Genset Performance 4

[redraven6]

Standby genset needed to supply motors (1.5 hp to 7.5 hp). Mfr A says his 40 kW set will supply the load; Mfr B says he must use a 100 kW. Why the huge difference ?

 

[ScottyUK]

Unusual to see a generator rated in kW and not kVA.

How many motors, can you sequence the motor starts or do they start together, can you start the biggest ones first, how big a voltage excusion is acceptable during starting, what type of AVR and governor...?

Too many unknowns at present. Maybe manufacturer B has a 100kW set with an indifferent governor and AVR, maybe he just wants to sell you the biggest set he can get away with.

Post your loads and performance requirements and I'll run them through the sizing software provided by one of the OEM's if I get time (and if they list sets as small as 40kW).


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

Unusual to see a generator rated in kW and not kVA.

Actually Scotty, it's not unusual at all, just one of the many IEEE/ANSI v. IEC differences.

 

[redraven6]

Thanks ScottyUK

Max starting voltage dip 35%
Max freq dip 6 Hz
Altitude 3000 ft
Max ambient 95F
Duty = standby
Diesel

All motors 208V, 3 ph, rated in hp

Step 1
7.5
5
5
3
2

Step 2
7.5
5
2

Step 3
3
3
1.5

 

[Marke]

there are two important components of gensets that have a major impact when starting motors.
1. AVR. Many low cost machines use single phase (two phase) peak reading half wave AVRs. These are slow responding and prone to instability with step changes such as motor starting. The three phase averaging AVR is a much better option whith motor starting.
2. Excitation. Self excited Alternators are essentially current limiting and have very poor overload characteristics so must be sized for the start current.Typical maximum currents are in the order of 120%. Permanent Magnet Generator excitation allows for overloads typically in the order of 300% during motor starting so you can reduce the alternator size.
If the AVR is single phase peak reading and the alternator is self excited, then you need to size on maximum start current. If the alternator is PMG excited and the AVR is three phase averaging, then you can size much closer to the motor load.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

 

[VTer]

see attached

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[ScottyUK]

Thanks David, I didn't know that. My little piece of learning for today is fulfilled!

VTer,

My report would have looked strangely similar. Thanks.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Also see

faq237-766

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

I have done some sizing for gensets in that range. I would use a 50 KW (62.5KVA) at first glance.
But your allowable voltage and frequency excursions are fairly forgiving, and the 40 KW (50 KVA) will probably be acceptable.
If you can get the Permanent Magnetic Generator (PMG) option for excitation, it will lessen the voltage dips. You can probably start that lineup acceptably with a self excited set.
Don't buy a used car from vendor B.
There is some art to sizing; For instance, if group 1 is running but not loaded, there will be more reserve to start group 2.
Some loads tend to over load the motor when first started. Without knowing your line-up and specific machines, our answers are applicable to average or typical loads but there are exceptions to average and typical.

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