Starting current of air conditioners

Discussion in 'General Electronics Chat' started by EngIntoHW, Aug 24, 2010.

  1. EngIntoHW

    Thread Starter Member

    Apr 24, 2010

    I have a power sensing unit, which senses the current drawn by an appliance.

    Appliances must be up to 16A rated (nominal current).

    However, as you know, appliance like air conditioner draws at their starting larger current than 16A.

    I'd like to switch an appliance OFF when I sense a current larger than 16A that lasts over X seconds.

    My question is, how long does the starting current "last"? (until the current goes down to its nominal level - below 16A).

  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    It is different for each motor based on the rotating inertia/torque, speed,current curves..but I'd guess no more than a few seconds.
    I'd be worried about how long your "power sensing unit" could take the starting current though if at all
    EngIntoHW likes this.
  3. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
    The current is sensed by a current sense resistor, which its 2 terminals are connected to the power sensing unit.

    The shunt is 31.6A rated, is it enough for a starting current?

    What is the level of a starting current of an 16A rated air-conditioner? (What's the range?).
  4. someonesdad

    Senior Member

    Jul 7, 2009
    There is no single answer to your question because different electrical devices behave differently. The usual starting current's time profile is determined by the start-up of an electrical motor and is determined by how long it takes for the motor to come up to speed (as mcgyvr indicated).

    The best way to get an answer is to measure it yourself. Unfortunately, this requires an oscilloscope and a current transformer or an appropriate shunt resistor. Both techniques can be hazardous and require you to know what you're doing. If you use a shunt, then I'd also recommend using a suitable differential amplifier.

    You say the resistor is rated to 31.6 A -- is this its continuous current rating? If so, then it's possible that double that current level for a second or so wouldn't hurt it. But the only way to know for sure is to test it. The usual shunt on a device like a DMM or Kill-a-Watt is a thick chunk of wire soldered to a PC board. An over-current condition can melt the solder, lift the traces, and cause other problems.
  5. EngIntoHW

    Thread Starter Member

    Apr 24, 2010
    Thank you very much.

    I'm not into knowing the exact time it takes the current to decrease.
    Just wanted to get a magnitude of oder.

    Is it less than say 10 seconds?

    All PCB traces are design to nominally handle 16A, as we haven't thought about the starting current when we designed the PCB.
    However, they weren't damaged, at least not evidently.
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    If it only draws 16A it isn't that large of a unit and it will be up to full operating speed long before 10 seconds has elapsed.

    One sure way to tell - plug a light bulb into the same socket (or two in series if it's a 240V unit) and see how long it dims after starting the unit.
  7. kingdano

    Senior Member

    Apr 14, 2010
    isnt a rule of thumb with starting torque for motors that it can draw up to 6X the rated current?

    so it would be 16*6 = 96 A peak power surge if that is the case.

  8. marshallf3

    Well-Known Member

    Jul 26, 2010
    There isn't much of a rule of thumb due to there being so many motor types. A window AC unit is going to be a capacitor start/run type, the compressor will be the largest part of the load and design of the unit will play a lot in how much the compresssor takes to kick in.

    Anywhere from 2 - 6 does sound like a reasonable number though.
  9. SgtWookie


    Jul 17, 2007
    You might wish to examine how circuit breakers are constructed.

    Just a couple of types;
    Those used in aviation are of a thermal trip type. Basically, there is a bimetal strip that is engaged when the spring-loaded breaker shaft is pushed in. If too much current passes through the bimetal for an extended period of time, the breaker shaft is released, which opens the contacts.

    Home-type breakers, for example the breakers that are used in Square [D] QO load centers, have a dual mode; an electromagnet that causes the contacts to open instantly if the momentary threshold is exceeded, and a bimetal strip that causes the contacts to open if an overload exists for an extended period of time.

    Keep in mind that attempting to break a heavily loaded mains-powered circuit is not a trivial task. If not done properly, you can wind up with a plasma fireball that will consume anything and everything nearby, including you.

    Use equipment that is already certified for such conditions rather than trying to build your own.
  10. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    hermatically sealed motors typically have a locked rotor current on thier nameplate. Failing that, considered 6x FLA as thier peak draw.
  11. PackratKing

    Well-Known Member

    Jul 13, 2008
    Yep.......for sure ..........I deal with this every day. -- 6x rated current is normal, only if it is a pulse of - say - 2-5 seconds.
    If attaining rated rpm takes longer, or the motor groans and complains, or starts to smoke, there's some serious issues to address.
    Bear in mind, starting and stopping either a cap-start, or PSC motor repeatedly, is cumulative, and will raise cain with windings in short order.
    Last edited: Apr 27, 2011