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RPC's and really small load motors

Rob Russell

Plastic
Joined
Feb 3, 2003
The rotary phase convertor (RPC) I'm building is sized to run woodworking machinery that's about the same size as the idler. I've got a 7.5HP saw/shaper, 7.5 HP joiner/planer, 7.5 HP sander and 5.5 HP dust collector. I'd expect to always be running 1 machine + the DC, so running HP will be in the 12HP range. My idler motor is 15HP. I've sized breakers/contactors/overloads so I can run 2 machines + DC and just wouldn't start the machines at the same time.

Question - can I run a small load on this RPC? For example, I'm now looking at line boring equipment that has small (1.5-2 HP) motors in relation to the RPC. Is that a problem? Or - is this the reason to have multiple idlers that get pulled in as HP needs increase?

I don't really want to go back and start the scrounge process all over again to swap out the 15HP idler and all the controlling gear to go to a dual-7.5 HP idler setup.
 
Absolutely!

If your three leg voltages are reasonably balanced when the 15 hp idler is running unloaded, you are all set. If you hooked up a 1/4 hp three phase load motor, it'll be in heaven.

One reason I like dual idlers is that you can shut one OFF if you don't need the power (or just don't turn it on). But it never hurts to have an oversized idler(s); for some applications such as regenerative braking it is even necessary.
 
Consider the pathological case of an "infinite bus", which is what a way, way oversized converter would simulate.

If the phase voltages are indeed balanced while "idling", application of an infinitely small load wouldn't change the condition of the bus voltages.

Only when the load on the bus becomes "significant" [ * ] are the bus voltages significantly affected.

[ * ] This is non-specific, I agree, and depends upon how well balanced the converter is and how "stiff" it is.
 
peterh5322,

What do you mean by "stiff", in terms of the RPC. How well the phase balancing holds up under load?

Rob
 
By "stiff", I mean a very low impedance.

This is the case where a small change in load has no, or almost no perceivable affect on the phase voltages.

The proverbial "infinite bus" is about as stiff as it gets, as no matter how much the load is increased, the phase voltages will stay the same.

Probably the antithesis of a stiff phase converter is where the "idler" is equal to or smaller than the size of the load motor.
 








 
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