From: Neon John <johngdNOSPAM@bellsouth.net> Subject: Re: A tale of 2 generators Message-ID: <3917E164.7559BBBF@bellsouth.net> Date: Tue, 09 May 2000 05:59:00 EDT Newsgroups: rec.outdoors.rv-travel Lon VanOstran wrote: > > Scene: Brother's cabin in the north woods. No electricity. Onan Emerald > 5K (lp, 4 years old) generator. (actually rated at 120v and 40 amps > which computes to 4800 watts) > > Problem: 3 HP air compressor which works fine in lower MI with Edison > type electricity. Cold, it works *once* to pump up, but when preasure > drops and motor tries to start it lugs, motor becomes very hot, and > breaker then pops at cabin breaker panel. This makes me think the air compressor's air unloader isn't working. The unloader allows the compressor to start without load so that a cheaper motor/smaller branch can be used. Look for an aluminum air line running to the pressure switch. This line should vent to air when the switch trips off. It should close when the switch turns back on. If you're trying to manually turn the compressor on when it still has some air in the tank, then this is sure to be the problem. Most pressure switches have a metal lever to manually turn the compressor off. The arm will also activate the unloader valve. Turn the compressor off with this valve before trying to start it on the generator. If your compressor really is 3 hp (and not just 3 spec writer HP), then you're really on the edge for that generator anyway. I suspect that it is not 3 HP because a true 3 hp motor will have a hard time starting on a 20 amp, 120 volt branch. My wheeled 2 hp Speedaire industrial compressor will just barely start on my 4kw Yamaha generator on 240 volts. > > Cabin is wired to code, complete with 12-2 w/ground wiring. Garage 150' > from cabin with it's own 60 amp breaker box. 6-Gauge underground cable > to garage. > > I went to our campsite 300 yards from cabin and retrieved Good Governor. > 121v 61hz in cabin. 124v +62hz in garage. We shut off every other draw > on the generator and tried again. The Good Governor showed a voltage > drop to about 56 when the motor tried to start until the breaker popped > again. > > We hauled the compressor to my motorhome where I have an Onan Marquis > (gas, new) 5k generator. (again, 120v and 40 amp which translates to > 4800 w) > Energy management system gave us lots more info than we could get at the > cabin. Upon firing up the generator, I see a load of 5 amps (fridge and > converter) which I left alone and plugged in the compressor. Load jumped > to 43 amps for about 1 second or less and then dropped to 24 amps and > stayed. We cycled the compressor several times with the same exact > results. Perfect performance to my way of thinking. If it really is pulling 24 amps, then it couldn't run on a 20 amp branch. You'd have to have a 30 amp dedicated plug. I wonder if they really did make a compressor that requires a special branch or if the MH panel is in error. What does the motor nameplate say? > Clearly my brother's generator is suffering from a lack of available > amps since the volts are there. > > Questions: > 1.Does anyone have suggestions regarding the poor performance of > brother's generator? > 2.Probably not since the volts are there, but: Could this be caused by > running his exhaust underground via 2" pipe and bringing it up about 25 > feet from the cabin in the back yard? (tractor type flap on the end of > pipe to prevent water infiltration) > 3. Is lp generator less effective than gas? What does the frequency do? If the frequency is staying on target, then the engine has enough oomph. What you're seeing really isn't that odd. Small generators just don't have the iron and copper to supply huge inrush. If you need to make a given generator run a given compressor, there are some tricks to try. First thing to try is to try adding some capacitance to the starting side. More capacitance will increase the torque and will usually reduce the inrush current. We use that trick in the refrigeration field to make compressors start on low voltage. Try approximately doubling the start capacitance. A large run cap across the main terminals of the motor will help with the very low power factor when starting. In other words, it will supply the reactive VARs that the generator can't. I'd start with about a 160 microfarad motor RUN (different from a start cap). Having another motor running, particularly if it has a heavy rotor and/or flywheel, will also help. When the compressor comes on, the other motor will act as an induction generator and supply power to the starting motor. This is the trick I use to get my restaurant's walk-in freezer compressor to start on my emergency generator. An old motor is a lot cheaper than a new generator :-). Motor should be larger than the compressor motor. The bigger the better. Hope that gives you some ideas. To get any more detailed, we'd have to instrument the thing and see in detail what's happening. John -- John De Armond johngdSPAMNOT@bellsouth.net http://personal.bellsouth.net/~johngd/ Neon John's Custom Neon Cleveland, TN "Bendin' Glass 'n Passin' Gas" From: Neon John <johngdNOSPAM@bellsouth.net> Subject: Re: Autoformers Message-ID: <392A333B.4E8C0D50@bellsouth.net> Date: Tue, 23 May 2000 03:28:59 EDT Newsgroups: rec.outdoors.rv-travel Owlman wrote: > > Thanx for your comments. Anything in the $2K area is out of reach for us. How > can so many parks (maybe 60+% of those we've gone to) be so far below standard > and yet everyone still seems to use the substandard electrical current? Are we > being unduly concerned? > > Generally, the lower readings will be at or near 115 V and then go below that > level when you turn the A/C on. It would seem that a gain of a set 10% would > take care of most of our situations. Your AC should start on 115 volts. It should start as low as 105-109 volts. If it isn't, you need to run down the cause. One cause could be a defective start capacitor (if it has one) or starting relay (again, if it has one). Need to check that out first. Another cause may be excessive voltage drop during the starting inrush. You need to hook your RV to a good, solid power source and then take a digital voltmeter with peak/hold and track down the starting voltage at each step. (the peak/hold will store the peak and the valley voltage during the test interval) First, check the voltage at the compressor terminals. If the voltage is OK there, then you need to look elsewhere for the problem. Hook up the voltmeter, activate the peak/hold function and start the unit. If the voltage at the compressor stays above about 105 volts during starting, it is OK. If not, work back. Test the voltage at the unit's terminals. A high resistance thermostat contact or selector switch contact could drop enough voltage to cause the problem. If the voltage is OK there, go back to the breaker panel. If the voltage is still low here, it could be a bad power cord. Does your power plug get warm when the AC is on? If so, there's voltage drop there. You could have a tight compressor. Not a frequent occurrence but it does happen. This manifests itself as difficult starting, high starting current and perhaps a bit higher run current. The start current should be less than the locked rotor current (LRA) listed on the nameplate. If it's at or near the LRA, then either the compressor is tight or the start device is defective. Takes an amp-clamp with peak/hold to see this. If the voltage at the compressor is marginal and/or the compressor is tight, a possible solution is a hard start pack. This is a capacitor and relay assembly that is designed to aid undervoltaged or tight compressors start. Commonly known by the trademark "KickStarter". Available at HVAC supply stores. Installation is straightforward - just hook across the compressor terminals. The kickstarter has the added benefit of reducing the starting current. John -- John De Armond johngdSPAMNOT@bellsouth.net http://personal.bellsouth.net/~johngd/ Neon John's Custom Neon Cleveland, TN "Bendin' Glass 'n Passin' Gas" |