Using a 25hp VFD on a 30hp motor

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jim dungar

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petersonra said:
does a VFD actually "make" ....
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Seeing how the VFD is the source of the motor current, it would be really hard to say it is not making it.;)
Remember a controller is defined as performing normal starting and stopping, not for disconnecting.
The controller deals with the actual motor current, as opposed to a pushbutton that deals with control circuit current.
 
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petersonra

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Bill Wessels said:
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
Click to expand...

It does seem clear.

jim dungar said:
Seeing how the VFD is the source of the motor current, it would be really hard to say it is not making it.;)
Remember a controller is defined as performing normal starting and stopping, not for disconnecting.
The controller deals with the actual motor current, as opposed to a pushbutton that deals with control circuit current.
Click to expand...

I am just not used to thinking of a VFD as making or breaking current. More as applying current. I have never seen a make or break rating on a VFD, but they are common on dry contacts of various types.
 
weressl

weressl

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jim dungar said:
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.
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Hm....controllers are usually rated by current and HP is (really) only listed as a reference.

So I have a drive that is rated 38A, but the manufacturer lists it as 25HP normal and 20HP heavy duty. The NEC table 430-120 tells me that a 30HP motor is curent is 40A, but I have a high efficieny 30HP motor that is nameplated @ 33A.

Can I or can't?

Here is the other scenario:

Same drive as above. I have a 40HP motor on an existing fan. The motor namaplate reads 45A, but it is drawing 35A on the day of highest humidity. It may be violation of the NEC, but it is clear that under engineering control in an industrial facility that assures that no engineering parameters will be changed the installation would be safe.
 
weressl

weressl

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jim dungar said:
Seeing how the VFD is the source of the motor current, it would be really hard to say it is not making it.;)
Remember a controller is defined as performing normal starting and stopping, not for disconnecting.
The controller deals with the actual motor current, as opposed to a pushbutton that deals with control circuit current.
Click to expand...

I think the distinction may be better made if we say that the controller is not for interrupting faults, although it does interrupt locked rotor.
 
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Cow

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jim dungar

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petersonra said:
I am just not used to thinking of a VFD as making or breaking current. More as applying current. I have never seen a make or break rating on a VFD, but they are common on dry contacts of various types.
Click to expand...

The NEC says a controller is the thing that starts and stop a motor by manipulating its line current.

If you have a fusible disconnect feeding a VFD feeding a motor, which one is the 'controller', as defined by the NEC?
 
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jim dungar

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weressl said:
I think the distinction may be better made if we say that the controller is not for interrupting faults, although it does interrupt locked rotor.
Click to expand...

The definition is right out of the NEC: a controller normally starts and stops the motor.
 
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jim dungar

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weressl said:
Hm....controllers are usually rated by current and HP is (really) only listed as a reference.

So I have a drive that is rated 38A, but the manufacturer lists it as 25HP normal and 20HP heavy duty. The NEC table 430-120 tells me that a 30HP motor is curent is 40A, but I have a high efficieny 30HP motor that is nameplated @ 33A.

Can I or can't?
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The NEC simply says the nameplate data is to be used to determine the suitability of the controller. The NEC does not tell you how to do it.
 
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petersonra

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Haji said:
More than that. When there is no agreement among you, guys, as to what constitute the NEC ( such as 80% capacity utilization of equipment as stated by authors from US, see http://books.google.co.in/books/abo...cial_power_systems.html?id=k-NSAAAAMAAJ&hl=en), people from other countries need to step in to bring order.
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I don't care what a book someone wrote may say. If it claims something that is not actually in the code, it still is not in the code.

And there is a huge difference between someone's design criteria and what the code actually requires.
 
weressl

weressl

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jim dungar

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weressl said:
It is also within its normal function to interrupt an overload, up to and including locked rotor.
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Funcitonally maybe, that actually is the role of the overload device. I am just following the definition in Article 430 and its diagram showing the different parts of a motor circuit.
 
iwire

iwire

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In my humble opinion 110.3(A)(7) gives the AHJ all the ammunition they need to turn this down.

110.3 Examination, Identification, Installation, and Use
of Equipment.

(A) Examination. In judging equipment, considerations
such as the following shall be evaluated:

(7) Classification by type, size, voltage, current capacity,
and specific use
Click to expand...
 
Dennis Alwon

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I agree with Bob as we never install a device that is not rated for the load.

I am also quite surprised at some of the unnecessary comments in this thread. Please lets keep it professional.
 
weressl

weressl

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jim dungar said:
The NEC says a controller is the thing that starts and stop a motor by manipulating its line current.

If you have a fusible disconnect feeding a VFD feeding a motor, which one is the 'controller', as defined by the NEC?
Click to expand...

A contactor is a binary controller. It does NOT control current, it controls voltage. The current is the resultant product and it depends on the connected circuit characteristics.

An ASD or a SSRV is an analog or more accurately incremental digital controller. One controls a single parameter of the supply and the other actually chnges the supply and creates a completely different supply that then it manipulates.
 
kwired

kwired

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Besoeker said:
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?
Click to expand...

Good reply.

ptonsparky said:
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.
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Also good reply.

petersonra said:
It does seem clear.



I am just not used to thinking of a VFD as making or breaking current. More as applying current. I have never seen a make or break rating on a VFD, but they are common on dry contacts of various types.
Click to expand...

Think of it as a complex device that is a power supply, controller, overload, and sometimes additional options all in one package.

jim dungar said:
The NEC says a controller is the thing that starts and stop a motor by manipulating its line current.

If you have a fusible disconnect feeding a VFD feeding a motor, which one is the 'controller', as defined by the NEC?
Click to expand...

Potentially both.

ptonsparky said:
Does the NEC state that you cannot have more than one controller?
Click to expand...
No.
 
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