I'm a new graduate and fresh hired Electrical Engineer who works in a company building magnetic separators for general industry (including mining, recycling etc.)

Normally, we build magnetic seperators with permanent magnets but yesterday, the management brought a new type of separator from a nearby laboratory for R&D purposes which runs the separation operation with 2 identical but opposite coils creating an electromagnet on the middle.

The coils are armored and insulated and I can't see the diameter of the wire inside. I'll include the image of the coils. I've been tasked to find the number of turns in one of the coils and the diameter of the wire used on the winding.

I have the following measurements:

  • The resistance of 1 coil is 16,5 ohms.
  • The machine runs on 150V and draws 7A on that voltage.
  • The core of the coil is iron which the diameter of it is 274mm and coil length is 420mm.
  • The winding height is 114mm.
  • Total diameter of the coil and winding is 515mm.

I need to find the number of turns and the wire's diameter.

Please help me and let me know if I can measure anything else for the computation I'm trying to do.


2 opposite coils creating the electromagnet. The closer one is armored, the other one unarmored which the armor can be seen.

enter image description here

Electrical box:

enter image description here

EDIT - 11th October

Hey all again, I was able to get the product catalogue and got some more information about the machine. It actually is a Jones type wet high intensity magnetic seperator (WHIMS). The model is P40. I leave some details down on the pictures below. We were able to measure 15000 Gauss on the rotor and grooved plate boxes part with a Gaussmeter. What I need to understand now is that what if I want to expand the rotor diameter from 400mm to 1000-1200mm? How would that affect the coils on the machine? Will I need more windings causing the wiring diameter to be bigger? Just trying to make sense on the whole operation about magnetic seperators here. I also calculated the turns with the given calculators and here are the results; enter image description here

Also the data from the catalogue;

enter image description here

General image of the Jones P40;

enter image description here

Does these makes sense for the given magnetic ? All help would be appreciated.

  • \$\begingroup\$ I would ask the "nearby laboratory" for those details, assuming they made it or had it made. \$\endgroup\$ Oct 1, 2021 at 15:45
  • \$\begingroup\$ @PeterBennett I can only assume "nearby laboratory" means a competitor. \$\endgroup\$
    – DKNguyen
    Oct 1, 2021 at 15:50
  • \$\begingroup\$ @PeterBennett It's from 1991 and writings on the machine is German so we're talking about foreign company. \$\endgroup\$ Oct 1, 2021 at 15:55
  • \$\begingroup\$ @DKNguyen It's been brought from a university in other city. It's been brought for R&D purposes. \$\endgroup\$ Oct 1, 2021 at 15:56
  • \$\begingroup\$ Well, good luck. Mystery coils are a major PITA. \$\endgroup\$
    – DKNguyen
    Oct 1, 2021 at 15:57

1 Answer 1


Turn that inductor into a transformer.

Wind a secondary winding over it.

Apply a low voltage (AC) to the solenoid ( Primary.)

Measure secondary voltage. That will get you very close for the number of turns. (Transformer equations)

Do the transformer test using the "unarmored" unit. It looks like the armor you mention is a steel casing around the solenoid (coil).

  • \$\begingroup\$ If it can't be turned into a transformer then this is a pretty unreasonable task without an X-ray or cutting it up. I don't know if a time-domain reflectometer would be any good either. Can they detect length through all the interwinding-capacitance and inductance? In any case, equipment unavailable to most. \$\endgroup\$
    – DKNguyen
    Oct 1, 2021 at 15:52
  • 1
    \$\begingroup\$ @DKNguyen I think two windings, mutually coupled constitutes a transformer. Maybe not great coupling, but with secondary open the voltage measurement should do well. \$\endgroup\$
    – Marla
    Oct 1, 2021 at 16:00
  • \$\begingroup\$ Is it pointless trying using the Inductance formula? I am at lost sorry If I'm talking nonsense. \$\endgroup\$ Oct 1, 2021 at 16:24
  • \$\begingroup\$ @MertÜnlüsoy . You could measure the inductance and compare with what you calculate. I don't think you are going to get exact results no matter how you proceed (non-destructive testing). \$\endgroup\$
    – Marla
    Oct 1, 2021 at 16:28
  • \$\begingroup\$ @MertÜnlüsoy For an inductance based method to work you would basically need to model everything else about the coil: dimensions, materials, and all. And...either wire diameter or number of turns to find the other one. Doh. \$\endgroup\$
    – DKNguyen
    Oct 1, 2021 at 18:03

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