Hi Declan, What I was doing is not a 2 to 3 phase inverter. I connect all coils to the mains and don't tap a "third" phase from it. Neither is there a high impedance phase.
Your practical comments are valueable and will hopefully improve the theoretic approach. I think, theory will provide a starting point so you will not start in full uncertainty about the order of magnetude of the capacitor. Regards, harry > > I have worked on some of these single to three phase inverters. It's a > very imprecise science. > > They use a phase shifted neutral, and all measurements in the '3 phase' > section of the circuit are with reference to the phase shifted neutral. > That loses me already. I'm just back from a holiday, and not up to this > at all. > > Doing it this way, they get two pahses and a single (capacitative) very > high impedance third phase. Theory doesn't matter; A 5A motor will never > run at 5A anyhow. 'Experts'then just add capacitance until they get > desired results, and keep all control phases away from the high > impedance phase. Measurement is of volts with reference to the the > neutral, and each other phase, while on full load, and performance is > lousy. Then check things don't go crazy when you are off load. The real > art is making sure nothing important is on the phase that does vary. > > > With best Regards, > > > Declan Moriarty. > > Aug 31, 2003 at 05:39:21AM -0800, H.C. Croon enlightened us thusly > > > Hi Edi, > > > > > I have a 3-phase motor witch I'd would like to run with only 1 phase, so I > > > need a capacitor to "generate" the missing phase. > > > > > > How can I determine the value of the capacitor? > > > > > > The motor has following type label: > > > > > > 220/380V, 1,3/0,75A > > > 0,35PS, 0,25kW 50Hz > > > cos phi 0,70 > > > 1400 U/min > > > > From a theoretical point of view I am comming to the following > > approach to the problem. As my experience with motors is very > > restricted, talk with practically skilled guys too. > > > > Driving a three phase motor with one phase > > 1. Analysis of the motor > > Motor data: > > 220/380V, 1,3/0,75A > > 0,35PS, 0,25kW 50Hz > > cos phi 0,70 > > 1400 U/min > > > > The motor must be star connected for 380V and triangle connected for > > 220V. That means that each motor coil in all cases recieves 220V and > > the voltages of the coils have mutual phase differences of 120 > > degrees. The current through each coil is 0.75A. > > > > The power data of motors are normaly the mechanical power delivered > > to the mechanical load. So, not knowing the efficiency, we can not > > determine the power consumption out of these data. But we can use the > > cos phi data. > > > > As the motor has been build up symmetrically, this cos phi is valid > > for each coil. > > As a model for a working coil we use the series circuit of a resistor > > and an inductance. The resistor is a combination of the ohmic > > resistance of the coil (the minor part) and the induced resistance > > from the cage rotor, which is a short circuited construction. Coil > > and rotor act as a transformer working at slip frequency and the > > power is transfered to the rotor, inducing a virtual resistance in > > the coils. > > > > Drawing a triangle with the resistor voltage, the inductance voltage > > and the total voltage, which triangle has angles of 90 degrees, 45.6 > > degrees (arccos 0.7) and 44.4 degrees, we get: > > Vr=Vtot.cos phi=220*0.7=154V > > and > > VL=Vtot.sin phi=220*0.71=157V > > This gives an equivalent R=154/0.75=205Ohm, and > > an equivalent inductance of L=157/(2*pi*50*0.75)=0,666H. > > > > Using these values we can calculate the consumption of electric power > > as > > Pe=3*R*I^2=3*205*0.75^2=346W. So the efficiency is 250/346=72%, which > > is a reasonable value. > > > > 2. One phase application > > We can consider different topologies, where the best seems to be: > > coil 1: starpoint to N, hot point to L > > coil 2: starpoint to capacitor 1, hot point to N, other side > > capacitor 1 to L > > coil 3: starpoint to N, hot point to capacitor 2, other side > > capacitor 2 to L > > By exchanging coils 2 and 3 or by changing the polarisation of coil 1 > > the motor direction can be reversed. > > > > In this configuration the capacitors have to cause capacitive > > currents of 0.75A through the coils. Be aware that the model used is > > based on a 3-phase situation for the rotor induction. Now this > > situation has been changed and the model of section 1 will only be > > approximately true. Nevertheless we use the model of section 1. The > > real model for this case being different, some experimental work, > > observing the currents in the coils when loading the motor, can > > improve the circuit. > > > > To achieve a current of 0.75A we compute the capacitor value from: > > 220/SQRT(R^2+(?L-1/?C)^2)=0.75. R and L being known this yields: C=0 > > (inductive behaviour) and C=7.6 uF (capacitive behaviour). The phase > > difference between the currents in coils 2 and 3 with respect to the > > current in coil 1 is approximately 90 degrees. > > > > A theoretical possibility of connecting the coils in parallel and > > using only one series capacitor of 15.2 uF will yield not understood > > interactions between the coils as a consequence of the induced > > voltages by the rotor, so this is not recommanded > > > > The resulting direction of the magnetic field of coils 2 and 3 is > > orthogonal to the magnetic field of the coil 1. The strength of the > > field of coils 2 and 3 is SQRT(3) times the strength of the field of > > coil 1, so the magnetic field is elliptical. > > > > Where this motor can be connected as well in star as in triangle all > > the coils are separately connectable. So the proposed circuit can be > > realised. > > > > For some aditional stuff see > > http://www.dpaonthenet.net/drives/drives_aug0203.htm > > > > Please give me some feedback when you succeed. > > > > Best regards, Harry > > -- > > Author: H.C. Croon > > INET: [EMAIL PROTECTED] > > > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > > San Diego, California -- Mailing list and web hosting services > > --------------------------------------------------------------------- > > To REMOVE yourself from this mailing list, send an E-Mail message > > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > > the message BODY, include a line containing: UNSUB CHIPDIR-L > > (or the name of mailing list you want to be removed from). You may > > also send the HELP command for other information (like subscribing). > > > > -- > > With best Regards, > > > Declan Moriarty. > -- > Author: Declan Moriarty > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). > -- Author: H.C. Croon INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
