Can I use a 1:1 isolation power transformer for this

Discussion in 'The Projects Forum' started by hobbyist, Jan 24, 2011.

  1. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Hello,
    I am asking a question, about using an isolation transformer.

    I have a DC motor (100 volt) from my micro milldrill machine, I am tired of replacing the PWM controle board, I seen a block diagram of how this board works, and from the diagram, the mains is connected to the H-bridge rectifier, and filtered and then sent to the motor,
    I have a thoro understanding of the diagram, I don't want to use the comercial way of doing it by hooking the rectifier directly to the mains.

    I am going to build my own PWM circuit for it, however,

    Here is my question, Is it safe and can I use a 1:1 isolation transformer to isolate the main power, and use the 120v.off of the secondary to rectifiy and bring power to the motor / circuit.

    I have built numerous power supplies from scratch, using step down power transformers, for low voltage applications, so I am proficient in designing power supplies, I just never built a supply larger than 30v., and was wondering if using the 1:1 isolation power transformer would be safe.

    If not I will either try to run the motor at a lower voltage, or purchase a lower voltage motor, to build a linear supply for,

    Thanks for any help.
     
    Last edited: Jan 24, 2011
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    It should be safe, nearly required, to use an isolation transformer for line power, assuming the board is designed for 120VAC input.

    Can you post the schematic, or manual, or any other info you may have on it? (.PNG images or .PDF documents are the strongly preferred formats)
     
  3. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Hi,
    Thanks for the reply.

    All I have at the moment is this block diagram, put out by gershings labrotories, website, this person was very kind enough to put out the block diagrams of these controllers.

    PWMControl.jpg
     
  4. kvandergriff

    New Member

    Jan 21, 2011
    1
    0
    An isolation transformer is a good idea. You would still have 120 V AC on the secondary though, I think you said your motor wanted 100 VDC. You would have to deal with getting 100 V instead of 120V in your rectifier/filter/voltage regulator circuit.
     
  5. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Thankyou for the reply,

    Yeh, I would have to regulate the voltage down to whatever meets the motor specs.,
    But you both answered my main question, it is safe to use a isolation 1:1 transformer, to be able to get a high enough voltage at the rectifier for this motor, without having to build a linear supply with less voltage.

    I need to do a little research and testing to see what for voltage this motor operates at the most efficiently, continuously.

    Then take it from there for the voltage regulation, and circuit design for the PWM, ect...

    Thankyou both, for taking the time to confirm a question about this.

     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,009
    1,530
    Don't forget that when using a full wave bridge rectifier you need to multiply the input voltage by 1.414. So - 120VAC x 1.414 = 169.68 VDC
     
  7. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Thanks shortbus,

    Yeh, before I even attempt to design and build this, I'll go through my course books, and do a lot of reading and research until I am convinced I have a thoro understanding, concerning this kind of supply, as well as the motor drive circuitry.

    Thanks everyone for your help.
     
  8. bdd4

    New Member

    Jan 27, 2011
    3
    0
    1.Be sure to consider the output current capacity of your 1:1 xfmr. You will need to know for certain what current your motor draws. Select a xfmr that can supply some reserve.

    2. Curious -You said you are tired of replacing PWM boards. Is the input to your board use protected or current limited.? Or possibly is the H-Bridge blowing out because of the inductive dump of your motor?
     
  9. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Hi bdd4,

    Thanks for the reminder of the safety measure needed for the choice of transformer,

    These boards, have current limiting, and failsafe circuits built in.

    The main problem seems to be the speed regulation goes erratic after long periods of use.

    I just purchased another board, so I can get my mill back up and running, that way I can take my time, and build a PWM unit, that will suit my needs for the future use with these machines.

    My mini lathe, works beautifully with it's regulation of speed, never replaced a board in it yet. It is just the micromill.

    Now that I'm switching over from gear driven to belt and pulley drive, I can pretty much put any motor I want on it, so I can experiment with different designs for a controle board.
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Sounds like a heat problem. Have you checked the temperature of the components on the board? An addition of a "Stick On" Heat Sink may solve that problem if you identify the component(s) running too hot to touch. My "rule of burnt finger" is if you can't hold your finger on the device (IC, Transistor, etc) for 5 seconds comfortably, you should cool it down.
     
  11. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Thanks,
    thatoneguy,

    When I get this new board put in, I'll keep that in mind, to check for overheating.
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    What type of motor, universal, shunt field, brush, or PMag?? Might consider phase control via SCR or TRIAC. There are several ckts in GE SCR manual, mine is 1972 edition, might find something newer.
     
  13. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Hi Bernard,

    I'm pretty sure it's a brush DC motor.

    However thanks for the idea about phase control with a triac, that might even be more easier than PWM, but it would require a AC motor.

    That's something I could consider too.

    Thanks.
     
  14. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    DC motor OK, now is it permanent magnet, do you feel cogging when turning shaft slowley??
     
  15. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Hi bernard,

    Yeh, It does have a cogging, when I turn the motor shaft slowly, I guess this indicates a permanent magnet motor?
     
  16. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    From GE SCR Manual, 1972: As motor power is not stated, bridge rectifier & diodes should be selected to match, D2 1N4002, D3, across motor should = motor current, SUS. silicon unilaterlal switch, 2N4987 or NTE6404. R3 is speed control.
     
  17. hobbyist

    Thread Starter Distinguished Member

    Aug 10, 2008
    764
    56
    Thanks for the schematic.

    I will study up on this, until I get a good thoro understanding of it's operation.

    Before I do any building.
     
  18. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Might also follow post by takeitapart, dc motor control.
     
Loading...