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]
>
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--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com
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