What is the use of choke coils on a DC motor

I have a pancake motor for a powerfeed motor on my Mikron WF3SA milling machine It needs a new drive
Now somebody is mentioning that it should have 2 choke coils in between the motor and the drive
Whats its use and how to calculate ???
A picture of the motor name plate

The inductors would cut the possible spikes, and more importantly for many applications, will smooth the current and reduce torque pulses, so there is less vibration. Pancake motors, depending on exact type, are often low mass rotors, and can pass through torque variations that are due to current variations.

They have to not saturate at the max current, and they need to have sufficient inductance to smooth the current "enough" for the application.

What size does that well enough depends on the motor, the drive type, and the application. The most troublesome drives are generally SCR types (including triacs), which produce large spikes and very discontinuous current. The least trouble are usually PWM types, which often include internal "spike suppressor" inductors.

If inductors are required, there often is some indication of the values required.

What sort of drive do you have?

Chokes are often used on DC motors to suppress brush noise. If you've ever seen the sparks that brushes make when they run, those sparks emit broadband electrical noise. It can be bad for nearby electronics. The chokes don't get rid of it, but keep it down a bit.

I've never used a pancake motor, so all of JST's comments could be correct, and maybe a consequence of a very low inductance motor. For brush noise, just about anything works, such as a couple of turns of the motor wire through a ferrite bead, for what JST is describing, and more substantial inductance is required.

I have never done any rigorous testing of the brush noise thing, but include it as "best practices" after having spent a lot of time diagnosing noise problems near motors. Other people also use a capacitor in parallel for the same purpose.

I have not decided o
Originally it had a BBC axcodyn drive without chokes
I am looking for a Unitek Also looking into this one
https://www.kb-controls.com/product.sc?productId=79
The cheap version with wich we have some experience with a siemens permanent magnet motor With that combination the lowest speed is problematic
Peter

How does one find the right sized choke rating?
amps obvious but correct or optimal mh?
Scope?

It's to help with commutator notching. Usually the reactors (chokes) are built in to the drive - at least from the ones I dealt with. I know this is a phenomena in 3 phase drives, but I think it really does not apply on single phase dives, but not really sure. Would have to think about that some more.

PMDC motor drives come in a variety of control archetectures, the least energy-efficient is a fully linear design (like a Class A audio amplifier)... and most efficient (power wise) being the most noise-generating switching-types.

The permanent magnet DC motor's impedence character changes with armature angle, brush position, torque load, and speed...

Most times, for the motor controller to be 'stable' under load variation means the controller has to be able to get a pretty accurate clue of what the motor's current really is, and this is frequently done by the controller having a feedback loop reading the drive's output... but this depends on the signal actually being very 'clean'... so switching noise, both in the motor's solid state, and also in the DC motor's brushes or slip rings... is problematic... so inductors and capacitors outside the motor casing helps snub out that racket.

If you take the drive apart, you'll probably find very similar, if not identical RF chokes on the drive output, just prior to departing the drive, so that transmission span in between is usually pretty 'clean'... and it's frequently just past the drive's output chokes... that the feedback readpoints are connected.

While the various external effects can be important, that is likely not the main issue. A powerfeed is intended to smoothly move the table. If the tqble does not move smoothly, the finish may suffer. Many DC motors suffer from pulsing when run from rectified DC at slow speeds using the typical SCR (or triac) control. These controls provide the power as pulses at a rate of 120 Hz.

When at low speed, the average voltage is low, and the pulses are short. The current flows during the pulse, and stops soon after the pulse is over. The result can be "cogging", a pulsing of the movement. An inductor can smooth out the current, extending the conduction time so that it is closer to continuous DC.

For instance, the typical older treadmill had a DC motor, and SCR type (triac usually) control. They often drew about 10A maximum, and had a series inductor in the range of 10 mH or so. Obviously, a treadmill that was jerky and not smooth at lower speeds is very undesirable.

I do not know what the people who are suggesting the inductors are intending to solve with them. The inductance of the choke is determined by what they are for. If they are for interference, a very low value will work. If they are for reducing the "cogging", a considerably larger value will be needed.

Selecting an inductance value is going to mean understanding just what the inductor is supposed to do. And it will depend on what sort of drive is being used. We do not seem to know what the goal is here, nor do we know what type drive circuit is being used. There really is no answer without that knowledge.

Obviously, the motor speed is determined by the feed rates, feed screws, and reduction ratio. It may be more useful to determine the ratio of slowest to fastest feed (including rapids), which will determine the slowest speed the motor has to run at. There may be no real issue of "cogging", which typically happens only with very slow speeds.