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Phase converter sizing

Frenchy

Aluminum
Joined
Jan 25, 2013
Location
Tucson, AZ
I know this topic comes up a lot but I always hear to double the hp of the motor you are running for a rotary phase converter. I have a four post hydraulic press with a 3hp motor and I'm wondering if I can get away with using a 5hp motor for the phase converter because I already have that motor laying around. Thanks in advance for any info.
 
Certainly you can. If you're planning to roll your own just be aware that tuning is required, meaning adjustment of run caps to match your load and do power factor correction. Not hard but you fill need a clamp-on ammeter to do it (and a voltmeter - usually one in the same). Still, it's pretty easy. just stay away from "static" phase converters. These days you can buy a small VFD so cheap that you might want to go that way instead.
 
I was planning on buying one of the pre-made rotary converter panels from ebay that go for around $150. Would that need tuning still?
 
Never even seen one. I've built quite a few over the last 30 years but I can't see you getting away with no tuning since they have no idea what your load looks like. If you're not planning to hang other equipment
t on it just buy the small VFD and program it for the simple functions (just setting parameters). Set up your soft start/stop, fixed frequency, and control inputs - you're done. And if you get one from eBay or amazon (Chinese) the worst part will be the manual, but it's not that hard. And you don't really need to over-rate it (people always say that). Only the input to the power supply gets overloaded (because it's using 2 of the three diodes and the filter cap has to work harder to cover the ripple).
 
I have no idea what that last part means but it sounds like a VFD is going to be the way to go. Thanks for the info
 
An RPC is a lot more DIY than a VFD. And if you plan on adding any other machines, the existing RPC will cover that, whereas another VFD is needed for another machine. And if a VFD breaks, you're screwed. If an RPC breaks, less likely, it's way easier to get back up and running.

Skip the static converter, take the motor that you have, wrap a rope around the shaft to get it going and start pressing! Selfstarting, features, etc, can be added later.

5 hp may be enough to run your press without tuning with caps. Hook it up and try it. Just be sure to get the shaft started in the right direction so the hydraulic pump is also rotating in the right direction. If pump motor overloads trip, add caps.
 
I was planning on buying one of the pre-made rotary converter panels from ebay that go for around $150. Would that need tuning still?

Not if purchased to match the idler motor HP, no. The builder does that for you.

Will the "tuning" be perfect? Never quite. It is always a compromise because it changes with the power demanded by the LOAD motor.

But it isn't all that BAD of a compromise, so mostly RPC JFW even with a wide range of loads.

My Phase-Craft, Jim even tuned for the slight difference as to whether the idler motor to be used with was to be a 2 pole, 4-pole, etc.

As to HP:

1.5 times load motor HP is about the minimum, and expects the load motor will start UNLOADED. Eg has a clutch or at least a low/no initial load. Really HARD starting loads can call for an RPC idler HP MANY times as large as the load.

Even so, one of the nicest things about an RPC is that you can ADD ON to the idler after it is up and running.

So if your 5 HP has trouble starting the load? Just start another unloaded 3-P motor with it first. That can be a dedicated idler OR just some other 3-P tool motor that can run unloaded. A 2 HP added, your effective idler HP goes from 5 HP to 7 HP, 5+3 = 8, etc.

More test lab than "shop", my 'array' has a total of four idlers 3 HP to 28 HP "summed", many combinations in-between. I can pick and choose to suit a wide range of starting and running loads and tests.

Once past the heavy starting load, the supplementary idler(s) can be shut-off.
The Phase-Craft was ordered to start the largest (10 HP), it has NO problem starting one of the 7.5 HP or the 3 HP.

Supplementary idlers do not need their own starter/controller - just a 3-pole contactor or the ordinary mag-starter switch as if they were ordinary load motors. As they can be. Lot of folks start their RPC, then a table saw, then a mill, then switch-off the saw. Or similar.


VFDs work. But.

... two machines need two VFD, four machines need four VFD. six need six, etc.

And each VFD has to be wired directly to its ONE load motor, so the controls must change, too.

One RPC can power many machines. No change to their controls. No extra wiring or switches.

Not a lot of advance planning needed as your machines and loads change by the hour or by the year.
 








 
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