Well, here are my two eurocents:
At 17/12/05 01:10, Darcy Davis wrote:
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.
As I understand American utility power, 208V three-phase will be a
different net from 240V single phase alltogether. It is basically the
same net as the 120V domestic one with 208V between phases and 120V
between phase and neutral.
The 240V single phase will have 420V (actually 416V) between the
phases and comes from a different transformer.
Using a phase cobverter is an option, but you will need a
transformer. Use a 120V converter and a single phase step-down
transformer, or a 240V converter and a three-phase transformer. I
would go for the latter, it will be easier on the converter.
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.
Those powers sounds about right.
We did some experiments with a hydraulic unit with a small 1.5kW
motor (we had to construct a better controller for the process). That
little one took (at 380V) 'only' 15 Amps to startup (that is 5.7kW)....
Any bigger a motor and we would have to devise a star-delta startup
controller. That starts the motor in a star connection (lower
voltage and hence lower current per winding) and switches to the
delta connection when the initial inertia is overcome, usually after
a second or so.
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.
I would change the motor relay to an automated start-delta relay.
Then it really doesn't matter much if you use your 20kW generator or
a phase converter with a step-down transformer....
I hope this is useful.
Happy routing,
Leo Potjewijd
hardware designer
Integrated Engineering
Paasheuvelweg 20
1105 BJ Amsterdam
The Netherlands
T +31 (0)20 46 20 755
F +31 (0)20 46 20 756
E [EMAIL PROTECTED]
I http://www.smart-ID.com
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