VFD size

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DARUSA

Senior Member
Location
New York City
Do any body know were I can find in the code a back-up statement to justified that a 30hp VFD can not be installed in a 40hp pump? is my understood that the motor input and the VFD output need to match. HVAC company buck-up his statement in that they will not run the motor in a full speed but I see a BY-Pass option on the VFD and is clear that in a by-pass mode motor will run a full speed. I read in the code about sizing conductor,overload protection but nothing that clear says that HP need to match. I know that you can use one size up VFD but small look as a no no for me. Thanks in advance for your help.
 
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DARUSA

Senior Member
Location
New York City
I did not actually open the VFD box due to I was waiting for the guys replace it for a 40hp drive!!! do you think that it is possible that a 30 hp VFD handle a 40hp pump?
What about overload inside when it is running in a BY-Pass mode?
I'm sure 40hp VFD will cost more money but I think that matching HP IS THE PROPER INSTALLATION!!!. Need something from the code to backup my statement .thx
 
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I sure as heck wouldn't try to do it that way but if the drive manufacturer gives you their blessing.......

I'm the opposite, I like to oversize the drive a little if I can...:)
 
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DARUSA

Senior Member
Location
New York City
Cow said:
I sure as heck wouldn't try to do it that way but if the drive manufacturer gives you their blessing.......

I'm the opposite, I like to oversize the drive a little if I can...:)
Click to expand...



Definite I agree with you !!! One size up in the VFD will not create any problem with overload settings but I not sure if you going more than one size up you will be abble to set-up the proper overload settings !!!
 
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DARUSA

Senior Member
Location
New York City
ptonsparky said:
You are required to install equipment as per it's instructions. Something about labeling and listing. If the drive mfg says their 30 hp setup is good for 40, go for it. I would think if it was able to handle the 40 hp...it would say 40 hp on the box.
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Right why would you limit your VFD with 30 if it can handle 40hp.
 
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
Well, some drive manufacturers have a general duty HP rating and a heavy duty HP rating. No way to know without checking into it.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
this is a lot like using an undersized contactor. the thing is that if the motor is properly protected by the overload, the contactor won't likely ever have a problem, and neither will the VFD. If it needs more current than the drive can put out, it will just safely trip.

whether this is code legal or not is dubious despite likely being safe.

incidentally, this is not that unusual of a situation. a lot of places only stock certain size motors as spares and if they need a motor and don't have one the same size as was originally installed they will just install the next size up that they do have.
 
tom baker

tom baker

First Chief Moderator
Staff member
Location
Bremerton, Washington
Drives can be sized to the load, either variable torque of constant torque (or standard duty and heavy duty). A variable torque load is a centrifugal pump or fan where the load varies with the flow (look up affinty laws), The constant torque load would be a rock crusher or similar. The mfg will have tables that state the given size drive for a motor. The drive itself will not be overloaded as the controls limit it to the max current you enter into the program.
My recommendation is to always go with a constant torque sized drive.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Okay, put the 40hp motor on the 30 hp vfd. Granted the drive most likely will limit the current etc to what it can safely deliver without destroying itself. The motor also will not develop its full 40hp output. We don't care but make sure your feeder is still sized for the 40.
 
kwired

kwired

Electron manager
Location
NE Nebraska
petersonra said:
this is a lot like using an undersized contactor. the thing is that if the motor is properly protected by the overload, the contactor won't likely ever have a problem, and neither will the VFD. If it needs more current than the drive can put out, it will just safely trip.

whether this is code legal or not is dubious despite likely being safe.

incidentally, this is not that unusual of a situation. a lot of places only stock certain size motors as spares and if they need a motor and don't have one the same size as was originally installed they will just install the next size up that they do have.
Click to expand...
A vfd starts with reduced voltage and current, across the line starting will be the same inrush current for a 40 hp motor whether or not there is 30 or 40 Hp of load coupled to the motor. Fortunately when it comes to NEMA contactors 30 and 40 HP both use the same contactor @ 480 volts anyway.

ptonsparky said:
Okay, put the 40hp motor on the 30 hp vfd. Granted the drive most likely will limit the current etc to what it can safely deliver without destroying itself. The motor also will not develop its full 40hp output. We don't care but make sure your feeder is still sized for the 40.
Click to expand...
And if the user ever finds out they have a 40 hp motor driving a load and is set to trip at 30 hp levels, they will want to see an increase in production from that motor every time.
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
ptonsparky said:
430.122(B)(2)
430.6
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430.122 Conductors ? Minimum Size and Ampacity.(A) Branch/Feeder Circuit Conductors.​
​
​
​
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Circuit conductorssupplying power conversion equipment included aspart of an adjustable-speed drive system shall have an ampacitynot less than 125 percent of the rated input current to​
the power conversion equipment.

The bypass option is IN the ASD manual and not what the HVAC supplier designed. Keep focus and ignore the tertiary issues and what if's..... Consequently 430.6 isn't applicable.


 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Does the installation have a 40 hp motor? If it does, how do you get buy without running properly sized wires to it? Now if you can satisfy both with the 30hp vfd requirement, good.

I cant reduce the wire size to motor because someone puts a flow restriction in the piping. Whyy is this different? I believe my inspectors would require the additional sizing.
 
D

DARUSA

Senior Member
Location
New York City
ptonsparky said:
Does the installation have a 40 hp motor? If it does, how do you get buy without running properly sized wires to it? Now if you can satisfy both with the 30hp vfd requirement, good.

I cant reduce the wire size to motor because someone puts a flow restriction in the piping. Whyy is this different? I believe my inspectors would require the additional sizing.
Click to expand...

I will agree with you in refer to the wire size but no matter that you provide the proper wire size for the rating of the motor plus an external over load protection to meet the OL requirements of article 430 you will still in violation of 110.3B because the listing of the VFD is for 30hp not for 40hp.


430.122 Conductors ? Minimum Size and Ampacity.

A)Branch/Feeder Circuit Conductors. Circuit conductors supplying power conversion
quipment included as part of an adjustable-speed drive system shall have an ampacity not less
han 125 percent of the rated input to the power conversion equipment.

B)Bypass Device. For an adjustable speed drive system that utilizes a bypass device, the
onductor ampacity shall not be less than required by 430.6. The ampacity of circuit conductors
upplying power conversion equipment included as part of an adjustable-speed drive system
hat utilizes a bypass device shall be the larger of either of the following:

1) 125 percent of the rated input to the power conversion equipment

2) 125 percent of the motor full-load current rating as determined by 430.6

 
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