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When our neighbors turns on a motor, a bulb in our home turns off for a very very short time and then it turns on again. It is something like a quick flash but a man eye can notice it. Would you tell me why that happens, please? What could be the reasons? Is that because of the coils inside the motor? or the motor needs much power to start?

At larger scale, If a factory turns on a lot of large motors at the morning, Does that affect the nearest city or district?

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    \$\begingroup\$ In a word: transients. There are whole books about it; to pick one nearly at random: books.google.com/books?id=17NYBQAAQBAJ \$\endgroup\$ – Fizz Oct 19 '15 at 18:35
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    \$\begingroup\$ And the particular kind you ask about in the first paragraph is called a sag. The latter kind (2nd paragraph) could cause a brownout if the grid is poorly designed. The latter happens in some countries, but I think rarely in the West. We had a question about something like that happening in India (regularly in a certain area) not so long ago. \$\endgroup\$ – Fizz Oct 19 '15 at 18:53
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Firstly, common types of motors take a lot of current when they're stationary, as they are when you first turn them on. This isn't really 'because of the coils' particularly - in fact, a pure inductance, which is what one tends to think of as a coil has exactly the opposite behaviour. It's because they're frantically drawing current to try to get themselves up to speed.

So, your neighbours draw a lot of current when they turn on a motor, and because you and they clearly share a cable back to some more major part of the mains system, this high current creates a voltage drop across that cable which you can see on your lights.

You're right to think that a similar effect happens at all levels of the supply network, but two things lessen the visible effect:

  • Even big industrial loads tend to be a small proportion of the total load averaged over lots of properties
  • The supply for a big area has an incredibly low impedance, so that normal loads don't cause much effect on voltage.

Electricity suppliers do have other tricks to regulate the voltage they supply - they change tappings on transformers, and they manipulate the flow of reactive power through the system, both of which affect the voltage at their customers. But mostly the reason you don't see flicker at the level of a whole town is just that the supply is very stiff relative to the individual loads.

Really, really massive single point loads do have to liaise with the network before switching, but those don't tend to be at the level of a few motors in factories.

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    \$\begingroup\$ The sudden turn-on of everything alluded to in the question tends to blow circuit breakers. Plants will subsequently learn to stagger their start ups. \$\endgroup\$ – gbarry Oct 19 '15 at 19:04
  • \$\begingroup\$ Plants tend to stagger starting of various pieces of equipment within the plant. That generally takes care of most of the problems. There are places with extremely large pieces of equipment that need to notify the power supply company to schedule starting. \$\endgroup\$ – Charles Cowie Oct 19 '15 at 20:48
  • \$\begingroup\$ I once worked at a place with a 5,000 kW grinding mill - they had a direct phone line to the power station control room, so they could co-ordinate starting the mill. \$\endgroup\$ – Li-aung Yip Nov 1 '15 at 3:43
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Most motors have a high inrush current when they start. It is the combined effect of the motor becoming magnetized and the power required to accelerate the inertia of the load from a standstill to full speed. The current causes a voltage drop in the wiring to the power source that you share with your neighbor. It is unusual for that to be noticeable. The wiring is usually sized large enough to make the voltage drop too small to notice. Larger motors have a larger effect, but they also have large wire to minimize the effect. At the start of business in the morning, you might be able to see the voltage drop with a sensitive meter, but it would not likely be otherwise noticeable.

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