Soft Starter Settings 2

[Inder76]

Hi,

I have to specify a soft starter for a 125 HP vent fan application. Just to make my self acquainted with the basic requirements, I read this 95 page ABB soft starter manual.
It talked a lot about pumps and cetrifgual fans but nothing about Ven fans used in mining applications. Anyways, I noticed some thing interesting: The recommendations in manual are :
"For softstarters designed for nomal start,
select one size larger than the rated motor
power."

"For softstarters designed for heavy duty
start, select according to the rated motor
power"

Is there any argument to justify this?

Q#2, Are consulting engineers supposed to satify any parametes to contractor or they just program as per default typical values. The manual says :

Start ramp: 10 sec.
Stop ramp: 0 sec.
Initial voltage: 30 %
It also talks about the change in settings with and with out current limit function being activated?



Q#3 This issue of with and without a by pass contactor. This changes the size of softstarter size selection.

I am sure, these are a lot of questions. If you could just give me a brief direction, I should be fine to venture to explore my self.

Thanks

 

[itsmoked]

You want a bypass contactor for a SS otherwise the semiconductors would have to remain in circuit. Most SS now have a bypass contactor build into them.

As a consultant you look at each application, some you would want faster ramps others slower. The speed of the ramp relates to the peak current seen during a start. The size, rating, and price are interdependent. So a SS that may start a motor quicker can do it only because it's current ability is greater and hence it will be costing more. Hence, you can't often turn the start time way short with a particular SS.

Of course a motor may not want to take 20 seconds to get up to speed as that may be a greater period of over current that will heat it up.

Often you start with the typical 10s then do a start and see what you think. Don't like the results? Change the setting, WAIT THE REQUIRED RESTART PERIOD and try it again.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

It sounds like they have the normal and heavy duty start descriptions mixed up.

That starter uses a voltage ramp with current limit? A good soft-starter these days would use either a full current ramp or some form or power/torque control algorithm for ramping. It would also provide a full suite of motor protection features and include a bypass contactor.

For the most part, setting up the start is fairly simple and Keith is right on the try it approach to programming, but it takes a bit of experience. Someone who's set-up a few should be able to hit the right settings the first or second time after just seeing the application.

Alternatively, the manufacturer should be able to supply a start simulation if you provide details of the motor and load. This simulation would also confirm that the correct soft-starter is selected.

 

[jraef]

Good responses.

I'll only add my voice to the issue of where the specifier's job ends. My experience is that the specifying engineer is not typically expected to pre-select the starter settings as far as ramp parameters are concerned; it is virtually impossible to know in advance what they will end up needing in the field. Analysis tools as LionelHutz mentioned are useful, but even they should not be allowed to override adjustments in the field that may be needed to make it work; with the possible exception of current limit under some circumstances.

If you have run a starting analysis (i.e. a Transient Motor Starting Analysis using SKM, ETAP or the like) and it says that you can achieve a start at a specific maximum current limit setting while avoiding a voltage drop beyond specified limits, then that should be a hard spec. If you haven't done that kind of analysis and don't intend to because you are not concerned, then stay away from specifying the ramp settings. Just specify that the installer is responsible for properly commissioning the soft starter to your satisfaction.

I for one would like to see more engineers get involved in specifying the protection settings however. Installers tend to skip over all but the basic Overload parameters, probably out of haste to finish, leaving some very useful features disabled. Later, after a failure that could have been prevented or more quickly diagnosed, the end user suffers from discovering that the tools were ignored.

 

[dpc]

Jeff,

One reason the specifier doesn't always provide all the protection settings is that the actual starter type and manufacturer may not be known when the spec is written. Ideally the engineer's scope would include a protective device coordination study that would be done after the equipment is purchased that should include the actual settings for a particular starter or relay.

But I agree that trying to specify the starter configuration is probably not a good idea unless there are some really specific issues that have to be dealt with.

 

[jraef]

Good point dpc. But often I see specifications for advanced starters, drives and protection relays that are very specific as to which protection features will be included, but then nobody takes responsibility for deciding what the settings should be. So it's often left to the vendor to do it at commissioning. This isn't a good idea for exactly the reason you stated; coordination. I have been involved in many coordination studies and I see that a lot of Engineering or Certified Testing firms who conduct them have fill-out sheets which again, only address the Overload Relay settings, nothing else. I say if you are going to specify it, be prepared to implement it or have it implemented.

 

[Inder76]

Thaks everybody for your timely input.

I guess the last portion of this situation. I wrapped up the specs:

I specified the incoming fuse and disconnect combination as per size for 125 HP, 600 V

fuse Class J type 150 A but then I realized I have this cap bank as well, which is sized as 90% of I(No Load) of Motor.

Where as I(no load)=30% of I(full Load)

So this comes out 35 KVAR.

Just quick word from you guys:

Now the fuse will see 125 HP+ 35 KVAR( Is 150A fuse good ?)

You might be using these soft starters for ages:

If I have to look in to this Softstarter(Harmonic generating source) + Cap bank in paraller. Could there be a problem ?

Again thanks for all the information.

 

[nightfox1925]

You may consider also specifying the capacitor bank with series reactor tuned to mitigate the harmonic order output of your softstarter (harmonic generator). Harmonic currents will spill out to your system and capacitor bank as well.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[LionelHutz]

The capacitor should only be energized after the motor has reached full speed. As such, it won't see the harmonics from the starter because there will be none once full-speed is reached.

The 150A fuses sound really small. Typical fuse sizing is between 150% to 200% of FLA.

With capacitors connected to a motor your current will be lower, not higher.

Doing a simulation before purchasing can ensure the correct starter is purchased. This is important in the world of light duty soft-starters when you're dealing with a high inertia or hard to start load.