single phase to three phase

24 Nov 2004
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United Kingdom
is this (attached schematic) "phase converter" drawing correct?
I have attempted to draw out the circuit of a machine that I have.
I think it is a static phase converter, which as far as I can make out is essentially a start/run capacitor system for getting a 3-ph motor turning, at which point the motor falls back to being supplied by 2 phases

it does of course have a large transformer, wired as an autotransformer (I think) but that appears only to be increasing the voltage.

the machine has this "boost" function controlled by the voltage sensitive relay, which seems to be the start capacitor(s)

why is it called "boost"? I could understand if the boost function was increasing voltage but I can't see that happening in this arrangement


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From the manual for some similar equipment:

The initial surge required to start the motor is provided by the boost circuit of the converter. The “boost on” light indicates the
circuit is energised. The circuit is controlled automatically from within the converter, switching on whenever a motor is ready to
start and switching off once the motor has attained its full running speed. If the setting of the switch is too low the motor will
struggle to start and the boost light will stay on. If the setting is too high the motor will sound rough when running, again the boost
light may stay on.
The optimum setting for a given single or multi-motor application is determined by quantifying the phase to phase voltages at the
motor/starter with the boost light out and the motor(s) running. The voltmeter fitted to the converter indicates the voltage drawn
between L1 and L2. This is the “artificial” phase induced by the motor. When commissioning the driven machine, bear in mind that
this voltage is only established once the boost light is out and the motor is running. Adjust the selector switch to the setting where
this voltage is as close to 400/415 volt as possible. The best overall balance of voltages will correspond to the correct switch

So it seems to be boosting the L2 phase only to help with starting load with more capacitors.
Yes, that's the circuit - I havecone of those which I "disposed of" when it was being scrapped at a previous job.
The auto-transformer steps up the voltage which provides two of the output lines. In theory, if you spin up an induction motor and connect the two lines then it will act as a rotary converter and create the third line. However, it won't be well balanced - so adding variable amounts of capacitance as shown helps to balance the lines.
And as stated in the manual extract, it needs more adding to the third phase for starting. Starting is by far the hardest part of this sort of project.

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