Start-up current multiplier for soft-starters 8

[SINGHK]

Good day,

I need to match a genset to motors with soft-starters on them. What would be a good estimate to use as a multiplier and what references can I base this on.

Regards,
Kabir

 

[waross]

Do the motors start together or staggered? What current setting do you usually use on your soft starters?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TurbineGen]

It would also be helpful to know what the ratings of the motors are and if they must be started in any particular order or if you may start them in any order you want.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[davidbeach]

What results do you get from the gen set manufacturer's sizing software?

 

[DickDV]

The key piece of information is how much current does the motor require to break the load into motion. That would be the "softest" and lowest current start possible.

 

[SINGHK]

The motors are pumps in a fresh water pump station which lifts water into a reservoir for a gravity feed to customers.

There are 3 motors of 55kW each fed from 400V. The start-up is generally staggered IIRC and thats the arrangement required from the client. All I have is a soft-starter model number of GPS 110.

Note that the client requires only 2 pumps to be running on emergency generator power.

Kabir Singh

http://WWW.AFRICON.COM

 

[waross]

Size the generator for the running KVA of one motor plus the starting KVA of the second motor. The Permanent Magnet Generator option on the generator is recommended.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

If the pump starts with the valve closed then I typically expect around 300% rated current. However, there is not nearly enough data provided here to confirm that number.

If you have the motor nameplate data and the motor speed vs torque and speed vs current curve plus the load speed vs torque curve then a manufacturer could tell you what is required.


DickDV - That is not really true. Many motors and loads may begin to rotate at 50% to 100% of rated current but still require 200% to 400% rated current at some part speed (typically around 60-70% speed) to fully accelerate.

 

[SINGHK]

Hi,

The problem I have is that most of these motors are decades old. Nameplate data is not visible and some manufacturers don't exist anymore. There are no electrical single line diagrams as well.

We have decided to use a factor of 2 at this stage for soft-starters. Any idea of where I could find a paper on this issue?

We are using x7 for DOL setups. Would you say x2 for star-delta is also too little?



Kabir Singh

http://WWW.AFRICON.COM

 

[DickDV]

Lionel, with a softstarter in place, I can safely say that absolutely NO loads will start at the 50-100% current you claim. That is because the available torque is a function of the square of the voltage reduction. A motor will a normal full voltage inrush of 7 times nameplate will develop only 1/49 its rated torque at 1/7 voltage (which would be 100% nameplate amps). That will not be enough to start the motor much less any attached load.

If, on the other hand, you were to change the word "current" to "torque" in your last post, then I could agree. But, in order to get 50% starting torque you would need to reduce the voltage 1/sq rt 2 =.707 which, for a motor with full voltage inrush of 7 times, the reduced inrush would be 7 x .707 =4.95 or about 5 times nameplate amps.

After doing this for 20 years now, I am sure that these numbers can be trusted.

 

[LionelHutz]

DickDV - I've done this many years too. I've been to many sites for start-up. I wrote a program to simulate a motor start using a soft-starter. I have a very good understanding of what happens when starting a motor with a soft-starter.

I've seen many many loads where the breakaway current requirement is not the point with the highest current requirement. A pump is a typical example of this - a typical pump may begin to rotate at say 150% current yet not reach full speed unless the current is ramped up to say 250% current.

I've seen many applications that will begin to rotate at rated current and I've seen quite a few loads that require more than rated current to rotate as well. Every motor and load are different. Notice I said begin to rotate - I never claimed the load would reach full speed at rated current.

I'll somewhat guess and say you have about 30% chance an uncoupled motor will reach rated speed if started with rated current and almost every uncoupled motor will at least begin to rotate at rated current.

 

[waross]

Have you considered measuring the actual current drawn by the motors when starting?
This may be more accurate than any calculations and given the lack of data on the existing equipment you have a very strong reason to present to the customer to accept the extra expense of measuring.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

Well folks, since I hate to argue, I will just duck out of this discussion. You have my experience stated above and you may use it, contradict it, or ignore it. It's your choice.

But I'll keep using it because I've tested it and trust it. And I'll keep sharing it with the hope that it will be helpful to someone.

 

[TurbineGen]

I'd take Bill's (waross) suggestion. Set a meter for peak current and start a motor up. The add that peak (start up)current to the running current of the motor and multiply by 400 to get the kVA needed for your generator. This gives you enough power to start one motor while one is running without excessive voltage drop.

You can also factor in voltage drop if you can determine how much is acceptable and size a generator slightly smaller. I've found that a motor should draw nearly the same start up current with a voltage reduced by up to 20% for normal start up. I'm not certain if this holds true for soft start, but I imagine it would. So if a 10% voltage drop is acceptable then you would need a generator for Start up (Amps + running Amps) * (400*.9).

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[badservo]

Make sure your soft starters can handle the frequency and voltage variation of the generator when starting the motors, some soft satrters don't like generators.

 

[waross]

Hi TurbineGen;
You have overlooked the factor of 1.73 in your formula for three phase KVA. I am somewhat prone to similar mistakes.
Suggestion:
Cut and paste your last post and make corrections before posting it.
Then Red Flag your original post and this one. That will leave the thread looking neat and accurate.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

DickDV,
I know you have "ducked out", but methinks there was a semantics issue here. As I see it, LionelHutz was using the term "start" as in "Start to rotate", and I think you took "start" to mean "Start and accelerate to full speed". The way I see it you are both right, just about different things...

 

[Marke]

We have decided to use a factor of 2 at this stage for soft-starters

I would not expect to start a pump at 200% current. You would be much closer to allow for somewhere between 300% and 400% current for older motors unless they are deep well submersible type pumps in which case you would get away with 250%.
You can find some information on this subject at my web site at

http://www.lmphotonics.com/m_control.htm

I agree with waross

Size the generator for the running KVA of one motor plus the starting KVA of the second motor. The Permanent Magnet Generator option on the generator is recommended.

I have done this many times.
Best regards,

Mark Empson
L M Photonics Ltd

 

[SINGHK]

HI Gents,

Thank-you for your inputs. Please check out the attached p.51 of the ABB Softstarter Handbook. Centrifugal pumps would draw up to 3.5 times Steadystate current. We are currently debating this issue in the office. Hopefully we will have consensus on Monday

Enjoy the weekend & don't forget the Roses for Valentine's Day!!!


Kabir Singh
WWW.AFRICON.COM

 

[jraef]

Just about every soft starter manufacturer has a chart like that available, they are more marketing tools than engineering tools. But they are generally based on experiences over a wide variety of installations, so there is value from that standpoint. But be forewarned, a pump is not a pump is not a pump; every installation is a little different, so gross generalizations are only valuable to a point.

My best advice is to hope for the best, plan for the worst, then you you will leave yourself with a comfortable margin of error. If you purchased based only on the hope aspect and it doesn't work out, you will have to spend more than twice as much to correct it.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376