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Subject: Re: AC Motor Soft Starter
Date: Sat, 28 Dec 2002 14:54:43 -0500
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
If you are driving an induction motor, watch out for some of the pitfalls
with triacs. A triac has a very limited range and may not turn off properly.
For most industrial units a pair of anti-parallel SCRs are still used for
the improved dv/dt. Consistent phase angle is also a major concern. If the
triggering on positive and negative half cycles is not even, a net DC
results which causes your motor inductance to saturate (ie go to zero and
If it were me, I'd simply put a small Ohmage power resistor in series with
the motor and bypass it with a power relay. Relay would be driven by a
current sense or a time delay.
I worked in a plant once where we had a motor generator set to make 50Hz
from the 60Hz 3 phase mains. The motor was a 10 HP synchronous unit and was
supposed to be wired in with a reduced voltage starter. Somebody goofed and
used a full voltage starter. I would cringe every time I had to go and turn
this thing on, sort of hit the switch and run. It sounded like it was about
to rip itself from its moorings
For valid response address, remove the '1' following oppie
"Rudolf Usselmann" wrote in message
| "John Jardine" wrote in message
| > Be aware that an AC induction motor soft-start is not as 'soft' as what
| > may wish for. I.e it's not as soft as for a DC motor or as soft as an AC
| > motor fed via an electronic inverter. The soft start just takes the
| > edge off the mech/elec loading at switch on.
| All I need is to reduce/eliminate the initial current spike on the mains
| and also to somewhat reduce physical stress.
| I wonder why you say that it won't be as "soft" as I expect it to be ?
| I have decided now to build it on my own, commercial units seem to
| go for $450 USD, which is absolute nonsense ! I found a controller
| IC from ATMEL (U2008B) that sells for about $1.60 USD. All I need
| is a TRIAC (I'm thinking of using a BTA61-600 $5.50 USD) and a hand
| full of other discrete parts. Should be less than $20 in parts !
| Don't see how that can go wrong !
| Thanks for the feedback everybody !
| www.asics.ws - Solutions for your ASIC needs -
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