VFDs and Code Logic

[RussWatters]

Hi guys, first time poster. I'm an HVAC/mechanical engineer and my question relates to a VFD/motor sizing mismatch issue and how the NEC's logic (or lack thereof?) applies.

The issue is with a fan replacement utilizing an existing VFD. I decided the existing fan motor didn't have enough safety factor from the original design and bumped it from 50 HP to 60 HP. An apparent miscommunication between me and the electrical engineer resulted in them reporting back to me that the existing motor and VFD were 60 HP. We are resolving the issue by replacing the VFD (and all associated wiring, breaker, etc.).

It's more complicated than that, but close enough...

My question relates to the logic for matching VFD and motor sizes. My understanding is the VFD has to be rated for at least the horsepower of the motor, but can't be rated for less. Given that the existing system is functioning fine and drawing less than 50hp, just installing a 60hp motor and fan wouldn't actually change anything about the operation and the motor doesn't know its own size: it just spins and the load applies a torque to it. It appears to me that from a practical standpoint, nothing has changed.

Because motors are dumb, my usual concerns involve accidental over-amps. Wrong sheave size, turning on a fan without a duct attached -- ironically, too much static pressure safety factor -- accidental overspeed of a direct drive fan with a normal operating speed below 60 hz, etc. The conditions I put the motor on are what determine the break horsepower and therefore amp draw. The motor's actual size has very little to do with that: it either provides what the fan demands or fails trying (by its protection device).

The code says, so we do, but am I missing something in the logic? Is the code requirement here simply a matter of simplicity/consistency as opposed to combating an actual safety concern?

 

[iwire]

So you think it is illogical that the NEC requires a VFD to be sized at least for the load connected to it?

I am sorry but to me you are the one being illogical.

 

[GoldDigger]

Just be sure to tell the VFD itself that a smaller motor is connected to it. It will then do its secondary job of protecting the motor from overloads and short circuits. The circuit breaker no longer has any responsibility for that. Is now protecting a 60HP VFD, not a 50HP motor.

Sent from my XT1585 using Tapatalk

 

[ActionDave]

A lightly loaded motor will draw less than its rated amps but a loaded 60hp motor will draw more amps than a 50hp.

 

[RussWatters]

So you think it is illogical that the NEC requires a VFD to be sized at least for the load connected to it?

I'm really not trying to be dense here: I presume the "load" is defined as the nameplate rating on the motor. What I am suggesting is that, unlike a light bulb, the *actual* amp draw of a motor does not necessarily match the amperage listed on the motor. In this case, I see the VFD performing the same function as a governor on a car.


Again, I'm not trying to be dense: I am honestly asking what the actual harm could be. I'm not seeing it.

We can probably set this aside if you could address my question directly, but on a related note I would like to point out that the NEC lists required amperages for motor circuits that don't actually match the motor amperages of the real motors. They appear to be based on the assumption that you *might* use an old, inefficient motor on a circuit or, rather, that you are designing a circuit without knowing the motor FLA. For example (in this case), a premium efficient 60 HP motor has an FLA of ~70 but the circuit must be designed assuming you will install a motor with an FLA of 77.

There appears to me to be a difference in code or design philosophy between mechanical and electrical in that electrical forces you into assumptions that don't match operating conditions whereas mechanical (in general) requires you to design around actual operating conditions.

 

[RussWatters]

Just be sure to tell the VFD itself that a smaller motor is connected to it. It will then do its secondary job of protecting the motor from overloads and short circuits. The circuit breaker no longer has any responsibility for that. Is now protecting a 60HP VFD, not a 50HP motor.

Other way around. I (would, hypothetically) have a 50HP VFD protecting a 60HP motor. In essence, it means the VFD will limit the motor to well below its rated FLA. The circuit breaker would still protect the VFD as a 50HP VFD. The two practical impacts I see are:
1. The motor will be more lightly loaded and thus run a little less efficiently.
2. The motor will be more lightly loaded and thus last longer.


[by the way, for some reason my space bar isn't working in here. I'm having to type these messages in a text editor and then copy and paste them...]

 

[iwire]

Putting aside the NEC for a moment have you consulted the manufacturer of a VFD to see if they will warranty a 50 HP VFD running a 60 HP motor?

Going back to the NEC you would also have issues with 110.3(B).

 

[topgone]

Other way around. I (would, hypothetically) have a 50HP VFD protecting a 60HP motor. In essence, it means the VFD will limit the motor to well below its rated FLA. The circuit breaker would still protect the VFD as a 50HP VFD. The two practical impacts I see are:
1. The motor will be more lightly loaded and thus run a little less efficiently.
2. The motor will be more lightly loaded and thus last longer.


[by the way, for some reason my space bar isn't working in here. I'm having to type these messages in a text editor and then copy and paste them...]

What @GD said, your circuit breaker should be sized ccording to the 50HP VFD. Since you're load is a 60HP motor (VFD is 83% of motor rating), you will overload your 50HP VFD if you run your motor beyond 83% of full speed! 83% speed limit on the motor is even too generous a figure knowing that the 60Hp motor draws more magnetizing current compared to a 50HP. Besides, the next person who will be tinkering with that setup may think the VFD is rated 60HP and reconfigure the VFD setting differently.
It is technically possible to run that arrangement of yours but I think you will be violating NEC rules regarding sizing of VFD to load.

 

[RussWatters]

Putting aside the NEC for a moment have you consulted the manufacturer of a VFD to see if they will warranty a 50 HP VFD running a 60 HP motor?

I have, but at this point it was with a bit of a hypothetical: I got a confusingly worded email from my client that implied the 60HP motor has an FLA of 65, which would be below the FLA of the VFD, so I posed that question to the VFD vendor: does it matter what the nameplate HP is or just the FLA? He said just the FLA. In other words, regardless of what the catalog size of the drive and motor are, as long as the FLA of the motor is lower than the FLA of the drive, it will be fine.

...but the caveat is that the 65A FLA doesn't seem possible and I have asked for a photo of the motor nameplate. The fan submittal does not include the FLA, but quotes the NEC's table of 77A, which also can't possibly be correct. So the question I asked may not be exactly the right question.

Going back to the NEC you would also have issues with 110.3(B).

Of course. But I've already said we are going to follow the code and replace the drive. What I am asking for is help in understanding its logic. I'm not the type of person who is satisfied by a "do as I say" explanation; I'd like to understand why.

 

[RussWatters]

What @GD said, your circuit breaker should be sized according to the 50HP VFD.

Right, that's what I said.

Since you're load is a 60HP motor (VFD is 83% of motor rating), you will overload your 50HP VFD if you run your motor beyond 83% of full speed! 83% speed limit on the motor is even too generous a figure knowing that the 60Hp motor draws more magnetizing current compared to a 50HP.

Well, not quite. This is an HVAC fan so it is not a constant torque application. The actual speed vs hp can vary with the control scheme, but for an airflow controlled system, 94% of maximum speed and airflow yields 83% power. I suppose with power factor, the amperage might be higher...

...but now we're getting somewhere: Is magnetizing current the same as inrush current or is it different (I thought inrush was from high torque for acceleration?)? What you are saying is that when you start it up, a 60 hp motor will draw more "magnetizing current" than a 50 hp motor connected to the same load (same rpm, acceleration, torque, etc)? What is the reason? Is it because it has physically larger wires and/or coils and therefore the magnetic fields hold more energy? How significant is that? Can it be mitigated by reducing the acceleration?

Besides, the next person who will be tinkering with that setup may think the VFD is rated 60HP and reconfigure the VFD setting differently.

Right; which is pretty much how we got here in the first place. If the NEC logic is nothing more complicated than K.I.S.S., I can (kind of) accept that.

 

[Jraef]

Per your question, the VFD is sized by the amps only. HP is meaningless in the VFD world. Case in point, a VFD rated 50HP for a Constant Torque machine is capable of running a 60HP Variable Torque machine; there is ZERO difference in the hardware. The only difference in in the overload capacity of the drive, and in telling it (via programming) that the load it VT, the drive will lower its current limit functions accordingly; from 150% for 60 seconds to 110% for 30 seconds. Those, and a change to the output being V/Hz squared instead of V/Hz, are the only differences. In the past, UL didn't acknowledge this use, but more recently has embraced it as an alternate listing.

One thing you may need to do is to ensure in programming that you have accomplished those changes, if not, you can do them manually. If so, look through the programming to find the way the drive responds to an overload or over torque condition. What you want to do is to set it up for reducing Hz to limit current rather than trip off line, which will likely be the default setting. In a VT application, that is effectively limiting flow and thus load, effectively making it a 50HP drive and motor as you surmised. I have used this technique to use a 10HP VFD to turn a 100HP motor for testing. Works fine albeit limited to essentially 10HP of loading, which would make a 100HP motor a big waste of money.

 

[petersonra]

this is the way it is.

what you want to do will work fine.

it may not be code.

as jraef mentioned some VFDs have dual HP ratings that might accommodate what you want to do in this case. If not, what you want to do does not meet code even though it will work fine.

There are a lot of things that are perfectly safe but are not to code.

 

[Ingenieur]

....
The issue is with a fan replacement utilizing an existing VFD.

I decided the existing fan motor didn't have enough safety factor from the original design and bumped it from 50 HP to 60 HP.

An apparent miscommunication between me and the electrical engineer resulted in them reporting back to me that the existing motor and VFD were 60 HP.

We are resolving the issue by replacing the VFD (and all associated wiring, breaker, etc.)...

original existing in operation motor you thought was 50 needed increased to 60???
why? Was it running over nameplate? Did you change the fan or only the motor?
you also thought the vfd matched at 50 also? But the existing is actually a 60?

So both existing are actually 60? (As reported by the EE)

Then why is anything being changed?
existing motor 60 (you thought 50)
existing vfd is 60
???

what are you actually changing?
what size is the new motor and vfd?

 

[adamscb]

I'm with previous posts, why are we upgrading a 50hp to a 60hp? Was the motor drawing too much current, overheating, tripping on overload, etc? In my mind if everything is sized properly, and it's not tripping, then it's already safe, and no additional safety factors should be included.

 

[adamscb]

There appears to me to be a difference in code or design philosophy between mechanical and electrical in that electrical forces you into assumptions that don't match operating conditions whereas mechanical (in general) requires you to design around actual operating conditions.

And before you continue to bash the NEC, the only "assumption that doesn't match operating conditions" is your desire to upgrade to a 60hp because of this magical 'safety factor', unless like I said in my previous post it was tripping on overload or overheating. As long as it's running under FLA, then the motor is fine. In fact, it could probably run over nameplate FLA for short periods of time.

The electrical engineer is doing his job in saying everything should be up-sized. If he didn't, that would be cause for concern.