Rotary phase converter

MF294-4 · Oct 17, 2014

Have a rotary phase converter on my mill. If I was to look at the output of the converter with a scope, what would be the phase relationship to each other? What is the percentage of power available compared to utility co 3 phase.

snafflekid · Oct 18, 2014

If you have a three or four trace oscilloscope and you monitor all the output voltages of the phase converter you would see three sine waves each 120 degrees separated from its neighbor. The three phase power transmission can be more cost effective for the utility. If a single phase and three phase system is compared with the same line voltage, the three phase will transmit 3x the power of the single phase system using only one more conductor. Advantageous for the utility. So um your percentage increase of power would be 200%

buickanddeere · Oct 18, 2014

(quoted from post at 23:33:04 10/17/14) Have a rotary phase converter on my mill. If I was to look at the output of the converter with a scope, what would be the phase relationship to each other? What is the percentage of power available compared to utility co 3 phase.

Two sine waves with the same amplitude and degrees separation of 180.
The 3rd sine wave will not match , being lower, distorted and 90 degrees offset.
True three phase is stable like a tripod. A rotary converters output is "T" shaped.
Back in the bad old days a rotary converter or a single phase motor driving a three phase alternator was the choice .
I would still pick the single phase motor driving the three phase alternator over the rotary converter.
Of course today a VFD is the choice for simplicity, torque control, soft starts, reversing, efficiency and availability of continuous full rated power from the motor without over heating.

snafflekid · Oct 18, 2014

Sorry, I automatically thought "VFD" when I saw the original post. VFD is the cleanest way to produce three phase.

Geo-TH,In · Oct 18, 2014

B&D is more of an expert than I. But if I recall the old rotary phase converters may have been at best AROUND 70%. So a 10hp motor will only put out 7 hp give or take. They worked and that's all the farmers wanted. It was cheaper in the long run for them to make old school convertes if 3 phase power wasn't available or the power company wanted housands to run them 3 phase and a heafty monthly service charge even if the farmer only used it a few times a year. Keep in mind this was decades before VFD's vere invented.

Rotary phase converter

MF294-4

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