This should be simple, but...

I have a 24V 50Hz AC solenoid.

The DC resistance of the coil measures 70 Ohms at ambient temperature.

The solenoid has a U shaped iron core with a moveable armature.

When energised, the core and armature form a closed magnetic circuit.

The core includes a copper shading ring.

I measured the inductance with the armature closed using a Marconi bridge.

If I measure using the internal 1kHz oscillator I get 180mH.

If I use an external low frequency oscillator in the region of 50Hz, I get 440mH.

I assume the difference is due to the permeability of the iron core changing with frequency?

I want to calculate the expected rms current when the solenoid is connected to a 24V 50Hz AC supply.

Using the 440mH figure, the reactance of the coil at 50Hz is 138 Ohms.

That gives a total impedance of 155 Ohms, phase angle 63deg.

Hence I would expect the current to be 24 / 155 = 155mA rms.

When I hook up a test circuit I actually measure around 220mA rms (using an AVO which approximates rms).

The current drops to about 200mA as the coil warms up.

I don't understand why the measured current differs from the calculated result.

What have I overlooked?

Is it because the shading ring distorts the current waveform?

If it's no longer a true sine wave then the current as measured on an analogue moving coil ammeter won't be accurate.

I have a 24V 50Hz AC solenoid.

The DC resistance of the coil measures 70 Ohms at ambient temperature.

The solenoid has a U shaped iron core with a moveable armature.

When energised, the core and armature form a closed magnetic circuit.

The core includes a copper shading ring.

I measured the inductance with the armature closed using a Marconi bridge.

If I measure using the internal 1kHz oscillator I get 180mH.

If I use an external low frequency oscillator in the region of 50Hz, I get 440mH.

I assume the difference is due to the permeability of the iron core changing with frequency?

I want to calculate the expected rms current when the solenoid is connected to a 24V 50Hz AC supply.

Using the 440mH figure, the reactance of the coil at 50Hz is 138 Ohms.

That gives a total impedance of 155 Ohms, phase angle 63deg.

Hence I would expect the current to be 24 / 155 = 155mA rms.

When I hook up a test circuit I actually measure around 220mA rms (using an AVO which approximates rms).

The current drops to about 200mA as the coil warms up.

I don't understand why the measured current differs from the calculated result.

What have I overlooked?

Is it because the shading ring distorts the current waveform?

If it's no longer a true sine wave then the current as measured on an analogue moving coil ammeter won't be accurate.

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