bhigdog
Stainless
- Joined
- Jul 20, 2005
- Location
- Eastern PA
I have a home made rpc using a 5 HP motor. I have a 5 HP motor I want to see how it runs. Will I harm my RPC if I briefly try to run the tested motor.........Bob
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RPC sizing question
- Thread starter bhigdog
- Start date
- Replies 15
- Views 1,255
BT Fabrication
Titanium
- Joined
- Nov 3, 2019
- Location
- Ontario Canada
i probably wouldn't suggest it, there are things like efficency losses on the 3rd generated leg.
thermite
Diamond
- Joined
- Sep 21, 2011
Probably not.
Problem isn't RUNNING it. That could work OK .. so long as the load motor is only being asked for part of max HP, not full-load.
Problem is STARTING it.
Even so, just try it. The hit is very short.
If it doesn't trip the breakers but looks to be heating up with a too-slow or outright not-gonna at start? Just power it off, go find another idler to add-on to beef it up, start the main, add the supplementary, start the load. BFD.
It may well start and run it, but maybe not if there is much of a load on the motor. Usually 1 1/2 x the HP is a rule of thumb for ordinary loads. For anything that may be harder to start, 2x is likely to be better. Totally stupid loads may require a bigger ratio.
thermite
Diamond
- Joined
- Sep 21, 2011
Yazz.. 150% is good for MOST things we utilize.
One of the "turn key" commercial ready-made RPC vendors published a table.
When it got up to where some starting loads wanted idlers over TEN TIMES the load-motor's rating? ISTR they had a 14 X? Sheesh! That looked all TOO much like somebody "did the math", but ignored the practical reality!
If it were true? Why even try AC at all? That's slow-ramped massive torque Dee Cee turf! Or maybe even steam recip?
3 X load is the practical upper-bound, 2 X usually more than good enough, AFAICS.
When it gets to where starting the idler itself is as big a problem as starting the load?
It is simply a good time to go do something else.
bhigdog
Stainless
- Joined
- Jul 20, 2005
- Location
- Eastern PA
I'm more worried about damaging the RPC. I use caps to start/run the RPC motor. I guess the question is will I blow the caps by an overloaded start. What I'm actually testing is a used 5HP Champion compressor. I'd like to run the compressor pump before I commit to buying a new 5HP single phase motor. Guess I should have said all that first. My apologies. Thoughts/opinions.........Bob
thermite
Diamond
- Joined
- Sep 21, 2011
You might be able to damage an RPC with a four-duece mortar round, But you'd need a solid hit.
Otherwise, as said, it simply trips its breaker.
That said, compressors and their start-up cycling are one of the nastiest of loads, any source of power. That is partly because there are so MANY start cycles in a shift, day after day after day. Which is easily as hard on 1-P that need starters.
Heck, then rope start the 5 HP RPC if you have to, open the blow-off on the compressor, and try if it will start the thing.
Start caps should not be in it, though, the RPC has to be running before you dump a load on it. There is an issue, though, with a "potential" (voltage) relay they can be cut back in if voltage falls. Not good.
Other than that, if it starts, you know something about the compressor, and if it does not, you know to hit the switch.
bhigdog
Stainless
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- Jul 20, 2005
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- Eastern PA
If I do try it the plan would be to have the rpc up and running, then cut in the compressor motor. The compressor tank will be at 0 pressure. I just want to make sure the air pump is functional before I spring for a new and single phase motor. Likely be next week before I can try it.........Thanks........Bob
thermite
Diamond
- Joined
- Sep 21, 2011
Really - if you have the RPC supplying even two or three other loads already - bring it up, start all the other machines, no load.
You will probably have made your 5 HP RPC into a 7.5 HP "functionally" as each unloaded motor stiffens the 3-P whilst "idling", has a bit of "flywheel effect", same as the "idler" does.
That should start a(ny) unloaded 5 HP compressor, be it empty tank, unloader valves open, or belt-off, no sweat.
Where it gets hairier is later.
A compressor throwing the starting load across the supply "on demand" all though the day as air is used and some of those starts at times when other machines are working, maybe in the finish cut.
I'd suggest a supplementary idler or a BIGGER idler - or BOTH - say add another 5 HP or 7.5 HP and KEEP the 5 HP as well - "selectable", one, the other, or BOTH online, and stay with 3-P motor for the compressor.
1-P have starter switches and starter caps and windings that will have shorter lives than the simpler 3-P when started so very many times every working day.
jim rozen
Diamond
- Joined
- Feb 26, 2004
- Location
- peekskill, NY
My guess is the idler could stall if the load motor is too large or loaded down when it's put online.
1) breaker for idler indeed may trip,
2) have somebody standing by to shut the power off to the idler immediately if it stalls out.
1) breaker for idler indeed may trip,
2) have somebody standing by to shut the power off to the idler immediately if it stalls out.
bhigdog
Stainless
- Joined
- Jul 20, 2005
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- Eastern PA
Great idea. I'd forgotten that every motor in the RPC circuit acts as another rpc. I remember being in a guys shop and he couldn't run his biggest lathe unless he ran his smaller lathe too. Thanks......Bob
thermite
Diamond
- Joined
- Sep 21, 2011
Good on yah!
Neat "byproduct" is it gives you a much better idea how much you have to add so you do NOT have to run the machines ..
You probably should have given more details in this first post that the “5HP motor I want to see how it runs” is not just a motor, but actually a motor driving a compressor...
Compressor motors. can have significant startup and run loads on them.
As has been suggested by others, add in other idlers to increase capacity.
The issue with the "added idler" is that it takes current also.
So while you get an assist on the generated leg when it drops, you also add to the base load coming in to the system at all times. The added current is the motor "exciting current", the current necessary to set up the magnetic field and run the motor at no load. It usually amounts to about 40% or a bit more, of the FLA on the added motor.
Then you of course have to supply the extra power to the generated leg as well. So any extra power out has to come in. That is added to the base load of 40% FLA.
That is not an issue if you have plenty of available current on the single phase input, but if you are closer to the edge on supply current, it might be an issue. If you are set up for a 5HP idler, then the added motor will end up a bit over what a 10 HP might draw, plus what is needed to run the load on the generated leg, possibly loading the supply side too much.
Usually one bigger motor is more efficient that way than two smaller ones adding to the same total HP.
So while you get an assist on the generated leg when it drops, you also add to the base load coming in to the system at all times. The added current is the motor "exciting current", the current necessary to set up the magnetic field and run the motor at no load. It usually amounts to about 40% or a bit more, of the FLA on the added motor.
Then you of course have to supply the extra power to the generated leg as well. So any extra power out has to come in. That is added to the base load of 40% FLA.
That is not an issue if you have plenty of available current on the single phase input, but if you are closer to the edge on supply current, it might be an issue. If you are set up for a 5HP idler, then the added motor will end up a bit over what a 10 HP might draw, plus what is needed to run the load on the generated leg, possibly loading the supply side too much.
Usually one bigger motor is more efficient that way than two smaller ones adding to the same total HP.
bhigdog
Stainless
- Joined
- Jul 20, 2005
- Location
- Eastern PA
I agree, that was my bad.
That said, you guys have given me good advice and the mission is accomplished. I wired the motor into the 3 phase circuit, stared the RPC unloaded, started up my lathe and shaper for an additional 5 HP and then cut in the compressor. Compressor started right up, no fire balls, no smoke.
Now I'm thinking the compressor is a bit tired. So I'll be chasing that now. It's a Champion R-15 pump HR-5-8 Compressor......Thanks all......Bob
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