3 phase converter

Discussion in 'General Electronics Chat' started by bladerunner, Aug 16, 2014.

  1. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
    I have read all there is to find about the Miller System (via its author on this site but none of the questions and answers answer this question directly. I have used the Miller system successfully on a few 3 phase motors mostly used in the lumber mills. However, I was asked if this would be done and the answer (my answer) was I don't know but will find out.

    Using single phase, I have a 'planning mill' that has 5 x 10HP motors 240 Volts 3 phase of which at least any three are running at one time. In some cases, all five are running. I would assume that under the Miller System, a board (set of capacitors set to the Miller System specs) could be set up for each individual motor. These could theoretically be set up so at least three of the motors start in consecutive order with the other two motors being turned on as needed individually.

    Is this something that con be accomplished with the Miller system or would I need to install a two motor rotary system which I really don't want to do.

    Thanks for any info.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,566
    2,379
    5 10hp 3ph motors on 1phase, how come this type of machine is set up in a 1ph environment in the first place??
    What is the 'Miller' system? Similar to RPC techniques for running a 3ph motor on 1ph with capacitor 3ph generation?
    Not something I would look forward to doing :confused:?
    Max.
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    A single phase 10hp motor of today's technology has a FLA of 46 amps. Lets assume the miller system converts single phase to 3 phase with > 100% efficiency and makes these (assumed antequated) motors draw the same as the linked motor. Where are you located, when you find yourself with 230A (realistically probably no less than 350 amps)worth of single phase juice to spare, but you don't also have 3 phase available?
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,041
    1,673
    The Miller system is a method I came up with about 20 years ago while in college. After that I started making custom units for people and locally it sort of just picked up my last name as the design that worked very well with most any old odd three phase motors powered by any halfway decent single phase power source.
    Granted over the years I have found other similar design online since buy hey 20 years ago we didn't really have much of an internet to do this sort of searching on anyway so locally the name stuck and I used it when I wrote up the designs on the ETO site a number of years ago.

    Anyway as asked by the OP the way I would be tempted to set the system up is to use the correct phase balancing capacitors for each motor then wire them all to a central control point where one starting circuit is used to get one motor running.

    After the first motor is running any of the others can be brought online or offline without causing any severe phase imbalance issues or high inrush currents being the other motor or motors will work as short term rotary units to help pull the motor thats starting up to speed letting the starting circuit stay off line in most situations.

    Power supply wise on a normal North AMerican 120/240 VAC single phase circuit the motors will take about 3.5 amps per HP so a 10 HP motor will usually have no trouble running off of a 40 amp service feed. Factory built 10 HP single phase motors tend to not have much of any power factor correction so their running amps tend to be higher than what mine do being there are two specially sized PFC capacitors on each motor that are doing the phase voltage and power balancing for the third phase that is being generated.

    Given the standard 746 watts per HP plus a typical 80% electrical efficiency with power factor correction a 10 HP motor will need about 39 amps at full load of which if any mechanical system is properly sized the motor will likely never be expected to carry that load continuously.

    I have 5 HP units in my shop that have been running off 240 VAC 20 amp breakers for years without issue. Same as with my 15 HP industrial air compressor that runs off a standard 60 amp circuit. Full load just before they shut off they are pulling just short of 90% of their breakers ratings.

    As far as wiring, coding and related stuff goes I leave the OP to deal with that themselves. ;)
     
    Last edited: Aug 16, 2014
    bladerunner likes this.
  5. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
     
  6. bladerunner

    Thread Starter Member

    Apr 15, 2012
    83
    4
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,041
    1,673
    No apologies needed.

    Depending on what you are doing and how heavily you are loading all the motors at the same time plus if your given electrical service can actually support a near 200 amp short term load without excessive voltage drop using a single starting circuit on its own 40 amp line followed by multiple three phase contactors that switch each motor and its independent 40 amp source online plus connect the third phase to the start circuit you more than likely could make the systems work just fine.

    To me the system you are implying you have could be done a number of ways. Personally to me if multiple motors are going to be running all at once tying them together so that they share their third phase connections when online will help further balance the phase generating and load balancing more evenly throughout the system.
     
    bladerunner likes this.
Loading...