Rpc I've Built Won't Get Motor Up To Speed.

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I've taken a small RPC that's been in the family for several years and used to power 3 to 5 HP motors. Currently have this set up with a 3 HP pony motor and trying to start a 2 HP that is located about 56 feet from the RPC. The motor I'm trying to start is under somewhat of an load. That is a variable speed belt drive on my lathe. When I hit the start button on the lathe, the motor wants to run but doesn't gain full speed and finally trips the heaters on the starter. I've double checked the motor leads to make sure they are connected correctly and they are correct. I'll have to open up the RPC controller and get the capacitor ratings, I will say they are small compared to the others I've used in my past.
So my first question is , Do I need a larger poney motor? Like a 5 HP?
Nest question is, Do I need larger capacitors for the 56 foot run of cable between the RPC and the lathe?
I can move the RPC closer to the lathe but will require some rewiring of the heavy cable supplying power to the RPC. Open for any suggestions. Ken
 
Hi Ken,
add a momentary start (NO) button and a 300+ uf cap in parallel to the 2hp start circuit, i guarantee if the motor is right, it will fire right off
 
Mike,

I agree with you on adding a capacitor to the motor starting circuit on the lathe. The whole exercise to this was starting the lathe with the RPC without doing anything else to the motor circuit on the lathe. One thing I didn't mentioned in my starting post was the voltages I'm getting coming out of the RPC. Which is 196-198 volts off any two of three legs coming out of the RPC to the lathe. I get about 1 volt drop at the end of the 56 foot run of cable. Also running 12 ga wire on this 56 foot run.
I wonder if upsizing the oilfield capacitors would help?
 
196-198 volts seems kind of light. What's the input voltage to the RPC?
 
your volts are low, added capacitance will help
if you have 10 uf capacitance try going 20uf and try again
measure the difference between the 2 set ups and then you can gauge what the circuit need from there
56 ft is a long run, but 12 gauge should be fine @2hp

i'm wondering if we have an open capacitor???
 
I'll have to check the incoming voltage. At the moment it is a pain in the rear to check safely without risking the chance of getting zapped. Out in this subdivision I live in, voltage drops is an issue. That's another thread I'll have to post on later. I'll report later on what I find out. Ken
 
Ok, checked the voltages a moment ago. Without the pony motor running, I get L1 = 115 volts to ground, L2 = 115 volts to ground. I know that's not proper to check to ground, I don't have a neutral run on this unit since I don't need it. Across L1-L2 I get 220 volts.
Now with the pony motor running I get 220, 217, 198 volts, much better than I was getting last week end. Later today, I'll hook up and try to start the motor again and see what happens.

Mike, the open capacitor may be a possiblity. There is two of them in this unit wired in parallel.
Ken

Here's a inside picture of the RPC before I added the wiring for the motors. Plus I took out the 200 amp lugs these people put in this when they built it over twenty years ago and replaced with some Buchanan terminal blocks I use in most of my installations i do on my machines where needed.

20160812_153810.jpg
 
looks like a nicely built system, but i would bring up the low leg -
20 volts is not a lot of inbalance, but when you are starting 2hp with long run , you'll need all that you can get
imo

don't be afraid to add a 3rd balancing cap (in parallel) :grin:
 
Is it in need of a large idler motor? I seem to remember that hard to start (high start load) machines should have a larger rpc.
I run a 10hp air compressor with a 15hp rpc and it was not enough to start it. I have 240 at the outlets here. I may not have things quite right like wire size etc, but it's all in one place and short wires. I just changed the pulley on the motor and live with it.
I realize that's no help with the lathe. IT makes me think of a suburban tool video where Don was running an old lathe and he would help it start by hand over hand on the chuck.

Is it the only 3 phase motor on the system? If not I think I remember that once one is running more can be added. Only help that would be is if you could start another piece of equipment and while it is running start the lathe and maybe it would start easier, then the extra piece of equipment could be turned off.
Someone correct me if I'm just being stupid....I have tied this and it seemed to work. I have 7hp worth of machines on my 3hp idler. I'm only one in the shop so rarely is more than 1 machine going. I don't have a hard to start machine (air compressor is on it's own) to test that but after reading that I had to try turning more machines on and they all seemed to run normal. Course they weren't doing any work when I tried that.
 
Guys, got the motor running on the lathe. Had to take the tension off the belt, lifted up the motor mount and placed a block of iron under it. That let the motor start up and get to operating speed. Once the motor was running, pulled the block out from under the motor mount, lathe came alive. Motor is a little rough running, probably in need of a new set of bearings. Also need replace the variable pitch belts, they are in sad condition.
If I continue to use the RPC in this condition, I'll do what Mike said to do by increasing the number of oil filled capacitors to the circuit. This is strictly a temporary situation until I can make room for the lathe in my garage. Anybody want a 20" Lodge & Shipley lathe that takes up half of my garage?
 
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