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

 

[LionelHutz]

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.

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....

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.

 

[Marke]

Hi LionelHutz

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

 

[LionelHutz]

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.

 

[waross]

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
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"Why not the best?"
Jimmy Carter