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  1. #1
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    Default Converting 3-phase compressor to single phase

    There have been several threads on the forum regarding converting 3 phase
    machinery to single phase.

    In some cases the suggestion was that this was easy, just replace the
    3-phase motor with a single phase motor.

    However, there are traps for the unwary, as my son has just found out and I
    would be interested in suggestions/comments from the engineers/electrical
    engineers/electricians among you.

    My son was give a compressor with a 3-phase motor but does not have 3-phase
    power in his shed.

    He then investigated the possibility of converting to a single phase motor and
    one of the engineers at his work came up with some calculations to indicate
    that this was possible ( see below ).

    He then went out and bought the single phase motor, fitted the new pulleys,
    and everything worked.

    Yippee I hear you say.

    Well not quite, when he ran the compressor with the lights on in his shed (6x2
    4ft fluoros) the lights "pulsed", ie dimmed and brightened, in time with the
    compressor stroke!

    I'll let him tell the story from here:



    Anyway calcs as folows.

    Base Formula:

    Power (HP) = Torque (ftlb) x RPM / 5252

    Therefore

    Torque = Power x 5252 / RPM

    In the case of the Copressor at standard - Fitted motor is 5.5HP, 2865RPM
    with a 85 od pulley
    Copressor pump has a 290 od pulley
    So using Dia*RPM=Dia*RPM to determine Pump speed = 85*2865=290*RPM,
    RPM=85*2865/290 = 839.75RPM
    So Torque at the drive shaft = 5.5x5252/2865 = 10.08ftlb
    Torque at the driven shaft or pump = 5.5*5252/839.75 = 34.39ftlb

    All good so far?

    Converted to single phase motor 3HP, 1400RPM
    Torque at the drive shaft = 3*5252/1400 = 11.25ftlb
    If I was to matain the same pulley ratios I would have 85*1400/290 = 410RPM
    at the pump. This is below the minimum RPM of 460RPM listed on the spec
    sheet and would only give me 10.86 FAD (Free Air Delivery) With that in Mind
    I thought I would alter to give myself around thee 15CFM FAD.

    How Did I work this out?

    Comressor has FAD of 22CFM at 830 RPM according to the spec sheet. Therefore
    22/830*410 = 10.86CFM FAD

    So I went to a 118 OD Pulley on the Drive giving me 118*1400/290 = 569.65RPM
    at the pump. Therefore 22/830*569.65 = 15.1CFM

    With all that in mind, back to the torque:

    3*5252/569.65 = 27.65ftlb (this may be my issue, but considering I still
    have flickering at nil pressure start I think all is in vain anyway.)

    To maintain original torque at the pump I would need as follows

    3*5252/34.39 = 458.15RPM (Say 460 Minimum) which would give me 22/830*460 =
    12.19CFM FAD.

    Hows your head so far??

    Now the electrics:

    3HP motor has a rating of 12.4 Full load amps.

    When I put the Clamp Meter on the active lead, nominal readings ranged from
    about 5amps at nil pressure start to about 11.4 at 110PSI add to that the
    the motor showed no signs of struggle at either nill pressure or 100PSI
    start (Adjusted the pressure swith to test to 100PSI rather than 75PSI
    start).

    All was great I thought! so I forged ahead with the final fitment, taking me
    in to the early evening. That's where all went pear shaped. Lights pulsed in
    time with the compressor. Faint at nil pressure start encreasing intensity
    as pressure and load increased. You wouldn't want to be epileptic!!!

    Free run, Motor runs at about 3.6 amps.

    As the pulsing was in time with the compressor My guess is that the motor
    load is mini spiking with each compression stroke. This I believe is
    confirmed by activating the peak hold on the clamp meter. Readings gained
    now range from 9 to 17+ amps nil to 110PSI.

    I understand the mechanics without a problem, it's the electrics where I
    fall behind. If there are any electrical engineers on the forum that could
    explain where I went wrong, it would be great, even if to put my brain to
    rest!!!

    Shed is powered by a 32amp circuit from the house board
    using a 25m run of 6mmsq orange circ cable.

    At no point did the 20amp breaker for the compressor circuit trip, nor did
    the motor thermal protection. While testing yesterday, I had the compressor
    cycling over and over for about 1/2-3/4 of an hour. Motor got warm enough to
    be uncomfortanble to put hand on but not burning hot.


    If you are still with us this far, he would appreciate any comments suggestions as to either what causes this "pulsing" or how to overcome it.

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  3. #2
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    Default

    Quote Originally Posted by Big Shed View Post
    Shed is powered by a 32amp circuit from the house board
    using a 25m run of 6mmsq orange circ cable.

    At no point did the 20amp breaker for the compressor circuit trip, nor did
    the motor thermal protection.

    If you are still with us this far, he would appreciate any comments suggestions as to either what causes this "pulsing" or how to overcome it.
    Fred, glad to help out but I have a question about the above.....

    You have a sub board in the shed? So a 32 amp circuit in the house board that goes to a sub board that includes multiple circuits?
    With how many circuits?
    Is the light circuit on its own 10 amp breaker? From the text above you are drawing about 2AMPs for the fluros....

    Or is it a mixed circuit?

  4. #3
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    Default

    Warren, sorry for delayed reply, have been out all afternoon.

    Here is my son's reply:

    Circuit as follows 32amp breaker at house board
    Sub board in shed contains:
    All power fed through 63Amp / 30mA RCD to:
    1 x 10A Breaker - Internal Lighting (4 x triple fluro & 2 x single fluro)
    1 x 10A Breaker - External Lighting (None connected at this point)
    1 x 16A Breaker - 4 double 10A Outlets
    1 x 16A Breaker - 2 x double 10A Outlets
    1 x 20A Breaker - 1 x single 15A Outlet (Welder)
    1 x 20A Breaker - 1 x single 15A Outlet (Compressor)
    Just in case, all internal shed wiring is 2.5mmsq braided white flat.
    Length of run to 15A outlets less than 1m as they are mounted on the sub board.


    Not being an electrician or electrical engineer, it has us baffled!


    Thanks for your help


    Fred



  5. #4
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    Default Flicker rate problem

    Big Shed,

    Any chance of getting a measurement of the voltage at the shed board without the compressor running and at the house board for the same condition? Also need the same measurements but for the compressor running?

    I want to see if the problem is with excessive voltage drop in the cable between the house and the shed or combined with the voltage drop at the house. It may be a problem that requires running an independant cable from the house for this circuit. My knowledge on the operation of fluorescent lights is limited but I will try to ask some of the guys in my home office if there is something about these lights that could be part of the problem and could be resolved with changes to them. This may take a couple of days with our holiday on Wednesday.

    I do design large power systems and have to carryout calculations to avoid these sorts of problems but it is typically at startup and not during operation of the motors.

    Cheers,

  6. #5
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    Default

    I know it is a bit different but what about a heavy fly wheel on the motor / compressor to smooth out the load surges?

  7. #6
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    Default

    Hi Big Shed,
    I'm not an electrician but I've been in the air compressor repair business for 34 years.Just a few weeks ago I had a spare pump (Swan ,exactly the same as yours) left over and I figured this would make a nice compressor to use in my woodworking shop at home.The only problem was that I only had a 1.75 Hp motor left over from another job.No problem..I just have to slow it down so I don't overload the motor.So I did some calculations,fitted a smaller pulley to the motor and ran the compressor at a much slower speed. Beauty I thought...nice and quiet. Just what you want at home.But...when I run the compressor in my shop the lights are flickering very badly. And I think this is the problem: Running the compressor too slow. To fix the problem I fitted a 3Hp motor and I'm running the compressor at the speed it was designed for. And the lights??? No more flickering. Or at least it happens so fast you can't see it.

  8. #7
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    Default

    Thanks Walro, that seems to mirror my sons' experience. His problem would be that he can't go to a bigger size motor!

  9. #8
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    Quote Originally Posted by DavidG View Post
    I know it is a bit different but what about a heavy fly wheel on the motor / compressor to smooth out the load surges?
    That is a possibilty, but I would be concerned with the extra load on start up, it is already quite high. Quite a creative thought though

  10. #9
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    Default

    Quote Originally Posted by Blackout View Post
    Big Shed,

    Any chance of getting a measurement of the voltage at the shed board without the compressor running and at the house board for the same condition? Also need the same measurements but for the compressor running?
    Blackout, he has measured 233V at various points around the house, same in the shed with no load.

    With the load it drops to 231-232V, so that appears to be within reason to my unqualified eye?

  11. #10
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    Default

    If you can not put the fly wheel on the same motor put it on another one which you have running at the same time.
    It will idle along until the voltage tries to drop, then it will generate and carry the load.

    Last straw but that is what the snowy scheme does on the grid.

    Apart from that you can use a 50hz tuned cct to electronically do the same thing. Big and expensive choke and capacitors.

    Or change the lights to incandescent.

    Or run the lights from a filtered supply.

    Or run the lights from a battery supply with a charger.

  12. #11
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    Here is some fuel to the compressor fire, this is a reply received from an electronics engineer in response to my sons' question.

    Firstly, I don't think the peak reading that your getting with the clamp meter is a problem, because the nominal rating for the motor will be an rms (root mean square) value. The peak of an rms value, is root 2 (1.41) times the rms value. Therefore 12.4 * 1.41 = 17.5 amps peak at Full Load should be OK.
    I also don't think it will be a voltage drop problem, because they have to install cable that will not drop the voltage by more than 5% (12 Volts) at the rated current. Your well below the 32 amps, so it should be fine. (as a ROUGH guide the max current for wire is about 10 times the cross sectional area, approx 60 Amps.)
    But to check the voltage drop, you can use the following formulas:
    To determine the resistance of the wire:
    R = ρL/A
    where
    ρ = resistivity of the wire material, (the resistivity of copper @ 20 deg C is approximately 1.724 x 10^-8 ohms.m)
    L = length of the wire, and (25 m)
    A = area of the conducting cross section of the wire. [ 6mm^2 = (6 x 10^-3)^2 = 6 x 10^-6 ]

    Therefore the resistance of the wire (R) = 1.724 x 10^-8 x 25 / 6 x 10^-6 = 0.07183 ohms. And I think you need to multiply this by two to allow for the return wire so; 2 x 0.07183 = 0.1436 ohms.

    Voltage Drop, V = I * R, so at the Full Load current the Voltage Drop will be: 12.4 * 0.1436 = 1.78 Volts. Hardly worth the effort really was it??


    I don't think it will be a harmonics problem, because harmonics are usually only a result of distorting the supply voltage. This is more of a problem when you have switching loads, or speed control circuits for motors etc..

    It could be something to do with the back emf produced by the motor?? (motor acting as a generator) As the current changes the inductance of the motor can produce some high voltages. Not sure about this, the electrical side of things is not really my forte. Have you got a meter that you can connect into a power point to check the voltage variations? (I could loan you one if you want) It would be interesting to see if the voltage is increasing or decreasing when the motor is running.

  13. #12
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    This is the point I drag the oscilloscope out the cupboard.
    Pity I am not closer.

  14. #13
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    Hi Big Shed,

    apart from all the calculations (which look very impressive), doesn't your compressor just need 5,5 HP since the original motor had 5,5 HP? In that case, it would stand to reason that a 3 HP motor has to strain very much at every compression stroke and the resulting high drawn single phase current (probably approaching starting or stalling Amp values, which can be six times nominal use) would show in your lighting brightness. Just a suggestion, i wasn't much in maths, you know...

    Regards,

    Gerhard Schreurs
    Netherlands

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