On Tuesday, July 11, 2017 at 5:40:08 PM UTC-4, Dave, I can't do that wrote:
Hi, we recently installed a window version of a swamp cooler. As such, it plugs into a wall outlet in the lounge-room. It has a new 1/3-hp (252-Watt) two speed motor from HomeDepot.

When it turns on either in Low or High, the voltage dips for almost a second from 125vac to 117vac then back up to 125vac. The dimming is maybe 20% longer for the High speed.

Is there some way we can soft-start this or what? It makes the lights dim in the house for that short period.

Before anyone launches into "house wiring," etc rest assured it is 11-years old, copper, in metal conduits and all up to code with 200-Amp coming into the house.

I used a 14g, 3-wire extension lead in a bunch of other outlets on different circuits and still get the same issue. I then hard wired the extension into the main breaker panel. Same again.

I have cleaned and oiled the fan bearings, adjusted the belt and the fan turns quite freely by hand.

What can be done to alleviate this momentary voltage drop?

Thanks

First, 117V is well within specs. That said, a 1/3 HP motor shouldn't load down your system like that. There is likely either a defective feed cable or a loose connection from the utility. You could check in the panel to make sure the feed cables are tight. Beyond that, you'll have to get the utility company to check it out. Around here, at least, they're usually pretty cooperative.