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I have an issue with my air conditioner. When it clicks on, the lights in my apartment dim for half a second.

If this was a car audio system, I would just wire some large caps up to the battery to smooth out those current spikes.

However, this is an AC circuit and a cap won't work here. So what's the equivalent?

What can I install on my circuit to help smooth out these spikes and prevent that annoying voltage sag?

Changing the wiring of my house etc is not an option, I need to fix this myself, so please don't suggest that.

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  • \$\begingroup\$ A sufficiently large UPS? en.wikipedia.org/wiki/Uninterruptible_power_supply \$\endgroup\$
    – Dampmaskin
    Jul 31, 2018 at 22:20
  • \$\begingroup\$ The same thing happens in modern homes, including mine. There is nothing that can be done about it. The voltage may briefly drop at the transformer supplying your power, so the onus is on the power utility to keep these voltage drops small and brief. That is all they can do. \$\endgroup\$
    – user105652
    Jul 31, 2018 at 22:22
  • \$\begingroup\$ @Dampmaskin. It would take an expensive UPS rated for motor (inductive) loads. About $3000 USD. \$\endgroup\$
    – user105652
    Jul 31, 2018 at 22:24
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    \$\begingroup\$ UPS is not viable, it's not rating for an appliance like an AC. But I don't believe that "nothing can be done" :) we are engineers after all! \$\endgroup\$
    – cat pants
    Jul 31, 2018 at 22:31
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    \$\begingroup\$ could use a large induction motor driving a large flywheel, this is probably not practical. \$\endgroup\$ Aug 1, 2018 at 7:16

4 Answers 4

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You can choose to use a separate breaker and power distribution cable, but if the ACU draws excessive surge starting current and your Voltage line drops more than 10% , all you can do is use regulated LED lighting to reduce the effects.

Capacitors perform the job of stabilizing voltage and providing the necessary jolt ( combined with higher current ) to start the motor in the first place. In an air conditioner, capacitors are connected to the compressor motor for start and run operation.

Your residence wiring or ACU load is sub-standard during startup and this may be outside your control.

ACU compressors often draw 300% to 500% of rated power during startup. Check the start-cap for failure. Ref

enter image description here

Essentially the Start Cap must be very big to act as a Power Factor (pf) Correction to the low inductance starter winding and thus reduce the reactive load current on the line causing voltage sag during startup. It must be rated for large ripple current and high voltage. It reduces the line current loss thus raising the voltage to improve start times to normal and reduce light dimming effects. In other words, the motor startup has higher reactive current which is partly supplied by the negative reactance of the caps. Ideally all of it, but this may be costly and very bulky.

I do not know the exact pf criteria they choose for residential start motor caps but the intent is to reduce the surge ratio above rated load current.

  • Anyone?

But I found this enter image description here

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  • \$\begingroup\$ Hi Tony, I notice you don't mention the inrush limiting option that I do in my answer. Although I've given a reference for an example of what seems a suitable product, you're an expert and I'm not - please comment on whether my answer is appropriate and I'll remove it if it isn't. \$\endgroup\$
    – user133493
    Jul 31, 2018 at 23:18
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    \$\begingroup\$ The problem with the air conditioner is the motor needs that high inrush current to speed up. If you limit the current, the motor needs more time to speed up and you extend the period of agony. \$\endgroup\$
    – Janka
    Jul 31, 2018 at 23:28
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Open your breaker panel tighten every single screw to every breaker especially the ones for the main breaker. Request to have the screws checked at your meter as well. Often this will fix the issue, especially if your house is being fed with alluminum wires. They tend to loosen up over time.

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Ask around your building to see if it also happens in other apartments. If it happens only in your apartment then maybe your air conditioner is too powerful for your electricity or only one connection from the main electrical panel to your apartment is faulty and dangerous (it might cause a fire).

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  • \$\begingroup\$ I mean nominally it draws 800 watts....my circuit is 20 amps or 2400 watts. It's definitely fine, but the lights dimming are annoying when the compressor kicks on. \$\endgroup\$
    – cat pants
    Jul 31, 2018 at 22:30
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If the inrush current were only a problem when turning on the appliance, a soft starter would make sense. Unfortunately, in this case, the appliance may turn itself on unsupervised.

A large thermistor can be used with air conditioners to tame inrush current, which can easily be over 10x the steady-state current. Needless to say, with the power level involved, it must be selected and installed carefully. As it would become part of the permanent installation, in many countries a licensed electrician would need to install it: if a fire resulted from do-it-yourself installation, you would not be able to claim insurance. As for selecting the thermistor, you could contact an applications engineer at a suitable manufacturer for advice.

There are several suitable manufacturers. Ametherm is one example. They have a guide to selection for air conditioners on their website here.

Added See the comments below this answer. Despite what manufacturers would have you believe, this isn't an appropriate solution!

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    \$\begingroup\$ Limiting the start current will extend starting problem so the lights will dim less but longer. This is a compromise and may trigger a breaker fault if not matched to the load and under the breaker I*t response curve. \$\endgroup\$ Jul 31, 2018 at 23:24
  • \$\begingroup\$ The problem with the air conditioner is the motor needs that high inrush current to speed up. If you limit the current, the motor needs more time to speed up and you extend the period of agony. – \$\endgroup\$
    – Janka
    Jul 31, 2018 at 23:29
  • \$\begingroup\$ Thanks for the comments. I'll leave my answer here as an example of how product advertising (e.g. the thermistor design guide page linked above) can lead you astray. \$\endgroup\$
    – user133493
    Jul 31, 2018 at 23:34

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