Does anybody out there know what is the absolute smallest generator in terms of kilowatts that is necessary to run a 16K BTU Marine Air A/C. The only thing the unit would run is this A/C unit at a given time. Marine Air told me that when running, the 16K BTU unit uses 1287 watts, but when starting it requires a peak power of 7370 watts. I know that generators can hold a surge of up to 3 times (well, at least Yamaha do) their rated power production for about 3 seconds. So theoretically, a 2.5, 2.8 or 3.0 KW generator could start it and run it. Has anyone made these calculations and installed such generators for this purpose? Does anybody know how small of a unit is installed out there to run such an A/C unit? Help on this matter is appreciated. Gil
Re-posting - Generator and 16K BTU A/C
- Thread starter gnarvaez17515
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LRA
Starting currents (LRA = Locked Rotor Amps) generally run between 5 and 10 times running current (FLA = Full Load Amps), with the duration of LRA primarilly determined by motor design & loading. The starting current is represented by a curve: that starts out very high (5-10 x FLA) then (with time) declines to FLA. Often, unloaded motors will start in under a second. Unfortunately, loaded motors (like compressors) often take much longer. Another interesting starting benchmark is 200% at 10 seconds, a rating that NEMA DC motor starters must comply with. This assumes that DC motors may draw 2 times running current for as long as 10 seconds. You indicate a LRA (starting current) of about 573% of FLA, (7370 watts starting ÷ 1287 watts running = 5.73) which is pretty decent. We can extropolate a worst case curve (200% for 10 sec), and make the (dangerous) assumption that your Generator will have to put out at least 7400 W for under 2-3 seconds, then 2600 w for the next 7-8 seconds. Ergo: ... theoretically, a 2.5 KW (and 2.8 or 3.0) generator could start it and run it ... To be certain, you’d really want to compare the Refer’ Motor Starting Curve to the Generator Loading curve; but it appears likely that the Yamaha 2.5 Kw Gen’ should do the job. BTW: I've pasted this discussion to http://www.kollmann-marine.com/phpBB/ , where Richard Kollmann is a real expert. HTH Gord
Starting currents (LRA = Locked Rotor Amps) generally run between 5 and 10 times running current (FLA = Full Load Amps), with the duration of LRA primarilly determined by motor design & loading. The starting current is represented by a curve: that starts out very high (5-10 x FLA) then (with time) declines to FLA. Often, unloaded motors will start in under a second. Unfortunately, loaded motors (like compressors) often take much longer. Another interesting starting benchmark is 200% at 10 seconds, a rating that NEMA DC motor starters must comply with. This assumes that DC motors may draw 2 times running current for as long as 10 seconds. You indicate a LRA (starting current) of about 573% of FLA, (7370 watts starting ÷ 1287 watts running = 5.73) which is pretty decent. We can extropolate a worst case curve (200% for 10 sec), and make the (dangerous) assumption that your Generator will have to put out at least 7400 W for under 2-3 seconds, then 2600 w for the next 7-8 seconds. Ergo: ... theoretically, a 2.5 KW (and 2.8 or 3.0) generator could start it and run it ... To be certain, you’d really want to compare the Refer’ Motor Starting Curve to the Generator Loading curve; but it appears likely that the Yamaha 2.5 Kw Gen’ should do the job. BTW: I've pasted this discussion to http://www.kollmann-marine.com/phpBB/ , where Richard Kollmann is a real expert. HTH Gord
A caution
Mistakes - yep, I admitted I was wronge once - turned out I was right after all. I'll never make that mistake again.
Actually - perhaps I was a little hasty in advising that a 2500W Gen should start the 16,000 Btuh Air-Cond: I’ve looked at the spec’s I could locate on-line, and caution that my previous opinion is really cutting it fine. I’d suggest you contact your Air-Conditioning dealer, and get the A-C starting curves. Alternatively, find out the Motor make & model, and go directly to them (probably Danfoss) for the info’. Unfortunately Marine Air doesn’t offer starting CURVES, just LRA (64 A 115V = 7360 Watts - but no start duration listed): http://www.marineair.com/marineair/pdfs/L-2124.pdf You’ll note that neither Yamaha Generator (listed below) actually meets the 64 A starting current (LRA) listed above. It’s going to be a very close call! From Yamaha @ http://www.yamaha-motor.com/products/gen/ef2600.html Model EF-2600: MAXIMUM AC OUTPUT: 2600 watts RATED AC OUTPUT: 2300 watts RATED / MAXIMUM AC CURRENT: 19.2 Rated / 21.7 Max. amps @ 120V Ergo: 19.2 x 3 = 57.6 Amps for 3 seconds Model YG2800i (Inverter) - This is a new one on me - but it “reads” well MAXIMUM AC OUTPUT: 2800 watts RATED AC OUTPUT: 2500 watts RATED / MAXIMUM AC CURRENT: 20.8 Rated / 23.3 Max. amps @ 120V Ergo: 20.8 x 3 = 62.4 Amps for up to 3 seconds. From the Yamaha Sizing Guide: http://www.yamaha-motor.com/products/gen/sizing.html ”...Normally, you won't need to consider motor starting requirements when using a Yamaha generator. When a Yamaha generator is properly sized for a tool's or appliance's running requirements, Yamaha's surge capability or 3 times the rated output for three seconds usually is sufficient to handle the motor's starting surge needs....” HTH Gord
Mistakes - yep, I admitted I was wronge once - turned out I was right after all. I'll never make that mistake again.
Actually - perhaps I was a little hasty in advising that a 2500W Gen should start the 16,000 Btuh Air-Cond: I’ve looked at the spec’s I could locate on-line, and caution that my previous opinion is really cutting it fine. I’d suggest you contact your Air-Conditioning dealer, and get the A-C starting curves. Alternatively, find out the Motor make & model, and go directly to them (probably Danfoss) for the info’. Unfortunately Marine Air doesn’t offer starting CURVES, just LRA (64 A 115V = 7360 Watts - but no start duration listed): http://www.marineair.com/marineair/pdfs/L-2124.pdf You’ll note that neither Yamaha Generator (listed below) actually meets the 64 A starting current (LRA) listed above. It’s going to be a very close call! From Yamaha @ http://www.yamaha-motor.com/products/gen/ef2600.html Model EF-2600: MAXIMUM AC OUTPUT: 2600 watts RATED AC OUTPUT: 2300 watts RATED / MAXIMUM AC CURRENT: 19.2 Rated / 21.7 Max. amps @ 120V Ergo: 19.2 x 3 = 57.6 Amps for 3 seconds Model YG2800i (Inverter) - This is a new one on me - but it “reads” well MAXIMUM AC OUTPUT: 2800 watts RATED AC OUTPUT: 2500 watts RATED / MAXIMUM AC CURRENT: 20.8 Rated / 23.3 Max. amps @ 120V Ergo: 20.8 x 3 = 62.4 Amps for up to 3 seconds. From the Yamaha Sizing Guide: http://www.yamaha-motor.com/products/gen/sizing.html ”...Normally, you won't need to consider motor starting requirements when using a Yamaha generator. When a Yamaha generator is properly sized for a tool's or appliance's running requirements, Yamaha's surge capability or 3 times the rated output for three seconds usually is sufficient to handle the motor's starting surge needs....” HTH GordAdding Start Capacitors
Adding Additional Start Capacitors (Parallel): Many of the newer A-C Compressor Motors now use PTCR’s (Positive Temperature Coefficient Resistors) in lieu of “Start” Capacitors (they still use “Run” Capacitors). You don’t want to add Start Capacitors to a motor utilizing a PTCR soft-start device. Care must also be exercised when paralleling additional Capacitors to a Capacitor Start Motor. Too much reactance can cause start relay (or centrifugal switch) failure, overheating and premature motor failure. I’d suggest that “customizing” your motor start circuitry is not an average person’s DIY project. Fred advises: “add as many 'hard start' capacitors to the AC as needed to get it to roll over the LRA hump. I have three on my home heat-pump so it will work with the standby generator.” I suggest he’s either very knowledgeable, very lucky, or about to find out he’s neither. Sorry Fred - and after you were so kind (to me). Gord
Adding Additional Start Capacitors (Parallel): Many of the newer A-C Compressor Motors now use PTCR’s (Positive Temperature Coefficient Resistors) in lieu of “Start” Capacitors (they still use “Run” Capacitors). You don’t want to add Start Capacitors to a motor utilizing a PTCR soft-start device. Care must also be exercised when paralleling additional Capacitors to a Capacitor Start Motor. Too much reactance can cause start relay (or centrifugal switch) failure, overheating and premature motor failure. I’d suggest that “customizing” your motor start circuitry is not an average person’s DIY project. Fred advises: “add as many 'hard start' capacitors to the AC as needed to get it to roll over the LRA hump. I have three on my home heat-pump so it will work with the standby generator.” I suggest he’s either very knowledgeable, very lucky, or about to find out he’s neither. Sorry Fred - and after you were so kind (to me). Gord
Caution Against Undersizing Generator
Repeated starts with a generator that is undersized will cause premature failure of either the generator and/or the air conditioner so caution is advised. Just because it works one time or a few times doesn't necessarily mean it's a good match. If it were me I'd err on the side of a larger wattage generator and get a type of generator that works only as hard as needed to produce the power needed. For example, like the Honda EU series. Sure, these are more expensive than, say, a Coleman, but the life cycle cost and other advantages, like less noise, less fuel consumption, etc. would be worth it. Salt air and salt water: another note of caution; the Hondas not a marine generator and are not designed for salt air environments so this should be taken into consideration, too.
Repeated starts with a generator that is undersized will cause premature failure of either the generator and/or the air conditioner so caution is advised. Just because it works one time or a few times doesn't necessarily mean it's a good match. If it were me I'd err on the side of a larger wattage generator and get a type of generator that works only as hard as needed to produce the power needed. For example, like the Honda EU series. Sure, these are more expensive than, say, a Coleman, but the life cycle cost and other advantages, like less noise, less fuel consumption, etc. would be worth it. Salt air and salt water: another note of caution; the Hondas not a marine generator and are not designed for salt air environments so this should be taken into consideration, too.
Manufacturer's advice
I asked Marine Air (sales@marineair.com) what they thought about adding start capacitors, given your 16,000 Btuh specifics. Reply came from Dometic, as follows: Mr. May, I'm sorry it took so long to respond, but I only got back to the office today. I wouldn't run another cap in parallel, I don't believe that it's necessary. Based on what you've told me, what you're buying should be able to handle the load. Comments below in red. Regards, Barry Cyr Dometic Environmental Senior Customer Service Representative 954-973-2477 ex. 133 954-633-3133 Barry.Cyr@dometicusa.com Q:What is the starting duration of your compressors? - A. Less than 3 seconds. Q: Will it reach 75% full speed in 3 seconds (or less)? - A. Yes
I asked Marine Air (sales@marineair.com) what they thought about adding start capacitors, given your 16,000 Btuh specifics. Reply came from Dometic, as follows: Mr. May, I'm sorry it took so long to respond, but I only got back to the office today. I wouldn't run another cap in parallel, I don't believe that it's necessary. Based on what you've told me, what you're buying should be able to handle the load. Comments below in red. Regards, Barry Cyr Dometic Environmental Senior Customer Service Representative 954-973-2477 ex. 133 954-633-3133 Barry.Cyr@dometicusa.com Q:What is the starting duration of your compressors? - A. Less than 3 seconds. Q: Will it reach 75% full speed in 3 seconds (or less)? - A. Yes
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