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).
