RE: Three phase conversion In Rush current Concern.

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



	-----Original Message-----
	From: Tesla list [mailto:tesla@xxxxxxxxxx]
	Sent: Fri 29/09/2006 10:52 PM
	To: tesla@xxxxxxxxxx
	Cc:
	Subject: RE: Three phase conversion In Rush current Concern.
	
	

	Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
	
	At 03:22 PM 9/29/2006, you wrote:
	>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).
	
	They'll run a motor (and relatively smooth running too), but the
	filter capacitance usually isn't enough to hold the dc bus voltage up
	between half cycles.

	

	<LC>Very true, but as long as you de-rate the drive by/to 50% the DC 
link filter components are large enough. I have 8 of these things 
from 2 HP to 50 HP running in my own shop; most of them being over 5 
HP. I live several kM away from 3 phase, so I step up the 240V single 
phase to 575 VAC single phase, and power the drives from that. My 
lathe is 25 HP spindle + roughly 5 HP of hydraulics, feed motors and 
coolant pump, and it runs from a single 50 HP Allen Bradley Powerflex 
700VC. Allen Bradley actually stands behind the product as long as it 
is de-rated by 50%, and you follow their connection requirements 
(internal cooling fans are power off of a particular pair of line 
input terminals, so it matters which phase you leave unconnected).

	
	
	
	>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.
	
	How many steps do you need.. One resistor (or light bulb) in series
	with a big relay might be enough.
	
	
	>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.