Using a 25hp VFD on a 30hp motor

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bbaumer

Senior Member
Location
Indiana
I was asked early this morning if it was OK to use a 25 hp rated variable speed drive on a 30 hp motor as "the motor won't ever see full load for the application".

I poked around art. 430 and the sections that apply to adjustable speed drives and did not see that this would be prohibited.

Any opinions on this or did I miss something?

Thanks.
 

bbaumer

Senior Member
Location
Indiana
I will also add this is already installed this way. The drive is being used on a supply fan. The installer used a drive already on hand to get it up and running quick and to avoid having to purchase a new drive.

It was brought to my attention this morning and I am trying to find a code reference to back up what will be my direction to replace the drive with the "right" one.

"Because I said so" isn't a very good reason. It will be hard to argue overload when the drive has built in overload protection for one and two, the drive will likely cut out on the high duct static pressure safety circuit before it is overloaded anyway.

Thanks.

BTW I am looking for an NEC reference, not Punjabi or Delhi installation standards

SuperStock_1606-129228.jpg
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It is a standard practice throughout the world.

What standard practice handbook did you find that it?

I have not found anything in the code that would limit it. You might find something in the instruction manual for the drive if you look hard enough, but I just don't think there is anything really wrong with such an install, everything else being OK.

How is this any different than hooking up a VFD to a motor that is too small to drive the load at full speed, which is a common practice.
 
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Haji

Banned
Location
India
It will be hard to argue overload when the drive has built in overload protection for one and two, the drive will likely cut out on the high duct static pressure safety circuit before it is overloaded anyway.

It is just to prevent the unwanted tripping of the drive on overload sensing even during normal operation of the motor, the drive size is made at least equal to the motor size. It is just common sense.

Unfortunately, Punjabi or Delhi installation standards and even NEC can not teach common sense.
BTW I am looking for an NEC reference, not Punjabi or Delhi installation standards
 
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Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I was asked early this morning if it was OK to use a 25 hp rated variable speed drive on a 30 hp motor as "the motor won't ever see full load for the application".

I poked around art. 430 and the sections that apply to adjustable speed drives and did not see that this would be prohibited.

Any opinions on this or did I miss something?

Thanks.

You could tell them to put a 25HP motor on it. Then you have no issue using a 25HP drive. Or buy an AB, I'll bet they have a Powerflex 70 rated 30HP general duty, 25HP heavy duty. That would cover your bases.

I honestly think they are splitting hairs if they are worried about the slight cost difference between a 25HP and 30HP drive. Buying a 30HP and only lightly loading it, may last longer and be cheaper in the long run.
 
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shamsdebout

Senior Member
Location
Macon,GA
I will also add this is already installed this way. The drive is being used on a supply fan. The installer used a drive already on hand to get it up and running quick and to avoid having to purchase a new drive.

It was brought to my attention this morning and I am trying to find a code reference to back up what will be my direction to replace the drive with the "right" one.

"Because I said so" isn't a very good reason. It will be hard to argue overload when the drive has built in overload protection for one and two, the drive will likely cut out on the high duct static pressure safety circuit before it is overloaded anyway.

Thanks.

BTW I am looking for an NEC reference, not Punjabi or Delhi installation standards

View attachment 8128

Funny but a low blow lol.
 

bbaumer

Senior Member
Location
Indiana
You could tell them to put a 25HP motor on it. Then you have no issue using a 25HP drive. Or buy an AB, I'll bet they have a Powerflex 70 rated 30HP general duty, 25HP heavy duty. That would cover your bases.

I honestly think they are splitting hairs if they are worried about the slight cost difference between a 25HP and 30HP drive. Buying a 30HP and only lightly loading it, may last longer and be cheaper in the long run.

No hair splitting to be done. A "free" 25 hp drive you already have sitting on the shelf not being used, is cheaper than buying a new 30hp drive.
 

Besoeker

Senior Member
Location
UK
I was asked early this morning if it was OK to use a 25 hp rated variable speed drive on a 30 hp motor as "the motor won't ever see full load for the application".

I poked around art. 430 and the sections that apply to adjustable speed drives and did not see that this would be prohibited.

Any opinions on this or did I miss something?

Thanks.
I can't comment on code implications but, within some constraints, you can.

One thing to note is that a totally uncloaded cage motor typically takes around 30% of full load current. You can reduce that by running at a reduced V/f ration.
But what does "won't ever see full load" equate? if not 30HP, could it be 29HP? I think, without that sort of information, you have basis for making a informed comment.
 

Besoeker

Senior Member
Location
UK
I will also add this is already installed this way.
So your question of whether it is OK to do so is not whether you can do so but whether it is permissible to do so.
Clearly you can because you have.
And no one seems to be able to find any code/rules that deem it unacceptable.
On that basis, it seems to be OK on both counts.
And, what's the worst that could happen?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I will also add this is already installed this way. The drive is being used on a supply fan. The installer used a drive already on hand to get it up and running quick and to avoid having to purchase a new drive.
If it is running OK, why mess with it?

I would bet there are plenty of things that the resources could be better spent on than screwing around with something that is working OK.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Don't forget, that in general Article 430 applies even if a VFD is involved.

From the 2005 edition,
430.2 defines a controller as the device that 'is normally used to start and stop a motor by making and breaking motor circuit current'
430.8 requires marking, on the controller, so that the suitablity of the installation can be determined.
430.83 requires controllers to be rated 'not lower than the HP of the motor'.
430.120 says the rest of 430 applies to VFD's, unless noted in 'Part X' which does not address this issue.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
If it is running OK, why mess with it?

I would bet there are plenty of things that the resources could be better spent on than screwing around with something that is working OK.

It took me a lot of years to start picking the battles I wanted to fight. We see so many things that are "wrong" we just can't fix them all, nor do most customers want to pay to correct something that has been successfully wrong for years.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Don't forget, that in general Article 430 applies even if a VFD is involved.

From the 2005 edition,
430.2 defines a controller as the device that 'is normally used to start and stop a motor by making and breaking motor circuit current'
430.8 requires marking, on the controller, so that the suitablity of the installation can be determined.
430.83 requires controllers to be rated 'not lower than the HP of the motor'.
430.120 says the rest of 430 applies to VFD's, unless noted in 'Part X' which does not address this issue.

does a VFD actually "make" and "break"? that is normal thought of as something a Cb or switch or contactor would do, i.e.- dry contacts.
 

Bill Wessels

New member
Bill Wessels

Bill Wessels

What about 2011 NEC 430.83(A)(1)? I think a motor controller (VFD) needs to have a horsepower rating not lower than the horsepowerrating of the motor.

Thanks
Bill Wessels
 
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