Frequency changer for compressor

Discussion in 'General Electronics Chat' started by leafar, Apr 2, 2012.

  1. leafar

    Thread Starter New Member

    Jun 3, 2011
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    Hi. First of all sorry for possible mistakes, english is not my native language.
    I'm working on a frecuency changer for a compressor (fridge). I know that to start the compressor is neccesary a high current and that´s the reason there is a "start capacitor".

    My question is: If the inverter of the frecuency changer can't handle the start current is possible start the compressor normally and then switch to the inverter without problems?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Is the purpose of the inverter to operate a 60Hz motor from a 50Hz line?
     
  3. leafar

    Thread Starter New Member

    Jun 3, 2011
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    No is not. why?
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    If not, then why do you need a frequency changer? Are you converting from 0Hz (DC) to 50 or 6Hz? How would you "start normally"?
     
  5. leafar

    Thread Starter New Member

    Jun 3, 2011
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    I don't know, maybe i'm using the incorrect term. I want to control the compressor's motor speed. For that i read that is necessary a inverter (full bridge with IGBTs), a control stage (for that i'm using a dspic with a SPWM modulation program) and a driver (IR2110) that connect the both stages. The inverter would take as input a dc voltage. If all that is a frecuency changer i guess i'm converting from DC to 60 HZ then.

    With start normally i mean connect the fridge directly to the electric network. When the compressor motor has started i want to switch to the inverter (with a relay or a contactor). ¿could be some damage or something wrong making that? ¿what do you suggest me?

     
  6. crutschow

    Expert

    Mar 14, 2008
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    You should be able to start with the mains voltage and then switch to the inverter voltage with a DPDT relay. Just be certain the relay is rated for the voltage and starting current of the motor.
     
  7. leafar

    Thread Starter New Member

    Jun 3, 2011
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    Thanks for your help.
     
  8. BSomer

    Member

    Dec 28, 2011
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    So if I understand you correctly, you want to build a type of VFD(Variable Frequency Drive). These are commonly used in industrial applications to control the speed of a motor for stuff like conveyor belts and pumps, etc...
     
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    And only work with three phase motors. There are small ones that take a single phase input but the output and motor are still three phase.

    Have never been able to find a VFD that will run a SINGLE phase motor, and I have looked for years.
     
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  10. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    What do think you will achieve by running the compressor at a different speed than what it was designed for? This sounds like one of those "power saving" schemes.

    Ken
     
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  11. leafar

    Thread Starter New Member

    Jun 3, 2011
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    Yes you are right.

    Well, i want to control de temperature of the fridge. This is part of a project for conservation of fruits controlling variables such as O2, CO2 concentration and temperature. First there was a simple on/off control for temperature but the compressor got damage because of that.

    I find something interesting related with my question:

    So i think i'll need two DPDT relays.
     
  12. somlioy

    Member

    Jul 28, 2010
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    Isnt that exactly how a fridge work in the first place?
     
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  13. strantor

    AAC Fanatic!

    Oct 3, 2010
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    You can't vary the frequency of a single phase motor to adjust the speed. the reason is because the capacitor is sized for the intended frequency. If you wanted to change the speed by varying the frequency, you would need to vary the size of the capacitor proportionally & simultaneously. This is why there are no comercially available VFDs (or frequency changers as you say) for single phase motors. You are wasting your time trying to build one. it won't work.
     
  14. leafar

    Thread Starter New Member

    Jun 3, 2011
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    Yes, but it use a Thermostat normally. Instead of that the control that the compressor had, connected and disconnected the mains voltage.

    I didn't know that. Although it seems that exist commercially VFD's for single motors they have many limitations so you are right.

    Thanks for your help
     
  15. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Holy Crap, Shortbus, did you see that? I knew if I kept saying they didn't exist, eventually someone would prove me wrong. And now they have. Congrats leafar, you are the first. Many who have gone before you have failed, including me. It sucks though, as it can only work with shaded pole and split winding capacitor run motors. But it's still a single phase VFD.
     
  16. shortbus

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    Wow, I'll have to reword my answer when it comes to this!!
     
  17. leafar

    Thread Starter New Member

    Jun 3, 2011
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    Well thanks [​IMG]. Now i have two new questions. There is a possibility of use another fridge but this have a motor that have a PTC starter. Since this motor doesn't have a start capacitor, a VFD should works without problem right?

    I have another question with respect to the inverter of the VFD. I´m going to use the Ir2110 driver and the bootstrap method, with VCC=18. When the bootstrap capacitor is charging i read somewhere that could there be a high peak current, if i use a 7818 regulator can it get damage because of that? Do i need a disipator for that regulator?

    [​IMG]
    [​IMG]
     
  18. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    The PTC is a solid-state thermal switch that replaces an electromechanical switch. It doesn't necessarily preclude start or run capacitors.

    Ken
     
  19. shortbus

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    Another problem with varying motor speed is the actual compressor. They are made to be run at a certain speed. If facing the problem you are having (which I don't quite understand, what that problem is) I would look at changing the refrigeration system size. It seems that your unit is too large for the needs.
     
  20. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Yes, I got so caught up in the single phase VFD thing, I didn't realize the failed logic behind the initial question. I just counted 3 or 4 times it's been brought up, but I'll bring it up again...

    Why are you doing this? There is no benefit to what you are doing. Actually, your fridge will most likely be LESS efficient due to all the added complexity. You said in post #1 that the compressor was damaged by switching on & off. How did you come to the conclusion as to the this being the cause? Refrigerators switch on & off - that's how they work. They are DESIGNED to work this way. They are also not designed to work for an eternity. Things break, you replace them. If the engine in my car starts knocking, I don't jump to the conclusion that the starter caused it, and design some kind of complicated system to make it run at really low RPMs while sitting the driveway. I fix the engine, restore it to original configuration.
     
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