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!

  • \$\begingroup\$ Autotransformers have only a single winding. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 27 '18 at 21:53
  • 1
    \$\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

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.


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

  • \$\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

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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