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Thread: Motor speed question

  1. #1
    Super Moderator Javaphile's Avatar
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    Motor speed question

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Heres one for all you electrical heads out there.

    For a motor designed to run on 2 phase 208 volts at 50Hz what frequency would it need to be run at when powered by 2 phase 240 volts to achieve the same rpms?


    Java "Inquiring minds want to know!" phile

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    Re: Motor speed question

    Java,

    A two phase motor is a synchronous motor..... that is speed is determined by the number of polls in the motor and the frequency of the voltage applied - unlike the series motors (brushed motors) where voltage also determines the speed....

    So the answer is 50 Hz.... but the motor will develop more power..... and will also run somewhat hotter on 240V...... It might even run marginally faster under full load as there is always a little slippage with synchronous motors.... but that is only a few RPM - unless the motor is very lossy.

  3. #3
    Super Moderator Javaphile's Avatar
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    Re: Motor speed question

    Hhhmmm......Running the motor at 50Hz the roaster sounds like its on the verge of liftoff. :o

    42Hz seems to give a nice speed with plenty of draft but the beans do tend to get thrown hard to the left when dumped leaving me to think the rpms are too high. I cant imagine how far theyd fly if it was running at 50Hz!

    Hhhmmm....I suppose Ill have to rig something up to actually count the motors rpm.


    Java "Arent projects fun?!" phile

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    Re: Motor speed question

    Java,

    When you say two phase - is it a capacitor run motor?

    Most of these have two sets of polls so run at 50/2 RPS * 60 for RPM

    So Id expect about 1500 RPM at the shaft.... in reality a bit less - probably 1400 RPM or so under load....

    Some motors are designed to be more lossy and drop speed more as the load is increased....

    By the way, if running at lower than recommended frequency, the current drawn (and hence temperature of the motor) will increase- might be worth dropping the volts a bit....

    That is a problem for people over here who buy 110V 60Hz appliances with big motors.... a transformer will get the volts right... but the frequency is too low---- hence it gets HOT.... especially as many are run right on their limit- assuming correct voltage and frequency of supply.

  5. #5
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    Re: Motor speed question

    Theres no Caps on this motor and I meant 3 phase, not 2 phase (if there even is such a thing). I also got its voltage mixed up with a diff motor I was looking at using.

    Heres the info from the motors label:

    The name on the motors plate is Georggi Elektro-Motoren App...and the rest is unreadable.

    Other information on the plate is:
    Type: KRD214
    Motor number: 496962 or 495962
    127/220 Volts
    1420 RPM
    50Hz
    200 watts maybe?

    The terminal block has 6 posts in 2 rows of 3 and they are labeled:
    Z Y X
    U V W

    With it wired for 220 3-phase the top 3 posts were shorted together with the bottom 3 posts being used to connect each leg of the 3-phase too.

    Im using a microdrive to power it which converts the 120v single phase to 230v 3-phase with the frequency being totally adjustable from 0 to 240Hz.

    Im just finding it hard to believe that this thing was ever designed to run at the rpms achieved when cranked up to 50Hz on the microdrive.

    Looks like Ill have to do a direct measurement of its speed and see how it compares to the original spec of 1420.


    Java "Always the wondering one" phile

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    Re: Motor speed question

    Yep, there are such things as two phase motors - quite common....

    They use a capacitor to generate 90 deg phase shift (hence the rotating field) and that capacitor is left in circuit - capacitor run - (unlike capacitor start where it is switched out by a centrifugal switch once the motor is rotating). You can even swap the capacitor to the other winding - and reverse the motors direction.

    Three phase is of course much better. 1420 RPM yep, 6 terminals allow you to delta connect (127 volt) or star connect the motor to 3 phase (the way it is now is star connect 240V- each phase goes to the point of the star with the mid point all joined (and are sometimes taken back to neutral (but it isnt necessary)....

    Maybe they just liked spinning the beans - real fast ;D ;D

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    Re: Motor speed question

    Oh, I see.

    --Robusto

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    Re: Motor speed question

    I had a laugh robusto...I love the response....

    Dont worry JavaB I understand....reading all those electrical engineering and mechatronics books do help...

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    Re: Motor speed question

    Quote Originally Posted by Javaphile link=1170193228/0#4 date=1170398179
    Im using a micro-drive to power it which converts the 120v single phase to 230v 3-phase
    Does your micro-drive have a facility to adjust the output terminal voltage at all Java? If so, it would be helpful to reduce the nominal Terminal Voltage down to that stated on the motors nameplate viz-a-viz 220V. Its only a 10V difference I know but it will be stressing the motor insulation just that little bit, which may affect the longevity to some extent. If the voltage can not be adjusted, well not to worry, its just a minor thing but every little bit helps with older equipment....

    Cheers mate,
    Mal.

  10. #10
    Mocha_Zarba
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    Re: Motor speed question

    This motor can be connected in star or delta, with the top 3 post shorted, tells me that itís star connected. Your micro driveís out put is 230 v 3 phase so I believe this motor should be connected in delta. A star connected motor would need 127 volts 3 phase, a delta connected motor would need 220 volts 3 phase to get the 1420 RPM required, In stead of having the 3 top posts connected, you should connect Z & U, Y & V, X & W. this configuration will give you a delta connection. With the micro drive you should be able to crank the pot up a tad to say around 55 to 60 Hz to get the required RPM,. When using a micro drive the motor should have a shielded cable supplying the 3 phase, with the shield connected to earth at the motor end, at the micro drive end, the earth wire connected to an earth connection in the micro drive labeled PE, itís important that is done correctly, it wonít affect the running of the motor but if itís not connected this way, it will generate a lot of electrical noise and interference with inn the 240 Volt supply!! And could cause problems to sensitive electronic equipment close by. Hope this will helpÖ



    MZ

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    Re: Motor speed question

    Mocha Zorba....

    Well star connection is usually used for the higher voltage range and delta for the lower.... (star applies voltage to windings in series where delta it is applied to each winding)

    See: www.inverter.co.uk/wiring.PDF

    And : http://www.usmotors.com/products/ProFacts/1-120-7.htm

    Assuming the inverter voltage Java is quoting is phase to phase and not phase to neutral..... it should be correct.

  12. #12
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    Re: Motor speed question

    The output voltage on the microdrive I have is nonadjustable and is determined by the input voltage Ive been told. So even though the microdrive specs at 230v output it may actually be putting out 240v given that its input is 120v. One more thing to measure.

    In re delta or star when I got it it was wired as it is now with Z, Y, and X being shorted together and with V being connected to neutral and U and W each having one leg of the hot 120v feed. When I was looking at possibly rebuilding the motor to the 60Hz standard I brought it to a rewiring company and they told me that was the correct way to wire it and it has been running that way for well over a year now with no problems.

    The motor appears to be way over powered for the work it does. I have a motor with the same hp rating that runs a printing press where its moving several thousand pounds of steel. The work this motor does in comparision is puny, It only moves perhaps 50 pounds of mass. Power is not an issue with it, mearly its speed. :)

    As far as the temp of the motor is concerned it never gets more than barely warm to the touch, even after 3 hours of non-stop roasting. :)


    Java "Always looking for understanding and perfection." phile

  13. #13
    Mocha_Zarba
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    Re: Motor speed question

    Yea good point JavaB,
    If the output voltage is phase to phase not phase to neutral the star connection would be correct..
    If Javas input voltage is 120 v then the output on his inverter will be 240v . From a Zarba to a JavaB & Javaphile, good luck..


    MZ

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    Re: Motor speed question

    I had a laugh And in the end were right back where I started with my head wrapped around this issue. I was hoping there was something Id overlooked but this discussion confirms what I already thought to be the case. That the speed of the motor is determined by the frequency.

    Now Im afraid Im going to have to do an actual speed test of some kind on the motor. Which is what I figured would be the case. ::)

    Hhhhmmm....Ive got a simple one in mind. I think Ill go give it a shot.


    Java "BBIAB!" phile

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    Re: Motor speed question

    Java,

    Piece of aluminium self adhesive tape wrapped around half way on the shaft..... Infrared LED and photo-transistor as a detector..... and use a frequency meter or measure the period on a CRO (and calculate the frequency) - or you could use a calibrated stobe....

    But a divining rod wont work ;D ;D ;D

  16. #16
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    Re: Motor speed question

    How about a divining zipstrip? ;D

    The motor (pictures and story of the motor/roaster for those not familiar with it here: http://coffeesnobs.com.au/YaBB.pl?num=1121040424) drives a belt which drives a larger pully wheel (which drives the shaft which drives the gear that drives the barrels {all of which Im sure Robusto understood quite clearly! ;D} of the roaster). While I cant keep track of something moving at 1400+ rpm given the size difference between the pully on the motors shaft and the pully on the drive shaft for the motors I thought I might be able to keep up with its rpms.

    Accordingly I attached a medium length zipstrip to the end of one of the larger pully wheels spokes making sure to angle it in the appropriate direction so it would be less likely to flay the skin from my finger.

    I then powered up the roaster and set the microdrive for 40Hz and let the roaster settle down into a constant speed. I then bravely and for the sake of Coffee Scientists everywhere stuck my finger into the path of the madly spinning zipstrip.

    Phew! The thwacking of the zipstrip didnt instantly lay my finger bare to the bone and each revolution was discernable. Even better I could keep up a running count in my head of the times my finger was getting hit. WhooHoo!! Science was marching right along!!

    I then pulled out the handy stopwatch and timed how many revolutions the big wheel made in one miinute. 200. How nice. A nice round number thats easy to work with.

    Now. Given that the pully on the motors shaft had a diameter of 2 1/16" and the larger pully that it was driving was 11 1/2" accross applying basic geometry we find that the drive ratio was 5.57:1. Now that we know the rpm of one of the wheels and the drive ratio between the two wheels we can determine the rpms of the 2nd wheel. In this case the motor. 200 * 5.57 gives us a motor rpm of 1115 @ 40Hz. Multiplying that times 1.25 to adjust to 50Hz gives us 1394. Certainly close enough to verify that the motor does in fact run at its rated 1420 rpm @ 50Hz, or at least is close enough to it to make no practical difference.

    Darn, now I suppose Im going to have to do up a roast or two at 50Hz just to see what happens when I dump the beans.


    Java "Constantly being chased by that darn science bug" phile

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    Re: Motor speed question

    Java,

    Gotta love that science in action ..... with just a tad of ingenuity! ;D

    Well all is well in coffee roaster land..... they are just trying to centrifuge the oils "outa them thar beans." - a novel approach ;)

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    Re: Motor speed question

    Behmor Brazen - $249 - Free Freight
    Quote Originally Posted by JavaB link=1170193228/15#16 date=1170493917
    Java,

    Gotta love that science in action ..... with just a tad of ingenuity! ;D
    Indeed! :)

    Well all is well in coffee roaster land..... they are just trying to centrifuge the oils "outa them thar beans." - a novel approach ;)
    No kidding. As I recall I ended up with a barrel speed of about 60 rpm when running at 40Hz, so running at the full 50Hz would yield about 75 rpms on the barrels. I think Im going to have a nice big deflector shield handy when I dump the beans at 50Hz! Those poor beans are really gonna be flying and Id rather they ended up in the cooling pan than on the floor or embedded into the wall! :)


    Java "Ah the things we do for our cuppa!" phile



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