Generator won't power compressor....ideas?

[Boeing]

Steve,
Just so I'm clear here....I'm sure that there is a wiring diag on the wiring plate. It MAY be for 3 wires though, it's an older unit. With 4 wires I'm guessing that black is power, red is power, white will remain neutral and ground is ground.....
Now, you said that the added capacitor would go on the SAME banonets that the existing capacitor is on? BOTH...capacitors wired together? I do not remove the orig one? thanks

 

[Soundguy]

wired together as in parallel ( not series ), as in adding capacity.

that's why it would be a starting 'AID' capacitor. ie.. helper.

 

[Qapla]

Keep in mind that they call them a "hard start" capacitor for a reason. I would try to relieve the pressure from the head while in start mode before I sent additional "kick" to the motor. It just seems to me that if the compressor work fine from household power it does not need the added "hard start". What it needs is "relief" while in start mode.

 

[Soundguy]

would be eay to rig a manual unload for 0 start pressure...

 

[jenkinsph]

Steve,
Just so I'm clear here....I'm sure that there is a wiring diag on the wiring plate. It MAY be for 3 wires though, it's an older unit. With 4 wires I'm guessing that black is power, red is power, white will remain neutral and ground is ground.....

I don't think you need the white nuetral wire on an electric motor if wired for 240v. If it were an appliance with 120 and 240v circuits you would need it. You will need a ground and two hot wires. You will need to change up the existing wiring of the motor leads inside the junction box too.

 

[chim]

here.. i'll throw a bone.. root mean square is .707 of peak..

e=IR
P=IE

those are dc measurements and don't take into account for inductance vs resistance.. but it's 'horseshoes and thermonukes close'

and under 'perfect' non real worl conditions that don't take into account fractional losses.. a hp = 746w

the math puts it very close from there.. a 4hp peak listing drops you into somewhere in the 2.8 range...
convert to w = 2088
derive current = 17.4a

a very realistic #

I appreciate the bone and your patience, but your approach is totally bogus. The 17.4A is certainly not "very close" at nearly 8% off the target. As soon as I see someone posting on a non-electrical forum and throwing out terms they picked up at an adult ed evening course instead of real numbers that work, the flags go up. I don't know your background, but it doesn't appear to be in power. Don't feel badly. I couldn't design a pair of earbuds without more schooling.

 

[Boeing]

I'm getting there.....Steve, if I don't NEED the white neutral (which will be in the 10/3 wire from the 4 plug generator)....I can just clip it or cap it and connect the black/red and ground, RIGHT?
And.....unloading from Soundguy...."easy to rig a manual unload".....give me a suggestion. This is a belt driven unit. I wish I could rig a clutch or belt idler like auto Air Conditioners use??? I like the long coiled tube idea that was suggested early in this post; but now that I think about it, it would need a one way check valve at the tank or the tube would have the same backpressure that the tank has......hummm?

 

[DarkBlack]

I'm enjoying all this too I think one poster is confusing BS peak hp ratings with peak sine wave voltage-not the same.
Just think, elec motor are so predictable that they ae allowed to publish power based on the fuel they consume...electricity. Imagine if it was done that way with combustion engines. Instead of cheating with locked rotor amps, they would be putting in oversized jets in the carbs to run richer, so they could advertise more HP, based on fuel consumption. I'm surprised nobody engaged me on the mention of rotor slip? Go buy a "cummins tool sale" motorized tool that runs slow and heats up in seconds, and you'll have yourself an introduction into slip and why it matters.

 

[Qapla]

... I like the long coiled tube idea that was suggested early in this post; but now that I think about it, it would need a one way check valve at the tank or the tube would have the same backpressure that the tank has...

The compressor should have some sort of check valve built in (perhaps part of the relief valve) or the air would leak back/out of the head every time it turns off.

 

[Boeing]

Wouldn't that Compressor check valve be in the compressor itself PRIOR to the copper tube that was to be lengthened....per suggestion? Once the air is pushed out of the compressor it goes thru the copper line and into the tank. I can't see how LENGTHENING this tube will help anything without de-pressurizing that tube. What am I missing?

 

[Raspy]

Wouldn't that Compressor check valve be in the compressor itself PRIOR to the copper tube that was to be lengthened....per suggestion? Once the air is pushed out of the compressor it goes thru the copper line and into the tank. I can't see how LENGTHENING this tube will help anything without de-pressurizing that tube. What am I missing?

There are various ways to do this, but the most common, and certainly on larger compressors, is to have a check valve at the tank where the delivery line enters the tank. Then the line can be depressurized at shutoff. It's the hiss you hear when they stop. The pressure switch often has a vent to the outside that opens when the high limit pressure is reached and the switch opens. This clears the delivery line and prepares the compressor for the next start. There is also a check valve in the compressor on the output port feeding the delivery line. The compressor feeds air through it's check valve into the delivery line. When that pressure is higher than the tank pressure, the air flows into the tank. On the intake stroke the air is stopped at the exhaust port and cannot re-enter the compressor. But during unload, the air in the cylinder can flow out the check and out to atmosphere through the unloader.

So, if you increase the volume of the delivery line the compressor, on it's first compression stroke, won't build as high of pressure in the delivery line and therefore starts with less resistance. Increasing length increases volume. Or it could be a larger diameter, but then you'd have to adapt it at each end to the existing fittings. Longer is simpler. It's good idea, if that is the problem. You just need more power and changing the voltage is the easiest way and the best way for your generator. Do that first.

There's no need to over think the electrical wiring in the motor either. Simply read the wiring diagram in or on the motor and follow that. Forget all this deep electrical theory and the "bone" you might get thrown, or the book you have to go read. Sheesh. Just get more watts to the compressor motor from your source. Re-wire to 240 volts, use the appropriate feed wires, and try it out. Probably two number twelve wires and a ground is all it needs.

,

 

[Boeing]

John,
Thanks for the clairification.......I don't THINK that I have a check valve AT THE TANK. Where can I get one, it sounds like an excellent idea.
With a check valve, a longer coiled tube and 240V it ought to run great.
I am sure that it already has one or even TWO start capacitors....there are two covered "humps" on the top of the motor.
Thanks again, Rob

 

[Qapla]

...I am sure that it already has one or even TWO start capacitors....there are two covered "humps" on the top of the motor.

One of those is probably a "run" capacitor and the other is the "start" capacitor.

Also, most compressors I have dealt with have some sort of unloading built in. This is to relieve the force needed to start the motor and compressor before beginning to pressurize the tank. However, when running on a generator or a long electrical run sometimes the compressor needs a little more relief, thus the longer supply from the head to the tank.

 

[Boeing]

Are there larger or more powerful start capacitors? If I have a pressure relief valve somewhere should I just add this longer coiled tube?

 

[tshep]

I don't want to add confusion but...

Apparently you have some control circuits (and thus wiring and devices) that are made to be operated at 120 volts (as exists).
If you change motor only to 240, they will still be 120 circuits. You will need that neutral!
Wire all 4 wires in new cord and plug, move old motor neutral feed to new other hot leg feed as directed in motor cover for 240 changeover - leave other devices on original neutral - add chassis ground if needed.
(Or you could verify every wire and device is 240 rated - we already know they are 120 rated - sleeping dogs,etc)

This is the fix - step one and 99.9% probability last step. Forget add-on unloaders, hard start etc - fix the current flow!

 

[Boeing]

Guys, I can't say THANK YOU enough.........
I don't get back to Va. for about a month but I have the ducks in order now.

 

[pat32rf]

The only devices MIGHT be pilot lights and magnetic hold-in contactors, neither which are likely to be found on a portable compressor. Anything else in the circuit such as pressure switches will operate OK on 240V. Just change the plug to a 240V unit, and motor connections to 240V and you should be good to go, leaving the 110v genny outlets free for lighting, skil saws etc

 

[jenkinsph]

I don't want to add confusion but...

Apparently you have some control circuits (and thus wiring and devices) that are made to be operated at 120 volts (as exists).
If you change motor only to 240, they will still be 120 circuits. You will need that neutral!
Wire all 4 wires in new cord and plug, move old motor neutral feed to new other hot leg feed as directed in motor cover for 240 changeover - leave other devices on original neutral - add chassis ground if needed.
(Or you could verify every wire and device is 240 rated - we already know they are 120 rated - sleeping dogs,etc)

This is the fix - step one and 99.9% probability last step. Forget add-on unloaders, hard start etc - fix the current flow!

No offense but you are adding to the confusion, this is a simple motor hookup to a pressure switch. There are no lights bells whistles or other diagnostic circuits to deal with.

 

[BertZegers]

Hope to make this clear and simple.

The power (HP) mentioned on a electric motor is mechanical power on the shaft. This is what is available for driving something. 24 hours a day 365 days a year. This is what the mechanical engineer needs to know.

The amps and voltage on a motor is what the electricien wants to know.

The formula for a single fase motor: Watts (Power)=Volts*Amps*power factor* efficiency.

This means for a single fase 4 HP motor = 3000wats on 230 volt: 3000=230*amps*0.8*0.9 Amps =3000/165.6=18.1 Amps
This means for a single fase 4 HP motor = 3000wats on 115 volt: 3000=115*amps*0.8*0.9 Amps =3000/82.8 =36.2 Amps

So If you connect to 115 or 230 volts, the mechanical output of the motor is the same.

If you want to know the power of a 1 phase motor: HP= volts*amps*power factor*efficiency/750
for example 230Volt*18.1Amps*0.8PF*0.9E/750=4 HP

If you connect to a 115/230 generator, then wire for 230 volts.

The correct numbers for power factor and efficiency you should find on the motor data shield.

 

[Raspy]

Bert,

I'm afraid you are missing he point. The "4 HP" number is bogus. It's not a 4 hp motor. That is an exaggerated advertising claim.

The specs say it is 115V and 16A.

The question here is how to get it to start, not whether the motor output is the same on 120 as it is on 240. We all know it is the same.

It seems clear that it is much more likely to start on 240 volts than 120 volts from the generator because of the generator design and the wire size.

No need to re-invent the wheel or delve into electrical theory here. Just give the thing more starting watts and that's it. The generator can do that if the compressor is hooked up to, and wired for 240 volts.