My first RPC 5-10HP is Fitch still the design to use? I have parts galore to build it - Page 4
Close
Login to Your Account
Page 4 of 4 FirstFirst ... 234
Results 61 to 68 of 68
  1. #61
    Join Date
    Aug 2014
    Location
    bainbridge island
    Posts
    1,129
    Post Thanks / Like
    Likes (Given)
    213
    Likes (Received)
    267

    Default

    the reason for the capacitor to boost the voltage is simply that the current is leading the voltage, and it is increasing the air gap flux in the machine.


    at a minimum, the 30% of FLA that is required to run an induction motor when powered from 3 phase, must be provided by something in order to get the volts at the machine up to par, it may very well be that a 5hp induction motor, which might consume 4-5 amps no load, needs 4-5 amps reactive current flowing through the unused coil when used as an RPC. and, coincidentally 4-5 amps at 240 volts is about 40-50 uf.. which fits the rules of thumb of 10 uf per hp. regarding the voltage regulation of this system, when the load motor bogs down the idler, the voltage drops and so does the current flowing through the capacitors, reducing their effectiveness.

    i wonder if a magnetic amplifier could be rigged up to control a variable capacitor.



    btw i loaded my motor with the welder because i had a vague memory of it consuming 8 amps at no load.. that memory was associated with a different welder, so i was disappointed to find it only consumed 3 amps lagging.


    given that the aux coil of my scroll compressor delivers 204 volts into a 9.5 amp resistive load (with no capacitive assist) it may indeed work just fine as an rpc, with the aux coil connected back to the neutral, to deliver 3 phase via the scott-t transform. i was assuming it would produce too high a voltage and i would need to wire up an auto transformer to reduce the voltage, which is why i had not tried this experiment some time ago.


    using a large 2 phase motor as an example of how the impedances work out makes the math a lot easier to think about.


    Edit: The 10 to 11 amps locked rotor current of my 3 phase 1/2 hp 3600 rpm bench grinder pulls the generated leg from 230vac down to 185 volts. (208 volts would represent balanced 3 phase 240vac. ) this voltage drop is consistent with 4 ohms impedance.

    at no load the grinder consumes about 1 amp on two of the lines and the third line pulls .25 amps, this is due to the voltage at the generated phase is too high, being 224vac relative to the neutral of the 120/240v system, (should be 208) when measuring the phase to phase voltages there is a bit of a phase shift as expected, the voltages measure about 245/255/260 volts.
    Last edited by johansen; 06-22-2019 at 07:47 PM.

  2. #62
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    24,214
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4410

    Default

    "(208 volts would represent balanced 3 phase 240vac. )"

    Not sure where that comes from.

  3. #63
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,180
    Post Thanks / Like
    Likes (Given)
    1799
    Likes (Received)
    3257

    Default

    Quote Originally Posted by johansen View Post
    i wonder if a magnetic amplifier could be rigged up to control a variable capacitor.
    I tried it with a core that had 4 legs, a coil on each end and a pair in the middle that I thought I could feed DC to saturate the middle of the core and electrically separate the end coils but I couldn't get enough power through it to matter. You definitely can make a variable capacitor with a Variac. I could vary the phase shift with it and optimize the motor at any load. It would be easy to make a phase detector controlling a servo motor to do that automatically but it likely would wear out the Variac with constant movement. A more practical approach would be something like a stepper switch with a phase detector controlling it.

    I suspect that it wold be possible with something similar to some welder's controls that have a cruciform core with the center leg moveable. You might be able to make a moving piece that had very small clearance but not touching the main core to keep the air gap small.

    A variation on that would be a cut core with coils on each piece and a capacitor connected to one coil. The other would be inserted in the place of a run capacitor and you could vary the amount of capacitor coupled to the circuit by moving a coil out of alignment.

    I wanted to do it with no moving parts but have not yet achieved that state of grace.

    A conventional magamp would not work because it does not vary the sine wave the way a Variac does, but delivers a waveform similar to an SCR, controlling by the firing point in the cycle rather than overall amplitude.

    Bill

  4. #64
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,180
    Post Thanks / Like
    Likes (Given)
    1799
    Likes (Received)
    3257

    Default

    Quote Originally Posted by jim rozen View Post
    "(208 volts would represent balanced 3 phase 240vac. )"

    Not sure where that comes from.
    Sketch a high leg delta service. The common is at the center of a coil instead of one corner. A triangle with sides 240 each has an altitude of 208. A 120-0-102 line with a capacitor feeding a third leg looks electrically just like the high leg service.

    Bill

  5. #65
    Join Date
    Feb 2004
    Location
    peekskill, NY
    Posts
    24,214
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4410

    Default

    Never found that number very useful in analyzing rotary converters, I always
    stick to the line-to-line voltage, much better diagnostic I believe.

    208 being the line-to-line in a star configuration. I always thought the high leg
    in a CT service was something else, 277?

  6. #66
    Join Date
    Aug 2014
    Location
    bainbridge island
    Posts
    1,129
    Post Thanks / Like
    Likes (Given)
    213
    Likes (Received)
    267

    Default

    all an RPC needs to do is provide 208 volts at 90 degree phase shift relative to your existing 120/240v service.


    you can sometimes access the midpoint of an induction motor's windings and you could connect the aux coil of a capacitor run motor, to the midpoint of the run winding and you would have no need to access the neutral of your 120/240v supply.


    my scroll compressor with its 227 volt open circuit aux winding, connected back to the neutral results in the 3 phase produced measuring 245/255/260 vac, so there is some phase shift its not exactly 90 degrees. i may find a way to measure the slip and see how much the rpm falls when loading the motor.

  7. #67
    Join Date
    Sep 2011
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    26,025
    Post Thanks / Like
    Likes (Given)
    7561
    Likes (Received)
    8212

    Default

    Quote Originally Posted by johansen View Post
    all an RPC needs to do is provide 208 volts at 90 degree phase shift relative to your existing 120/240v service.


    you can sometimes access the midpoint of an induction motor's windings and you could connect the aux coil of a capacitor run motor, to the midpoint of the run winding and you would have no need to access the neutral of your 120/240v supply.


    my scroll compressor with its 227 volt open circuit aux winding, connected back to the neutral results in the 3 phase produced measuring 245/255/260 vac, so there is some phase shift its not exactly 90 degrees. i may find a way to measure the slip and see how much the rpm falls when loading the motor.

    More interesting way to waste a Sunday afternoon than running the weed-whacker, but ISTR I actually read the dead-tree (in the era..) books over half a century ago. It was what it was then. It still is, now.

    So best I put on the bug spray and safety sunglasses ...and go make war on weeds... before some "progressive" comes along and tries to issue them a driving license and a free pass for medical care.


  8. #68
    Join Date
    Nov 2004
    Location
    Webster Groves, MO
    Posts
    7,180
    Post Thanks / Like
    Likes (Given)
    1799
    Likes (Received)
    3257

    Default

    Quote Originally Posted by jim rozen View Post
    Never found that number very useful in analyzing rotary converters, I always
    stick to the line-to-line voltage, much better diagnostic I believe.

    208 being the line-to-line in a star configuration. I always thought the high leg
    in a CT service was something else, 277?
    277 is the line to common of a 480 star. Draw the vector diagram for a high leg service and you will see the 208 altitude of the triangle. Square root of (240 sq -120sq} =208 roughly.

    Bill


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •