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While disassembling the power for the spindle of my hobby CNC mill, I came across something peculiar which I think warrants a question on this site.

The input is standard 220V AC.

  • Phase 1 (brown) from source is fed directly to spindle phase 1.
  • Neutral (blue) from source is fed to transformer primary coil in.
  • Transformer primary coil out is fed to spindle neutral.

Please see photos below.

gray cable to right is source, black cable to left is spindle, bottom gray cable is transformer

  • Gray cable to right is source
  • Black cable to left is spindle
  • Bottom gray cable is transformer

Only connectors for primary stage on this side and name is "drossel" (throttle?)

  • Only connectors for primary stage on this side
  • Name of this transformer is "drossel", which according to google means "throttle"

No connectors here (or anywhere else for that matter)

  • No connectors here (or anywhere else for that matter)

So it appears that the transformer has only a primary coil and no secondary. In other words, it is functionally equivalent to a giant coil.

My question is, what purpose does this transformer serve?

My naive guess as a novice is that it is used as some kind of filter or storage, like a capacitor.

Any hints are welcome!

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  • \$\begingroup\$ Autotransformers have only a single winding. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 27 '18 at 21:53
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    \$\begingroup\$ @IgnacioVazquez-Abrams: but an autotransformer will have more than two connections to its single winding. \$\endgroup\$ – Peter Bennett Jul 27 '18 at 22:47
  • \$\begingroup\$ Could you please draw a schematic of the circuitry and add it to your post? \$\endgroup\$ – try-catch-finally Jul 28 '18 at 8:10
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It's choke, a.k.a. an inductor. The nameplate says 125mH 6A.

From inspecting the wiring, only two power wires go to the spindle, so it's single-phase and the inductor is in series.

Its purpose is to limit the current that can flow to the spindle motor; this limits the torque the spindle motor can develop. It also much reduces the peak current to the spindle motor when starting or stalled.

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It is also translated as choke.

If your motor is 3-phase then this is most likely used to create phase-shift. Kinda ad-hoc starter for asynchronous motor.

enter image description here

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  • \$\begingroup\$ Don't forget the question, what purpose does this transformer serve? \$\endgroup\$ – KingDuken Jul 27 '18 at 21:39
  • \$\begingroup\$ @KingDuken Better? :) \$\endgroup\$ – Maple Jul 27 '18 at 22:08
  • \$\begingroup\$ When you need a high reactance small capacitors work, but when you want a low reactance (i.e the currents are fairly high), you need a big capacitance. Big capacitors are much more viable today than they were in the past, which is probably why a choke is being used when today a motor run capacitor might be. \$\endgroup\$ – Henry Crun Jul 27 '18 at 22:32
  • \$\begingroup\$ @HenryCrun Sure, single capacitor can do the same. Although nowadays both kinds of hacking can only raise the eyebrows. \$\endgroup\$ – Maple Jul 27 '18 at 22:40
  • \$\begingroup\$ nice idea, but it's not wired that way. there only 2 power wires going to the spindle motior \$\endgroup\$ – Jasen Jul 28 '18 at 3:05
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This kind of "drossels" or chokes have been used in massive C-L-C filters after standard AC rectification in old power supplies for tube-based equipment, to filter out 100 (or 120) Hz component in power rail. At that time (60-th and 70-th) there were no affordable means (linear stabilizers) for high-power supplies, so the massive chokes were used instead.

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    \$\begingroup\$ There are other advantages. By evening out the current demands they reduce the load on the rectifier and the transformer. \$\endgroup\$ – user207421 Jul 28 '18 at 1:13
  • \$\begingroup\$ @Jasen, yep, you are right, mental blip happens, sorry. Corrected. \$\endgroup\$ – Ale..chenski Jul 28 '18 at 3:26

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