On Tue, 11 Jul 2017 14:40:03 -0700 (PDT), "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.

The problem is the motor. The type of motor sold at H-D is designed
for low inertia loads that start very rapidly. This type of motor
draws tremendous starting current - I've seen as high as 80 amps - and
at low power factor.

The correct solution is to buy a fan duty motor. This type of motor
is designed for high inertia loads and is designed to draw only
moderate starting current. Your best bet would be to return that
motor if you can and buy the correct one from an electric motor shop.
If there isn't a shop near you, I recommend the shop where I used to
moon-light - Jerry's Electric Motor Shop, Cleveland, TN. 423
472-5959. Tell Jerry that I referred you.

If you're stuck with that motor, there are some things that you can
do. Foremost is to supply the starting current from a capacitor. The
starting current is very low lagging power factor. A motor start
capacitor will supply the leading power factor current to offset the
motor's lagging current demand so that mostly only real power (low
amps) is drawn from the line. The capacitor is simply connected in
parallel with the motor's terminals through a current relay.

The capacitor can't be left in the circuit. IT must be disconnected
once the current draw has dropped sufficiently. This is normally done
with a current mode starting relay.

This type of relay is normally open with a few heavy turns of wire as
the coil. The motor current flows through it AFTER the capacitor. The
contacts are connected in series with the motor start capacitor. The
inrush current draws in the relay, allowing the capacitor to supply
the imaginary power. When the current drops to say, 15 amp, the relay
drops out, disconnecting the capacitor.

If you're interested in tackling this kind of work, I can spec out the
components.

John
John DeArmond
http://www.neon-john.com
http://www.tnduction.com
Tellico Plains, Occupied TN
See website for email address