Sizing backup generators for VFD driven motors

[viking684]

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

 

[davidbeach]

VFDs aren't pleasant loads for a small generator. Is there anything else you could also put on the generator? Could you run the pumps without the VFDs when on the generator? I'd contact a generator vendor and get their input on the unit sizing. Cable sizing is simply per code.

 

[viking684]

Thanks davidbeach. In regards to the motor controls, unfortunately there are no other controls aside from the VFD. Also, I had an electrician tell me that I would need a size 4 AWG Cu wire to wire the 2 wells, not sure if this is correct though.

 

[ashtree]

The generator supplier will also need to know whether or not you have to start both wells at the same time. Starting the 30hp first will result in a smaller gen set than if you have to start the 20 hp first and way smaller than if you have to start both together. davidbeach is correct in his comment about VFDs and gensets and what he meant was , is there other loads other than these motors with VFDs that can be run at the same time , preferably before you start these motors. Ideally these would be a straight resistance type load, heating systems , or incandescent lighting network, or similar, but other motors without VFDs would be okay. Some VFDs will be more suitable than others for gen set operation and some generators will be better than others as well.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[itsmoked]

I've run some just sized big-enough generators running VFD wells and nothing else with no problems at all. So I would just check with the generator supplier until I found one who's generators work correctly.

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

 

[waross]

It's my understanding that it is the AVR that doesn't like VFD loads. I would be looking for a VFD compatible AVR.
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

 

[jraef]

Assuming from your voltages that you are in the U.S., NEC 430.122(?) will require that the conductors be sized for 125% of the VFD maximum input current, not the motor current. So because your VFDs are over sized, that affects your conductor sizing.

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

 

[itsmoked]

Excellent points Jeff about the wire size and purchase agreement.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[catserveng]

Give this a read, you may find it helpful in understanding the issues,

https://www.trane.com/content/dam/T...lectrical-considerations/admapn019en_0106.pdf

A little more general in nature from Cummins,

http://power.cummins.com/sites/default/files/literature/technicalpapers/PT-7007-SizingGensets-en.pdf

Here is a bit more technical discussion not manufacturer specific,

http://www.mid-island.com/downloads/pdfs/Drive_on_generator.pdf.pdf

Here is an old Caterpillar document,

https://www.idc-online.com/technica...ngineering/Generators_and_nonlinear_loads.pdf

General article from ECM,

http://ecmweb.com/basics/sizing-gen-sets-large-motor-starting

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.