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
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Start-up current multiplier for soft-starters
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Start-up current multiplier for soft-starters
RE: Start-up current multiplier for soft-starters
RE: Start-up current multiplier for soft-starters
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Start-up current multiplier for soft-starters
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
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
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
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
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
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
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
RE: Start-up current multiplier for soft-starters
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
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
You can find some information on this subject at my web site at http://www.lmphotonics.com/m_control.htm
I agree with waross I have done this many times.Best regards,
Mark Empson
L M Photonics Ltd
RE: Start-up current multiplier for soft-starters
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
RE: Start-up current multiplier for soft-starters
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
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RE: Start-up current multiplier for soft-starters
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.
I had thought I clearly put "begin to rotate" in my descriptions....
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
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
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
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