Limiting startup current 2

[roobarb]

Hello Folks,
what practical (and cheap) solutions are there to limit the startup current of a 0.8MW fan? It is directly connected to the network and the high startup current causes a lot of trouble for me and customer. I have been thinking of inserting some resistance during startup and removing them once the fan is stable but did not find any detailed technical information of that. Any thoughts appreciated.
R

 

[gingell]

Hi,

With an 800KW fan i'd be surprised if the fan supplier did not have some ideas for you so its worth trying them.

If its supplied from some sort of VSD then you could maybe code in some kind of soft start.

Alternatively, switching out resistors just after start-up as you say sounds good.

 

[roobarb]

Thanks gingell,

Do you happen to have (or know of) some online technical information on resistor switching technique? (Dimensioning etc.)

 

[dpc]

Solid state Reduced voltage starter is the best solution these days.

You have to make sure the motor will provide enough torque at reduced voltage regardless of what starting method you choose.

 

[electricpete]

One small thing, it is sometimes possible to do things on the mechanical side that reduce the torque load during startup (although there's not much you can do about the inertia). For example a pure radial flow pump starts faster with the discharge shut and a pure axial flow pump starts faster with the discharge path wide open (there may be other factors restricting position of valve such as water hammer and pump minimum flow requirements).

If the pump is connected in wye, then a delta-wye starting method will usually reduce the starting current - might possibly be cheaper than the electronic soft start although also can cause some problems with switching transients and incorrect timing of the switchover.

To get more ideas, google "reduced voltage start" and you should get a bunch of ideas. I defer to people like dpc and others on the forum on which is the best. My experience is limited to power plants where no reduced voltage starts are required.

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[davidbeach]

If the pump is connected in wye, then a delta-wye starting method will usually reduce the starting current - might possibly be cheaper than the electronic soft start although also can cause some problems with switching transients and incorrect timing of the switchover.

No, not quite like that. It is wye start - delta run, so you would have to start off with a motor that presently runs in delta, not wye, and you would have to be able to bring out both ends of all windings. The typical NEMA only brings out 9 of the 12 ends and can not be converted to wye-delta starting. Since the OP gives the rating as 0.8MW rather than in HP, it is likely that he is dealing with an IEC motor rather than a NEMA motor. My understanding is that IEC motors are often set up to run at two different voltages, one in delta and one sqrt(3) times higher in wye. If you are using the delta voltage it might be possible to use wye-delta start (only consider closed transition despite the additional cost). If the motor is at the wye voltage the option doesn't exist.

 

[electricpete]

yes, you're absolutely right david

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[LionelHutz]

+1 for solid-state starter. Pretty cheap these days and much more adjustable than any other method. The only way to provide a smoother load on the power grid is to use a VFD. Y-delta and autotransformer starters still produce some transients that could cause flicker issues.

But, you need to study the system to see if a soft-starter can reduce the transients enough to fix the problems.

 

[electrolitic]

Roobarth
Sometimes old technologies sound better.
O,8 MW fan looks to be a medium voltage motor and a fan requires more severe starting torque.
Please check an autotransformer starter (closed transition=Kondorfer connection)

 

[swgrmfg]

Mr Roobarb:

Late reply, but the 'cheapest way' would be to add a series, iron-core current limiting reactor with a bypass contactor. (A resistor would be too big and expensive and doesn't give you di/dt advantage). The bypass contactor can either be timer, or current sensing switched. Generally speaking, you can look at the speed-torque curves of the motor and the fan and design the reactor voltage drop so as to maintain acceleration torque until the point of bypass. Inrush current is usually limited to 50% or less.

Regards.

 

[slavag]

Hi.
Intresting idea of Swgrmfg.
It's also old-technologies as AT starter. Are still used?
Regards.
Slava

 

[jraef]

For medium voltage motors (this is different for low voltage because of component costs).

Options in order of ability to limit voltage drop:
VFD
Autotransformer
Solid State Soft Starter
Reactor
Star-Delta

Options in order of initial installed cost (lowest to highest):
Open Transition Star-Delta (assuming the motor is wound for it)
Reactor
Closed Transition Star-Delta
Autotransformer
Solid State Soft Starter (ofetn similar in price to AT)
VFD

Options in order of flexibility to adjust to unknown starting parameters:
VFD
Soft Starter
Autotransformer (limited)
All others are not adaptable at all

I tend to still recommend Soft Starters because of the ability to adapt to unknown starting parameters, but it depends greatly on your mains capacity. What you really need to do it so gather all pertinent data on your load, motor and power system, then do a Transient Motor Starting Analysis using the available choices outlined above. On these high cost medium voltage systems, guessing can be an expensive endeavor.

 

[RRaghunath]

roobarb,
I will be concerned, if I have to use any reduced voltage start for a system originally designed to start DOL. More so, when the driven equipment is a Fan with high torque requirement.
It is true by clsong the input dampers before starting the fan you can reduce the torque reqiurement. May be it is worthwhile talking to the Fan vendor and motor vendor and have the curves ready for analysis.
I guess it would be worthwhile to order a different motor that comes with start current limited to say 3.5pu. Such motors are available in the market. The demerits of this option could be cost (when compared to standard motor) and the efficiency. But you have advantage with regard to reliability and O&M costs.