For our senior project we are making an electronic door lock controlled by a raspberry pi. We created a proposal which was approved for all the specs for creating the device which we have to abide by. In our proposal we specified that we would use a DC Motor for a broad range of options.

We found that a solenoid was a great way to accomplish our project. However when we asked for approval our advisors repsonse was:

Not sure how a solenoid is a DC motor. Please explain.

So I looked up the definition of an electric motor which is:

An electric motor is an electric machine that converts electrical energy into mechanical energy

Which I think fully describes a solenoid because a solenoid works by using current through the coil (electric energy) which creates a maganetic field which creates linear motion (mechanical enegry) to move the metal post.

Would you agree or disagree with this assessment?

  • 2
    \$\begingroup\$ The definition you quote is from wikipedia. If you are looking to convince your advisor, I would strongly suggest finding a better source. \$\endgroup\$
    – us2012
    Aug 7 '13 at 13:35
  • \$\begingroup\$ Agreed but this was just a quick google search, I have a physics text book I can use as a source. \$\endgroup\$
    – Nick
    Aug 7 '13 at 13:40
  • \$\begingroup\$ As your advisor, I would disagree with your assessment - a solenoid is not what you meant when you wrote DC Motor in your proposal, even if it absolutely is the best way to control a door lock. \$\endgroup\$
    – medivh
    Aug 7 '13 at 13:47
  • \$\begingroup\$ When creating the proposal we used the term "DC Motor" as a broad generic term to allow for a range of options so we weren't limited for just this reason. \$\endgroup\$
    – Nick
    Aug 7 '13 at 14:03
  • 1
    \$\begingroup\$ If your project supervision is not allowing you to switch to an alternative which you have found to be superior to what you had originally planned, something is seriously wrong with the process, in a way that makes the answer to your question irrelevant. \$\endgroup\$ Aug 7 '13 at 14:34

You are on thin ice. I agree with your advisor.

What you should do is go back to your advisor and say that now that you have gotten further into the design, experimentation, and testing, that you feel it is more appropriate to use a solenoid instead of a motor. Depending on what lesson he is trying to teach, he might say that's fine, just revise your documentation accordingly, or no you can't do that since a motor was already specified and it's too late to change that.

If the second answer, suck it up and use a motor. Then learn from that to not write tight specs next time until you have done proper design, experimentation, and testing to know what exactly to spec. Writing specs properly is not trivial, and may in fact be what he is trying to have you learn. This sounds like a good lesson that early on in the process you spec what needs to be accomplished, not how. The how part happens later as part of the design.

  • \$\begingroup\$ I completely agree and the lesson he is trying to teach us what you had mentioned at the end of the post about specing what not how. I posted to get some opinions on matter to make a more educated response in hopes that my advisor would allow for the change. \$\endgroup\$
    – Nick
    Aug 7 '13 at 15:17

Let's put it the other way around: why would you call the solenoid a DC motor? Although you are definitely right in saying that by some definitions the term accurately describes one, most people (likely including your advisor) have a mental image of this when you mention a DC motor:

DC motor

Most people would not consider the broader definition you intended.

If you absolutely must keep the specification of the attached device broad, you could use the term "electromagnetic actuator".


Most people would think it has to be rotational mechanical energy, but that's not true. A linear induction motor has linear motion just like the solenoid. I think the difference is that a motor has a continuous motion, either linear or rotational, or at least pseudo-continuous, like for instance a stepper motor.
A solenoid is just on/off. (Also note that according to your definition a relay is also a motor.)

  • \$\begingroup\$ A linear motor will have a limit to how far it can travel, as would a solenoid; I would posit that what makes something a "motor" is the ability to repeat a "cycle". I would not use the term "motor" for a row of 64 independently-wired coils, but would use the term for a motor with 64 coils wired "ABCABCABC" etc. A solenoid wired with a ratchet to advance a wheel might be called a motor, but the solenoid itself would not. \$\endgroup\$
    – supercat
    Aug 7 '13 at 15:02
  • \$\begingroup\$ @supercat: then my linear motor isn't a motor? What would you call it then? (I think "linear motor" is a rather common term.) \$\endgroup\$
    – Johan.A
    Aug 7 '13 at 15:06
  • \$\begingroup\$ @Johan.A that it's a common term further underscores that an otherwise unqualified "electric motor" means a rotating motor. The linear is necessary precisely because it's not the canonical sense of the term. Merriam-Webster defines motor as "a rotating machine that transforms electrical energy into mechanical energy". \$\endgroup\$
    – Phil Frost
    Aug 7 '13 at 15:16

Your reasoning is flawed. Blue is a color. Does that mean all colors are blue?

An electric motor is an electric machine that converts electrical energy into mechanical energy. But so is a solenoid. So are piezoelectrics. So are speakers. So are railguns.

You can, if you want, argue that the definition of motor is sufficiently broad to include any device that produces motion. After all, it comes from the Latin movēre, meaning "to move". Of course, no one will understand you, because that's not what most people understand "electric motor" to mean.

XKCD comic

  • \$\begingroup\$ A solenoid is not a linear motor. It is a switch. It does not convert energy: It swtiches between two stationary states. It does not produce motion: it switches between two stationary states. Apart from that, everything you write is correct. \$\endgroup\$
    – david
    Aug 8 '13 at 8:16
  • 1
    \$\begingroup\$ Surely it's an actuator - not a switch! We're talking about an electro-mechanical solenoid with a moving, driven element - not just the coil. It moves so it does convert electrical to mechanical energy. \$\endgroup\$
    – Transistor
    Dec 6 '15 at 10:00

Firstly, a parts list is not a specification. In fact, it is reasonable to develop your specifications even before you select part number 1.

Despite that, this is a semantic argument that you're embarking on with your instructor, and certainly doesn't bode well for you. If when you wrote your document, you actually meant solenoid and thought that "DC motor" included solenoids, then you didn't communicate effectively. Effective communication is one of your most valuable engineering skills, and it sounds like this exercise is teaching you this. If I had an engineer working for me that sent a semantic argument like this on my desk, instead of explaining to me how this change would help me solve my problem better, I'd have some issues with his level of professionalism.

Your response should be "Ongoing research showed us that a solenoid will enable us to meet our specifications better than a DC motor", then make your case, show how it doesn't adversely impact your bottom line or your Gantt Chart, show how it will enable you to meet your actual specifications, and ask to use the solenoid. If you can meet your specs with a DC motor, and the change to a solenoid is at all inconvenient, the DC motor may well be the way to go. (As an aside, an engineering team that consistently exceeds specs instead of just meeting them might well be wasting time and money). Maybe your instructor is actually looking at a slightly bigger picture than you are, and for some reason thinks that the DC motor is a better option.

In any case, as one who teaches design, I suggest that you'll have a much better experience if you communicate with your instructor instead of arguing with him.

Note that in our class, we get actual customers with actual problems for our teams instead of just letting students select their own project, mainly to enhance customer interviewing, problem identification, and communication skills. Also, we try to pin down specifications and deliverables, but we realize that in a time compressed 8-month experience, expectations and achievements can differ and flexibility is required. Changes are carefully tracked, though. So far as a change like the one in this question goes, however, identifying an approach through research that results in a positive change before being absolutely committed to an expensive and time consuming option is to be applauded, not discouraged. Of course, we're seeing half a picture here, and this instructors approach might be optimal for this situation.


A DC motor's coil is a rather dense solenoid, is it not? So a DC motor coil can be considered a solenoid since it follows the same magnetic force equations. Just don't say a solenoid is a DC motor.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.