I posed a rather cluttered question earlier, by including the servo motor, which is, as pointed out by several of you, is not a specific kind of motor, but a closed loop control system involving servo mechanism, to control the speed or position of the motor being used(AC or DC).

However, the question of switched reluctance motor vs the stepper motor still remains ON.

All in all, I know is:- steppers generally are run on open loop and have high number of poles to allow for small and precise step changes, while SRM on the other hand requires a position feedback, at the very least; so that current pulses can be switched on to the stator windings in an orderly fashion, to generate some useful motoring or generating torque.

So, if someone have heard of srm and know the proper arguments to differentiate between them, it would really be helpful.

Please, correct me if I am wrong about the difference I have written.

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    \$\begingroup\$ What’s the difference between a banana and a lightbulb. How would anyone be able to reasonably answer that. \$\endgroup\$ – Andy aka Dec 8 '18 at 12:41
  • \$\begingroup\$ @Andy The difference between a banana and a light bulb is easy to explain and answer. What has that reasoning have anything to do with the asked question. I think it is a decent question given the amount of poor information on the web. Have you gone and looked at the Wikipedia page on Servomotor? It does not talk about the motor, just generally the implementation. If you voted down the question then I think you are wrong in doing that. It is a fair question. \$\endgroup\$ – Rudy Dec 8 '18 at 14:10
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    \$\begingroup\$ I haven't down voted your question and you haven't explained your base knowledge on all three motors and applications. So, where would one start beginning to answer your question - we neither know your base knowledge nor have any idea what applications you are considering. This is a question and answer site and YOU are advised to ask answerable questions. Also, I don't see that you've formally accepted any answers to your previous questions so how much do you realistically know about this site (BTW it's not a forum) in your 3 months..... \$\endgroup\$ – Andy aka Dec 8 '18 at 14:47
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    \$\begingroup\$ So, play the game and ask a reasonable question backed up with a few words about your baseline knowledge, and, formally accept a few answers to previously asked questions. Here's my down vote to encourage you down the righteous path. \$\endgroup\$ – Andy aka Dec 8 '18 at 14:49
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    \$\begingroup\$ @user58802 how do you define the term 'servo motor'? \$\endgroup\$ – Jon Dec 8 '18 at 15:05

I've never heared of a Switched Reluctance Motor, but the first line of its wiki is clear: "The switched reluctance motor (SRM) is a type of stepper motor". So those two have a subset relation.

A servo motor is not a motor but module that contains a motor, a position sensor, and a feedback (correction) circuit. I've never seen a servo that contained a stepper motor, but in theory it could. Or it could be any other type of motor.


  • All three have winding free rotors.

A servo contains some kind of motor. I wouldn't know why it has to be a winding-free-rotor-motor. I think most (hobby) servo's I have lying aound in my lab a simple low-cost DC motors with static magnet, a wound rotor, and a commutator.

The SRM wiki shows a winding-free rotor. I think the steppers I have seen all had a winding-free rotor for practical reasons, but I don't think it is a must.

  • Servo Motor and SRM, both require position feedback in some form to run.

A servo has positional feedback built in, so it is a bit weird to say that it needs it to run. Do I require a brain to live? And a servo doesn't run the same way other motors can do: it can only go to a position, often from a limited set of degrees, it can't rotate continuously.

An SRM, being a stepper, can run quite happily without any feedback. In fact, this is the main point of a stepper: if it steps, it will step to a very defined position, so often no feedback is needed nor used.

  • Servo and Stepper have rotor made of permanent magnet, while in SRM, it's just a salient pole iron piece.

As said, an SRM is a kind of stepper, so it would be weird if they have a distinctive feature. And a servo contains some (unspecified) type of motor.

  • \$\begingroup\$ I suspect they mean 'brushless permanent magnet motor with sinusoidal back EMF' when they use the term 'Servo Motor'. This is what the motion control industry tends to refer to these motors as. It is a rather confusing term though. \$\endgroup\$ – Jon Dec 8 '18 at 14:50
  • \$\begingroup\$ Also SRMs as a class are not stepper motors, but you can get stepper motors that are SRMs (they are quite common). The first SRMs were traction motors for electric tramways. \$\endgroup\$ – Jon Dec 8 '18 at 14:54
  • \$\begingroup\$ As Andy implies in his comments: a sloppy question gets the answer it deserves. For me a servo is a hobby-servo. That might well be a very limited interpretation. \$\endgroup\$ – Wouter van Ooijen Dec 8 '18 at 14:58
  • \$\begingroup\$ Of course one require a brain to live. SRM is a kind of stepper, saying that dosen't make it the same. No literature says so, and SRM shouldn't be made to run on open loop. It always requires \$\endgroup\$ – user58802 Dec 9 '18 at 11:01
  • \$\begingroup\$ a position feedback, so winding currents can be switched precisely. \$\endgroup\$ – user58802 Dec 9 '18 at 11:01

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