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I am trying to understand how the shading ring in solenoid valves is able to produce a 90° out phase magnetic field compared to the coil's magnetic field. The logic seems different from a shaded pole motor.

The solenoid valve is composed of a magnetic plunger (E in picture) surrounded by a coil (A in picture), a shading ring (ring of copper or aluminium, C in picture) placed at the end of the plunger and a string.

The solenoid being AC powered, the magnetic field produced by the coil collapses periodically as the magnetic force. During this moment the plunger could chatter due to the spring. The shading ring role is to create a 90° out phase magnetic field that will compensate the collapse of the coil magnetic field and keep the plunger attracted.

As I see it, the magnetic field created by the solenoid coil will travel trough the shading ring. Then according to Lenz's law, the shading ring magnetic field will oppose to the coil flux. Therefore, should it not be 180° out of phase instead of 90°?

How does 90° phase difference is acheived ?

enter image description here

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    \$\begingroup\$ In transformers, the primary and secondary currents are 180 degrees out of phase (adding clarity). \$\endgroup\$
    – Andy aka
    May 2, 2023 at 10:43
  • \$\begingroup\$ I made it clearer, thanks ;) \$\endgroup\$
    – Bim
    May 2, 2023 at 13:09
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    \$\begingroup\$ Unfortunately you made it a tad confusing by mentioning magnetic fields. The magnetic field in a transformer core is not due to secondary load current. Any field it produces will be cancelled by the field from equivalent load current that flows in the primary. The field in the core is purely due to the primary voltage and the primary magnetization inductance i.e. the magnetization current; loading doesn't produce a magnetic field. \$\endgroup\$
    – Andy aka
    May 2, 2023 at 13:15
  • \$\begingroup\$ @Andyaka, well, i didn't want to talk about the flux of the core created by the magnetizing current but the flux of the primary and secondary that cancel each other when there is a load on the secondary. From my point of vue the secondary coil does produce a magnetic field but it is continously canceled by the primary coil as you mentioned. In order to cancel, those magnetic fields have to be 180 out of phase. As i see it, the same logic shoud apply with the shading ring. But you are right, i will rewrite it. \$\endgroup\$
    – Bim
    May 2, 2023 at 14:36
  • \$\begingroup\$ I'm no expert on shaded poles/rings by the way so, I'm thinking that the ring is acting more like a resistor and hence, produces a current that is properly out-of-phase with the magnetization current and, somehow, this keeps the magnetic field topped-up when the peaks of applied voltage cause the proper magnetization current to fall to zero. I'm guessing here but, I am keen for you to get an answer. I think @SpehroPefhany once described it in an answer but I can't find it. \$\endgroup\$
    – Andy aka
    May 2, 2023 at 14:40

1 Answer 1

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It isn't 90°. As explained in Wikipedia, the difference in phase angle of the shaded pole is small, but it's enough to get the desired effect: a starting torque in a motor, or a reduction of chatter in a solenoid.

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    \$\begingroup\$ This has more info. \$\endgroup\$
    – rdtsc
    May 2, 2023 at 19:13

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