Capacitors to ease startup of a 10HP 3 phase compressor motor powered by generator

Discussion in 'Automotive Electronics' started by colinkris1, Apr 20, 2016.

  1. colinkris1

    Thread Starter New Member

    Apr 17, 2016
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    So I reviewed the "restricted topics" section and saw "transformer less power supplies". Does this refer to a generator? My question is as follows: I have a 10HP 3 phase air compressor. I do not have 3 phase at my home. I have a 12/15 KW 4cyl diesel generator. The GE 10hp motor draws 27.2 A FLC at the 230 V taps service factor is 1.15. The generator puts out roughly 42A per leg tapped to 120/240 Delta 3 phase. It works without tripping motor overload or breaker on generator, but is hard starting. I was wondering if I could connect capacitors to ease inrush current load at startup? The motor is code H which someone told me can be looked up to determine the KVA per HP required under locked rotor conditions with nameplate voltage applied. I am working on that code value now. Is this an appropriate question for this forum? If so how do I go about sizing my caps and how should I connect - A-B, B-C, A-C or A-N(G), B-N(G), C-N(G)? Thanks in advance!
     
  2. colinkris1

    Thread Starter New Member

    Apr 17, 2016
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    0
    With the generator tapped this way I have 120/120/208 to ground 240 phase to phase. I can't tap to the 480V because I use the two 120V to ground legs as backup power for my house. So if I were to connect a capacitor to ground on each leg one leg is 208V (to ground) while the other two are 120V - not sure if this is an issue? Seems likely I would be doing the phase to phase setup for continuity of voltage.... Thanks again for any input!
     
  3. elec_mech

    Senior Member

    Nov 12, 2008
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    Welcome to AAC. I can't answer the capacitor question, but I can give you a little more information on your system. A 10HP motor requires between 63 to 71 locked KVA to start where the voltage won't drop more than 30% during that fraction of a second at start-up by industry standard. A 15-17kW generator likely has a locked KVA of 25-30KVA. I would therefore assume the voltage is dropping well below 30% at start-up, hence the hard starting.

    I'd suggest looking into a variable frequency drive (VFD) or adjustable speed drive (ASD). You'd have to talk to a manufacturer or supplier to be sure, but this should reduce the in-rush current to the compressor and avoid the hard starting. Alternately, you might consider using 240V, single-phase from your house with a phase converter. The second option will probably cost more to implement though.

    This is just my two cents until you get an answer from a more experienced member here in this field.
     
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  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    Google transformer starters.
     
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  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I don't know why this is posted in the automotive section being if it had been anywhere else I would have gotten to you the first day.

    Anyway.

    Capacitors only work as reserve power devices in DC systems. In AC systems they only counteract inductive effects so they would do no good on a AC induction motor in terms of giving any measurable amount of reserve power. .

    Now for running a three phase induction motor from a standard single phase system I do that ll the time and in fact have a 15 HP commercial unit in my shop that runs off of a standard 240 VAC 60 amp line and has been doing so for around a decade now without problems.

    So if you want to know how that done here you go. Your 10 HP should work just fine off a standard 40 amp 240 volt line set provided that you have adequate wire size and capacity to and from your main power distribution point being the startup load will be around 80 - 100 amps for the first few seconds while it gets the flywheel up to speed.

    At worst if you have a odd cheap motor that has a low or no reserve service factor and your AC power is limited you may have to reduce the drive pully on the motor by 10 - 30% in order to improve its starting ability and or reduce its starting amp draw.

    http://www.electro-tech-online.com/threads/3-phase-converter-schematic-miller-system.100563/
     
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  6. tranzz4md

    Member

    Apr 10, 2015
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    Hello colinkris! What's the word?

    I know lots of folks are doing this kind thing with VFDs too.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Don't know about all VFD's, but the manual that came with my Teco VFD's says they are only usable to 5HP on single phase input.
     
  9. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Most are limited on single phase due to their input rectification system and main capacitor bank not being large enough to handle the higher current and ripple associated with runing at full power form a single phase feed.

    I've modified a number of older three phase only input VFD units to work at full output on single phase by replacing the stock rectifier system doubling or tripling the rectifier capacity and main capacitor bank size.
     
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  10. crutschow

    Expert

    Mar 14, 2008
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    One off-the-wall thought would be to use a centrifugal clutch pulley on the motor so it would be partially up to speed before the compressor is engaged.
    It probably could be a light-duty type since the compressor doesn't normally start and stop that often.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If a dual voltage motor, it may be possible to set up Star-Delta starting, used to be very popular at one time, only needs a couple of contactors, and a time delay.
    Max.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Presumably you have looked at ensuring the compressor is unloaded when starting? This can be done simply with a dual acting valve.
    Max.
     
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