless steel. The resulting electromagnetic force strives to reduce this magnetic resistance and to close the air gap. This pulls the moving magnetic plunger of the solenoid valve to the pole. The electromagnetic force of the magnet system is calculated using a simplified approach and considering:

• the coil current I,
• the number of turns of the coil N,
• the length of the air gap or the stroke of the solenoid valve δ,
• the area A_iron of the pole or the magnetic core,

to

The induced electromagnetic force is therefore proportional to the square of

• the current in the coil I,
• the number of turns N,
• the reciprocal of the air gap length or core stroke .

Current consumption of a coil

The current consumption of a DC coil is determined by the supply voltage and the copper resistance of the winding R_dc. In case of AC coils, in addition to the ohmic resistance R, a so-called reactance X_L must be taken into account, the size of which is determined by the frequency f of the AC voltage and the inductance L of the coil:

The resistance in case of alternating current is therefore calculated as follows:

Two cases must be considered for the inductance L of the coil. In the first case, the magnetic core is not tightened to the pole and thus there is an air gap. In the second case, there is no air gap and the core is therefore in direct contact with the pole. Since magnetic stainless steel has significantly better magnetic conductivity than air, the inductance of the coil is significantly higher when the plunger is attracted than when the plunger is not attracted, because there is an air gap here.

Impact on pull-in power

Let’s look at a direct current coil and an alternating current coil with the same current consumption I_Hold and the plunger attracted:

Since the inductance L of the AC coil is significantly smaller at the moment of switch-on than when the plunger is attracted and thus also the complex resistance Z  from formula 3, the pull-in current at the moment of switch-on and according to formula 1 therefore also the electromagnetic force are significantly larger than in the holding state and also significantly larger compared to the electromagnetic force of the DC coil:

This allows a valve with an AC coil to handle greater differential pressures than a valve with an DC coil

with the same current consumption in the holding state. This mainly affects valves with a large core lift (air gap) such as the SVS type series 27A: