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Combination RPC 5hp and 20hp idlers

Joined
Apr 17, 2020
Hello all!

I finally got my 16in pacemaker ready to run, just need power now!

The lathe is 220v 3phase 7.5hp. I have had a 5hp 3phase motor I took off a compressor years ago when I swapped to a single phase motor. Kept it because, well, I keep everything!

So I've done tons of reading, and I have decided to build a combo RPC, because I got a like new 20hp motor for free.

I've started building my box, with the plan as follows- start the 5hp idler like any other RPC, single phase with a timed contactor with a start capacitor. Once 5hp is running, use the 3phase generated by the 5hp to start the 20hp. Once the 20hp is online, I can either leave the 5hp running for better phase balance/generation, or shut it off. The 5hp will have run capacitors to balance it as a single RPC, so if I get a 3hp or less machine I can run just it. The 20hp will have run capacitors as well when it is online.

So, onto my questions/concerns- my 20hp is a 3600rpm motor, with delta connection high and low voltage. The 5hp motor, is a 1800rpm, I can't find the wiring online for it but the plate just has a Y in between the low and high voltage wiring diagrams, so I assume is wired in a wye.

Will these two idlers be ok together? I have read about delta RPCs running wye wired equipment, and visa versa, so I assume (dreaded word) they should be fine. They will not be physically connected.

Anyone have any thoughts or advice? I really wanted to use the 5hp to start the 20hp to reduce the stress of getting the 20hp going on single phase, and so I can have a small RPC for light loads, or a nice large size one for my eventual equipment expansion.

Thanks!20201213_132032.jpg20201213_132039.jpg20201213_132052.jpg20201213_132103.jpg
 
So I've done tons of reading, and I have decided to build a combo RPC, because I got a like new 20hp motor for free.

You are basing a design on something just because it is free. If the 20hp motor is dead quiet then good. Otherwise it will be like a nagging girlfriend
complaining that it is either too cold or too hot.
 
I've run both motors at work on 3phase and they seemed similar noisewise to me, definitely not near nagging girlfriend decibels, or annoyance. :D They are both going outside of the main shop in the air compressor room anyway.
 
I've run both motors at work on 3phase and they seemed similar noisewise to me, definitely not near nagging girlfriend decibels, or annoyance. :D They are both going outside of the main shop in the air compressor room anyway.

Not sure a 5 HP idler will start a 20 HP without some help? Have you tried this, yet?

Easier for me with a mix, 3, 5, 7.5, and the 10 HP as the largest and the very one the Phase-Craft control was built to start and run, back when it was sole player.
 
they both look to be Wye wound motors, delta will usually only be one voltage unless wound delta Wye. best to ask the manufacturer to be sure.

and running 2 separate motors will need 2 separate contactors and capacitors to keep it balanced. AKA another control box is needed.
 
... running 2 separate motors will need 2 separate contactors and capacitors to keep it balanced. AKA another control box is needed.

In the case of the widely disparate idlers, here, yes, he may need two different values of "start" cap as well as run/balance.

With finer granularity, not necessarily.

"Box" yes. Stuff has to live SOMEWHERE. Zorro has "NK" (No Knockout) Wiegmann ones affordably priced.

Put the run/balance caps motor-side of each contactor though, yah need a "control" AKA RPC starter box only capable of starting the heaviest one.

So basically only an extra contactor for each.

"Control" is otherwise just a row of switches to select which contactors are to put their associated motor/cap combo to work ... or drop it out.

ISTR another member had a nice neat schematic for his one?

I do not. Can't even recall where I left my India ink...

:D
 
I have three contactors, one for each motor, and one for the starting cap for the 5hp. I also have a disconnect for everything, and a box for just the run caps. Here is the wiring diagram for the 20hp from the manufacturer.image.jpg
 
I have three contactors, one for each motor, and one for the starting cap for the 5hp. I also have a disconnect for everything, and a box for just the run caps. Here is the wiring diagram for the 20hp from the manufacturer.View attachment 307625

Bog-common 2XX/4XX connectable for either Voltage, and straight Delta wound.

No mysteries to them.
 
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Got everything but the amp coils set up, and wired the motors temporarily. The picture is with the 5hp only running. From left to right, A-C, B-C, A-B. It will start up the 20hp, but I have the system plugged into my welding outlet to test, and it gets probably halfway up to speed and trips the 50amp breaker. I have a 70amp breaker I will try later today. The 20hp might start better with the run caps installed for the 5hp. The contactor for the 5hp idler is noisy as heck. I have 2 more like it I can test and swap in if the are better.20201215_191306.jpg
 
With some run caps installed on the 5hp, bringing the ghost leg up, the 20hp will start easy with the 5hp running on my 50amp welder circuit. The combined idlers will just start the lathe pinning up but it trips the breaker before it is up to speed. The 50amp welder circuit isn't enough for the two idlers and starting the 7.5hp lathe.

So I am going to hook up the amp coils so I can see better what everything is drawing. I also need to work out the run caps for the 20hp. Getting there!
 
I have a 70amp breaker

How about - #4 Copper, I think it is? #6 is about 60 Amps? Temperatures, run-length, conduit crowding/not.. all enter in to the figures.. See handy tables, online and not-only for "Ampacity" and suitable types.

Not that your are pushing it that hard, YET, but you don't want to use the smaller wire as EITHER of "ballast resistor", "space heater", nor "fuse", as that's what it will do, unasked. "Ohmic losses" becoming part of your power bill.

Sez he who keeps five colours of #4 THHN/W on hundred-foot rolls "handy.." just on principle.

Well. a minor "investment", too? Given that Copper prices have been reliably going UP, not down, over long years!

:D
 
With some run caps installed on the 5hp, bringing the ghost leg up, the 20hp will start easy with the 5hp running on my 50amp welder circuit. The combined idlers will just start the lathe pinning up but it trips the breaker before it is up to speed. The 50amp welder circuit isn't enough for the two idlers and starting the 7.5hp lathe.

So I am going to hook up the amp coils so I can see better what everything is drawing. I also need to work out the run caps for the 20hp. Getting there!

the 20HP is usually rated for 49A at 3Ph, so in single phase its pulling 85A and start up is more like 150A inrush

so no a 50A 220V 1 ph breaker wont do it. 20HP needs at least a minimum 100A at the least and ideally 110+Amps.
 
the 20HP is usually rated for 49A at 3Ph, so in single phase its pulling 85A and start up is more like 150A inrush

so no a 50A 220V 1 ph breaker wont do it. 20HP needs at least a minimum 100A at the least and ideally 110+Amps.

80A will probably do it OK, "real world" with the 5 HP already up to speed and assisting as if a "flywheel". 70A "only maybe".

The 20 HP idler will not jump-up and hit speed "instantly", though.

THIS .. is one of the reasons why I have FOUR "stagger-started" idlers to total a max of 28 HP. But only the largest is even 10 HP!

Starting and running the loads is easy, even though none have a clutch.

The RPC's OWN startup, OTOH, can otherwise be a ball-buster!

"QED"

:D
 
the 20HP is usually rated for 49A at 3Ph, so in single phase its pulling 85A and start up is more like 150A inrush

so no a 50A 220V 1 ph breaker wont do it. 20HP needs at least a minimum 100A at the least and ideally 110+Amps.

The 5hp is drawing about 8amps just idling, it will easily start the 20hp on the 50amp breaker without tripping it in about 1 sec to full speed, and the 20hp is only drawing about 28amps idling. So 36amps combined idling, not enough headspace to handle the startup of the 7.5hp lathe motor on the 50amp circuit. As Thermite stated, the wiring is also a limiting factor. I have a roll of #4 and an 80amp breaker. Going to install a 100amp industrial plug I have to connect the RPC to the main panel, with #4 to the RPC as well.
 
So i would probably suggest larger diameter wire, as #4 is usually rated about 70A, and putting that on an 80A breaker is asking for a fire.........

No, nothing REMOTELY that dramatic.

First-off, #4 has more than one rating. It gets into temperature, conduit & crowding vs open-air, insulation type, wet, dry, etc. turf.

Talking COPPER, I hope, so the REAL danger zone, best-case, is up over 100 A.

Can be good for sustained 85 A @ 75 C f'rinstance. And HIS use is peak, actually, not sustained.

The wire will carry markings. Check 'em out, look it up online, stay within code.

It is no harder than NOT adhering to Code and seldom even costs enough to be BOTHERED trying to find a shortcut or cheat.

JFDI and move on to other needfuls. Proven path has value.
 
I have the system installed and tested many times now. Used #4 wire to feed the RPC, with a 70amp breaker. After balancing more with an oscilloscope, I have been able to get 0, 88, and 180 phase balance with the lathe running taking a 1/4 cut in 12l14. With the lathe taking a cut the system draws about 41amps on each leg of the 240 single phase. Way better than the 60 it was drawing. I found I needed the 70amp size for start up surge only. It would occasionally trip a sixty so I put in the 70 and all has been well. The interesting thing is if I shut off the 5hp after the 20hp and lathe are running, it will draw 48smps a leg. Just figured I'd let everyone know how it went.
 








 
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