Single Phase AC Motor Drive

Discussion in 'The Projects Forum' started by Maketronic, Oct 30, 2011.

  1. Maketronic

    Thread Starter Active Member

    Mar 21, 2009
    49
    0
    Hi All

    I am about to embark on a AC Single Phase Motor Drive project to control the speed of my drill press. In New Zealand where I am from domestic single phase power supply is 230VAC @ 50Hz. The Drill press Motor in question is approx 400W.

    I have done quite a bit of reading and plan to use a H Bridge to switch rectified and filtered 230 VAC and then output it to the motor from here.
    I will use a PIC Micro and a lookup table for generating the pwm duties for creating the sine wave. I am aware that I will have to also change output voltage as well as frequency (of my pseudo sine wave)

    My biggest question is regarding filtering, I plan to use a pwm frequency of maybe 15 KHz or more and I know I will need to include filters to stop it from causing interference.
    Will I need to filter both the AC side to stop the switching frequency from entering back into the supply system or will I also need to filter the motor side too?

    I have spent quite a bit of time searching forums and Google and are having very little luck finding information regarding the filter side of this project so any suggestions that people can provide will be greatly appreciated.

    Regards

    Bruce
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    This is done by adding reactors to the input or output lines--generally, not both. With the motor leads in grounded conduit and the motor frame grounded, little interference will be radiated from the output--so my suggestion is that you add two reactors (or three phase reactor with one leg not connected) to the AC mains.

    These are generally sized for 3 to 5% impedance (3 or 5% of the applied line voltage is developed across the reactor)--not all that critical. In other words, with the drive running at full load, the line voltage measured at the input to the control, will be 3 or 5% lower than the voltage measured at the input to the reactors.

    Check out this application note:
    http://www.precision-elec.com/produ...oad-reactor-with-smvector-inverter-drives.pdf

    One thing they generally do not mention in these discussions is carrier frequency--this is a bigger issue as low frequency line harmonics generally do not bother anyone outside the industrial environment--esp. at this low power level. The rectangular carrier waveform generates harmonics up into the mHZ region.

    Here are three reactors that are available on ebay:
    http://www.ebay.com/itm/Hammond-Pow...235?pt=LH_DefaultDomain_0&hash=item414d7eb6eb

    http://www.ebay.com/itm/1-used-3-Ph...833?pt=LH_DefaultDomain_0&hash=item4a97ec7c49

    http://www.ebay.com/itm/SOLA-SLR-SE...t=BI_Control_Systems_PLCs&hash=item588c78134e

    Or you can calculate the required inductance and size two discrete inductors:
    Using your AC line current (essentially your existing full load motor current) calculate the 50hZ reactance required to develop 1.5% of your line voltage--then calc the inductance.

    Ambitious project--wish you well...
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
    1,527
    I wish you well on your project, but a single phase motor(other than a 'universal motor with brushes) is not going to be easy. There are no commercial variable frequency drives that can be found for a single phase motor. The reason is that a single phase induction motor is not self starting, like a three phase motor. All single phase induction motors use a starting winding or a capacitor start. This makes it hard or impossible to use a VFD on single phase.

    Even the first link that "jimkeith" gave shows only three phase motors.
     
  4. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    I agree with shortbus--the single phase motor is carefully optimized for 50 /60HZ 1450 /1750RPM--these parameters may be deviated from somewhat provided that it not running rated torque. Aside from the starting issue, the adjustable speed range is limited to perhaps 2:1.

    Regarding starting, typical across the line starting current may range from 500 to 1000% FLA--that calls for oversize power semiconductors--starting with an unloaded motor at 50% freq & voltage may reduce this somewhat. Operating slower than this may interfere with the centrifugal starting switch. All kinds of issues--makes a poor general purpose drive--that is why it is commercially unfeasible.
     
  5. tonigau

    New Member

    May 2, 2011
    2
    0
    Hi Bruce, I was thinking along the same lines as your self, but decided to go DC motor, 24V for the drill press(s) & 48V for the lathe & mill.
    Lots of reasonably priced DC motor speed controllers around.
    Having a future need to run from a battery supply did have an influence on my choice...

    The filters for switchmode supplies typically consist of a compensated choke (2 opposed windings on the came core) & 2 Y capacitors, A series inductor & a couple of X capacitors. If you open up an old PC power supply (or just about any switchmode supply) you can anatomise the makeup of the filter.

    That aside, have a look for an old UPS a few KW or so, it will need to be dual conversion sinewave output. You may well be able to use some of it intact.
    The dual conversion means it is running the inverter all the time so its rated for continuous use. These are of course designed for IT equipment loads but if there is plenty of power headroom it may be a ok for motor running & starting, You might be able to soft start the motor, but in high spindle speed selection this might be a bit harder.
    Induction motors have a poor power factor when lightly loaded so your PWM inverter will have to supply the full VA, motor starting will be significant !!.
    Also the inrush current to your capacitor bank will need some limiting via either a NTC or relay & charge resistor, the latter preferable if you are switching the supply frequently.
    You mentioned 400W, is this nameplate rating.
    My lathe motor for example is a 0.37kw CSCR motor, the power meter shows 234W low gear spindle load only, amps = 2.4 so VA = 576

    Equipment W A PF % VA calc W calc
    Mill 390 5.94 29 1425.6 413.424
    Lathe 234 2.4 40 576 230.4
    Drill 203 0.85 98 204 199.92

    Blown Inverter welders are a good source of design info (& parts).
    I have just saved a UPS from the scrap bin last week- Powerware 9125-2000i, 2kw & there's lots of goodies inside. Even has a plugin control card that I could substitute.
    This UPS just has a blown IGBT & some PCB corrosion.

    Looking in this UPS (photo attached). PWM filter is 1 of the Large toroid inductors (red plastic shield around) & a 1uF capacitor just to the right.
    (there are 2 output inductors as the output is duplicated). There is also a current compensated choke & a couple of Y caps (common mode filter) shown top right of photo.
    The other choke & cap just below is the input filter.
    A simple sketch of the output is attached.

    Please let us know your progress as I also am looking at using a PIC18F2510 with sine tables to generate PWM AC voltage for inverter application.

    There is a company here in oz that make a VSD (or preset lower speed) for swimming pool pumps, they are getting reasonable saving for customers power bill just by running the pump motor at a lower speed, optimised for the filter setup.

    Toni
     
  6. Maketronic

    Thread Starter Active Member

    Mar 21, 2009
    49
    0
    Hi All

    Thanks for your replies.
    To be honest the project is as much about learning as it is about the end result (which I know I may not achieve).
    One interesting concept I saw in one application note was to dispense with the run capacitor and use the vfd to create the phase shift for the aux winding.
    I am quite in favor of this idea because it would create a psudeo rotating magnetic field and may well be self starting.

    I did think about grabbing parts from a ups but I was appalled by the horid waveform, (unloaded)

    With my experimenting I plan to start with a low voltage half bridge config and work my way up to a full h bridge.
    I was planning on starting with coarse pwm for the sine wave (maybe 15degrees between steps) and work up from here.

    I welcome and thank you for your help.

    Regards

    Bruce
     
  7. Maketronic

    Thread Starter Active Member

    Mar 21, 2009
    49
    0
  8. tonigau

    New Member

    May 2, 2011
    2
    0
    Sorry, correction...
    The X cap before the common mode choke is 10uF on this UPS & there is a 0.47uf X cap after the choke as well.

    Microchip have some good app notes on PWM power sine wave generation/induction motor, AN843, AN1279, AN887
     
Loading...