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Help getting air compressor motor over "the hump" on generator power

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fastline

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I feel like a rookie but I threw a mobile air compressor and generator in the truck that I have. Genny is known good, compressor was newish but not tested. Compressor is 14A FLA, 240V, 1P, genny is 5000W cont/6500peak.

Compressor would start but found that it was pulling 30A, dragging on the genny, and rpm on the motor was 2500 when spec is 3450. Everything tested out good so I knew either the pump had issue or I was not getting enough power to the load.

Irush on the genny was 46A.

I plumbed into to house power and inrush was 76A, cont was 9.6 with no air pressure. SOLID!

So now I know that the genny will not start the load but should handle it. So.... Might I be able to stack some capacitance on the start cap to help this issue? I really think if I can get over the hump, it will work. Just not getting the motor quite up to speed and the start circuit is not kicking out.
 

dr pepper

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'better' and commercial compressors have a valve that unloads the pump, for the first second or so the pump just squirts to atmosphere, I seem to recall import 7cfm ones having this built into the pressure switch, the switch emptying the line from pump to tank.
If you allready have this then you might have trouble getting it to work, upping thr start capcitance will increase phase shift, and may well increase start up torque, however that will also worsen the inrush.
Try unscrewing the pump outlet and see if it'll fire up.
 

crutschow

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A couple of other solutions:
  • Reduce the motor pulley size to reduce the motor startup load. (That, of course, will reduce the maximum air flow from the compressor).
  • Add a centrifugal clutch pulley to the motor.
 

fastline

Member
Thanks guys but i already have things unloaded well. Pump outlets are to atmosphere already. No way to further unload it. Oem start cap is 200uf. Thoughts on more? Have a bunch of oil filled run caps around
 

crutschow

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Thanks guys but i already have things unloaded well. Pump outlets are to atmosphere already. No way to further unload it. Oem start cap is 200uf. Thoughts on more? Have a bunch of oil filled run caps around
I don't think a higher start cap will help.
Their value is generally optimized to give maximum starting torque.

What about a smaller pulley or a centrifugal clutch as I mentioned?
 

fastline

Member
Directly driven. Actually motor and pump are integral.

Sounds like the best way is running a bigger genny..... Frustrating.
 

dr pepper

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These days its possible the cap is on the small side to save cost, like I said increasing it might make things worse.
Are you sure the pump and bearings are Ok, your gen ought to have enough beef to get it started.
Maybe get yourself a pull start petrol engine and drive the compressor direct.
 

Les Jones

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One thing that MIGHT just work is a push to break push button switch in series with the start capacitor. The idea is based on the fact that once a single phase induction motor has started rotating at a reasonable speed it does not need the start winding. It could be disconnecting the start winding may be just enough to reduce the current so the generator voltage came up enough for the motor got up to full speed. Once it was up to full speed the centrifugal switch would be open so the push button could be released. Even if this worked there is the problem would be that the compressor would need to stop and start to control the pressure.

Les.
 
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ClydeCrashKop

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How about running a compressor off the generator engine with belts & an electric clutch pulley to control it.
 

fastline

Member
Les, I like your idea and just might work. I think before that I might first try ramping up the engine rpms on the genny to add a little more capacity and see if the compressor will take it.

The problem is I also have another 1/2HP load to throw on there too but I can start that after the compressor is running I suppose.
 

kinarfi

Well-Known Member
If you have access the the gen wiring and it's a 120/240 gen, reconnect the windings to give 120 with twice the current to get past the hump, then reconnect for 240. Problems are that you have to be at the gen every time you need to start the pump unless you have a way to let the pump free wheel.
following up on Les' idea, try getting it spinning on 120, less volt, less current, less engine load, then switch to 240
 

Dr_Doggy

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One thing I was holding off on suggesting is a 5-10 second delayed trigger made with a few components to switch a relay to give the generator some "warmup" time... but IDK how this would fair .... just a doggy guess... maybe wait for opinions!
 

spec

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This may sound obvious, but perhaps the start capacitor is faulty- worth a check.:)

spec
 

tcmtech

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Theres not a lot you can do here given what you have. As your generator stands yo only have a ~2:1 peak supply current limit over the motor running current and your motor needs at lest 4 - 5:1 to start.

There's a limit to how far you can cheat the physics behind how a single phase motor starts andy simply do not have the capacity to cheat far enough with either the supply or the load end as they stand.

The most effective but not simple and cheap route to keep thing all electric would be to set the compressor up with a three phase motor and VFD unit to ramp the compressor up to speed without overloading the generator.

Otherwise the compressor needs to be set up with a gas engine and the necessary pressure unloader valving and engine control related components.
 

fastline

Member
Well, I determined that if I ramp up the RPM on the genny a bit, I could get the compressor to come up fine BUT once the compressor had some pressure, it would not work. I was right on the edge and just not worth burning something out or having to watch it constantly so I just grabbed a bigger genny. We own several among friends. I was using a 5000/6500, now I have a 6500/8500 in the truck. I think that should get us there.
 

kinarfi

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THUMBS UP
 

tcmtech

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Years ago when I worked at a local welding supply store as a service tech I had a customer come in wanting a welder generator set that had enough auxiliary power capacity to start an electric 3 HP air compressor he had on his service truck.

I offered to do a up front test of our units to see if any had the capability to start it. Sure enough our bigger Miller Bobcat Diesel sets with a honest rated 10K auxiliary power output could start the thing right up without issue.

I didn't see him for a while until one day he shows up all mad at me because his new Miller Bobcat welder genset was not starting his air compressor like it had when we did the test run. Well it turns out that instead of buying the unit with the big Diesel he went with the cheaper 8000 watt (almost half the price) one that ran off the ~16 HP Onan gas engine and those welders with those cheap junk engines had absolutely no reserve power left in them at the 8K load point compared to the 10K diesel sets that were just getting a good lug going at 10K and weren't maxed out until 11+K.

I still don't know how that was suposed to be my fault but he figured it was.
 

fastline

Member
Well, we might have to dig more into this to see if there is any science or specs that we can look at to spec the right genny for a compressor load. I really thought I had this licked by grabbing a 6500w cont/8500w peak genny. NOPE! I actually think this one is even worse! I could at least start the comp with no air pressure on the 5000w. In despiration, I put a manual switch on it to kick out the start cap once it was spinning. Works like a charm but I cannot stand at the compressor to do that all the time. It works in a pinch though!

Can I parallel two gennies? Does that even work or do they fight each other? I figure the frequency variance alone could be an issue?
 

ronsimpson

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There is a saying at the parts store. You need a generator 2X bigger than you think to start a compressor.
 
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