Motor

Dear JRaef

Thank you for your reply

Samsung

<...No, there is no reason why an inverter duty motor cannot ....>

All those negatives. So is that a very weak 'yes', or what?

Sooooooo confusing....

Yes, that is a no.

I agree with JRaef. Furthermore, pump motors, especially on centrifs, are usually sized with plenty of margin (sometimes even for end-of-curve), so you should not get anywhere near SF. Have you asked your electrical engineer why he needs 1.15?

i am perplexed why you want to use a VFD motor for what is really a simple squirrel cage application....as JRaef says, it seems an overkill.

Maybe a simple motor (SF 1 or 1.15 is a moot point) will do the job ? Maybe the vendor is desperate to meet his top/bottom line ?

I don't know if a SF is specifically required by the electric code where you are (or where I am for that matter), but one possibility for the EE to require a SF of 1.15 might not have to do with the motor, but the motor protective devices.

For example, if the breaker spec'd is already near it's low end and the SF is reduced from 1.15 to 1.0, then the breaker would have to adjusted to the lower value. If it is already near the bottom end, it might not be able to be adjusted to the new lower value and thus not provide the protection the motor requires.

I'm not an electrical engineer (and those who are please chime in!)...

You are incorrect about this, at least as far as North America is concerned (and outside of North America, SF doesn't exist). SF is not to be used in the selection of circuit protective devices. Circuit breakers and / or fuses are required to be selected from tables in the National Electric Code based upon motor HP and voltage. Overload relays / heater elements are to be selected based on motor nameplate FLA, with allowances for SF only if it is used continuously; which as I mentioned is a bad idea.

ok... first understand that I am just trying to understand. That being said, I broke out the NEC to learn a bit more and found the following with regards to motor protective devices:

"A separate overload device that is responsive to motor current. This device shall be selected to trip or rated at no more than the following percent of the motor nameplate full-load current rating." ... and then goes on to list the various motor markings (with SF) and the applicable percent rating. (i.e. motors with a marked service factor not less than 1.15 125%, and so on)

As I said, I'm not an EE. But this says to me that if my nameplate reads a full load amperage of 100A and has a SF of 1.15, then the breaker can be sized/adjusted for 125A. Again, I'm not trying to argue just understand.

You are reading the section on overload relays and applying it to circuit breakers. Read the section on circuit breakers, 430.52 I think.

Thanks for pointing me in the right direction. And you are in fact correct. In addition to researching the NEC a little more I took the liberty of discussing this with a friend who is an EE. His take on it was this...

(paraphrased) Most EE's will spec a motor based on non-overloading conditions over the entire operating range of the attached equipment and in doing such the SF makes no difference and doesn't come into any consideration for sizing anything. The SF will only make a difference if the system has been designed to operate in an overload condition (operating above it's designed output), which is a non-conservative approach.

I think this is pretty much the sum of what you have said earlier as well.

In regards to the OP this would say to me (as obvious as this sounds) that if the engineer has required a SF the best thing to do is to adhere to it.

In my opinion there is still a little doubt. The opening question was

• 1) if a VFD rated motor is suitable for a normal application without a VFD.
• 2) the motor will be used for a pump

In the past I have seen application where a motor was used with a frequency drive. But the output frequency of the motor was in the range 10 tot 87 Hz.

The motor was 230/400 Volt Delta/Wye 50Hz and was connected in 230V Delta

The frequency drive was connected to 400V. At 87 Hz the output voltage of the VFD was 400V.

This application went fine, but I would advice the topic opener the verify if the required speed of the motor and the supply voltage will be ok.

Last point, it is possible that the motor will match the speed and voltage. But for a smooth start and stop the pump vendor perhaps needs a VFD.

One thing to be aware of is inverter duty motors pull more current at start-up in "across the line" applications then standard motors. So starters should be sized accordingly.

Shawn

I've never seen this. But what you may have observed is that new higher efficiency motors, inverter duty or not, do indeed have higher magnetizing current than older designs, so they tend to cause nuisance tripping of instantaneous trips of circuit breakers. This is well documented and has been addressed (here in the US) in the more recent revisions of the NEC that now allow for upwards of 1700% setting for instantaneous trips under an exception. but one would not change the size of the motor starter.

The term "Inverter Duty" for motors has been unfortunately ill defined in the past and that lead to a lot of abuse and ultimately confusion in the market place. In the early days of widespread inverter use, a few manufacturers capitalized on the lack of definition and did shady things such as just re-label larger motors as "Inverter Duty" for a smaller HP rating; i.e. a 15HP standard motor became a 10HP "inverter rated" motor. That lead to some people figuring it out and over sizing protection devices and starters etc. But this was at its core a flawed design philosophy because thermal issues turned out to be a lesser problem than voltage issues, so it only served to add to the confusion.

What "Inverter Duty" means is now much more narrowly defined in the NEMA MG-1 Part 31 design specifications and most reputable manufacturers adhere to that. In a nutshell it means just higher insulation values on the windings. Thermal insulation values, external cooling for low speed operation, shaft grounding etc. etc. are all suggested, but not technically part of the specs. All are good ideas, so anyone thinking of using a motor on an inverter is advised to be very specific and clear as to what issues you want addressed, otherwise bottom feeder suppliers will only give you the minimum.

I'm not sure where you are located or your vendor's name, but I do know at least one US vendor that dual rates the same motor 1.0 SF on VFD power and 1.15 on Sine Wave Power...

It might be worth looking into whether your particular vendor does that as well.

Wow, it started as a simple pump motor, and we have all learnt quite a lot about Nema and SF and VFD... that is the beauty of CR4. Lots of collateral learning.

i wonder whether the OP is still watching and learning. i have no clue why (1) someone thought my post was OT and (2) why did the OP not answer.

Well, that's life.

Yes, I agree. There are many threads in CR4 that make the readers more competent in the profession with so much other related engineering and technical information. It is a great forum. For myself, I learned much more than I did contribute.

Whoever thought your comment # 4 is "Off Topic" can see it now – I voted for taking it off.

- MS

I think I may have made a mistake and thought I was voting for GA, but had a last second brain fart and clicked the wrong radio button. I'm not sure if this was the one I did that on, but I definitely did that at some point this week.

It doesn't let you "un-click" or vote again to cancel it out.

VFD suiatble motor can be used for non VFD application . you can very well use the motor with no harm .

Second regarding SF 1.15 for pump application why SF of 1.15 is required . plz check up what is operating range of pump in terms of input KW required & you can use same motor without any hitch .

From a motor manufacturer's perspective, they are including the inverter duty wire and insulation because this is, in all likelihood, an off the shelf motor. For this reason, it must be suitable for a variety of applications.

It doesn't hurt to have it though since you may decide in the future that you want to retrofit a drive to this application and the motor will be ready to handle it!

Dear samsung,

The answer, for your question, technically, is YES.

DHAYANANDHAN.S