On Mon, 14 Jun 2004 08:46:58 -0400, Brian Lawson
wrote:


And as just a follow-up to that, we used MG sets that were operated
Star-Delta. These were effectively 6 lead motors, at 575 to 600 volts
AC producing 0 to 240 volts DC, or sometimes PWM DC. The AC end of
the MG's had three "hot" leads at all times, and the motor is started
and run by switching the other three leads at the controller. That
way, a whole contactor, valued at from $50 to $250 (depending on size)
was "saved". Only problem is, these MG's sit for at least 50% of the
time in a shut-down state, but the 600 volts is still a potential on
the windings, and these MG's would blow the AC ends at a much higher
rate than the same models in a more conventional "all motor leads
open" type.

Okay, that was false economy in action... But I don't see new
equipment built that way anymore, we've learned better.

(Though I catch pool men "handymanning" their own electric and
single-pole switching 240V pump motors all the time because they
/don't/ know any better. When I get to a complex where the controls
are screwed up, they complain about blowing out new motors, and I
learn the Pool Man has worked on the wiring, they come /all/ the way
apart for a full inspection...)

An example - modern hydraulic elevator machines with open
"submersible" pump motors in the oil tank that are run with 208V
Star-Delta starters, and they have two contactors in the controller.
(Even if you don't have access to the equipment room, the second
"clunk!" is quite audible from outside.) One for total shutdown, the
other for the Delta switch when up to speed.

They do have a second contactor involved, but the complexity of the
starter must be outweighed by the longevity of the motor. Especially
since the hydraulic oil might get traces of moisture in it. That and
if the motor shorts out there's 50 or so gallons of oil there.

(There is also a sticker on the top of the tank to not use water
based hydraulic fluids. Well, Duh....) ;-P

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
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