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Participant

Join Date: Sep 2008
Posts: 1

Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/16/2008 12:40 AM

Hi

I have a 5 ton 1Ph 240 v central ac unit running current of 19.8 amps start up of 48 amps.

Living in Fl. storms cause a need for portable power,I have a 7800 watt gen with start up of 13,500 watts. just seems short on start up abilities can additional capacitors be wired in the circuit to help? And how would they be sized and wired?

Any help would be appreciated.

thanks

SNOOK!E

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Guru
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#1

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/16/2008 7:02 PM

Depending on the generator, you need about 3 times the starting amps capacity to start you A/C. 13000 watts/ 240 volts = 54 amps. So you would need an gen with a starting capacity of 48 x 3 = 144 amps x 240 volts = 34000 watts. Capacitors would not help. You just have the capacity to start the A/C.

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Anonymous Poster
#2
In reply to #1

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/16/2008 9:53 PM

Thanks for taking the time to reply.

Snook1e

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Join Date: Feb 2006
Location: Piney Flats, Tennessee
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#3
In reply to #1

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/16/2008 11:04 PM

thats the outside unit --- what about the 110 volts for the fan inside --- no much good to run the compressor outside and the fan if there is not fan running inside to reap the benifits ---

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Join Date: Jan 2007
Location: Los Angeles, California
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#4

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/16/2008 11:31 PM

Wareagle (Great Name!) may be right about the generator being too small, but here are some experiments to try before purchasing a new generator.

1) Have a technician install a properly sized start-relay and start-capacitor on the compressor. It cannot hurt, and may actually improve normal start-ups. Do not use a solid state start assist device. They are good, but a correctly sized start relay is better.

2) Your A/C system should have three motors: Indoor Fan, Outdoor Fan, and Compressor. Have the technician temporarily disable the first two motors, and see if the generator can start the compressor by itself. If the compressor does start by itself, have the technician install time delay relays, so that the the other two motors start several seconds after the compressor starts.

3) Try to avoid voltage drop in the external wiring between generator and air conditioner by using power wires as large and as short as possible. Good Luck!

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Power-User

Join Date: Jul 2006
Location: CA (Central Arkansas, USA)
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#10
In reply to #4

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/17/2008 12:26 PM

Also, add a time delay to make sure the pressures are equalized before the compressor trys to start. -- JHF

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Power-User

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#11
In reply to #4

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/17/2008 12:41 PM

I agree. I think what many posts are missing is that most A/C compressors are sent from the factory to operate as a permanent split capacitor motor and have no start relay or cap. Adding a proper start relay and cap may give you the edge you need. I would still delay the indoor and outdoor fans. I don't know much about PF correction, but I think the delays and what we call a start kit will help. Also be sure pressures are equalized before starting. A time delay will do this nicely. The valve idea is good, but is a little over kill in my opinion, but if the compressor still won't start, adding a valve would be much cheaper than a bigger gen set. -- JHF

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#12
In reply to #4

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/18/2008 2:04 AM

Definitely a GA only thing to add is, measure the current with a good tong tester at each stage of development. I have used the start relay and increased start cap method to start difficult compressors before. Reducing the start current and / time at maximum current is the key not merely PF as many posters have come up with. After the starting improvements are sorted out then start fiddling for a bit more improvement with PF. The difference is start Caps are in series with the start winding and pf Caps are in parallel with the entire motor or motors.

As another poster mentioned, a short cycle timer or a head pressure switch is a very good idea too in case of an unforseen dropout or if the natural cycle of the system isn't long enough to allow equalisation. Short cycling is not only hard on the Genset but can quickly destroy the compressor.

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Anonymous Poster
#5

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/16/2008 11:48 PM

The capacitor solution should help to certain extend though the DG suppliers usually discourage this becoz of the design considerations of alternators which cannot support leading power factor loads (while only capacitors are connected and actual load is OFF). Typical compressors in DOL Mode have 0.7 lagging power factor and as such adding about 10 A capacitors in parallel should help however the following two solutions seem better promising in this case (besides the possibility of increasing the DG size):

1. Convert compressor DOL starter to Star-delta Starter if at all AC supplier has option

2. Install a VFD for the Compressor.

Regards

Sanjeev

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Anonymous Poster
#6
In reply to #5

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/17/2008 4:32 AM

Hi just popped in and note that we are recommending star delta start, can't be done this is a single phase motor.

The real easy solution to this is off loaded start, install a bye pass solenoid valve (opens when energised) between the high and low sides of the compressor, wire via a delay off relay set for 6-7 seconds, size 1/2 inch should be about right. A bonus will be by removeing the high starting pressure and thus amps you could well extend the compressor life.

Regards

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#7

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/17/2008 10:02 AM

Look for a soft-starter with adjustable ramp-up, ramp down. They are available for single phase installation. Start the compressor first than each of the fans or both. Nothing is going to happened if they start couple of seconds later. once running, the soft starter is bypassed out of the circuit. As your gen set is pretty limited so you can even use three of them one for every circuit and connected in series . They are nothing more than a modified light dimmers (Europeans are made for 240A up to 10-15kW as shelf item)... dirt cheap and it works.

Wangito.

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Associate
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#8

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/17/2008 10:19 AM

Capacitors would be hard to use in this application, but a flywheel could help. You could build a fly wheel to assist starting large loads from a small source using a 4 pole motor (1800 rpm) of 3 to 5 horse power with a flywheel mass matched to the starting torque of the motor. The fly wheel would add inertia or stiffness to the small power system by supporting frequency and contributing reactive power when the large ac compressor starts. Second, the small power required to keep the flywheel spinning would keep the generator's engine off the governor and improve it's response.

just my random thought of the day

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Power-User

Join Date: Jun 2007
Posts: 141
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#9

Re: Will Additional Capacitors Help Start-up AC Unit from Portable Generator?

09/17/2008 12:24 PM

Hello Snookie,

If I understand your problem correctly, the ac draws as much as 48 amps when starting, and you have a generator capable of providing 13,500 watts for a short period to allow starting of equipment.

If you know the power factor of your unit, then you can calculate the size of capacitor necessary to bring the power factor back to unity. If the unit is 240 volts, draws 19.8 amps when running, and has a 70% pf, then you would about 3.4 kVAR of capacitance to bring it back to unity, wired such that when the compressor runs, the caps are also on. It would reduce the running amps to about 13.9 amps using this example. However, this may not provide any drastic improvement when it comes to starting the ac.

While 13,500 watts should be sufficient to start your ac, I suspect that as the ac is trying to start from your generator, it is drawing more than 48 amps. From what I've seen, the heavy load placed on the generator during start-up by a motor drops the voltage being supplied, causing the motor to draw more current in an attempt to start, dropping the voltage more... and so forth. The final result is the motor stalls, or takes so long the main on the generator trips.

If the gap is small enough between what your generator can supply and what is needed, perhaps the capacitor in combination with some of the other inventive solutions mentioned by the other posters, such as delaying the start of the other smaller motors in your ac, may be what is needed to bridge the gap.

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Anonymous Poster (3); Circuit Breaker (2); dadw5boys (1); Emjay4119 (1); Gadget Guy (1); jconn_ETEC_05 (1); Lendog (1); wangito (1); wareagle (1)

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