On Tue, 21 Jun 2005 07:45:07 -0400, "SimonLW" <(E-Mail Removed)> wrote:
>"SimonLW" <(E-Mail Removed)> wrote in message
>news:(E-Mail Removed)...
>> We have a very limited power from the utility. We can run the motor, but
>> can't start it. This motor turns a very large fan. Is it possible to start
>> without exceeding the run current? Since it is just a fan, the run-up time
>> can be longer. Thanks!
>>
>>
>Thanks for all the replies. We looked at VFDs, spinning it up with smaller
>motors and even uglier ideas. We finally told the client to talk to the
>utility. The problem is the supply to the plant is inadequate and the
>utility is complaining about the expense of the miles of wire and equipment
>to deliver the power.
>
The utility will most likely not be of much help. They tend to tell
users to either cough up a ton of money to upgrade the feed to their
new system, or they tell them to limit the starting kVA. Along those
lines, there are 3 choices.
1) A VFD, but at 4160V you will be looking at $250,000 by the time it
is all said and done. The advantage: a VFD can start the load at 100%
FLA, and since you alrerady know the system can handle 100% FLA, you
know in advance that the VFD will work.
2) Pony motor starting. This is the suggestion from above wherein you
can use a small motor to get the fan moving, then switch over to the
main motor after it is already spinning. Theoretically it could work,
but when the 4160V motor is connected, there is still a massive inrush
even if the fan is already moving, just smaller by a few percent. This
is very expensive to experiment with only to find out it didn't work!
They also tend to add mecahnical safety issues to the problem.
3) Reduced Voltage Starting. This is what was mentioned above as
either Autotransformer (RVAT) starting or Solid State (RVSS) starting.
In this HP size the RVAT will be slightly cheaper (if you ignore motor
protection issues), but riskier and very big and heavy in comparison.
Risky because again, you are not 100% sure it will work until you try
it. RVSS is going to cost more up front, however most RVSS starters
now come with protection systems comparable to Multilin Relays
built-in, so if you add the cost of that to an RVAT, it comes out
even. The best part is that RVSS manufacturers will often do a
Transient Motor Starting analysis for you for free if you can provide
all the motor, power system and load data to them. I used Motortronics
on 3 projects last year and every one of them came out dead on to the
TMS analysis they did prior to my purchasing them. So I knew in
advance that it would work before I purchased them. On a 4th project,
the utility restrictions were too severe so I had to use a VFD.
Motortronics ran the TMS study for me and told me that any RVSS would
not work, so essentially they lost the order. So at $25,000 each for
the RVSS's and $250,000 for the VFD, I looked at it as though
Motortronics saved me $225,000 each on 3 projects! The Motortronics is
the one sold by ABB as well as mentioned above. The first ones I used
5 years ago came from ABB as part of a package deal, and I would still
go that way again if the project called for it, but when I just wanted
the starters alone, I went directly to the manufacturer.
If you want to get more opinions on this, try logging onto
http://eng-tips.com and go to the Electric Motors and Controls forum
to post a question. Great resource, free as long as you register, and
they DO NOT sell your email address.
Similar Threads
Linear Mode
