Cooling tower fan varying duty?

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WPiper

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I have a customer who insists that a cooling tower fan on a variable speed drive falls under the classification of a continous rated motor being used for varying duty service as defined in Table 430.22(E) in the 2005 NEC.

I believe that the statement under this table:
"Note: Any motor application shall be considered as continuous duty unless the nature of the apparatus it drives is such that the motor will not operate continuously with load under any condition of use."
makes it pretty clear that something like an automatically controlled fan motor is not what the writers had in mind when they are talking about varying duty. Obviously the fan motor on a tower will operate continuously with load under most conditions of use. In fact, that is the main reason for using a variable speed drive on the tower - to eliminate cycling and the temperature swings it creates and force the motor to operate continuously, ableit at reduced speed.

He also made the following statement to me:
"The wiring, contactors, terminal blocks from the VSD output to the motor need to be rated for 200% of the motor FLA. This is because of the standing wave the drive can generate in the wiring per NEC code for variable duty running of the motor."
I'm not an EE. Is this what the code is about? "Standing waves"?
 
WPiper said:
"The wiring, contactors, terminal blocks from the VSD output to the motor need to be rated for 200% of the motor FLA. This is because of the standing wave the drive can generate in the wiring per NEC code for variable duty running of the motor."
I'm not an EE. Is this what the code is about? "Standing waves"?
Click to expand...

I'm inclined to agree that any application involving a cooling tower is probably not really continuous unless a VFD is used to make the motor run all the time instead of turning on and off based on a temperature switch.

Why would you need a contactor if you have a VFD in the first place? The code does not require upsizing of any downstream components when using a VFD. And there is generally no need to have a contactor on a VFD, nor is it an especially good idea.
 
contactor on the vfd output

contactor on the vfd output

The contactor is part of a bypass circuit to allow the fan to operate on either the vfd or a conventional motor starter. It is interlocked with the vfd so that the output of the drive is turned off if the contactor is opened, and also interlocked with the starter to prevent both from energizing simultaneously.

So, based on your answer, do you think that a tower fan motor is the type of application the NEC means when they say "varying duty application of a continuous rated motor"? Have you ever seen a cooling tower fan motor wired based on 200% of the normal FLA? Most tower fans cycle on and off although you certainly try to limit it to 4 or 6 starts per hour to prevent damage to the motor and avoid tripping the overload relay or blowing fuses. Eliminating the cycling is certainly one of the reasons for using a VSD for this.

What about the "standing wave?" What is that about?
 
This customer sounds like an expert on VFD's with half a jar of knowledge, knows a couple of impressive terms and is now an expert.
You do not need to upsize the conductors on the output of the VFD. The motor has a rated full load amps. Size the drive, conductors bypass and overloads based on the motor ampacity and lead length.

All of the tower fan applications I have designed and started up are rated for continuous duty. Period. During peak cooling times, you do not want to fans out the service because the loads are undersized. The starting FLA of the motor on the drive is greatly reduced, reduced belt and pulley wear. All good things with the VFD.

Hope this helps
 
The standing wave is based on harmonics, which I will not try to explain as it is not really required to answer your question.

Refer to Article 100, Duty, Varying, whereby I would think your application may apply. Therefore the 200% could be required. Since the Owner is wiling to accept that, and it meets NEC, I would give him what he wnats.
 
Just an observation, most of the inverters I've worked with had the capacity to 'peg out' at 200% for a short while, say for motor acceleration. This may be where the 200% number was picked up from.
 
200%

200%

The 200% comes from the NEC I believe. He is looking at table 430.22(E).

I don't think the code writers meant that table to apply to something like a cooling tower fan. A fan may indeed by cycled on and off and controlling the rate is part of good control design. But the footnote to that table says: "Note: Any motor application shall be considered as continuous duty unless the nature of the apparatus it drives is such that the motor will not operate continuously with load under any condition of use."
To me, that note says that any application that COULD POSSIBLY operate continuously should be considered a continuous load. The implication of that to me is that a continuous load is "worst case" in the context of the table. Duty cycle rated motors are SMALLER than continous duty rated motor for the same load because they get cool down periods between operations.

I don't think that it really has anything to do with VFDs.

In any case, I will supply his system with 200% wire and components between the drive and motor. Its certainly safe since that is oversized, but I've never hear of it anywhere else. I'm just trying to understand his criteria and figure out if its legitimate or the result of misunderstanding the code.
 
Bump because nobody seems to have ever answered

Bump because nobody seems to have ever answered

What does the NEC mean when it lists a continuous rated motor used for varying duty?

Are they talking about any application that may result in the motor being turned on and off? What about an air handling unit that cycles the fan off when the demand for cooling is satisfied? A booster pump that only runs when there is demand for additional flow in a system? Ventilation fans controlled based on room temperature?

These are all motor applications that typically use continous rated motors that may be turned on or off depending on control requirements. In all cases, the motor would be sized for the full load requirement of the device.

Should all of these situations be considered "varying duty" as definded by the NEC? What about the note under the table that says "Any motor application shall be considered as continuous duty unless the nature of the apparatus it drives is such that the motor will not operate continuously with load under any condition of use." All of the situations described above fit the description in the note - that is they could operate continously under some conditions of use.

Anybody know what this table really applies to?
 
I think that a variable speed drive for a fan on a cooling tower, which is intended to keep the motor turning with speed adjusted to maintain the cooling, is a continuously operated motor falling under the requirements of 430.22(A) which requires ampacity of 125% of rated full load current.

The increased ampacity requirements for Duty-Cycle Service described in Table 430.22(E) reflect the heating associated with frequent starts from zero speed.

The definition of continuous duty in the note to Table 430.22(E) is consistent with the usual operation of a variable speed drive on cooling tower service.

Furthermore, operation of a fan at reduced speed reduces the torque, and therefore current, by approximately the square of the speed.
 
Lets ignore the VFD for a moment. While increasingly common, they are not universal for these applications.

Is a cooling tower fan being cycled by a temperature controller a variable duty application UNDER THE MEANING OF THE TABLE? What about all the other applications I listed? All have the potential to start and stop frequently. It has always been my experience that the overload relays on a motor starter protect the circuit against overheating due to frequent starting since they are thermally sensitive. I have never encountered a job where the contractor, engineer or inspector required wiring from the starter to the motor to be sized at 200% based on NEC 430.22(E). My experience is almost entirely with pumps and fans, and although they sometimes are generally expected to run continuously, there is never anything to prevent them from being stopped and started as often as the end user might want or a bad control setup might cause.
 
A cooling tower fan is a continuous duty application. I approach all industrial applications as continuous.
VFD's work great on cooling towers. Just be sure to set the minimum speed for the motor at a reasonable rpm. I would hate to see a motor running at 2 rpm for days, unless it was a forced blower inverter duty motor.
The 200% is BULL.
 
WPiper,

I think 430.120 thru 430.128 should be used for your situation, section X.

Adjustable-Speed Drive Systems, have special rules for drives. JMO

Also,most of these motors are 'inverter duty' leaving 430.22 out of the picture.
 
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The standing wave has nothing to do with the 200% duty issue.A VFD gernerates a higher voltage than a normal 60hz 460 motor, if RMS this is 700 volts or so peak to peak, if I recall. The VFD can generate standing wave of 1200 or 1500 volts. This wave has to be absorbed into the motor windings, and if the motor is not "invertor duty spike resistant", the first few windings of the motor will burn up. Your VFD mfg or NEMA will have more information on this.
Motor circuits are sized at 125% and pumps are sized at 140%. The VFD will limit the motor current depending on how its programmed. The VFDs I work on won't let the motor over amp.
I don't believe there is a NEC basis for the 200%. Motor conductors are over sized as the motor will draw Locked Rotor current, which is why motor starts are limited. But this is not an issue with, and why VFDs are used.
 
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