Motor start windings confusion

Discussion in 'General Electronics Chat' started by sherlock ohms, Dec 31, 2015.

  1. sherlock ohms

    Thread Starter New Member

    Dec 24, 2013
    25
    2
    Knowledgeable repliers to an earlier post of mine helped me understand the role of a ptc thermistor in the compressor start windings of a refridgerator.
    But the more i learn, the more i know i dont know.
    Question: in a clothes dryer, the start windings are de-energized once the motor is up to speed and the centrifugal switch opens up and it's just run windings from there on.
    However, on a fridge compressor, the start windins are de-energized once the ptc heats up and resistance ultimately rises to stop the current through the start windings, and similar to the dryer, the compreesor operates soley on the run windins. But won't the thermistor cool back down in a while and thus offer the low/no resistance path again through the start windings when the compressor doesnt need it because it's already running??
    What happens then, or what keeps this from happening?
    Again, many advance thanks to you.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Once the PTC heats up the resistance increases and is kept that way while still connected to the winding.
    Hence constant very low current.
    [​IMG]
    Max.
     
    Last edited: Dec 31, 2015
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Personally I always thought there was a PTC to prevent instant stop-start in a 'fridge compressor.
    Max.
     
    Last edited: Dec 31, 2015
  5. sherlock ohms

    Thread Starter New Member

    Dec 24, 2013
    25
    2
    Thanks for the reply. I'm pretty much a rookie with this, so the diagram is over my head. But i think what you wrote is that once the thermistor heats and R rises, it blocks most of the current flow to the starter windings, but NOT ALL of it? So there's not enough current flowing through the ptc to energize the start windings, but there is enough to keep the ptc warm, R up, and therefore unenergized start windings?
    Have I got it???????
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    That has been my take on it, @#12 may confirm it.
    Max.
     
  7. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    Correct.

    Remember I said the PTC kind of disconnects the start winding. It really reaches an equilibrium between heat and current such that the current is insignificant as long as power is applied and the compressor is running. Only when the thermostat opens for at least one minute does the PTC cool down enough to allow another start surge.
     
    sherlock ohms likes this.
  8. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    It's been a long time since I was in a fridge compressor. At that time they used a bi-metal thermal in one of the legs. This would prevent compressor from starting against pressure.

    The PTC is a neat little gadget. Once the voltage drop is developed, it remains in the low current predicament, until the voltage is removed and cooled.

    I wonder if you sprayed a cooling aerosol on it while it was locked, if it would reset?

    According to Mike's article, I believe the current would jump.

    And I didn't see any reference to noise, if there is any, in the unlocked state.

    The PTC doesn't seem to be as abrupt with the current as a mechanical thermal would be.

    A smoother action.
     
  9. sherlock ohms

    Thread Starter New Member

    Dec 24, 2013
    25
    2
    Thank you very much for your knowledgeable info. I feel i fully grasp this concept now!

    Ay theenk oive gaught ett. Thanks so much.
     
    Last edited by a moderator: Jan 1, 2016
    #12 likes this.
  10. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    I feel a need to elaborate on that. (ps, the leg to which you refer is the pin which is common to both windings.)

    In the case called, "short cycling", which means trying to re-start too soon after the compressor was turned off (usually by the thermostat, sometimes by a short power outage), the Freon pressure prevents the compressor from starting. The bi-metal device is an over-current sensor that clicks open after a few seconds of high current (Locked Rotor Amps) flowing in the combined amount of the start winding and the run winding. The pressures will equalize during the time the bi-metal is cooling down. When the bi-metal clicks back to the "on" condition, the compressor should start.

    Yes. If you can take the ceramic down to room temperature, it will allow the start winding to have full current for a few seconds, or as long as you can keep it at (or below) room temperature. The difficulty is that while you are spraying coolant on the PTC it is changing towards lower resistance, current is increasing, and that current is heating the PTC. That means a fight. If you have some R-22 @ -40 degrees, you can probably win, if winning is defined as powering up the start winding on a compressor that is already running.

    According to Mike's article, you can make a bi-stable oscillator with a constant current source, which is a stretch of the imagination for how a compressor motor winding works. You might consider air to be the fluid in that article, but there is almost no air movement in the motor controller cavity. You might consider the thermal mass of the compressor to be the fluid, but the thermal transfer capability of the compressor's electrical connectors is rather low. It is difficult for me to find the relevance of the article, and I did this refrigeration thing for 40 years. I don't expect a beginner to grasp the concept.

    You've got me stumped there. Electrical noise? Audio noise? Unlocked state of what?

    The PTC creates a smoother change in the start winding current, but I don't see any advantage to that, compared to a, "click-off" device like a current controlled start relay. It's just cheaper to build. The PTC actually causes other problems. A short re-start time will find the PTC still in a warm condition. If it's warm enough to defeat sufficient current to the start winding, it will re-heat without the compressor starting, and the compressor will be stuck in a perpetual "no start" condition. That is when the bi-metal saves the day. It will click off as an over-current (LRA) safety and give the PTC time to cool down. Only then, after both the bi-metal and the PTC have cooled down, will the compressor re-start. The bi-metal serves the same purpose for a current relay on the start winding. A quick restart will energize the start relay, but the Freon pressures will keep the motor from accomplishing any rotary velocity. Stuck again! The bi-metal allows time for the pressures to equalize, and when the bimetal resets to the "on" condition, the start relay will again energize the start winding.

    The bi-metal over-current safety device is critical to saving the compressor from having to use its internal over-heat safety switch. If that one gets stuck open, the (hermetically sealed) compressor is dead...un-repairable. Fortunately, the bi-metal switches rarely fail because they are rarely needed. The bi-metal might only have to save the compressor once or twice a year. That is not very many life cycles compared to the PTC or a start relay. If the machine is designed without a bi-metal, the compressor has to use its internal safety switch to let the pressures equalize and let the PTC cool down. It is a very expensive event when the internal safety fails.
     
    Last edited: Jan 1, 2016
    BR-549 likes this.
Loading...