Surge Demand Consequences

Discussion in 'General Electronics Chat' started by SOLARE, Apr 10, 2013.

  1. SOLARE

    Thread Starter New Member

    Apr 9, 2013
    2
    0
    Hi All,

    My first post here, hopefully its in the right spot!

    I am planning a stand-alone rural domestic solar system near Sydney Australia (240 AC). I want to set it up with minimal battery supply, enough to get me through one night only. I calculate the nightly energy usage at 2kWh.

    I don’t want this energy raided through the day by daylight appliances on a dull winters day, so I plan to have basically 2 separate switchable supply ccts for the house – one battery powered (night), and the other, daylight, running off an AC bus drawing power from PV panels via an inverter, with NO battery backup. There will also be a genset for the occasional heavy loads, breakdowns, and battery boosting in lieu of “days of autonomy”.

    My question relates to the daylight cct and its loads, specifically the small aircon we use during summer (presently powered by 6kW genset). I want to power this from the AC bus during the day. My clamp meter tells me it runs at 1100 watts continuous (4.6A at 240VAC). AS 4509.2 suggests a surge factor of 7 for induction motors when they switch on, meaning, as I understand it, a momentary current draw of about 32 amps at 240VAC.

    If that surge demand is not met from the panels at startup time because of cloudy weather, etc, or the 4.6A cts is rising and falling above and below that number, what are the consequences for the aircon? Will it get going, but slowly? Will it burn out? Should some cut-out device be installed to protect it?
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    The compressor will fail to start. The usual sequence is that it clicks out its internal overheat sensor in the windings. Much better to detect a start surge that lasts more than about 3 seconds and default to shut down.

    Can you do a current detect and switch-off circuit?
    Do you care to do that?

    ps, delaying the start of the fan motor for 5-10 seconds will give you an edge.
    pss, this almost surely means you will need a battery to provide the start surge power.
     
  3. timescope

    Member

    Dec 14, 2011
    298
    44
    The surge current you refer to is actually the starting current required by the motor. Failure to supply sufficient current to start the motor will result in it drawing a large current known as the locked rotor current. Air conditioners usually have overload protection.

    You would need a large number of solar panels to start an air conditioner without a battery and would probably need to use an inverter that is designed to work without batteries, for example a Grid Tied inverter, as normal inverters have a limited DC input range ( for example 10 - 15v for a 12v unit).

    The battery in a solar energy system regulates the system voltage and supplies the extra energy required to start motors etc.

    A 6 kW system I designed recently used 30 x 245 watt panels, 48v @ 1,400 Amp Hour batteries and a 6kW sine wave inverter /charger. It was designed to run a 12,000 BTU aircon for a few hours a day off the batteries and power all the normal domestic appliances.

    Timescope
     
    Last edited: Apr 10, 2013
  4. SOLARE

    Thread Starter New Member

    Apr 9, 2013
    2
    0
    Thanks for that. Ive got an understanding now. I plugged the aircon into the genset again, to check the surge again with the clamp meter (at the genset GPO), as I hadnt really seen any real change earlier, thinking it was so brief I missed it the first time I tested it. I switched it off again, then back on within a couple of seconds, this time getting a clear reading over 20A for a few seconds, then it dropped to 4A. The genset didnt like it at all, the power being demanded so soon under full load like that. The aircon may be "soft start" -- explaining why it made no real heavy demand when it first came on, but certainly I doubt it would handle on-off, on-off, at 20A all thru an overcast day. I expect it would cook the unit and probably the wiring as well. I like your idea of delaying the fan, & timing the surge with default shutdown (& timed auto-restart after say 10mins?)

    No. I wouldnt know where to start, but happy to try -- If its a simple enough project.

    Well, batteries I plan to do without for this application (cost), and simply throw kilowatts of PV (+ inverter) at the problem, maybe 15-17 kW. We get subsidised for panels+inverter, but not batteries. Once it is installed, we can play round with what it can reasonably run in the way of daytime additional farm equipment without irritating brown-outs.

    This is a part of Australia where its rarely hot when its overcast, so aircon unlikely to be needed on such days. Different if we were in Nth Qld in places where solar power struggles -- where its often cloudy, hot and muggy, and it rains every couple of days with annual rainfall measured in feet, not inches.
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,718
    4,788
    Sounds like the classic case of poorly thought out government subsidies that throw other people's money at problems encouraging the people that get other people's money to make poor engineering decisions because, to them, it appears "cheaper".

    Not saying I wouldn't do the same thing in the same shoes.
     
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