canman47 Posted July 19, 2014 Share Posted July 19, 2014 My roof AC runs on shore power but it won't run on the generator. It almost stalls the generator every time the compressor tries to start. The generator is supposed to handle a 13,000 btu AC and this one's only 7,100 so it shouldn't be overloaded. Volt meter shows 120V output which drops to 80 when the compressor tries to start. Could it be a start capacitor or something else? Quote Link to comment Share on other sites More sharing options...
linda s Posted July 19, 2014 Share Posted July 19, 2014 Possible your AC doesn't have a easy start capacitator installed. You should be able to add one Linda S Quote Link to comment Share on other sites More sharing options...
Derek up North Posted July 20, 2014 Share Posted July 20, 2014 http://www.modmyrv.com/2009/05/27/rv-air-conditioner-hard-start-capacitor Quote Link to comment Share on other sites More sharing options...
zero Posted July 20, 2014 Share Posted July 20, 2014 My roof AC runs on shore power but it won't run on the generator. It almost stalls the generator every time the compressor tries to start. The generator is supposed to handle a 13,000 btu AC and this one's only 7,100 so it shouldn't be overloaded. Volt meter shows 120V output which drops to 80 when the compressor tries to start. Could it be a start capacitor or something else? It would behoove you to buy or borrow a cheap Kill-A-Watt meter. You can get them new for $20 if you shop around. That will eliminate any guessing at your problem. Your low voltage reading can be a result of excessive current draw from your AC unit or low output from your generator . Just plug the meter in and you'll know right away what the problem is. Quote Link to comment Share on other sites More sharing options...
zero Posted July 20, 2014 Share Posted July 20, 2014 By the way - what sort of shore power is working for you? An outlet on a household type 20 amp circuit - or a campground type outlet rated at 30 amps? Quote Link to comment Share on other sites More sharing options...
Maineah Posted July 20, 2014 Share Posted July 20, 2014 How many watts is the generator? Quote Link to comment Share on other sites More sharing options...
Maineah Posted July 20, 2014 Share Posted July 20, 2014 The kill-a-watt is a great tool but if you have issues with the A/C the meter is only rated 1800 watts. I don't know how well it will take a hit in excess of 1800 watts. Quote Link to comment Share on other sites More sharing options...
canman47 Posted July 20, 2014 Author Share Posted July 20, 2014 The generator is 2.5kW. My shore power is a 20 amp circuit at home which seems to run the AC ok. The AC is small - just 7100btu and Coleman specifies a 20 amp fuse so a 20 amp circuit should be safe. I think I'll get one of those Kill a Watt meters but it sounds like I could burn it up if the AC is pulling too much current. The booster start capacitor looks promising. Have others here tried that? Quote Link to comment Share on other sites More sharing options...
WME Posted July 20, 2014 Share Posted July 20, 2014 I had a 2.8 Onan and a 13.5 AC. The only way to make the AC run was with one of the Hard Start caps. Quote Link to comment Share on other sites More sharing options...
zero Posted July 20, 2014 Share Posted July 20, 2014 The generator is 2.5kW. My shore power is a 20 amp circuit at home which seems to run the AC ok. The AC is small- just 7100btu and Coleman specifies a 20 amp fuse so a 20 amp circuit should be safe. I think I'll get one of those Kill a Watt meters but it sounds like I could burn it up if the AC is pulling too much current. The booster start capacitor looks promising. Have others here tried that? Not all generators sold as "2.5 KW" are the same. There are big differences. Differences in surge rating. voltage (the kind you can't read with a standard voltmeter), etc. Many small gensets are known for having low voltage peaks which can make motor starting harder then when on grid power. What make and model is it? The 7100 BTU Coleman AC runs on less then 10 amps once going but can take over 20 amps to first start the motor. Note that a standard household 20 amp circuit can handle over 30 amps for quite a few seconds. To trip a 20 amp circuit breaker right away, it takes up near 40 amps (depends on actual spec of breaker). Many generators sold as 2.5KW can't put out more then 18-20 amps even for just a second or two and those cannot handle the start-surge of many small AC units. Quote Link to comment Share on other sites More sharing options...
linda s Posted July 20, 2014 Share Posted July 20, 2014 If his still has the original generator and it's the Kohler 2500 then it was supposed to be able to run a 13,000 BTU AC unit according to the manual. His AC is far smaller. Maybe the generator is just giving out and not pumping juice like it used to Linda S Quote Link to comment Share on other sites More sharing options...
Maineah Posted July 20, 2014 Share Posted July 20, 2014 The generator is 2.5kW. My shore power is a 20 amp circuit at home which seems to run the AC ok. The AC is small- just 7100btu and Coleman specifies a 20 amp fuse so a 20 amp circuit should be safe. I think I'll get one of those Kill a Watt meters but it sounds like I could burn it up if the AC is pulling too much current. The booster start capacitor looks promising. Have others here tried that? If it's the stock Kohler that would not surprise me they are not the best. Regardless of how you power the A/C a start cap would be a step in the right direction. Quote Link to comment Share on other sites More sharing options...
zero Posted July 20, 2014 Share Posted July 20, 2014 The generator is 2.5kW. My shore power is a 20 amp circuit at home which seems to run the AC ok. The AC is small - just 7100btu and Coleman specifies a 20 amp fuse so a 20 amp circuit should be safe. I think I'll get one of those Kill a Watt meters but it sounds like I could burn it up if the AC is pulling too much current. The booster start capacitor looks promising. Have others here tried that? Maine AH might be correct about the Kill-A-Watt meter. I've never run mine to the max so don't really know how high it can read. It's rated for 1800 watts continuous use but I have no idea just how high it can actually read. When in doubt- you can borrow . . or buy a clamp-on inductive amp-meter for $14 and it can read up to 1000 amps. Great thing to have but I don't know if you care about owning one. Hard to go wrong with a model 95652 for $11.99 from Harbor Freight. Note that with a clamp-on inductive unit - there are no wire hook-ups needed. Main problem is . . it has to be clamped on to just one wire carrying power. Your lead cord has two. So you cannot just clamp such a meter on a power cord to get a reading. Easy remedy is to get a cheap extension cord- cut the insulation so you can separate the neutral from the hot. Then when taking amp readings - use that cord and clamp it on the hot black wire only. Your 7100 BTU air-conditioner draws 4000 watts (34) amps when it first starts. Your household 20 amp circuit can likely handle 40 amps for a few seconds. The AC only does that though for less then a second. Then tapers way down to below 1200 watts. I've had the same size AC unit plugged into my Honeywell 2000i inverter-generator. Starts it OK (labors a little) and during the first second or two - AC voltage drops to 100 volts. If your voltage is dropping into the 80s - you've either got an amp draw at or over 40 amps - or the generator is inadequate. And note LOW voltage in small gensets is very common - even when they are new. A good starting capacitor will shorten that high surge-time . If you really want to know without guessing - you need a surge-amp-reading. How much and for how long. 34 amps @ 100-120 volts for up to one full second is normal. Beyond that - and the AC unit has problems. Quote Link to comment Share on other sites More sharing options...
canman47 Posted July 20, 2014 Author Share Posted July 20, 2014 The generator is an Onan 2.5CMZ. It's rated at 20.8 amps. Seems like maybe that's not enough to start this AC. Does the hard start capacitor reduce that load or just the duration of the load? I'm going to get an inductive ammeter. I've wanted one anyway. The Kill-a-Watt sounds like a good thing to have too. Thanks for all the info. This site is a great resource. Quote Link to comment Share on other sites More sharing options...
linda s Posted July 20, 2014 Share Posted July 20, 2014 The generator is an Onan 2.5CMZ. It's rated at 20.8 amps. Seems like maybe that's not enough to start this AC. Does the hard start capacitor reduce that load? Or could my existing capacitor be defective? It should have been enough to start it when new. Lots of people are running those AC's with a Honda 2000 with no hard start capacitator. I would look into getting my generator serviced to see why it's bogging down. Linda S CMZ would make it a Kohler not Onan. Look again at your manual if you have one Quote Link to comment Share on other sites More sharing options...
zero Posted July 21, 2014 Share Posted July 21, 2014 . Does the hard start capacitor reduce that load or just the duration of the load? The starting capacitor - regardless if the OEM cap, or a "hard start" kit that's been added - doubles or triples the voltage to the motor for a second or two. The reason is to give the motor more torque to get started more quickly and out of "start" mode. A sign of a bad starting capacitor is when the motor draws excess current for excess time when starting. So it does not reduce the load. It increases the initial torque and that shortens the duration of start-time. There are other devices that reduce load on the compressor at start - not related to the electronics. Quote Link to comment Share on other sites More sharing options...
linda s Posted July 21, 2014 Share Posted July 21, 2014 He seems to have an original generator and original air conditioner. Pretty sure they didn't come with hard start capacitators and darn sure that they all worked together properly when first sold. Either one of them is drawing too much power or the other isn't putting out enough. Could a bad wire in between cause this problem? Bad ground? Linda S Quote Link to comment Share on other sites More sharing options...
zero Posted July 21, 2014 Share Posted July 21, 2014 He seems to have an original generator and original air conditioner. Pretty sure they didn't come with hard start capacitators and darn sure that they all worked together properly when first sold. Either one of them is drawing too much power or the other isn't putting out enough. Could a bad wire in between cause this problem? Bad ground? Linda S They all came with starting capacitors when new. Many have "running" capacitors too. Caps go bad over time. He might just have a weak OEM cap. We used to fix a lot of cooler units at dairy farms. A start problem was often just a weak cap. In this guy's case - the unit works with shore-power but not with his generator. That could still be just a weak factory capacitor. Weak enough that the the household circuit can make up for the excessive draw and his generator can not. A properly wired household circuit rated for 20 amps can have almost twice the capacity of a 2.5 KW portable generator - to start a motor. Air conditioners in RVs just about always (that I know of) have cap-start induction-run motors, or cap-start-cap-run motors. Both versions MUST have a good starting capacitor to start properly with normal draw. Adding a "hard start cap kit" just adds an additional cap to supplement the OEM one, or to make up for a dead OEM cap. I've got a Coleman 7100 sitting in my junkpile that still works. I'll take a look later and see what rating starting cap it has. That all being said - without testing - we don't know if he has a weak cap, a compressor turning overly hard at first, or a weak generator. With the generator - the first thing that should be done is to check it's RPM. It must run at full rated RPM to make it's rating. That RPM is controlled by a simple governor spring and they all get tired over time. Very easy to adjust and get the RPMs back. Easy to check RPM with a cheap hand-held vibration tachometer. No hook-up and no electronics. Simple, easy and cheap if you shop carefully. I'll add that I know of many people with small generators that make poor power that just raised the RPMs a little over the rated speed and they worked much better. The only downside is that changes the Hertz Cycles and any clocks plugged in will read a little fast. Quote Link to comment Share on other sites More sharing options...
Maineah Posted July 21, 2014 Share Posted July 21, 2014 There is another issue with starting A/C units when the weather is hot and the A/C cycles the head pressure is higher that it would be from a cold start so it will need all the help it can get to restart. Some of the better DVM's these days have the ability to read capacitance. The caps are cheap so if there is any doubt just change it they do get tired with age. Quote Link to comment Share on other sites More sharing options...
zero Posted July 21, 2014 Share Posted July 21, 2014 New starting cap for a Mini-Mach 7100 is $10 if bought generic. 21-25 MFD and the original is rating 165 volts. A new replacement needs the same MFD (plus or minus 10%) but the voltage can be higher. Coleman part # for a high-priced Mallory cap is # 61632. Caps can be checked with a good multimeter or a cap-meter. Here's some AC cap info for anyone that cares. Quote Link to comment Share on other sites More sharing options...
canman47 Posted July 21, 2014 Author Share Posted July 21, 2014 Oops, yes it's a Kohler. Onan is my other RV. Quote Link to comment Share on other sites More sharing options...
canman47 Posted July 22, 2014 Author Share Posted July 22, 2014 So today the AC will run on the generator. The only difference is that it's much cooler today. When it was stalling the generator it was 107 in the shade and much hotter in the sun baked RV. Today it's only 85 which is a freak cold spell here in the Sacramento Valley. Does that make sense? Lacking any high current reading meter I used the smart meter on my power pole. I don't think it can read the surge start current but I briefly saw 1800 watts and then it dropped to 1400. I'm going to go ahead and replace or add a capacitor since it's 22 years old. So I guess I can use the AC as long as it's not too hot! Quote Link to comment Share on other sites More sharing options...
WME Posted July 22, 2014 Share Posted July 22, 2014 The higher the outside temp the more current the AC draws on a restart. Classic sign of a weak starter cap is one good start then it won't restart when the AC cycles. Quote Link to comment Share on other sites More sharing options...
Odyssey 4x4 Posted July 22, 2014 Share Posted July 22, 2014 It might be a little off topic but when my generator was running low on oil if i tried to start the AC the generator would bog down and/ or stall. The generator would then eventually not start at all due to the low oil switch. Filling it with fresh oil took care of the problem. Quote Link to comment Share on other sites More sharing options...
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