Air compressor and other 3phases stuff

  1. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Yesterday I got an air compressor and a bench grinder both are three phase 400V
    I ran them using my genset and they works
    But now I need to run them without the generator
    I first thought to use a capacitor and this can work on the bench grinder but not on the air compressor
    So I was thinking to use a VFD but the output has the same voltage of the input so 230V
    I would like to give 230V single phase and obtain 400V three phase ,an I found a very expensive VFD that does that
    After little bit of searching about I understood that the majority of the induction motor build before 2000's can't work driven by an inverter , because winding insulation can't manage the spikes and electrical impulses generated by a VFD
    Because the VFD doesn't make a clean concatenated sinusoidal but it's more like what we obtain with a PWM or SCR controller
    So this option has to fail because is all 70's or 80's stuff
    Was reading about rotary phase converter but I can't really understand how this thing works
    A bunch of capacitors and a motor ,live and neutral are L1 and L3 ,capacitor starting winding will become L2
    You connect the motor you want to run in delta configuration and it should work in 230V 3~

    I don't really know what to do because they will give me also a pillar drill and other 3 phase tools
    Have you any experience ,advice or other that can help me to understand what to do ?
     
  2. CoValent

    CoValent Member

    Messages:
    143
    Location:
    Limerick, Ireland
    Can you give some indication as to the size of the compressor and motor? A picture of the motor plate would be helpful.

    TBH, unless it's very small I think you're going to be disappointed with any of the single-to-three phase solutions, based on cost and performance, due to the high startup demands of a compressor. You might be better advised to investigate a motor swap for a single phase unit.
     
    skotl and eLuSiVeMiTe like this.
  3. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    I'm running a rotary converter fed from a modified oil-cooled welder used as a transformer (wound a new secondary coil to raise the voltage to 400+), it's capable of about 8 KVA fed from a 32A type-C breaker on 240v. There's lots of info on Practical Machinist, they have a sub-forum just for phase converters and VFDs, well worth a look.
    I use a modified ABB VFD on my lathe, which also needs 415v 3-phase, part of that installation was to DIY some "load reactors" - simple chokes/inductors that go between VFD and (dustbin-sized, unique and irreplaceable 3-speed) motor to limit the spikes.

    I have to head off to work, I'll check in later with some details!

    Dave H. (the other one)
     
    eLuSiVeMiTe and CoValent like this.
  4. CoValent

    CoValent Member

    Messages:
    143
    Location:
    Limerick, Ireland
    Not wishing to drag this thread off into the weeds, but have you written this up anywhere? It would be very interesting to hear more about that build!

    Have an Oxford welder that I can't bear to scrap. :)
     
    Hopefuldave and eLuSiVeMiTe like this.
  5. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Overload protection is rathed for 4A so it's 1600W ,the plate is missing
     
  6. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    How does this thing work?
     
  7. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    OK... they can be really simple, mine is fairly complicated - because I'm like that!

    I used a Pickhill oil-cooled arc welder as a starting point, the low-voltage (welding) winding was removed and a new coil wound on a 3D printed former:
    Coil windingf.jpg

    That was then installed in place of the original low-voltage winding - the number of turns was calculated by lifting the transformer out of the oil, wrapping 5 turns through the core and measuring the voltage - I measured about 8.7 volts on 5 turns, so roughly root3 v/turn - convenient, that's the voltage ratio I wanted, so I would on 240 turns of 3mm (diameter, NOT cross-section-area) enamelled copper wire. The former was then bagged up with some airline hose to apply vacuum from Lucky Bleeder and feed in epoxy resin, vacuumed down then impregnated with the resin (took a 720 g pack) and left to harden.

    The new winding was then installed in place of the original, this involved unclamping the transformer core laminations and tapping carefully, then re-stacking through the new coil's former - like this:

    20190630_203214.jpg

    I also wound 5v and 10v windings for fine adjustment if needed, sleeved the ends and brought them out to a sturdy terminal block:
    20190630_203231.jpg

    That completes the foundation, more next post!

    Dave H. (the other one)
     
    CoValent and Memmeddu like this.
  8. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Hope to see other features of your converter soon
     
  9. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    I bought a 5.5 kw 3-phase motor on EvilBay for not too much money, and had a 3 kw hiding under the bench - these would do nicely! Both were wired for 415v already (the 5.5 in delta, in star it would take 700v), so just needed wiring in parallel - in the "power plane" drawing below they're shown as a single motor, as I was looking for an 8 kw when the 5.5 came along!

    240-415 phase convertor v2 Power Plane updated.JPG

    This shows the power connections ONLY, I still haven't got around to drawing the "control and alarm plane" diagram!

    So... incoming 240v passes through an ammeter (not essential), a 32A typeC circuit breaker (pretty essential!) and a hefty contactor to the both the 24v transformer for the control / contactor coils and the mains contactor which switches the power to the transformer.

    The 415v side of the transformer connects through the motor contactor to two of the motor's 3 phases, the other phase is connected to the transformer through the start contactor which puts a capacitor on one leg of mains, "generated" leg of 3-phase when starting up - the capacitor shifts the phase of the incoming mains 415v enough to start the motor rotating.

    Once rotating, the start contactor opens, the motor keeps rotating as it's generating its own third phase.

    So... we now have 415v direct from the transformer on 2 of the motor's phases, it generates the third - the third isn't quite right, the voltage is likely to be off so more capacitors are added from the generated phase to the others to "balance" the voltage - these need to be suitable for the 415v they're running at, so I use pairs of 400v rated RUN (NOT START) rated capacitors in series to get the voltage needed, just double the values as putting them in series halves it.

    Everything to the right of the motor / start circuit is just me being over the top :)

    Dave H. (the other one)
     
    eLuSiVeMiTe, CoValent and Memmeddu like this.
  10. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    @CoValent - this is about all the write-up there is...

    Dave H. (the other one)
     
    CoValent likes this.
  11. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Is quite difficult ,a question for you....
    Why you didn't used the primary side to make 400V ,there are generally 2 windings in series both made to manage 230V , but both togheter can manage 400V
    So all you had to do was connect to use 230V and take the phase knob and the third wire of the winding , this way you should have an autotransformer without using the secondary side
    But if you did not use this method i think that you did this to prevent disturbing the other electrical users
     
  12. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    The Pickhill only had a single 240v primary winding (I was surprised, there were 400 and 415 terminals in the connection box but no wiring to them!) And also for safety I wanted an isolated source - the 415v isn't connected to earth (through the supply neutral), reducing shock risk.

    Dave H. (the other one)
     
    Memmeddu likes this.
  13. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Insulation transformer are good but if it goes to electrified the motor housing this might be dangerous
     
  14. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Capacitor capacity and wiring diagram of them ?
    If you can ...
    I'm searching the motor and a welder from the scrapyard (I have a friend that sell metal garbage )
     
  15. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    The mains earth is carried through to the sockets so there's protective earth available.

    Capacitors:On mine, I started with 60 uf per motor HP for start capacitors (450v), the balancing capacitors start with 2 x 10 uf 450v in series per HP (which gives you effectively 5 uf per HP at 900v rating) between generated phase and one 415v fed phase (the one that's also switched through the start capacitor), 2 x 2 uf 450v per HP in series between generated phase and the other 415v phase (so the start capacitors are put in parallel with the larger balancing capacitors to get it spinning).
    The start capacitors are only used to spin the motor up switched out of circuit once it's coming up to speed.

    These capacitor values are just a starting point, you'll need to adjust values for the motor and load...

    If you haven't already, I recommend reading through the DIY rotary converter thread on the Practical Machinist site, there are much more knowledgeable people than me on there, lots of constructional posts worth reading!

    Dave H. (the other one)
     
    Memmeddu likes this.
  16. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Ok after struggling searching every where (didn't visit practical machinist yet) I found two YouTube channels where they build two rotary converter and seems to be something so easy but so complicated at the same identical time
    The working principle is that
    You take your live and neutral you connect them to the motor
    You have to spin the motor ,you can do this by wiring in delta connection (contactors) and use a start capacitor ,or use another smaller motor to make it spin and have the motor permanently in star configuration
    The winding that is connected directly to the main (230V ) will be the primary coil of a transformer (the motor itself will work as a Variac)
    Because the motor is spinning this will cause phase cicle
    The working is easy to understand with minimal electrical knowledge (and I'm a professional electrician ) ,the building is little bit trickier because you have to use at least 3 contactors (delta ,star , capacitor exciting battery ) and related controls
    But essentially is something very easy in working terms
     
  17. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    Correct, the principle is quite simple, but a warning, the delta-star switching convertors are A VERY inefficient way to go from 240 to 415, perhaps only 30% of the power in is delivered as 415 3-phase, so you need a huge motor - and high power 240 delta / 415 star motors are hard to find!
    In the U.K. anything over 3 or 4 HP is likely to be 415 delta, 700 star...

    A transformer will be 90% or more efficient, and lets you use a smaller 3-phase motor, you'll want about 1.5 times the biggest tool motor for reliable starting (with the delta-star voltage conversion you'll need 5 times...)

    Dave H. (the other one)
     
  18. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    Considering that I have only 3kW what size of motor you might use if been in the same situation?
     
  19. Hopefuldave Member

    Messages:
    772
    Location:
    The Shed f Danger, surrey, England
    If that 3 kw is the machine's motor, go for at least 4 kw idler motor in the convertor, 5 kw would be better - once the idler's running any other *unloaded* motors will add, so if you have e.g. a 3 kw idler and start a 2 kw machine *and don't load it* you have the equivalent of a 5 kw idler. Or you can use multiple motors for the idler, I'm using a 5.5 kw and a 3 kw to give the same effect as a 8.5 kw idler - just wire the motors in parallel, both wired for the same voltage.

    Read the Practical Machinist forum! NOW! Before you start buying parts!

    Dave H. (the other one)
     
    eLuSiVeMiTe likes this.
  20. Memmeddu

    Memmeddu Member

    Messages:
    652
    Location:
    Italia Sardegna
    My power forniture Is 3kW .
    I will buy only capacitors and switches ,I have a good friend which can give me some motor
    The big problems is calculate capacitor for HP , because the general rule is 50uF for HP using 230V 50Hz , but I found that 1 HP motor can run smoothly using a 25uF HP
    Other than how balancing capacitors needs to be calculate ? In function of my out put or they must all the same capacity ?
    I would like that you explain me all the attempts you did before have success because I know that practical machinist is an usa based forum so all what I can read there isn't suitable for 400V .
    Otherwise the main line is always the same I guess
     
Advertisements