RE: Three phase conversion In Rush current Concern.

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>

Hi Jim,

I was originally going to reply to the effect of one of the other 
replies that suggested that since the output was to be rectified, why 
bother with three phase, and merely beef up the DC link filtering. I 
snipped that part out when I vaguely remembered that Jim had a ready 
built DC power supply, which likely had a three phase transformer. So 
now that you have confirmed my memory of the facts at hand, I would 
suggest that the three phase output currents from the rotary 
converter (without the generator) are going to be close enough that 
as long as you don't run your power supply to the limit, life will be 
good. Your kVA rating is going to decrease, because common core three 
phase transformer designs bank on the "flux sharing" of sorts to 
achieve better core utilization. That is why they are much more 
compact than the equivalent circuit using three separate single phase 
transformers (which utitlities do use sometimes for really large kVA 
ratings because otherwise things get too large to practically build or move).

If and when you add the generator to your converter, the output will 
be about as perfect as perfect can be, and this problem will be a 
thing of the past.

Regardless of how balanced your three phase ends up however, of 
course we know that power in is going to be somehwat more than power 
out, because even state of the art rotary converters (M-G sets) are 
less than 85% efficient. (A good drive will beat this without any 
moving parts BTW).


Cheers,

Leigh

	-----Original Message-----
	From: Tesla list [mailto:tesla@xxxxxxxxxx]
	Sent: Fri 29/09/2006 10:33 PM
	To: tesla@xxxxxxxxxx
	Cc:
	Subject: RE: Three phase conversion In Rush current Concern.
	
	

	Original poster: "Jim Mora" <jmora@xxxxxxxxxxx>
	
	Hi Leigh et al,
	
	On phasing the output, I have a three 1256d's. Perhaps they could have some
	form of balancing chokes for three phase? This setup is supposed to work
	well driving more induction motors, it is hard for me to assume how it will
	work with a three Phase 208 wye high voltage transformer supply...which
	would be manually connected by another contactor in the control console once
	the idler motor is up to speed.
	
	As Always,
	Thanks for the feedback and ideas
	
	-----Original Message-----
	From: Tesla list [mailto:tesla@xxxxxxxxxx]
	Sent: Friday, September 29, 2006 3:23 PM
	To: tesla@xxxxxxxxxx
	Subject: RE: Three phase conversion In Rush current Concern.
	
	Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>
	
	Hi Jim,
	
	
	Depending on your budget, a 3 phase variable speed drive (there is a 50
	HP unit on e-bay right now for $330 for example) would not only allow
	you to control start up, but it would create true 3 phase from single
	phase. Most of the drives out there use a static rectifier (diode, not
	SCR or transistor) and will run on single phase provided that you
	de-rate them accordingly (50% is typical).
	
	Otherwise, a pair of TRIACs slowly ramped up would be effective as a
	soft start.
	
	Simpler, but more expensive would be a tapped autotransformer or
	reactor, or even a resistor bank, stepped out with contactors on a time
	delay or current basis. All of this was the norm for starting large
	motors 30+ years ago, and is still in use today in many cases.
	
	You may find starting a bit troublesome (although you won't require much
	starting torque) with reduced voltage techniques however, given that you
	are faking your 3 phase with capacitors.
	
	Cheers,
	
	Leigh
	
	-----Original Message-----
	From: Tesla list [mailto:tesla@xxxxxxxxxx]
	Sent: September 29, 2006 12:51 PM
	To: tesla@xxxxxxxxxx
	Subject: Three phase conversion In Rush current Concern.
	
	Original poster: "Jim Mora" <jmora@xxxxxxxxxxx>
	
	Hello All,
	
	Here is the schematic of my proposed three phase converter to power my
	Raytheon 3 phase 24 KV DC power supply (I also have a generator in
	progress). The capacitors are mid-range based on other phase converters
	documentation and are subject to trail and error, to achieve balanced
	output.
	
	http://hot-streamer.com/temp/JimMora/3PhaseStaticIdlerMotor.pdf
	
	If anyone has experience in this arena I could use some help.
	
	The motor will run at ~3425 rpm which has the benefit of handling sudden
	change in current draw (think spark gap) and the disadvantage of inrush
	current to start it up.
	
	The motor will draw around 49 amp in static state load conditions. The
	LR
	rating @ 240v is 290 amps, yikes. I have seen a few schematics of "soft
	start" with triac's or SCR's.
	
	I think  my neighbors to the right will see a burnout as will their
	computers as we are a distance from the pig and the service wire is
	1950's #
	4 I think. I have upgraded to 100 amp service. Beef connected to turkey
	at
	the Mast head.
	
	Any suggestions would be appreciated, I am a novice in poly phase, yet
	know
	the complexities of a seeming simple circuit.
	
	Thanks Much,
	Jim Mora