Soft starting a 1,200 HP 4160 volt motor

[SimonLW]

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!

 

[SQLit]

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!

Sure this is done all of the time. Best check with your motor manufacture
first about this application.
Then call ABB, I have seen more of their medium voltage stuff than anyother
manufacture in this kind of application.

Seems pretty weird that your company would install equipment that they can
not start. Or is this something your looking to buy?

 

[SimonLW]

Sure this is done all of the time. Best check with your motor manufacture
first about this application.
Then call ABB, I have seen more of their medium voltage stuff than anyother
manufacture in this kind of application.

Seems pretty weird that your company would install equipment that they can
not start. Or is this something your looking to buy?

Actually it is a client. It is a fan for a flash dryer and they are limited
by what the utility supplies. They are looking to install this equipment. We
normally don't deal with this aspect of the design.
Thanks -S

 

[SQLit]

Actually it is a client. It is a fan for a flash dryer and they are limited
by what the utility supplies. They are looking to install this equipment. We
normally don't deal with this aspect of the design.
Thanks -S

If they are considering it. Then look into a axial fan and controls. The
motor starts with the fan blades closed so it is almost a "no load" start on
the motor. Then as time progresses the controls can open the blades and load
up the motor. Did this on a 480v 3phase motor for smoke removal. Starting
amps was an issue with the generator.

 

[Brian]

Try using a freq drive. That is if the motor will allow the freq change. Works great for chillers.


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!

 

[operator jay]

Try using a freq drive. That is if the motor will allow the freq change. Works great for chillers.


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!

A huge frequency drive on a high impedance system could give voltage
distortion problems. And maybe even liability problems.

j

 

[SimonLW]

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.

 

[Bob]

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.

 

[SQLit]

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.

Reduced voltage starters that I have worked on will not limit the current
much. Starting current values on our 4160 chillers are 3/4 of full load.
'course that could be the operators. I just look at the load data once a
month.

No one ever said that medium voltage was inexpensive.

One other thing that you should be aware of,
Unless the technology has gotten a lot better in the last couple of years.
Medium voltage motor controls really do not like heat. I know of a water
pumping plant in N. Phoenix that has a 10 ton air conditioner sitting on top
of their medium voltage VFD's. In the summer time the room runs constantly
at about 85-90 F. So if your application in in a warm place best check with
the manufacture before going very far. Eaton/Cutler-Hammer used to produce
low voltage VFD's and Softstarts that were rated at 40 C. When I called they
thought I was crazed until I sent the temp information for my desert. We
ended up ducting a/c directly into the bottom of the vfds to keep them
running in the summer months.

 

[operator jay]

Reduced voltage starters that I have worked on will not limit the current
much. Starting current values on our 4160 chillers are 3/4 of full load.
'course that could be the operators. I just look at the load data once a
month.

Across the line starting current is normally given as 600% or so of full
load current. Often soft starters limit current to 300% of full load
current, or maybe 150% of full load. Dropping starting current to 75% of
full load sounds very low. There are instances where this wouldn't even
produce enough torque to get the motor running.

j

 

[SQLit]

Across the line starting current is normally given as 600% or so of full
load current. Often soft starters limit current to 300% of full load
current, or maybe 150% of full load. Dropping starting current to 75% of
full load sounds very low. There are instances where this wouldn't even
produce enough torque to get the motor running.

j

I was not speaking of a soft start.
Reduced voltage stuff we have at 4160 v has an auto-transformer in the
circuit until the start windings are used.

 

[Mike Lamond]

I was not speaking of a soft start.
Reduced voltage stuff we have at 4160 v has an auto-transformer in the
circuit until the start windings are used.

I learned about the starting torque problem a few years into my career
while programming PLC controls for centrigual chillers. Fortunately, the
motor controls weren't my problem. One site, a new central utility plant,
had two 4500 HP and one 1750 HP chillers with RVAT starters. They
started one big chiller with no problem and put a load on it. When they
went to start the second one, the entire building lost power almost
immediately after the start contactor closed. It turned out someone at the
engineering firm had underestimated the starting demand, and the brand
new substation had tripped on overcurrent. When asked about switching
the autotransformer tap from 65% to 50%, the chiller manufacturer's
engineers determined that the motor wouldn't have enough torque to
start turning the motor, speed increaser (1:3.2 gearbox) and compresssor.
The owner and engineer were forced to put in a larger service.

Closing the intake dampers on startup will also help. I watched the
contractors go mad trying to start a 100 HP multistage centrigual
aeration blower with the intake valve fully open and not trip the solid
state starter. They were convinced that pulling a vacuum against a
closed valve took more current than moving air.

Here's another thought - is it any less expensive to install and operate
cogeneration at the plant than to pay the utility to install more new
equipment with more capacity? It's more complex, and depends on
the availability of fuel, usually natural gas, oil or excess plant steam.
If a 1200 HP load pushes the existing supply to its limits, where will
the power come from for any future growth?

Mike

 

[operator jay]

I was not speaking of a soft start.
Reduced voltage stuff we have at 4160 v has an auto-transformer in the
circuit until the start windings are used.

I tend to refer to autotransformer starters as soft start too. I have not
looked into medium voltage applications but at low voltages, the lowest taps
I see on Reduced Voltage Auto Transformer starters are 50%, giving 25% of
across-the-line starting current at the primary -- however across-the-line
starting current is typically 600% of full load current, so RVAT starting
current is still 150% of full load current. A starter that dropped starting
current to 75% of full load current would be great for starting a motor
without jolting the mechanical load. Or for starting a motor on generator
power. I am surprised to see a number that low (75%), is all.

j