rotary phase converter question

[slowcoach]

Hi all,

Need a bit of advice. I want to power a 3 phase mill off a rotary converter. The mill motor is 2.5hp and the rotary converter idler motor is rated at 2.2kw / 3hp. My question is, will the mill run ok being so close to the rotary's 3hp limit?

Cheers
Rob 

[enginenut]

IIRC the load motor can only be 2/3 the size of the RPC idler motor due to the fact you are using 2 of the idler fields as a motor and the other generates the wild third phase. I think, possibly, I ain't no expert, YMMV etc.......

[John Hill]

The motor on your mill is only 2.5HP when it is fully loaded, otherwise it is something less.

[Lew_Merrick_PE]

Rob,

The technically correct analysis is: 2.5 HP * 3/2 factor = 3.75 HP.  There are 746 W/HP, so 3.75 HP * 0.746 kW/HP = 2.8 kW as a direct conversion.  The problem with this is that a motor starting under load (say a heavy flycutter) can easily draw 300% of the rated current for a short period of time.  To a certain degree (and I am not an electrical engineer, so put the appropriate amount of salt on my opinion) the motor/generator in your rotary phase converter has nearly the same over-current capability as the motor on your mill.  The flip side of this is that, if you are careful about starting loads you would almost never draw the full rated power in a cut except by stalling the motor.

Your mileage may vary...

[Jonny]

Spot on Rob and why i had to go for a 7.5kw to run a 2.2kw motor!
Current draw was like starting up a 9hp motor.

Its not only the startup current but the running load, ok in theory but in practice make the machine worthless.
I have a rotary convertor 5kw and did have a 5.5kw digi invertor. Digi no chance of powering up without gearing down and maybe a push start. Rotary groaned like no tomorrow really scary.
Heres the key, both rotary 5kw and 5.5kw digi inverter couldnt cope with normal machine use. Motor was too easy to slow down with menial cuts 1/16" deep, even worse was the rotary sounding like it was gonna explode, noise unbearable.
Using a 7.5kw inverter i cant get the spindle on same machine to slow, not even for 1/2 a sec facing off 3/8" cut in 6082 on 5 1/4" dia, thats the difference.

[slowcoach]

Thanks guys for the information you posted  I don't understand all of it, but the bit that I do makes sense. Jonny I have a inverter and yep, it slows down a tad when taking heavy cuts.

Rob