Just a thought: They make 3-phase motors with large flywheels for surge
buffering. This is wired in parallel with the power source (generator) and
is physically separate from the real motor you want to use. It runs
continuously and uses very little power even though it is a similarly sized
motor because the flywheel offers no running load. The purpose is to buffer
the line voltage during surges and handle large intermittent currents. When
you turn your vacuum motor on and the voltage just starts to dip a little,
energy stored in the flywheel is converted through back-EMF to 3-phase
current (synchronous with the source) pushed back onto the power lines to
bolster them up. They are use in many industrial applications where peak
loads are very short and high compared to the average. The buffer motor
rating should be equal to or higher than that of the vacuum motor, and the
flywheel momentum must be sufficiently large to not slow down significantly
during the startup surge. If you estimate the startup energy required for
the vacuum motor (volts x amps x time), the stored rotational kinetic
flywheel energy should be about ten times larger.
Jeff Condit
----- Original Message -----
From: "Alfonso Baz" <[EMAIL PROTECTED]>
To: "'Open Topic Forum'" <[email protected]>
Sent: Sunday, December 18, 2005 1:02 PM
Subject: RE: [OT] PCB router
Darcy,
The 5hp motor is around 3.7W at no load start-up current would be around 6
times the full load current, which in this case is around 107A (208V).
Your 20kw alternator would be rated at around 96A.
The alternator's irons shouldn't saturate at the 107A but the rather
abrupt
current drawn will produce a voltage dip.
The only way around this with the gear you have, if you're lucky, is to
see
what adjustments are available on the AVR. Most decent AVR's have at least
2
adjustments, response time, and stability. If you can get the AVR to
respond
quick enough you might get the voltage dip to lessen in it's severity but
you could end up with an alternator who's output voltage hunts. Another
problem you'll run into is the engines governor and the power rating of
the
engine. If the engine is weak compared to the load you are placing on it
then no matter how fast the governor responds the engine will not be able
to
keep up the revs and thus the frequency dip will also upset any electronic
controllers on the router.
An alternative to buying a larger alternator might by to by another
alternator and run it in parallel. With this set up though you will need
some kind of syncroscope to bring the 2 alternators on line and the avr's
should also have droop control so that you can keep a lid on circulating
currents (Power Factor or cos phi).
I believe the cheapest alternative would be to replace the motor with a
single phase one until you get 3 phase installed and the get the existing
motor/s rewound to what ever is the standard 3 phase voltage there (415V
in
Oz) going to a higher voltage will reduce the current drawn by the motor
but
you might have to upgrade contactors etc to handle higher voltages
Regards,
Alfonso
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
Behalf Of Darcy Davis
Sent: Saturday, 17 December 2005 11:10 AM
To: [email protected]
Subject: [OT] PCB router
Hey Guys,
We've found ourselves in somewhat of a bind. We've got a PCB router in
house
which is powered from 3-phase 208V. Its a very, very long story, but its
going to be 10wks till our utility supplies 3-phase power. Currently we
have
240V single phase power, so I've looked into a phase converter but we
still
need to bring the voltage down to 208V. The other option we've been trying
is to use a generator. Turns out that when the 5hp vacuum motor fires up,
the generator doesn't respond quickly enough to counteract the effect, so
we
suck the voltage (and frequency) through the floor. This of course causes
the motor controllers to complain. We've got a 20kW generator already (for
a
~4kW load), so I don't think a bigger generator will fix the problem. Is
there another way around this with a generator? (Ie, could we try to run
the
controllers from a UPS or will we kill the UPS?) If we were to run a
3-phase
motor load continuously while we run the router, would it tend to buffer
the
effect? Power systems is well out of my league, so any comments you have
would be welcome.
Thanks,
Darcy Davis
Design Engineer
=========================================================
Dynastream Innovations, Inc. Ph: (403) 932-9292 ext. 132
228 River Ave. Fax: (403) 932-6521
Cochrane, AB T4C 2C1 Web: www.dynastream.com
Canada
=========================================================
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