Conversion of Atlas Copco GA37 air compressor from star-delta start to soft starter - issues?

[thony999]

All, happy new year.

In one of our remote facilities, the gas genset providing power supply is rated 156kVA, while we intend bringing in a new Atlas Copco GA37 air compressor(rated 37kW) for provision of instrument air to the facility. The base load in the facility is about 40kW, while the new 37kW Atlas Copco compressor will operate in star-delta mode.

From preliminary estimates, based on the star-delta operating mode, the starting power requirement of the compressor will be 4-5 times the full load requirement i.e. 148kW-185kW. This clearly is above the rating of the genset or rule-of-thumb recommended 80% maximum loading.

To forestall this, we are exploring replacing the star-delta control system with a soft-starter system which will present a maximum load equal to 125% of the full-load rating of the 37kW compressor.

Does anyone envisage a problem with our proposal? Are the Atlas Copco GA37 specifically built for the installed star-delta operating mode or can easily be converted as proposed?

Feedback will be appreciated!!!

 

[LionelHutz]

Not sure where you're getting your information but it just seems plain wrong.

The full-voltage starting kVA would be about 6x the motor rated kVA. Same applies to the starting current, it also would be ablout 6x the motor rated current.

Technically, the star-delta will reduce the starting current to about 33% of the full-voltage starting current. This would mean the inrush kVA would be about 200%. However, if the starting requires the transition to occur before the motor is almost at full-speed then there is a surge during the transition and that could be 3-4 times the motor rated kVA.

The soft-starter will probably allow the motor to start with 2.5-3 times the rated motor current. So, the starting kVA will also be 2.5-3X the rated kVA. The advantage of the soft-starter is that there are no transients during the start.

I doubt your generator would have an issue with either start method. The generator should be capable of handling 1.5-2 times it's rated kVA for a short period of time.

 

[waross]

ISSUE #1; Power:KVA
Motor starting current is at a very low power factor.
Starting KVA may be about 6 times motor rated KVA. The current associated with the KVA load will cause voltage dip or drop. This may impact other lads.
Staring power (kW) may be 3 times rated motor power. This will cause a frequency dip. This may inpact other loads.
ISSUE #2; The generator, specifically the AVR may not like the wave form presented by the operation of the soft starter.
ISSUE #3; Generator rating;- Is this a prime rated set? If the set is prime rated then you may add 10% to the set rating for motor starting purposes. A prime rated set is based on a derated standby rated set. The prime set may have accesories that may not be provided on a standby rated set. Oil coolers and extra large lubricating oit sumps are two examples of added features on a prime rated set. A prime rated set allows a 10% overload for 1 hour out of 12 hours. More than enough time for motor starting.
Suggestions;
A rule of thumb that has served well in the past is that the reserve current of the generator must be 3 times the rated current of the motor for successful Direct On Line motor starting. This is based on motor electrical kW, not motor mechanical kW.
156 KVA - 40 KVA = 116 KVA.
37 KVA motor rating x 3 = 111 KVA capacity needed. You should be able to start this motor DOL. If DOL is possible, then star/delta start should work also.
A suggestion;- Consider two 20 kW compressors. Starting will be easy. The money saved from the price of a soft start may go far towards covering the extra cost of two smaller compressors.
Note; Cat and others provide free generator sizing software online. The software is generally conservative. One parameter is maximum allowable voltage dip when the motor starts. The software is based on an arbitrary conservative limit of maximum voltage drop. Using the 3 times rule of thumb may at times result in a voltage drop a little greater than the suggested limit in the sizing software. However some versions of generator sizing software allow you to calculate the actual voltage drop of a proposed generator-motor combination. This actual calculated voltage drop is generally acceptable.
A PMG (Permanent magnet Generator to supply the AVR) will aid in reducing the voltage dip when a motor starts.


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

 

[jraef]

The only thing I will add is that the issue of the AVR "not liking the waveform of the soft starter" was a problem, but has been solved by most of the major generator mfrs with some simple filtering of the sensing circuit for it. If yours is less that 10 years old, I wouldn't let that bother you.

Other than that, I too agree that you should not have a problem starting a 37.5 kW motor Across-The-Line (DOL) on a 156kVA source. But a soft starter is barely more money than the Star Delta starter and adds significant benefits in terms of reducing the mechanical stresses of Star-Delta starting. There is ALWAYS a torque spike at transition, sometimes it is just worse than others, but it takes a toll on mechanical components over time. Compressors are the #1 application for soft starters in the world, mostly for that reason. (It used to be pumps, but VFDs have cut into that market for them).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[thony999]

LionelHutz & Warsop, many thanks.this clears it for me