"Capitance"

[Kalee20]

Hi Jeffrey, indeed polyphase rectification (up to 30 phase) is common on some aircraft systems to improve power factor  and help reduce EMC effects of "current chopping").

Indeed! And I have been privileged to have met Dr. Ed in the 'day job' over such polyphase systems.

I once designed a 3-phase 400Hz 1kW transformer for a 30-pulse rectifier (15-phase output, 30 diodes). Just lots of secondary windings interconnected, lots of opportunities to get it wrong in Production...

But the input currents were pretty good, nearly sinusoidal. And that's good because not only are the harmonics small, but the load on the generator is steady. A balanced 3-phase system is constant power, meaning that the sum of the powers taken by all three phases is constant throughout the cycle - and so the load on the generator, gearboxes, etc is a constant drag with no 'cogging'.

[Ed_Dinning]

Well put Peter, it's good for everything in the system, improves powerfactor / efficiency, but does give the poor design and production Engineers some headaches

Ed

[reset]

Jeremy at the NBTVA sent me this curiosity this morning. He writes:

"I chanced upon a mystery word today: CAPITANCE. The source is 'the Practical Electrician's Pocket Book of 1961' and I have attached the scanned page. No other reference or index entry appears.

The nearest is Harmsworth's Wireless Encyclopaedia which gives CAPTANCE as a single word for capacitive reactance. But this doesn't seem relevant.

No dictionary I've tried, whether printed or online, or any electrical reference book, or web search has yielded anything at all on the actual word. From the context, all I can think is some mechanism of fault current limitation by the transmission-line behaviour of a close parallel conductors mains cable. But any thought would be most welcome."

Looking at the attached table, it does seem reasonable that the fatter the cable, the longer it would have to be to limit the short circuit current. Under these conditions, the circuit impedance would be very low, and as this is presumably only 50Hz, I don't see how shunt 'capacitance' (say) would be a factor.

Can anyone make sense of this?

Thank you!

Steve

GVR's own Amie used this method to create a transformer less heater supply. I posted a series on her project here:

http://radio.radiotrician.org/2020/02/ca...-lamp.html

In this simple circuit I series 2 15 volt bulbs and feed them thru thru the capacitor. If I short one the other continues to burn because it is a CURRENT limiting circuit. I used the method to build an amp. The high voltage feeding a rectifier circuit and the capitive circuit feeding the heaters.

In the world of wired telephone they require an amp alone the transmission path for this (and other) reason. Over long distance the lines length will create delays which are seen as phase shift which appears as "capacitive reactance" for the first 1/4 wave length. A bell lab paper from 100 years ago showed the losses and amplification required to overcome them. A 1/4 watt amp every so often opposed to one huge amp to do the same job. As memory serves it would take a 100 watt amp or 10 1/4 watt amp to satisfy the example circuit.
A plant using motors as the load will have inductive reactance loads and the plant engineer will add capacitor banks to counter that effect.