Author Topic: Understanding RPC's  (Read 157 times)

jpigg55

Understanding RPC's
« on: December 14, 2022, 08:40:07 PM »
I ran across and watched this video today: https://www.youtube.com/watch?v=mUXLxK_zI24
It gave me a better understanding between Wye & Delta windings and expected readings.
SB 9A, Clausing 8520, Sanford MG 612

4GSR

Re: Understanding RPC's
« Reply #1 on: December 14, 2022, 09:06:19 PM »
Watched his video this past weekend.  Very informative.  Back when my dad had his shop, we had 240v 3-phase power.  At the time I always thought it was Wye 120/208v even though across any two legs it measured 240 volts.  It was actually Delta-Wye 120/240v we had.  The high leg to ground was something like 190 volts from what I recall.

Talking about RPC's.  They actually produce a artificial "third leg", which creates real 3-phase power for most parts.  But all of this has to be balanced out with capacitors to make it 99% true.
Ken

jpigg55

Re: Understanding RPC's
« Reply #2 on: December 15, 2022, 07:39:37 AM »
Talking about RPC's.  They actually produce a artificial "third leg", which creates real 3-phase power for most parts.  But all of this has to be balanced out with capacitors to make it 99% true.
Yep, haven't found a good resource about choosing the type, rating, and number of capacitors that an RPC would require if one wanted to build their own.
The surface grinder I purchased quite a while back has a 3 phase motor so I purchased the 3 HP motor RPC he had as well. I still have them in storage and haven't hooked it up yet. This video will help me when I do.
SB 9A, Clausing 8520, Sanford MG 612

4GSR

Re: Understanding RPC's
« Reply #3 on: December 15, 2022, 08:54:10 AM »
On smaller motors, if your SG has a 1-1/2HP motor, choose a RPC that is double what the main motor is. So 3 HP RPC would be correct to use with your SG. 
Ken

Carpenter84

Re: Understanding RPC's
« Reply #4 on: December 15, 2022, 09:58:57 AM »
I have a 15hp RPC that runs my shop. The only issue I have with it is the voltage drop on the third leg when starting the lathe, which is the biggest draw machine in the shop. On a hard start the in-rush is nearly 30 amps @ 600v. With the 10" 4 jaw chuck in high speed the motor often single phases when trying to start up. Usually takes a few tries. But, I don't often run that fast for it to be much of a concern. I know there is a way to change/add caps to steady the voltage drop but I haven't looked into it. I should tho...
Shawn

4GSR

Re: Understanding RPC's
« Reply #5 on: December 16, 2022, 04:13:27 PM »
I've seen people run a vfd on hard to start motors ON 3-phase juice from RPC's.  Cheaper than buying a true soft start motor controller.
Ken

Carpenter84

Re: Understanding RPC's
« Reply #6 on: December 16, 2022, 05:04:36 PM »
Really...?! Because I was looking into soft starters for the lathe but stopped when they were very expensive, and actually had a surprisingly limiting cycle per hour limit - which I don't really understand. A vfd doesn't have that issue. Altho, 600v vfd's aren't cheap, it's probably better than the whipping the RPC usually takes, or whatever damage I may be causing the motor every time it single phases.

Good info, Ken. Thanks.

Carpenter84

Re: Understanding RPC's
« Reply #7 on: December 16, 2022, 05:09:12 PM »
However, I'm not sure how interested I am getting into a big wiring thing with the lathe (the high/low speed switch makes it a bit more complex). The lathe is too close too the wall to really work on the control box (ya ya, I know - limited shop space). And I'll have 3 phase hydro soon. I would have had it already but I couldn't get my ducks in line quickly enough before the ground froze too hard to dig a trench.
But maybe.... Variable speed would be nice...

Hmmmmmmmmmmmmmmmm... 🤔

Ken! Look what you started...