leitmotif
Member
I go along with Gil. The cost of 100% reliabilty is very high.
I think risk and consequence of failure must be taken into account. Two arbitrary examples:
If this is a 100HP circ pump in a production facility I am sure no one will get hurt if it fails. OK you dont produce for a day or so and you may lose raw material (food facility)
On other hand if this is a 10 HP unit that pumps oxygen to patients in a hospital then cost may be justified. If unit fails you have dead people.
Gil is also correct how are you going to completely deenergize a unit that is connected in parallel with another energized one? In this case I would consider using manual disconnects to switch out the VFD and switch in the DOL starter. They could also be locked out which I am sure a servicing electrician would find as a relief. Would require 4 disconnects.
You would ideally need two different enclosures to ensure electrician is not working hot.
FINALLY IF DOL is added the conductor from the power supply must be upsized.
Reason
on a VFD you can size wire much smaller than you need to size it with DOL. Example for 100 HP unit at 480 volt, full load amps about 100.
On a VFD setup you would only need 100 amp conductor
on a DOL you would need 300 amp capacity from source to the starter relay (old days was 300% now you go thru a couple charts in NEC with different results but 300% is easier for example here).
From contactor on you would only need 100 amp conductor unless a long run then I would apply larger conductor. Even on a short run I still use larger wire - cheap compared to burning out motor due to undervolage on start.
You would also need overload sensors sized for 100 amp.
I would seriously consider buying a spare VFD unless you need 100% reliabilty (life safety critical) but to assure 100% you have to have two completely independent units including pump etc etc (just like Gil recommended). This would probably cost just as much and would give more reliability.
Dan Bentler
I think risk and consequence of failure must be taken into account. Two arbitrary examples:
If this is a 100HP circ pump in a production facility I am sure no one will get hurt if it fails. OK you dont produce for a day or so and you may lose raw material (food facility)
On other hand if this is a 10 HP unit that pumps oxygen to patients in a hospital then cost may be justified. If unit fails you have dead people.
Gil is also correct how are you going to completely deenergize a unit that is connected in parallel with another energized one? In this case I would consider using manual disconnects to switch out the VFD and switch in the DOL starter. They could also be locked out which I am sure a servicing electrician would find as a relief. Would require 4 disconnects.
You would ideally need two different enclosures to ensure electrician is not working hot.
FINALLY IF DOL is added the conductor from the power supply must be upsized.
Reason
on a VFD you can size wire much smaller than you need to size it with DOL. Example for 100 HP unit at 480 volt, full load amps about 100.
On a VFD setup you would only need 100 amp conductor
on a DOL you would need 300 amp capacity from source to the starter relay (old days was 300% now you go thru a couple charts in NEC with different results but 300% is easier for example here).
From contactor on you would only need 100 amp conductor unless a long run then I would apply larger conductor. Even on a short run I still use larger wire - cheap compared to burning out motor due to undervolage on start.
You would also need overload sensors sized for 100 amp.
I would seriously consider buying a spare VFD unless you need 100% reliabilty (life safety critical) but to assure 100% you have to have two completely independent units including pump etc etc (just like Gil recommended). This would probably cost just as much and would give more reliability.
Dan Bentler