VFD sizing for motor loads

[brother]

Have anyone done much vfd sizing for motors?? I know you have to take torque and and horsepower into consideration alot. It appears most of the vfds I have seen tend to have larger conductors feeding it than one would calculate for a motor load without a vfd. Maybe its because the vfd is more oversized, or they used 150% NEC 2008 430.6(C). from the motor tables unlike only 125% sizing for conductors to the motor.
Any one have an instance where they had to size the vfd?? was the feed to the vfd allowed to be smaller than it would be for the motors itself?? How did you size it?

 

[don_resqcapt19]

The feed to the VFD must be rated at 125% of the VFD input amps. It has nothing to do with the horsepower of the motor that is connected to the load side of the VFD. 430.122(A)

 

[kwired]

The feed to the VFD must be rated at 125% of the VFD input amps. It has nothing to do with the horsepower of the motor that is connected to the load side of the VFD. 430.122(A)

I agree with that in general principle - then enters reality.

I do have an installation that has been running maybe close to 10 years now that would fail inspection per that section.

50 hp motor is what is driven, 250 hp drive happened to be available. this drive is supplied with original supply circuit that powered the 50 hp motor - IIRC it was #4AWG and 100A breaker.

250 hp motor was originally on at least 400 amp circuit maybe(probably) even larger, don't recall specifically what it was. Have never had a problem with this drive since installing it. Did have to rewind motor couple times before we got smart and got a motor designed for use with VFD, no trouble since then.

 

[texie]

I need help understanding the requirement that VFD input conductors be sized to the drive and not the load. It seems analogous to sizing conductors for an across the line starter based on the starter size and not the motor size which, of course, we don't do. Like the previous post with the 250 HP drive on a 50 HP motor with conductors sized to the motor, other than a NEC violation, is there a technical reason not to do this?

 

[infinity]

My guess is that this allows for changes to the motor in the future. Kind of like a 200 amp panel with one 20 amp branch circuit breaker.

 

[texie]

My guess is that this allows for changes to the motor in the future. Kind of like a 200 amp panel with one 20 amp branch circuit breaker.

I think you missed my point. The NEC does not require a 200 amp feeder to a 200 amp panel, just like my across the line starter comment. I'm try to understand why a VFD is treated differently.

 

[brother]

ok, heres a more clarity to my question. How do you size the vfd for a motor?? is based on the motors flc from the NEC table?? or is it both what the motor is used for and its flc??

So basically if you have a 25hp motor would it need a 25hp vfd?? or is more on the current of the motor??

 

[infinity]

I think you missed my point. The NEC does not require a 200 amp feeder to a 200 amp panel, just like my across the line starter comment. I'm try to understand why a VFD is treated differently.

Yup, not really the best example. :roll:

 

[kwired]

ok, heres a more clarity to my question. How do you size the vfd for a motor?? is based on the motors flc from the NEC table?? or is it both what the motor is used for and its flc??

So basically if you have a 25hp motor would it need a 25hp vfd?? or is more on the current of the motor??

I can not recall ever seeing a drive that did not specify maximum hp or kW. Usually they have both marked on them.

There are situations where you need to derate the drive, like when it is used with single phase input, or if there is frequent starting/stopping, acceleration/deceleration or even frequent reversing. Many times each of those instances could depend on the drive being used as far as what kind of deration is needed where a different mode with similar kW rating could be derated differently.

 

[kwired]

My guess is that this allows for changes to the motor in the future. Kind of like a 200 amp panel with one 20 amp branch circuit breaker.

My example of the 50 hp motor powered by a 250 hp drive you are not just going to put a 250 hp motor in place of the 50. First it will not fit in the machine. If you do find a way to fit it in there the machine is not going to draw any more load if operated at same speed and other conditions. If something changes to load it more the overcurrent protection settings also need changed or it will shut down on overcurrent.

If I had same situation without the VFD (across line motor starter) and someone changes the motor to a larger one we still have all the same issues don't we?

If the motor starter were large enough for the 250 hp motor but powered a 50 instead do I need to supply it with same size circuit as I would for the 250?

I don't really know why NEC thinks this drive would need supply circuit sized for the drive rating if the connected load is not even close to its rating.

I can understand because of loses the drive may require more ampacity in the supply circuit than the same motor may require if connected across the line. I also understand that the power factor with the drive will be lower than across the line and may actually compensate enough for other loses that little or no additional ampacity may be needed.

 

[KD8HE]

Mechanical load demand should be the primary consideration.
The VFD and cable should be sized to the motor based on the maximum current
required by the motor under peak torque demand?not on the basis of horsepower
considerations.

 

[kwired]

Mechanical load demand should be the primary consideration.
The VFD and cable should be sized to the motor based on the maximum current
required by the motor under peak torque demand?not on the basis of horsepower
considerations.

I will add that it isn't necessarily the mechanical load demand that needs consideration but rather the amount of demand that is allowable without shutdown by protective features.

Again this is not what NEC says, but NEC seems to assume a drive will have a motor rated for maximum drive output connected to it.

 

[Cow]

ok, heres a more clarity to my question. How do you size the vfd for a motor?? is based on the motors flc from the NEC table?? or is it both what the motor is used for and its flc??

So basically if you have a 25hp motor would it need a 25hp vfd?? or is more on the current of the motor??

When you order a drive they ask you for the motor HP. That's it, that's all there is to it. Just like buying a starter.

 

[weressl]

The feed to the VFD must be rated at 125% of the VFD input amps. It has nothing to do with the horsepower of the motor that is connected to the load side of the VFD. 430.122(A)

Consequently the use of the oversized drive may not be the 'economical' choice, even if it was just laying around there. Depends on the distance the cable cost difference could be significant, not to mention the cost of the larger disconnect/breaker front of it. One should also think cost of the higher losses of a larger drive, the higher harmonic content it will produce on the system - unnecessarily. If it was an older drive the cap's may be on their way out. How much does a 50HP drive cost, $3-5K? If there is any accuracy on the controls desired the drive may not function as well as a properly sized drive would.

 

[weressl]

ok, heres a more clarity to my question. How do you size the vfd for a motor?? is based on the motors flc from the NEC table?? or is it both what the motor is used for and its flc??

So basically if you have a 25hp motor would it need a 25hp vfd?? or is more on the current of the motor??

The drives primarily current rated devices, so as a general rule one needs to size those based on the motor nameplate FLC. The HP/kW rating on the drives is given for base reference, as the users are used to deal with HP of a motor, not FLC. Remember that ASD's are built - nowadays - for worldwide use and often have programmable input voltage range, while the current limit remains the same.