Motors & VFD


Senior Member
So basically I can size the VFD from the motor nameplate info?

Not quite.

Size VFD to the motor name plate info AND the application. Watch out for the duty cycle table for instance. Two have stated VT/CT aka ND/HD ratings and there are special attention applications like compressors and crushers and rock quarry screens where even the VT/CT rule doesn’t work.

Size cables to motor size as per normal 430 procedure…normally 125% but again duty cycle matters.

On the feed side you use 125% of the VFD rating (in amps) as per 430 for wiring. The VFD is the load. Ignore the motor. OCPD as mentioned must follow manufacturers guidelines. Fortunately fewer of them are specifying exotic fuses these days.


Electron manager
NE Nebraska
So the motor being 160A could actually have the OCPD smaller with a VFD in the circuit?
Maybe. maybe not.

Depends on some the thing that have been brought up here.

Drive output will carry the extra kVAR the motor demands because of power factor. Drive input will not really have any displacement power factor but can have some distortion power factor but is still somewhat negligible in comparison to power factor associated with the motor.


Senior Member
Don’t care about petro industry standard because the fluid properties are known. Try mineral slurries that are non-Newtonian or plastics. And this is just the start.

Say the process engineer calculates 100 HP. So it makes all the sense in the world to put a 100 HP VT motor there with say water. With a given pump the turndown ratio is limited. No matter what you do there is a limit to turn down ratios and it’s pure fluid dynamics that is the limit. Note that most comical engineers do NOT get this. All problems, even process ones, are electrical until proven otherwise.

So say they really need 150 HP. And in reality most systems need a certain amount of kilowatts per pound of material moved but we mess this up by getting lost in units like CFM or HP or Amps. So at first they crank it up to 60 Hz. Then they ask you to unlock it, get to say 65 Hz and it trips. It doesn’t matter what point this happens at. At some point they are going to hit the motor thermal limit. The thing is that with pumps and fans the affinity laws say power draw is proportional to the cube of speed. So they are going to play this game and find out it makes very, very little difference before we even get into issues with cavitation.

So if you had the budget to do it and went up 50% on the VFD or at least just size everything “HD”, no problem. They buy their bigger pump and motor and you just change the motor parameters in the drive. But if you sized to name plate now it’s changing the VFD, cables, breaker, and maybe transformer.

Now saying this there is a big difference between say buying a 20 HP VFD for a10-15 HP pump and buying a 750 HP VFD for a 500 HP pump.

And you should have a feel for it. The gas & oil and power generation industries tend to look at things in a more pure cost/benefit point of view and won’t hesitate to rip out a million dollar drive system and put in another million dollar drive system 6 months later. That’s far different from wood or food plants that will cry over an engineering mistake but live with it for years. And mining notoriously undersized things then may or may not fix it but will blame you for selling a “cheap/weak” VFD, especially quarries.

The best way to answer this is make sure you size for the application as a bare minimum. Offer a size upgrade too. Leave it on the customer. Often I will look at frame vs. model size and just offer the lframe size as the upgrade since it doesn’t change any dimensions drastically and often has only a small price increase.
FYI, we operate big PD pumps and maximize those at 125% ( demand change due to production requirements)! The VSD is 127% of the pump size! I have yet to experience your scare of tripping VSD's and these pumps are all handling slurry! Sizing can never go wrong with keen eyes on the driven pump performance curves.


Senior Member
Northern illinois
Personally I'm not sure the average electrician should be sizing motors or VFDs for the kind of applications where the sizing makes that much difference. The vast majority of VFDs are used on fans and pumps where it doesn't make any real difference. The one in 10 or 1 in 20 times where it might make a difference most electricians aren't really in a good position to make that choice.