Air Compressor Conversion

[orezok]

Sounds like a genny speed problem. Low frequency will cause slow running and will heat a motor up. Compounding that with a sticky start winding switch and youve got a smouldering fuze.

I've checked with my Fluke and both gens were on spec for both voltage and Hz. The centrifugal switch moves freely. For some reason, this motor doesn't like generators.

 

[CobyRupert]

Doesn't the CFM of the blow off valve have to be compared to the (worst case//maximum) CFM of the compressor?

(i.e. otherwise can you have a situation where the compressor can continue to supply more air than the valve can exhaust?)

 

[Deere Dude]

Doesn't the CFM of the blow off valve have to be compared to the (worst case//maximum) CFM of the compressor?

(i.e. otherwise can you have a situation where the compressor can continue to supply more air than the valve can exhaust?)

I think you are correct. But it would take a lot of HP to overcome what a relief valve could dump.

I have an air compressor from an old 8 HP John Deere garden tractor. I needed/wanted a gas operated compressor so I took the setup and put a new 5 HP Briggs motor on it. It just runs and pumps and dumps and repeats. It is noisy with the air dumping now and then, but works well.

 

[Mendonsy]

9kw genny should have no problem starting and running that motor...

Not necessarily. That motor should be a bit under 3kw running, but the starting current of that motor will be four to six times that much. A 9kw could be too small to start it.

 

[Soundguy]

Not necessarily. That motor should be a bit under 3kw running, but the starting current of that motor will be four to six times that much. A 9kw could be too small to start it.

My pocket chart says a 2hp motor should take approx 2000 running watts. ( less actual ) If you use actual #, that 9kw genny can fully do 6x running watts theoretical. If using the practical #, then it can do 5x.

Not to mention his genny is 9k running 10K+ starting...

 

[Mendonsy]

My pocket chart says a 2hp motor should take approx 2000 running watts. ( less actual ) If you use actual #, that 9kw genny can fully do 6x running watts theoretical. If using the practical #, then it can do 5x.

Not to mention his genny is 9k running 10K+ starting...

He says that his motor is rated at 12 amps, 230 volts. That is 2760 watts or 2.76kw. If the starting current is nominally 5x the running current then it draws 13800 watts to start it. All of that agrees with my handy Square D pocket calculator for a 2 hp, 230 volt motor.

 

[orezok]

Let me clarify some more. The gen "starts" the compressor with just a little bog down and the compressor is running at a good speed. What I don't know is if it is running at full RPM or less than 75% at where the start cap kicks out. Too much noise from both machines to tell. I've got the motor apart right now for inspection, but when I get it back together I'll put the Fluke on it and see what the inrush is and for how long.

 

[Soundguy]

He says that his motor is rated at 12 amps, 230 volts. That is 2760 watts or 2.76kw. If the starting current is nominally 5x the running current then it draws 13800 watts to start it. All of that agrees with my handy Square D pocket calculator for a 2 hp, 230 volt motor.

Lot of iffs.

5x is way on the high side.. "if" it is, I think I get a new motor, you can get better ones, vs horrible power hogs.

We start 2hp motors on jobsites with 9k gennies ( the 6.5k gennies would not start the motors )

Maybee the Genny is overhyped and the 9k # is fake sales speak.. Like stated as a peak#. See that all the time, electrical components advertised with generous peak ratings that mean little.

 

[SPYDERLK]

He says that his motor is rated at 12 amps, 230 volts. That is 2760 watts or 2.76kw. If the starting current is nominally 5x the running current then it draws 13800 watts to start it. All of that agrees with my handy Square D pocket calculator for a 2 hp, 230 volt motor.

If motor SF is above 1 that multiple is applied to motor rating

 

[aczlan]

Here is what you will need: 1/2" Female NPT Gas Air Compressor Unloader Check pilot Valve Combination & Bullwhip Throttle Control Included 8HP and Below Applications (14-175 Unloader & 18" Throttle Control Cable) - - Amazon.com
It has a throttle valve (to idle the engine down when not needed) and a unloader valve (to make it easier to start it back up).

Aaron Z

 

[Soundguy]

If motor SF is above 1 that multiple is applied to motor rating

I'd like to see the plate info. Fla vs sfa.

 

[SPYDERLK]

I'd like to see the plate info. Fla vs sfa.

Never seen it detailed but suspect FLA is SFA.

 

[orezok]

I'd like to see the plate info. Fla vs sfa.

For what it's worth.

 

[orezok]

Sorry for the delayed answers, but here's what happened. First, I decided to see if a replacement switch for the centrifugal switch was available for my McGiver'd switch on this 35 year old motor. I called tech support at Graingers and they said that they would have to contact the factory. They told me that while the motor was labeled Dayton, it was actually an Emerson motor. A day lated they reported back that parts were no longer available from the factory. No big surprise there.

They recommended that I contact carbonbrush.com aka Arrowhead Electric Co. Carbonbrush asked me to send them pictures and dimensions of the switch. I sent them and a day later they came back and stated that a switch from Doerr would work. Doerr is a German electric motor manufacturer. I ordered the switch and it is almost an exact replica of my original switch. Only $16.00! The only difference was that the central hole that the bearing had to pass through was slightly smaller. Hole saw fixed that problem.

I installed the switch and did some testing. When connected to shore power, the motor draws 30 amps inrush according to my Amprobe clamp on. It has a "hold" position for maximum inrush. Next I hooked it back up to the generator. Inrush on the generator was 40 amps, but the motor ran fine with no release of factory smoke.

For now my problems solved.

 

[buickanddeere]

I originally had a 3500 watt generator and tried to run the compressor. After about a minute it started to release the factory smoke. I stopped it and inspected the motor. One of the points on the centrifugal switch was completely melted off. I McGivered it as the motor is 35 years old and I couldn't find a replacement part. I dug through my junk box and found an old set of automotive points that had a large contact. Silver soldered it to a piece of brass shim material and then silver soldered that to the metal spring that contacted the centrifugal mechanism. The nylon "rubbing block" was also gone so I found a small piece ofnylon and epoxy'd it to the spring.

This has worked for about a year when I could access shop power. I got a new 9000 watt running 10000+ starting generator and thought it would do the trick for remote operation. I hooked it up and tried it again, closely watching the motor. After about 10 seconds,the factory smoke started to reappear. I quickly killed it. No noticeable damage this time. Tried it again on shop power and it runs OK.

The only thing I can think of on the generator power is that for some reason the motor is not geting up to full speed and the centrifical switch is not disengaging allowing the start capacitor to remain in circuit. This will cook the windings in short order.

The 9000 has more than 3 times the power needed to run this motor which has a RLA of 12 amps.

Since I just use this compressor a couple of times a year for small projects, I think I will just install a 120PSI blow off and let it run. This motor doesn't have a governor so there is no practical way to make it idle down. I'll just manually adjust the speed to keep the pressure close to blow off pressure.

If the FLA's is 12 then you need a generator rated for 60amps or more with an active voltage regulator . If the compressor motor lacks a beefy start capacitor then up that available current at rated voltage of 10times the motors 12FLA.

 

[buickanddeere]

A centrufugal clutch and an pressure activated active throttle is far superior to a dump valve .

 

[orezok]

If the FLA's is 12 then you need a generator rated for 60amps or more with an active voltage regulator . If the compressor motor lacks a beefy start capacitor then up that available current at rated voltage of 10times the motors 12FLA.

Did you read my post?

By ACTUAL testing the compressor only had an inrush of 40 amps on the generator and 30 amps on shore power. The 9000 generator has a run capacity of 37.5 amps and a start of 42 amps.

Where in the world did you come up with the 10x figure?

 

[buickanddeere]

The initial inrush for the first half some wave . The peak and hold function averages over 5 cycles or so.
The small generator just can't get over that first 1/2 cycle to allow starting current to drop to 5x or so of normal FLA.

 

[SPYDERLK]

The initial inrush for the first half some wave . The peak and hold function averages over 5 cycles or so.
The small generator just can't get over that first 1/2 cycle to allow starting current to drop to 5x or so of normal FLA.

Strangely enuf, I have started a 240V 3HP motor driving a 5HP Kellogg American compressor, and run it to 150 psi cutoff pressure using a Coleman 4000/5000 genny. The big compressor had an centrifugal unloader so it spun up freely to a point before starting to pump. By that time the genny had recovered apparently. -- About 21 Amps at cutoff.

 

[orezok]

The initial inrush for the first half some wave . The peak and hold function averages over 5 cycles or so.
The small generator just can't get over that first 1/2 cycle to allow starting current to drop to 5x or so of normal FLA.

Don't tell that to my small generator. It doesn't know that it can't start my compressor.