How to Size a Phase Converter

This discussion of phase converter sizing includes a brief, clear description of how phase converters work. April 21, 2011

Question
I've recently acquired a Weinig p22n 4 head 15hp top, 10hp bottom and 10hp shared for the sides, and 3 hp feed. I am told I need a minimum 30 hp phase converter. Iíve read on here that bigger moulders than mine are run on 20/25hp converters. My machine has an Allen Bradley SMC plus system that evidently allows for a ten second soft start. Can someone please educate me on the converter sizing issue?

Forum Responses
(Solid Wood Machining Forum)
From contributor Z:
I ran this model on a 50 hp phase converter. It had an extra head, and I needed it for the dust collector too, which is another 15 hp. Though it is possible I had more phase converter than I needed. Won't you also need a three phase dust collector on this? You might want the molder on 440 volt to bring the amperage draw down which means adding a transformer.



From the original questioner:
I am wondering how a 70hp 6 head moulder plus or minus the dc could be handled with a 25hp converter. Iíll also be running a 10hp 4 up 4 down collector.


From contributor Z:
You only start one motor at a time on a moulder. So once started, the load is low, and you can keep starting motors. Not sure when it maxes out.


From the original questioner:
Iíve already submitted the machines info and got sizing requirements. 30 hp is recommended. I had a 3 phase Logosol ph260 a year or two ago. This Weinig is my first real moulder. I want to do this right the first time.


From contributor O:
Within some limits one sizes rotary converters to balance the missing phase of the largest motor. In fact all the rotary phase converter is a self switching pair of capacitor banks and another three phase motor that is being used on two windings as a motor and as a generator on the third, which when coupled to the load motor the two motors cross supply each other the missing phase. That is why the converter is sized to the largest motor.

Additional motors, brought on line one at a time, can, with most converters, be added up to three times the rated hp of the converter and all they do is cross feed and support each other. The max load that the converter and its motor ever seeís is the imbalance in the largest motor.