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Thread: Help with my rpc set up?
01-12-2012, 10:27 PM #1
Help with my rpc set up?
Hello, I just bought a 16x40 lathe. It has a 3hp/4hp motor, three phase, mag start, controlled by a rotary switch for high and low speed. I received a 600 p-o-m series sfc with it. I have a twain 5hp motor for a rfc. When I switch to high speed it blows the 30 amp breaker. I spoke to p-o-m, and was told to increase the cap size to 400-480 because the higher load and length of load. Will this be enough?
And, how critical is wire size? I'm temporally running this off 12-4 S.O. cord that is about 30' in length.
I have a 7.5hp 1100rpm 3ph motor, but from what I have read the 1100rpm will not work. Can anyone shine some light on this for me?
Thanks in advance for your time. Be safe. schick
01-13-2012, 06:24 AM #2
I am some what confused as to what you have and what you want to do.
You have a p-o-m static phase converter.
Do you have a 5 hp rotary phase converter with an asian buit motor, or do you have a 5 hp motor you can use to make a 5 hp rotary phase converter?
You have a 7.5 hp, 1100 rpm motor but what do you want to do with it? If you are asking if it will work for a rotary phase converter (rpc), then the answer is Yes, it will work just fine.
I think you will need at least a 7.5 hp idler, rotary phase converter, for this lathe. The lathe motor can not deliver full horsepower when operated from a static. I am not familiar with the p-o-m and will look them up.
01-13-2012, 06:51 AM #3
I have a similar gearhead lathe running on a 7 1/2hp RPC and it is not enough to start in high gear unless the oil and grease has warmed up quite a bit.
And even then it makes a rather large fuss.
For a gearhead lathe you should have a RPC idler at least 3 times larger than the hp of your load.
To be happy the RPC would need to be 12-15hp.
01-13-2012, 10:08 AM #4
''For a gearhead lathe you should have a RPC idler at least 3 times larger than the hp of your load.
To be happy the RPC would need to be 12-15hp.''
You shouldn't have to go that big if you balance the power with capacitors to maintain voltage close to line on all three phases.
Check out the ''Delta vs. Star thread;
01-13-2012, 10:38 AM #5
Good morning, and thanks for the replies. I will try to clarify.
1. p-o-m, is Phase-O-Matic (sorry in looking up the model I see it is called Phase-A-Matic), static converter, The model # is PAM 600HD.
2. I'm using it to start a asian (5hp) motor. This combo will not run the lathe in the high speed mode. When in low and switch to high while the lathe is running, it tries to increase speed, but after about 5 seconds it pops the 30 amp breaker.
3. I do have a American made 7.5hp 3ph 1100rpm that is avaiable to use, but I have read and been told the rpm is too slow. I'm looking for verification if it will work? This one has been answered, thanks.
Are you running off a 30 amp breaker?
I hope this make my scenario clearer? With the information that has been given, I'm going to wire the 7.5hp motor with 8ga wire and 40 amp breakers, and up the cap size to the 400-480uMF to see how that works. I will keep you posted.
Thanks again for your time, and information. Be safe, schick
01-13-2012, 02:22 PM #6
I think you are going about it the wrong way and asking to burn out a perfectly good motor. Even if that static actually does something when switching to high range (not sure if the potential relay will do that), that is obviously a difficult start load even with full power and you are NOT getting full power with a static.
Raising the wire and breaker size to 40A is what is required for this load but IMO, you are asking for issues here. I would strongly recommend an RPC at a minimum. Sounds like even on an RPC, they struggle.
01-13-2012, 02:25 PM #7
Not exactly accurate. voltage balance is a function of load and that load is all over the map with the majority of loads being put on RPCs. Compressors are an exception. I can guarantee from rather recent and in depth testing that oversizing of an RPC is required in many applications.
01-13-2012, 03:56 PM #8
If I understand correctly, the static phase just add start up voltage (3rd leg) to get the idler running. Then your L1, L2 voltage runs threw it to the idler that generates the 3 phases.
If I'm missing something, please explain?
Thanks again, be safe. schick
01-13-2012, 04:11 PM #9
You are correct, your static is just acting, in this case, as glorified start caps for the RPC without balance caps. you MUST, repeat MUST install run caps IMO. I have done extensive testing with motors with imbalanced voltage. The more imbalance, the higher one leg or another will be. You can be pulling over FLA amps and not even know it if not protected and I sure hope that motor is protected by more than a breaker.
You should not be running a 7.5HP load with a 5HP converter. Here is what you need to do. Install switching to "start" your other 7.5HP idler motor after your 5HP. Then you need to follow the stickies above to get some run caps in circuit and tune voltages so A-C and B-C are about 10V over your single phase voltage. This will balance down with load.
When you run the 5HP and the 7.5HP, you will get the combined RPC performance of 12.5HP which will be about perfect for you. You do not need more start caps for the 7.5HP motor. I have a setup like this and use a mag contactor to bring in another idler when needed with a wall switch.
01-13-2012, 09:29 PM #10
From your discussion, I take it to mean that the wall switch adds an aditional idler AND its appropriately sized run caps? I like the though of optionally switching and want to add another 10PH to my current 5HP RPC.
01-13-2012, 09:31 PM #11
Viper, I hope you realize I was quoting Racer in my post you quoted. I do not believe the OP needs a 12-15hp RPC to run his 5hp lathe if the RPC is tuned to normal voltage on all three legs. I hope the OP reads; http://www.practicalmachinist.com/vb...0/#post1725178
and realizes he will have to add run caps to his RPC.
01-14-2012, 12:56 AM #12
Don't know who told you that an 1100rpm motor won't work as a phase converter, but they are FOS. I have been running a 5hp Baldor 1100rpm idler for several years now with no ill effects. I have a home brew cap start setup with balance capacitors on all phases. I can start my 5hp lathe motor with no drama whatsoever, but it is a clutched lathe, so inrush is minimal.
01-14-2012, 08:35 AM #13
Last edited by Newman109; 01-14-2012 at 02:57 PM.
01-14-2012, 11:40 AM #14
I read the thread you linked and it in no way addresses the oversize requirement for a very hard starting load-like a gearhead lathe in the highest speed.
The rule of thumb on this board is:
for easy start RPC=> load
medium hard start RPC=> 2*load
very hard start RPC=> 3*load
For the OP the 5hp idler would only be big enough for a 1 1/4 hp gearhead lathe when starting in high gear.
The 7 1/2hp idler would be ok up to almost 2hp in those conditions.
But combining the 5 and 7 1/2 would get them what they need for those conditions like mentioned earlier.
01-14-2012, 01:12 PM #15
What some fail to realize is if you match your load with RPC size, you WILL NOT maintain voltage balance worth a shit at all, your motor will run hot, and you will NOT get as much power due to the imbalance. Obviously you can tune an RPC for a specific load BUT typical loads are sanders, saws, metal working machines, etc. IE variable loads. Oversize reduces these balance issues. I have done extensive load and brake testing searching for the holy grail and it is leading me down a different path but RPCs still can get the job done if oversized correctly.
In this instance, 7.5HP might be enough, not sure, but I can guarantee 12.5HP will be better in every way regarding making the load happy.
01-14-2012, 07:42 PM #16
01-15-2012, 12:11 PM #17
Good Morning. I love the internet! Where else can a person get SO much information from the comfort of your sofa? Thanks for all the helpful information.
Well after some further investigation, I found a few things that I had/have to change.
- I had the T1,T2,T3 in the wrong locations. After changing the legs around it runs better, but will not in all gears in all speeds. The spool time on the faster speeds is lengthy till the starter in the lathe gives up but no longer blows the 30amp breaker.
-I have switched the start caps in the phase-a-matic to 400-480uMf, and installed the 7.5hp idler (the only idler). The cap change was on the direction of Phase-A-Matic, making the static starter from the PAM-600/3-5hp to the PAM-900/4-8hp series starter. I have yet to balance the voltages with run caps but will.
-Another issue I'm having is drive belts. This unit has a 3 belt sheave, 1 belt is missing. I sprayed the 2 belts with belt dressing and this did help. I have the new 3vx-710 belts on order.
-In checking the information on the motor placard, I discovered a possible flaw in my thinking? The motor HP says 4/3. With the 2 speed switch, I was thinking this meant 3hp or 4hp, being that a .75hp would be very small for a 16x40" lathe. Here is a picture of the placard.
This is a test run/install to confirm the lathe was operable before purchasing of all permanent installation equipment.
Any and all information is appreciated and welcomed.
Thanks again for your time. Be safe, schick
01-15-2012, 12:47 PM #18
That is an interesting motor tag, not too useful to me. Hopefully somebody will decode it for us.
FWIW I have a small gearhead lathe that was converted from 5hp 3phase to 3hp single phase, and it will not be able to use highest speed till well warmed up. I presume that there is a synthetic oil that will help because if it was the heat itself I would expect the tolerances to bind up rather than loosen.
01-15-2012, 01:24 PM #19
I see the posts regarding the required size rpc to run a heavily loaded lathe. I'm not opposed to having a larger idler. I just don't know where to get a used 3ph motor in the Stockton CA area that won't cost a bundle? I have a 5hp and 7.5hp why not have a 10hp in the family?
Thanks again. Be safe, schick
01-15-2012, 05:00 PM #20
I have hydraulic/transmission oil in mine, more like 20wt. The 7.5 will be fine once you add sufficient run caps on both new legs. Run caps can be purchased used for a few dollars each and do not normally expire unless voltage is not respected. The motor will have been happier with slipping belts. Belt dressing is rarely a good idea except as an emergency solution. It tends to tear old belts apart and suck HP in the process.
Make sure you buy a multimeter along with the run caps.