Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question on 3 phase motor comparison

  1. Feb 21, 2008 #1
    I'm not sure where to post this one but i'll start with the mechanical guys first.

    There are two different motors that share identical ratings but different sizes physically. They are represented as though they perform the same work, share a common HP and KW rating and the mechanical device they each drive are twins in every way, however one electric motor is much larger than the other (in terms of stator and rotor). The mechanical interface is smaller for one than the other (they are both flange-mount).

    My question is this, how can they both be the same horsepower and kw rating? I thought the smaller of the two, given the work is the same, would have to be of a lower hp than the bigger brother. Am I incorrect?
  2. jcsd
  3. Feb 21, 2008 #2


    User Avatar
    Science Advisor
    Gold Member

    There isn't really any fundamental reason a smaller motor will be less powerful than a larger motor. Just look at the advances in motor technology in the last 50 years- motors have gotten smaller and more powerful at the same time.

    That being said, it is possible that the two motors you are comparing have different torque curves and stall torques because of their differences in geometry. For example, if the larger motor has a higher stall torque but lower maximum speed, it's peak power output could still be the same as the smaller motor which may have a lower stall torque but a higher top speed.

    It's really not possible to tell if the motors are indeed excatly the same without more information about the specific motors you are referring to and their desired application.
  4. Feb 21, 2008 #3
    When I say they perform the same work they have identical mechanical outputs in flow rate for air, maximum pressure achieved and the mechanical devices they are attached to are identical in terms of all aspects with the exception of the flange mount. This is due to the described size difference of the electric motor that is driving the respective devices. The shaft size of the rotor, the bearing and even the impeller are identical. The housing containing the impeller is as well the same for both (excepting the flange interface point).

    In description the motors have identical Voltage, RPM, AMP, KW and HP ratings on the nameplate. As I said on paper they are twins but the motor housing, stator and rotor for one is very much larger than its supposed equal.

    My questions stem from my dealings with a company that quoted both to me at different times, with the same verbage for each in terms of performance almost to the letter. The visual appearance is so uncanny that if they weren't placed next to one another you would not be able to tell the physical difference; they look identical. They are billed as such but fail to perform as such.
  5. Feb 21, 2008 #4


    User Avatar
    Science Advisor

    No, the physical size doesn't determine the rating of the motor. There may be some indirect/coincidental relationship in a general sense, but that is not how motors are rated.

  6. Feb 21, 2008 #5
    So how are they rated?

    If that is they case why waste all that copper on windings if it can achieve the same thing with less materials?
  7. Feb 21, 2008 #6


    User Avatar

    Staff: Mentor

    Well for starters, one might have thicker wires than the other. The internal geometry may be different. The materials may be different.

    There are a host of physical differences that can affect the appearance, but still allow you to end up with the same horsepower. It isn't any different from how you can have two different looking car engines with the same horsepower. There are just different ways to skin the same cat.
    Last edited: Feb 21, 2008
  8. Feb 21, 2008 #7
    So how come one lasts longer than the other or maybe I should say how does one verify the ratings?
  9. Feb 21, 2008 #8


    User Avatar

    Staff: Mentor

    Well some of those things that may or may not affect the mechanical performance also may or may not affect other operating parameters - such as temperature tolerance or efficiency. About the only way to verify the ratings, though, is to test the motor. And buy a quality motor with a good warranty. Electric motors, though, are usually pretty reliable.

    Are you having an actual problem with a motor you need help with? Maybe some more specifics would help us help you.

    [btw, you mentioned different shaft diameters. That's also a common physical difference that has nothing to do with the power rating. Browse the motors at grainger.com - you'll find that there are a lot of common/standard optional physical features of motors. Ie, you can get the same hp motor in different shapes, with different shaft sizes, etc.]
  10. Feb 21, 2008 #9
    Thank you for the response; yes I am having trouble with a motor supplied to my company that hasn't performed well. I'm hesitant to go into too many specific details regarding the players but am planning on having the motor Dyno tested.

    I mentioned shaft diameter as a function to illustrate that the shaft is identical so the impellers among both mechanical devices can remain twins yet still facilitate a smaller actual rotor and stator width.

    Long story short:

    Regularly purchased many motors from a company for end-use on a manufactured product we sold

    Unit was of German manufacture up to DIN Standards

    Performed excellent considering conditions less-than favorable

    German manufacturer sold to larger concern

    Larger concern purchases most (if not all) with similar physical, electrical and mechanical properties as original units purchased

    At my customer's request (due to unit cost, etc) inquired about alternative replacement with the same characteristics to Large Concern (I should be calling them unconcern)

    They offer up motor with the exact same operating parameters as what we were buying before at a reduced cost

    Performance of alternative is dismal.

    (you now get three guesses as to the origin of "alternate" and the first two don't count)

    The size difference is no joke; in fact when I had the head of their engineering department out to my facility I placed the motor's next to one another and he remarked "holy S***". I said you build them both and didn't ever look at them next to each other?

    Asked for how the ratings were established and proof of same...........nothing; won't and haven't said and I've been told not to expect to get one.

    There are many, many other issues that would be relevant in the treatment and handling of this situation that basically serve to squash the theory that this country is moving into a "heavily service oriented" direction since customer, service and support are obviously not present once the check clears.

    Would the Dyno provide the uniform, standard baseline for comparison to an applied nameplate and promise to provide?

  11. Feb 22, 2008 #10


    User Avatar
    Science Advisor

    Why bother with a dyno? The manufacturer had better be able to provide a power curve for you. That should already be done. If that's not the case it's time to look for a new motor supplier. There are a ton of them out there.
  12. Feb 22, 2008 #11
    They are sole manufacturer of a unique style and purchased all competition previously making similar product. In any case we have reverted to purchasing the original motor. I am trying to examine things as they are in reference to the Uniform Commercial Code and how that may relate to the resources spent on a misrepresented product. I need to know what is provable and what is not. The details of my scenario are easily identified which is why I was cryptic to begin with.

    As I stated earlier I have repeatedly asked how the imported motor can even get in the country without some validation that would exclude it from the DOE efficiency standards; you would think a record of the ratings would be on file somewhere. Since they say no and tell me they don't have one and only offer the IP enclosure rating it sort of confirms what I suspect - the motor doesn't meet the nameplate. Also, these guys are BIG, really big so being David isn't fun but right is right. I aim to find out if I am and thought some ME's could add some insight that aren't directly involved.

    Further validation comes in the form of an insulting offer at a fraction of the liability if I'm right. Although the offer in and of itself doesn't admit fault, a condition of acceptance is that I stop researching the issue. I have some other engineering realted issues exclusive of the ratings that I used an independent lab to check up on; the results were not good for the supplier so they know I'm willing to find out when they won't. Stopping further "research" regarding the problem went a touch too far with me and their request doesn't pass the smell test.

    If it weren't for their utter arrogance in treatment I might have a different attitude; I'm small compared to most out there but my stock-in-trade is principle. The money is already gone and I'm still in business so what I do now is not for the money, it's for the point.
  13. Feb 22, 2008 #12


    User Avatar
    Science Advisor

    The UCC is only applicable in the US. If you are importing your motors, as you have stated, then there is no point in researching the UCC.

    You may certainly employ another manufacturer to design you a motor that meets your desired specification. What is so special about your specific motor?

    The design calculations will definetly be on file somewhere. They probably just don't want to give them to you.

    How are you determining the motors do not meet the nameplate data?

  14. Feb 22, 2008 #13


    User Avatar
    Science Advisor

    Yes, you can get a baseline by using a Dynomometer (along with some other test equipment). Keep in mind that the nameplate data will be the cacluated values, and probably not emperical (i.e. expect some deviations).

  15. Feb 22, 2008 #14
    They import the motors and as part of an assembly. My transaction took place in the U.S.

    There are several reasons - it's not just a "motor" it's a motor they apply to something and sell to me; I then apply it in a system. There are many, many of this style in service today as both the old style, the alternate and now back to the old anew. I can't just arbitrarily make design changes; there are many factors beyond my control, logistical concerns, physical restrictions and FAI's already submitted, approved and catalogued. Before I can properly address my situation with my customer I need to have my ducks in a row.

    There are excessive failures relative to the motor first supplied which matches pound-for-pound the alternate when quoted save one characteristic that should extend the life but isn't - insulation. Field-time is not agreeing with the ability of the two twins to act the same. So in the field we have a smaller stator that has a better insulating factor than the old-style. My logic says that since they perform the exact same function the small stator is already losing in terms of material to mitigate heat generated and handle other factors. The old-style still out-perform the alternate and the observation of winding failures are now seen too often, heat is a contributor (in reference to the alternate; I've still yet to see an old-style with the alternates ailments).

    Add to the mix that when we were quoted originally these guys were in the process of merging and buying each other like mad and I really don't think they have what they need to prove I'm wrong. Why not give it to me? That seems a bit childish after I spent a substantial amount of money with them; paper's cheap.
  16. Feb 22, 2008 #15


    User Avatar
    Science Advisor

    It's still just a motor...no matter how you use it. Therefore, you can certainly provide an alternate motor to use in whatever your system is that has the required specifications.

    What is the failure mode you are experiencing in the field (i.e. how is the motor failing)?

  17. Feb 22, 2008 #16
    The failure mode of the smaller is burnt windings; not a failure we have seen or are seeing with the older-style then or now. I am still baffled about the fact that the mechanical-driven portion of the device can be exactly the same for both, have exactly the same flow-rate, max pressure achieved, RPM and physical size yet one motor is at least 25% smaller on the electric motor and a higher insulation factor yet they are rated identical. I thought the windings were a function of distance.......if the size of the wire is larger in one than the other wouldn't the ability to dissipate heat be greater in the larger wire size and thus a higher rating would be achieved? Add to that the the smaller has better insulation consuming more winding space with less copper again.
  18. Feb 22, 2008 #17


    User Avatar

    Staff: Mentor

    I mentioned this possibility before and it sounds like a stronger possibility now. If the new motor is smaller because the wires are smaller, the wires will have a higher resistance and generate more heat and as a result, overheating failure will be more likely.
    Depends on what "rating" you mean. They get rated for temperature as well as horsepower. So they may have chosen to derate the temperature tolerance while keeping the performance the same. Do you have the manufacturer's temperature tolerance and heat dissipation (really just motor efficiency) data?
  19. Feb 22, 2008 #18


    User Avatar
    Gold Member

    Once in my past i priced copper wire, and was given different prices for what seemed the same wire, it was found that the thermal resistance of the insulation made a big difference, but then the purity of the wire was different in how it was made.

    One of many links that might help, and if the effort is worth the expense, you might have the copper analyzed.

    Cheap copper might be the best way to reduce cost of a product. Check the summary at the end of the page.

    http://www.copper.org/innovations/1997/12/wiremetallurgy.html [Broken]

    Last edited by a moderator: May 3, 2017
  20. Feb 23, 2008 #19


    User Avatar
    Science Advisor

    Sounds like the motor is being overloaded.

    As the developed torque in the motor increases it approaches a breakdown point. Beyond that it will decelerate to a locked rotor condition. As the load on the motor, and thus developed torque is increased, the rotor current increases as well. If the motor is allowed to continue to operate beyond or at the breakdown torque, it will burn out.

    Essentially, the locus of rotor current is a function of the slip. As the slip approaches unity, the rotor current increases. Typically, motor slip is between 0 and 0.03.

    Slip is defined mathematically as,

    [tex] s = \frac{n_s - n_r}{n_s} [/tex]


    [tex] n_s [/tex] is the synchronous speed
    [tex] n_r [/tex] is the rotor speed

    If you are indeed using the motor as it is rated, then it is rated incorrectly. You can verify the motor rating using a dynamometer and a multimeter.

    As Russ pointed out, motors are also given an ambient temperature rating. If you are operating the motor beyond the temperature rating, it will adversely affect the motor as well.

    To determine the efficiency of the motor,

    [tex] \eta = \frac{P_{shaft}}{P_{in}} [/tex]

    The mechanical power is determined by,

    [tex] P_{mech} = \frac{T_D \cdot n_r}{5252} [/tex] (Hp)


    [tex] P_{mech} = \frac{3I_{r}^2R_rn_r}{sn_s} [/tex] (W)


    [tex] T_D [/tex] is the developed torque
    [tex] R_r [/tex] is the actual resistance per phase of the rotor windings
    [tex] I_r [/tex] is the magnitude of the rotor current

    Note that the rotor resistance is dependent on the length, cross-sectional area, resistivity, and skin effect of the rotor conductors (and possibly any external rheostat resistance if the rotor is the wound type).

    To determine the shaft power,

    [tex] P_{shaft} = P_{mech} - P_{f,w} - P_{stray} [/tex]

    or simply

    [tex] P_{shaft} = \frac{T_{shaft} \cdot n_r}{5252} [/tex]

    Hope that helps.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook