Not pumping water, but pumping air

Scottg4001Scottg4001 Solar Expert Posts: 47
To keep this short, I have a small piston 24v BLDC air compressor directly connected to a panel for daytime use only. The problem I am having is that the compressor will not "slowly" start as the sun gradually makes greater contact on the panel. I have to manually switch the circuit off then back on for it to start. I think its something internal in the compressor that requires the circuit to be reset, not 100% sure, I have one more test to do tomorrow morning.

My question is this, other than using a time clock is there any other electronics available that would start the compressor when either there was enough current available or certain amount of watts/m2 available? The compressor only needs 3A to run and its a 7.9 Imp panel. Thanks, :cool:

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    Re: Not pumping water, but pumping air

    You might be overheating the motor as the sun rises/sets... Motor stalls, not enough panel current to start motor, which increases current flow and heating of the motor.

    Not cheap, but there are Linear Current Boosters that are designed to take the "high voltage/low current" and turn it into low voltage/high current needed to start/run motors from solar panels. That should help, if not fix, your starting problem.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Scottg4001Scottg4001 Solar Expert Posts: 47
    Re: Not pumping water, but pumping air

    BB, thanks. I actually have one of those linear current boosters currently connected inline between the panel and compressor and I have double checked the wiring configuration too. I will consider the overheating issue.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Not pumping water, but pumping air

    Starting a compressor (or other pump) against a load requires more current than running, regardless of the type of compressor.

    Other than the booster Bill mentioned there's no way to know what the current available is until it is 'tested' with a load. It might be possible to have the panel power a relay which would engage when the current is high enough to run the pump, but then half the power would be spent running the relay (although that too requires more current to engage the contacts than to keep them closed).
  • Scottg4001Scottg4001 Solar Expert Posts: 47
    Re: Not pumping water, but pumping air

    Carib- this is true. Compressors need a higher start current. Not worrying about power consumption, could you tell me what relay and contact I would need for something like this? I know what a contact is, so basically the relay. But what device would actually "read" the current? Or is that what the relay does? Sorry, just not familiar with relays.
  • BB.BB. Super Moderators, Administrators Posts: 31,085 admin
    Re: Not pumping water, but pumping air

    If the pump is not "over heating"--You could possibly get the auto restart by using a voltage controlled switch. Turn off the pump if the pump input voltage falls below (for example) 8 volts. The voltage will probably shoot right back up and turn on the switch again (if you can get/build a switch with a 5 minute turn on delay and a 5 second turn off delay--that might work well).

    This one has a 3 second on/off delay.

    This may be another option.

    There are other pumping controllers that may have options you can use (real pricey though).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Not pumping water, but pumping air
    Scottg4001 wrote: »
    Carib- this is true. Compressors need a higher start current. Not worrying about power consumption, could you tell me what relay and contact I would need for something like this? I know what a contact is, so basically the relay. But what device would actually "read" the current? Or is that what the relay does? Sorry, just not familiar with relays.

    I've never actually designed such a system so I can't be specific. Just thinking along the lines that if you know how much current you need to get the compressor started you could have a relay with a similar current requirement connected to the panel; when the current produced by the panel is sufficient to close the relay contacts the relay would then start the compressor. The obvious flaw here is that if all the power is going to the relay there's nothing for the pump, so you have to do a bit of experimenting to find the right combination of relay and perhaps resistor to make it work. A capacitor or two may have to be included in the circuit as well; they can be handy for controlling contact arcs and current surges.

    If there were an easy way to sketch up a schematic here I'd toss my idea out there for the people who are skilled at circuit design to laugh at and redo. :p
  • Scottg4001Scottg4001 Solar Expert Posts: 47
    Re: Not pumping water, but pumping air

    Does anyone know of a 24VDC photocell that is "closed" during the day so at night the circuit opens and shuts off the compressor and closes when it senses enough light to turn the compressor on?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Not pumping water, but pumping air
    Scottg4001 wrote: »
    Does anyone know of a 24VDC photocell that is "closed" during the day so at night the circuit opens and shuts off the compressor and closes when it senses enough light to turn the compressor on?

    Light isn't the problem. The problem is that nearly every such circuit relies on Voltage for trigger and getting Voltage from a PV is easy; current isn't.

    What I was thinking of is having a relay that is normally open and directing the PV's output through a resistor with the same value as the pump windings. Parallel to this would be the relay's windings and a balancing resistor so that this second circuit has equal resistance to the first. When the current flow across both becomes sufficient the relay closes, switching the output from R1 to the pump allowing it to run. It may also be necessary to charge a cap across the PV output to take the "sudden jolt" when the pump is switched in to the circuit in place of the resistor.

    On the other hand it's been one of those days so far so this whole post could be rambling nonsense. :p
  • Scottg4001Scottg4001 Solar Expert Posts: 47
    Re: Not pumping water, but pumping air

    current isn't the issue either. When I go out to the system before the sun has shined any light and turn the breaker off then back on the compressor will start moving when the sun does start to come out, but very slowly and obviously gains speed as more light makes contact to the panel because current is available. I am trying to avoid manually resetting the circuit. For some reason, something happens with the circuitry in that compressor when the sun goes down and forces me to reset the circuit everytime. The breaker doesnt trip, I just need to turn it off then back on and at anytime between dusk and dawn.
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Not pumping water, but pumping air

    WH not a voltge controlled switch coupled with a small battery? Set the voltage to turn on once the sun provides enough power, then the battery provides the pump the boost it needs to start, then charges from the PV?

    Is there really a problem turning it on manually? If not, are you in search of a solution for a problem that doesn't really exist?

    Tony
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Not pumping water, but pumping air

    Actually it is current and Voltage.
    With any amount of light on a PV it will produce Voc; enough Voltage to 'fool' an electronic circuit. Connect a load across the output and the Voltage instantly sinks as the panel tries to produce current. Technically a panel capable of 7 Amps can produce 7 Amps at Vmp or at 1 Volt; it doesn't care where the Voltage settles acting on the resistance applied. The pump motor, on the other hand, does. It wants to see 'X' Volts with a capacity of delivering 'Y' Amps, which is what you get from a battery. If the pump is powered by something which can deliver more current than it is designed to handle it will suffer.

    So the pump can take, say, 3 Amps @ 12 Volts. The panel can deliver 7 Amps @ 17.5 Volts. But the panel can also deliver 7 Amps @ 12 Volts or 7 Amps @ 1 Volt. It depends on the insolation of the panel and the resistance of the pump. The pump wants 36 Watts of power, and it wants it as 12 Volts @ 3 Amps. The panel can deliver 36 Watts of power as any combination of V * A, and if the Voltage to the pump is too low the pump does not spin fast enough - even though it is getting maximum current.

    What you need is a system that keeps the pump disconnected if there isn't that stable A @ V that will run the pump properly. That's why I outlined the circuit with the relay and resistor; with a load across the panel the Voltage remains low until the panel can produce enough power (Watts) to supply the V*A need of the pump.

    I don't know what's inside the pump so I'm speaking in generalities.
    Are you sure you don't want to get a controller and battery and just stabilize the whole power flow thing all at once? :D
  • Scottg4001Scottg4001 Solar Expert Posts: 47
    Re: Not pumping water, but pumping air

    I can't use batteries for legal reasons....don't ask why. True story though.
  • Scottg4001Scottg4001 Solar Expert Posts: 47
    Re: Not pumping water, but pumping air

    And I don't care if the pump starts slow then picks up speed. I just want it to start without having to manually reset the circuit every morning.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Not pumping water, but pumping air
    Scottg4001 wrote: »
    And I don't care if the pump starts slow then picks up speed. I just want it to start without having to manually reset the circuit every morning.

    There is a problem associated with starting, if the panels can produce more than the normal full-load current of the pump motor. If the pump uses 3 amps at 12 volts at full load, and the panels can supply 18 volts at 6 amps, that will not greatly overload a pump which is only delivering its rated output power. The pump will spin faster and then pull less current. Probably even less than 3 amps. Not the best situation, but workable. The starting surge of the motor may well use the full 6 amps.

    But if the panels can deliver 6 amps and the available current ramps up gradually, there is the risk that the pump motor will end up drawing more than 3 amps along the way before it even starts under load. A lot depends on whether it is a centrifugal pump (lower power consumption at lower volume, and seeing zero load at zero RPM) or a positive displacement pump, which will see the full back-pressure load as it is trying to start.

    A small circuit to not deliver current to the pump until a minimum current is available (short circuiting the panels until the current threshold is reached and then locking in the open circuit position until the current to the pump drops below a certain value or the voltage goes to zero during the night) should do the job.
    SMA SB 3000, old BP panels.
  • GreatBallofFireGreatBallofFire Registered Users Posts: 16
    Re: Not pumping water, but pumping air
    Scottg4001 wrote: »
    And I don't care if the pump starts slow then picks up speed. I just want it to start without having to manually reset the circuit every morning.

    If you're unable to use batteries to maintain voltage, the next best option is some form of capacitor. Is there an external controller box or built-in control circuit that is seeing the voltage drop off after the sun sets? If so, then a basic relay and capacitor circuit might be able to "trick" the controls so you don't have to reset it everyday. But, since I'm not really sure what you're working with, what I just said could be completely worthless.

    -Mark
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