Will old 8HP genset be enough for 6.5HP air compressor?

softdownsoftdown Solar Expert Posts: 3,247 ✭✭✭✭
It is a 70's vintage Briggs & Stratton 8HP genset. I believe it claims 3300 watts. Has a small 220 volt outlet. Loves to start - started on second pull after 5 years. I use good fuel additives.

It is a 1999 Ingersoll Rand 240 air compressor calling itself 6.5HP and 15 amps @ 240 volts. Has a large 220 volt plug that I installed in 2012. Sounded good when running. Was using a larger genset that is now 160' away.

Just wondering. Would be so convenient. This genset has wheels. But undersized gensets can also fubar machinery.

My inclination is "close shave". Less than ideal. Might work. Might be too small of genset.
First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,656 admin
    Generally, I would start with a genset that is ~2x larger than the driven electrical motor (i.e., 6.5 hp compressor => 13 hp or larger genset).

    Other ways to look at the issue:
    • 15 amps * 240 VAC = 3,600 VA
    • 15 amps * 240 VAC * 0.66 PF (cheap induction motor PF) = 2,376 Watts
    • 2,376 Watts / 746 Watts per hp = 3.2 HP minimum
    So, it is possible that the air compressor is closer to a 3-4 HP, and not really 6.5 HP. So it is possible that your 8 hp genset may run the air compressor.

    Starting with the tank at 0 psi--Probably no problem. Starting with a 100+ PSI in the tank, you now need to check the air compressor and what it uses for an unloader... Some unload when the pump stops spinning, some unload when the air pressure from the pump drops below tank pressure and unload then... Etc.

    If the unloader is working OK, then hopefully the pump gets up close to speed, and the does not bog down the genset.

    Another possibility, a continuous run unloader valve... The valve monitors the tank pressure, and either sends compressed air to the tank, or vents it to the atmosphere and lets the pump run against (almost) zero backpressure.

    https://www.amazon.com/Control-Devices-Brass-Continuous-Unloader/dp/B007GDXVK0 (don't know anything about this particular valve/company).

    I have used this type of valve multiple times... Both on smaller gensets running from a ~2,000 Watt generator, and from utility power (large genset that dimmed the lights when starting--So made continuous run).

    You can pair the valve with a standard pressure switch--And setup the compressor for continuous run (on genset), and start/stop (back at the shop).

    So--I would give it a shot--See if the old genset start/stops the compressor with a full tank, and if it does not (reliably), then look into a continuous run unloader.

    One compressor/genset combination was so "close", I had to make a motor mount hinge. I would lift the compressor motor and set slack in the compressor belt. Start the genset and electric motor (zero load), bring up to speed, then tilt the electric motor back down to tension the belt... And run as a continuous run/unloader valve. Worked great (the whole compressor was built out of bits and pieces).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,828 ✭✭✭✭✭
    Marginal.  With windage losses in both engine and alternator, the genset is going to be at full load.   It may not have enough "grunt" to even start the motor.  
    I'd look into wheels or a long cord
     Here's the motor draw specs. Even though they are for a well pump, a capacitor start motor is a capacitor start motor.
    Largest on the chart is 5Hp   good luck





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  • softdownsoftdown Solar Expert Posts: 3,247 ✭✭✭✭
    Bought a 3500 genset for resale - tried to make a go of a cabin solar business for awhile. This genset claims a 4300 watt surge capability. It is still in the box. If I use it - the value drops almost 50% - there is that. On paper - this would work. But does my rare need for a large air compressor justify dropping the value of a new genset by 50%?

    My Makita 120 volt compressor is very good - and completely portable. It kept up with three active roofers in Florida. So I may not use the 240 volt compressor unless the old genset can run it.
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
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