Stumped on Single Phase Motor

Discussion in 'General Electronics Chat' started by kewlrunnings, Dec 30, 2011.

  1. kewlrunnings

    Thread Starter New Member

    Dec 30, 2011
    5
    0
    A single phase motor question here where I’ve been stumped on the configuration of the power supply.

    I recently came across a 220V single-phase fan motor. It had a 16 uF (400V) cap installed and had 3 wires connected to the terminals.

    From my lack of experience with single-phase motors, I always thought that single-phase motors needed 2 supply wires and not 3, so you can see where I am stumped.

    It didn’t help that the cables were not labelled so I disconnected the 3 wires from the motor and measured the supply between them. Cable 1-2 = 220V, Cable 2-3 = 220V, Cable 1-3 = 0V, however when I measured the supply with respect to earth, each wire had a potential of around 120V. Meaning Cable 1-3 were the same phase, but I tested this by turning off the power and checking the continuity between Cable 1-3, much to my surprise there was no continuity. Wondering what is happening here.

    I have attached some pics of motor details and the final connections I made to the motor to get it running at the rated speed.

    I was curious to see the behaviour of the motor with only 2 supply wires connected so I connected ‘Cable 1 = U1’ and ‘Cable 2 = U2’ with the links up and down. The motor turned however not at the rated speed. I finally connected the motor as I first came across it, with ‘Cable 1 = U1’, ‘Cable 2 =U2’, ‘Cable 3 = Z1’ with the links up and down and voila back to rated speed.

    It’s been awhile since I have been in class and the fundamentals of single-phase motor with a start-cap, has faded from my memory a bit. If someone can lend me there expertise I will definitely be grateful and pass on the knowledge.
     
  2. Geoffr67

    Member

    Dec 5, 2011
    69
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    im interested to know why as well. Ill be watching this thread!
     
  3. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Recheck your work--this is a basic permanent split capacitor motor that requires only 2 power leads. My guess is that lead #3 is not connected to anything and is going only for the ride--try disconnecting #3 again and make sure that the link going to Z1 is tight.

    Question: Is the 16uf capacitor intact? or is lead #3 connected to an external capacitor somewhere else?

    Question: Does the application require reversing via an external switch or contactor?
     
  4. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    When I see 200-240 power. Power for this motor needs to originate from a double breaker in your power panel.
     
  5. kewlrunnings

    Thread Starter New Member

    Dec 30, 2011
    5
    0
    Follow Up:

    I slept on this problem and woke up this morning with a revelation that jimkeith mentioned, if there is an external capacitor connected. Final answer is yes. There was an external capacitor connected which justifies the mysterious 3rd cable. Some logical sleuthing around and discovered that this motor has been replaced, without disconnecting the capacitor of the original motor which I found to be in a separate control cabinet not by the breaker where the motor is fed from. My guess is whoever connected this motor, figured that if the original motor had 3 wires connected, this motor should too, without realizing the capacitor for the failed motor is in the control cabinet.

    Before I go on, I should correct myself in my original thread, where I mentioned the motor is running at rated speed. Since the external capacitor was connected, the motor actually ran faster, which I assumed was the correct speed. However the motor was incorrectly connected and the external capacitor was removed, with Cable 3 disconnected. Power was put back on and the motor ran slower, when connected correctly. Also the load was halved, from 3.3 A to 1.3A. You can only imagine how hot the enclosure was.

    I have attached a simplified circuit diagram of how the motor was connected with the external capacitor (7uF).

    The question I want to know is why does the motor run faster with this external capacitor connected? Clearer it is not a proper motor circuit but puzzled me to find the motor operating this way at a much faster speed.

    Thanks in advance for all your responses.
    [​IMG]
     
  6. roadey_carl

    Active Member

    Jun 5, 2009
    116
    5
    Maybe it has something to do with there being 2 capacitors wired in parallel allowing more current to flow though? e.g. 16uF+7uF = a 23uF cap..... the bigger the cap, the more current? I don't know, its just how I image it.
     
  7. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    My guess is that with the phase difference between the stator windings altered, it is operating as a 2 pole rather than a 4 pole motor. Analysis of the magnetic circuits would probably be a witch's brew...

    Two speed motors were sometimes used in old washing machines, but they have multiple stator windings that are switched in or out. The best washer I ever had was a 1970 vintage Maytag with a two speed motor--wish I still had it as the currently made junk cannot compare...

    Perhaps you have discovered a new two speed technique--call your patent attorney NOW!

    Glad to hear that you got it working.
     
  8. kewlrunnings

    Thread Starter New Member

    Dec 30, 2011
    5
    0
    Its actually parallel to the coil and capacitor, but I am not definite that it is exactly parallel because there is a cable supplying voltage between the coil and cap.

    Not sure what to call it, and the math, I didn't even imagine what it would look like.

    I figured someone else can muster something up and make sense of why it was running faster at max load.
     
  9. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    single cap run motors do have some speed/voltage variability, so your supply impedance is likely altered. Although you posted a schematic, it doesn't indicate how power was connected.

    I a bit confused with your term 'load'. Head is a little fuzzy this morning however. Happy New Year.
     
    Last edited: Jan 1, 2012
  10. kewlrunnings

    Thread Starter New Member

    Dec 30, 2011
    5
    0
    220V Power supply was hooked up to Cable 1 and Cable 2.

    Correction on the load comment, I meant current drawn by the motor.


    Happy New Years everyone!
     
  11. kingull

    New Member

    Jan 1, 2012
    6
    0
    Might be a three phase motor winding where lines go to 1 and 2 and capacitor to two and three. I was in motor tech sales.
    Check ohms accross each way. Equal resistance inndicates three phase design.
     
  12. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    Then you would have had a voltage divider set up between the caps, greatly reducing the voltage to the aux windings.
     
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