Sizing backup generators for VFD driven motors
Sizing backup generators for VFD driven motors
(OP)
Hello,
I have 2 water wells located approximately 300 feet apart. One well has a 30 Hp, 460 V, 3-phase motor controlled by a 40 Hp, 460 V, 3-phase VFD and the other well has a 20 Hp, 460 V, 3-phase motor, controlled by a 30 Hp, 460 V, 3-phase VFD. I am thinking of buying a backup generator for the 2 wells and would like to know how to calculate the cable size I need to use to connect the 2 wells and what size backup generator to get to power both wells. Any help will be appreciated.
Thanks
I have 2 water wells located approximately 300 feet apart. One well has a 30 Hp, 460 V, 3-phase motor controlled by a 40 Hp, 460 V, 3-phase VFD and the other well has a 20 Hp, 460 V, 3-phase motor, controlled by a 30 Hp, 460 V, 3-phase VFD. I am thinking of buying a backup generator for the 2 wells and would like to know how to calculate the cable size I need to use to connect the 2 wells and what size backup generator to get to power both wells. Any help will be appreciated.
Thanks





RE: Sizing backup generators for VFD driven motors
RE: Sizing backup generators for VFD driven motors
RE: Sizing backup generators for VFD driven motors
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
RE: Sizing backup generators for VFD driven motors
Since the generator is presumably to keep the water on in a disaster you might want to consider using two smaller ones close to the individual wells so you have some water even if you have a single generator failure. It would cost a few dollars more but you'd save a lot on the wire costs as a trade-off.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sizing backup generators for VFD driven motors
There is an old type of current fed AVR that should be able to ignore VFD loads.
A constant voltage transformer is used to provide enough excitation to give full voltage at no load. The transformer output is rectified and there is a trimming resistor to adjust the voltage. Then a CT on the load conductors creates a voltage across a dropping resistor that is proportional to the load. This voltage is rectified and added to the voltage from the constant voltage transformer. Again a trimming resistor is used to adjust the voltage under load. Another advantage of this type of AVR is that it may be set to over-excite under load to compensate for line loss on long lines.
For VFD starting, use the full load current of the smaller set and add the current limit setting of the larger set. If the pumps have different current limits, check the rated current of the larger set plus the current limit of the smaller set. There may not be enough difference in size to justify always starting the smaller pump first.
Given the distances involved, the wire size seems reasonable.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sizing backup generators for VFD driven motors
Bill is right, it's the AVRs that might not like the VFDs. Discuss this with any generator supplier. There are plenty of generators that have no problem running VFDs, but if you don't ask, you will not know what you are getting. Make it a spec requirement and if the genset has issues, the onus is on them to fix it.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Sizing backup generators for VFD driven motors
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sizing backup generators for VFD driven motors
https://www.trane.com/content/dam/Trane/Commercial...
A little more general in nature from Cummins, http://power.cummins.com/sites/default/files/liter...
Here is a bit more technical discussion not manufacturer specific, http://www.mid-island.com/downloads/pdfs/Drive_on_...
Here is an old Caterpillar document, https://www.idc-online.com/technical_references/pd...
General article from ECM, http://ecmweb.com/basics/sizing-gen-sets-large-mot...
There are a lot of issues involved, some are easy and can be taken care of with simple adjustments. Some require modifications or accessory components. Some combinations don't work worth a darn and end up costing someone lots of money.
Best bet, find an experienced engineer to write you a good, solid, clearly defined spec, don't accept exceptions or deviations without fully understanding their impact.
Hope that helps, MikeL.