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Start-up current multiplier for soft-starters
8

Start-up current multiplier for soft-starters

Start-up current multiplier for soft-starters

(OP)
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

RE: Start-up current multiplier for soft-starters

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

RE: Start-up current multiplier for soft-starters

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.

RE: Start-up current multiplier for soft-starters

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

RE: Start-up current multiplier for soft-starters

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.

RE: Start-up current multiplier for soft-starters

(OP)
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
WWW.AFRICON.COM

RE: Start-up current multiplier for soft-starters

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

RE: Start-up current multiplier for soft-starters

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.

 

RE: Start-up current multiplier for soft-starters

(OP)
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
WWW.AFRICON.COM

RE: Start-up current multiplier for soft-starters

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.

RE: Start-up current multiplier for soft-starters

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.

 

RE: Start-up current multiplier for soft-starters

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

RE: Start-up current multiplier for soft-starters

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.

RE: Start-up current multiplier for soft-starters

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.

RE: Start-up current multiplier for soft-starters

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.   

RE: Start-up current multiplier for soft-starters

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

RE: Start-up current multiplier for soft-starters

2
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...

RE: Start-up current multiplier for soft-starters

Quote:


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

Quote:

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

RE: Start-up current multiplier for soft-starters

(OP)
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 smile

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

Kabir Singh
WWW.AFRICON.COM

RE: Start-up current multiplier for soft-starters

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: Eng-Tips.com Forum Policies  

RE: Start-up current multiplier for soft-starters

Using 200% for the soft-starter is too low.

Using 200% for star-delta is also too low. A star-delta lowers the current by about 57%. So, if your DOL current is 700% you lower this to 399%.

Bill's suggestion of adding up the one pump running plus one pump starting is a good idea. If you can get an analysis from a soft-starter manufacturer then you can use that as a basis for the motor starting KVA.

A back-up generator can become very expensive when it is sized too small and it has to be replaced before it even sees service.


Quote:

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".

I had thought I clearly put "begin to rotate" in my descriptions....

Quote:

The key piece of information is how much current does the motor require to break the load into motion.

This is the part I didn't agree with. I typically see the highest motor current required at about 50%-70% speed when the torque dips and the load is requiring a lot more torque. Not always, but typically for pump and fan loads.

 

RE: Start-up current multiplier for soft-starters

Hi LionelHutz

Quote:

Using 200% for star-delta is also too low. A star-delta lowers the current by about 57%. So, if your DOL current is 700% you lower this to 399%.
This is true for the current through the winding, but the current in the incoming line is further reduced as in delta, the current in each line flows through two windings, In Star, the current flows through one winding.
The current in the line for a star connected motor is one third of the current for that same motor connected in delta when used with a star delta motor.
For an older motor with a locked rotor current of around 600%, a star current of around 200% was reasonable. Modern motors typically have a locked rotor current of 700 - 750% and so the star current would be closer to 250%.
The "start" current is the current required by the motor to get it to full speed and with a star delta starter, there is typically insufficient torque available to get the load to full speed in delta and so the motor effectively starts at full voltage in delta for part of the start period and the current goes up above 500%.

Best regards,

Mark Empson
L M Photonics Ltd

RE: Start-up current multiplier for soft-starters

You're right Mark. I forgot about the double sqrt(3) reduction making it 1/3.

I will agree with your last paragraph. Don't just expect a wye-delta to reach full speed in wye mode. Then, the current after the transition will depend on just how close to full speed the motor gets.

 

RE: Start-up current multiplier for soft-starters

Don't forget that the starting current is mostly reactive. It is drawing much more KVA than kW. The AVR, especially with a PMG will compensate by trying to hold the voltage up.
If the only load is motors and some voltage dip may be tolerated, the generator may be abused somewhat more.
If the real power demand of the starting motor exceeds the capacity of the prime mover, the speed and frequency will drop. If the set slows more than about 3Hz, then the "Under Frequency Roll Off" will operate. UFRO will reduce the voltage in proportion to the frequency drop (after the first 3 Hz drop). This acts as a VFD and reduces the real power demand from both the starting motor and the motors that are already running.
We try to avoid this reduction of frequency and voltage on mixed loads, however a dedicated pumping application can be operated in this mode.
Remember, while a generator does not have the overload capacity of a transformer, it does have a few tricks up its sleeve.
Transformer: The voltage drop is proportional to current.
Generator: Is able to self correct for the internal voltage drop.

Transformer: Sees the load as KVA only.
Generator: Sees the load as KVA and kW separately.

Transformer: Fixed frequency set by the supply frequency.
Generator: Able to drop both the frequency and the voltage to shed load.

I have a bit of direct experience with generators that were spec'ed by others too small to handle the connected motor starting load. I spent time at six different sites determining how many motors could be started on the sets. Later I was back, sizing and later installing six new, larger sets. That success led to contracts for 3 or 4 more installations.

I have some experience with soft starters on gensets, but not much, and that installation had some other interesting issues.

My gut feel is running load plus 200% of the rated KVA of the motor starting will be rock bottom minimum for soft start starting.
The absolute minimum for DOL starting of the second motor would be 250% and would result in a voltage and frequency dip. That would probably be acceptable on a pumping station.
 

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

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