FLA Questions - Industrial Control Panels

[jsmyth]

Hi, Regarding FLA calculations for industrial controls panels - would you calculate this based off your motor size or the max ability of your VFD. The reason i ask is because many plants require you to use "heavy duty" VFD sizing. This results in using a VFD that is able to power a motor 2 or 3 times the size of the motor you have. NEC 430, 409 and UL 508A do not specify enough for a clean answer. If anyone has a recommendation of how they do it please respond.

 

[Jraef]

... This results in using a VFD that is able to power a motor 2 or 3 times the size of the motor you have. ...

Wow, someone really has you or your buyers bamboozled! HD sizing of VFDs generally means one HP size larger than the "Normal Duty" (Variable Torque) rating and in fact for most drives 10HP and under there is only HD sizing.

There is no real conflict with UL, NFPA 79 and the NEC. You must be misinterpreting something here. Can you give an example?

 

[petersonra]

Hi, Regarding FLA calculations for industrial controls panels - would you calculate this based off your motor size or the max ability of your VFD. The reason i ask is because many plants require you to use "heavy duty" VFD sizing. This results in using a VFD that is able to power a motor 2 or 3 times the size of the motor you have. NEC 430, 409 and UL 508A do not specify enough for a clean answer. If anyone has a recommendation of how they do it please respond.

UL508a has a very clean description of how to calculate the FLA of the panel. If you have specific questions about it, feel free to ask but it is pretty straightforward. NEC motor feeder calculations do not require a FLA calculation, only an ampacity calculation (wire size).

Like Jraef said, a HD VFD is usually a size bigger so it can carry overload current for a little longer. I have never seen one sized 2 or 3 X.

 

[kwired]

I can see people specifying a size larger for 10 hp and below, I don't see many bean counters approving much increase for a 100 hp drive or larger - especially if it is going to be 2-3 times larger sized.

 

[jsmyth]

Wow, someone really has you or your buyers bamboozled! HD sizing of VFDs generally means one HP size larger than the "Normal Duty" (Variable Torque) rating and in fact for most drives 10HP and under there is only HD sizing.

There is no real conflict with UL, NFPA 79 and the NEC. You must be misinterpreting something here. Can you give an example?

A certain unnamed automobile manufacture required a 3hp VFD for a 1hp motor. It made no sense, but they what they wanted and paid for.

 

[petersonra]

A certain unnamed automobile manufacture required a 3hp VFD for a 1hp motor. It made no sense, but they what they wanted and paid for.

I have not seen that but it is not unusual to see a spec requirement to use nothing smaller than size 1 starters. I have also seen specs that do not allow fractional HP VFDs.

I also have seen specs that allow no single phase motors, regardless of size. Try to find a 1/20 HP 3 phase 480V motor sometime.

 

[Jraef]

A certain unnamed automobile manufacture required a 3hp VFD for a 1hp motor. It made no sense, but they what they wanted and paid for.

Sometimes you have to do this when you are using the VFD as a way to convert from a single phase service to run a 3 phase motor, something that VFDs are inherently able to do. So with that in mind, often times user will specify that oversizing for critical equipment so that if there is a phase LOSS on a 3 phase system, the VFD can keep humming along feeding the motor with 3 phase, none the wiser. The other possibility is just so that all small drives are exactly the same, regardless of what dive the motor is. It cuts down on spares inventory, and the difference in cost between a 1HP and a 3HP drive is worth it for that reason.

Most VFD mfrs will tell you not to exceed the motor FLA by more than 2x, mostly because of the accuracy resolution of the current sensors. But under about 12A, that becomes irrelevant because the sensor ranges only go down so far anyway.

As to how that affects your nameplate, if you are building the panel, forget the NEC articles, they are about installation, not construction. UL508A and NFPA 79 are the operative standards for that, and in both cases your nameplate must reflect the total maximum input power rating. So if your VFDs are all 3HP, that's what you base it on, not the motors behind them. The motors can change, and especially if the user looks at the ratings of the VFDs.