Gentlemen, using a "phase pusher" capacitor is a cheap and common workaround when a three phase supply isn't available. I have used this with a lathe for some time, since a machine like this doesn't need its full power most of the time and it always starts idling without load.
Ordinary three phase current runs 120 + 120 + 120 degrees to make a full revolution. If you have only two, this makes it 180 + 180 and won't run because there is no direction information in it. A capacitor connected to one leg of the two-phase system produces a 90 degree phase shift relative to this lead. Using this as a mock three phase system, you will have 180 + 90 +90 degrees for a revolution including a direction information, depending to which leg you connected the capacitor. The distribution is uneven which is the reason for reduced power, but better than nothing. Don't confuse this with a starter capacitor used to supply a direction information to a generic two phase motor! Those are for short time use with small motors only and blow their tops when used continously (because of faulty starter relay or so). Of course, the size of the capacitor depends on the amount of current it has to supply to the third leg. Having learned from practice, I used at least 70 microfarads per kW to achieve about half the power the motor can deliver on a real three phase system. More capacity doesn't contribute much. The capacitors need to have a voltage rating of mains voltage times sqrt of 2. The cheapest way to get them ( I was a student then) was cannibalizing the current compensation capacitors from old flourescent lamps at the junk yard. I needed about a dozen of them, but at zero expense. Watch for belly shaped tops, there is plenty of stinking smoke compressed by the factory in these aluminum cans! Perter Blodow andy pugh schrieb: > On 14 March 2011 15:30, gene heskett <ghesk...@wdtv.com> wrote: > > >> If I read it right, it is being said that perhaps a capacitor of the >> correct value to create a usable phase lead, from L1 or L2 to L3, will >> start a 3 phase motor on single phase power, direction of the rotation >> dependent on which 2 the capacitor is connected to. >> > > Yes, in fact this is how my coolant pump is wired. You don't get the > same power output, and the motor needs to have an external star point > so that it can be wired for the lower voltage, but it works fine. > > In fact, many of the cheaper single phase motors are exactly that, a > three-phase motor and permanently connected capacitor, with no > centrifugal switch. > I don't know if it is still the case, but the single-phase motor we > bought from Machine Mart was exactly this, and had far too little > starting torque for the vehicle lift we wanted to run. > http://www.machinemart.co.uk/shop/range/details/230v-110v-single-phase-motors/path/single-phase-electric-motors > > ------------------------------------------------------------------------------ Colocation vs. Managed Hosting A question and answer guide to determining the best fit for your organization - today and in the future. http://p.sf.net/sfu/internap-sfd2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
