VFD vs Soft Starter

[hanksmith]

Did a little search and did not find anything that really answered my question so I figured I would try a post.

Looking at an application for agitators, they will be agitating a high solids slurry and there is a possibility that if we lost power the slurry would settle out and then we would have to start from a sanded condition. Motors are sized at about 200HP.

First option was a full voltage reversing starter, pros - high starting torque, cons could possibly shear the agitator shaft.

Soft Starter - don't know much about these, they would reduce the voltage at starting and decrease the starting current, have read that they are not good for high torque loads, any body with some information on this?

VFD - Again I don't have much experiance with these. Their cost is high, we don't need speed control so this feature would not be used, do they provided high torque for starting high torque loads?

Anybody that has possible dealt with a similar installation and could offer me some helpful info, or even is someone could point me to some good documentation on VFD vs Soft Starter documentation.

Thanks
Hank

 

[jraef]

If you don't need to or cannot benefit from varying the speed, a VFD is for the most part,a waste. Both a VFD and a Soft Starter will reduce the starting torque, the soft starter just cannot vary the speed.

What you want to look for is a soft starter that offers two selectable starting ramp profiles. Use one with as gentle of a start as you can under normal every day conditions, then program a harder start for a re-start after a problem. That way you can dial in the right amount of torque necessary to agitate the sand but without unnecessarily stressing the mechanical components.

 

[Thedroid]

At my old job we had a seperator to seperate the fat from milk in order to get cream. It was a heavy load that took awhile to get started. For years the motor was coupled to the load with a Voith Turbo fluid coupling in order to start it slowly. The couplings were very expensive to replace or have rebuilt, and on average lasted through about a year of normal use, unless we had problems. There was a de sludge valve that would stick every so often, and if it did the coupling was toast. To keep this from happening we decided to go with a frequency drive. We coupled the motor directly to the load and set the drive for a 60s ramp time, and it worked flawlessly. If something happened with the de sludge valve then the drive would sense the overload, and shut down.

I'm not saying it's the right thing to do, but this application worked for out needs.

Cement Plant Electrician and
Instrumentation Technician

 

[Compositepro]

In a sanded condition you will need maximum torque to get started. If that torque will damage your shaft then you will have to get your shaft unstuck by some other means. You could pump fluid in though a bottom nozzle or nozzles to fluidize the sediment enough to allow the shaft to start. Even a vibrator on the tank or shaft could work. It all depends on the properties of your slurry.

 

[jraef]

Separators are centrifuges, and VFDs are indeed a good solution for starting centrifuges because the acceleration time cn be extended indefinitely so that the effects on the power system are minimal.

Mixers are not centrifuges. Generally, you need something between Locked Rotor Torque and Full Load Torque to get the material fluidized. Soft starters are an excellent choice for that, they are used extensively in that application (when, as I said earlier, there is no additional benefit from varying the speed).

Alternate means of fluidizing is also a good idea, but you will still need something to start the motor.

 

[DickDV]

Sounds like a good job for a NEMA Design C motor with a soft starter to make the high starting torque adjustable.

 

[ahung]

If you don´t need control the motor speed you shouldn´t use the VFD. In your specific case you need to move a big motor (200 HP) depending of voltage level may be the starting current is very high and is better use a soft starter, but with the soft starter you can´t get a 100 % full load torque to move the slurry.

Regards

Alberto J. Hung C
Caracas Venezuela

 

[roydm]

I worked in mining for many years, I don't recall that starting agitators after a power outage was a huge issue on the 30' tall leach tanks, I'm sure they were well sanded out. May have been a function of blade design. There must be a wealth of experience in that industry.
Regards
Roy

 

[LionelHutz]

It should be fairly easy to apply a soft-starter to this application and have good success.

but with the soft starter you can´t get a 100 % full load torque to move the slurry.

This is not true of all soft-starters, just some that have low current limits.

 

[ScottyUK]

Isn't keeping starting current down the main purpose of a soft starter?


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Yes Scotty, the main purpose is to lower the current and therefore the torque. However, that does not mean some soft-starters are not capable of pushing the motor to full current and full torque if you actually wanted to.

 

[jraef]

Absolutely. When selecting soft starters, you must ALWAYS be cognizant of the device ratings, usually referred to as "Overload Ratings" based on a percentage of FLC and a time period.If you consider that an asynchronous motor typically pulls 600% current at some point in a start, a good soft starter design should be capable of delivering that current if it has to. That's why I suggsted choosing one that has selectable starting ramps. One for normal start, another for "loaded start". If need be, you should be able to set up the "loaded start" ramp parameters to be capable of watever it takes to do the job.

Some manufacturers provide that information up front, some ignore it altogether. The ones who skirt the issue tend to have something to hide. There are a number of new soft starter designs out there which are being sold as "energy savers" and when you eventually dig through the marketing fluff, turn out to be rated so poorly that it's a wonder they can start anything. One very prominent one uses skimpy components that lead to it be rated for 200% current MAXIMUM for 30 seconds. That means it will have trouble starting almost any motor connected to a load, and only about once per hour (if you are lucky). Such a soft starter could NOT work in this scenario, it would shut itself down to protect its own components (hopefully).

 

[burnt2x]

hanksmith,
Sanded conditions are not really big problems if your process design already provided ways to do away with those conditions. We identified these problems during the design stage and the solution was to shut down the slurry tank, divert the operations to another backup tank. Our slurry is more sticky that, if after three tries of softstarting, and they can't make the agitator run, operators shift to the backup tank! NO shaft breaks, no agitator motor toasted, but we have to empty the "sticky mud" on the tank by other means!
I agree with jraef that the softstartes we have does its job most of the time- a few moments of overloads, i.e. drive the agitators even when some slurry are on top of its blades!