NEMA "F" Class Insulation Motor Wired for low voltage and VFD use

[RobsVette]

Hey guys,

at a project I'm doing now I have a NEMA class F motor that is able to be wired for 208V and 460V. Currently I have it wound at 208V and I would like to apply a VFD to this motor. Although the motor may not be specifically rated for inverter duty, I would think that I should be okay considering that the VFD is less than 5 ft from the motor and that I have the motor wired at 208 instead of 460 and the insulation on the motor should be good for 600 V.

I am thinking that the max DC voltage the drive would produce would be around 360V which should be fine since the motor can handle 480V.

I realize there are many other items that are different in a motor that is inverter duty rated, but I feel given the low voltage that is being applied I dont think it would be a problem.

Has anyone had any experience doing something like this?

Any insight would be appreciated.

Thanks,
Rob

 

[waross]

What size is the motor? How much will it cost to replace if you are wrong?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wayne440]

"How much will it cost to replace if you are wrong?"

With all due respect, that question is one of the first that should be considered when "I think" and "should be" are the basis for engineering decisions. I'm not an expert by any means, but a 208V motor on a 360V supply would have me checking the books for money to replace same in the near future, even if it did somehow initially work OK.

 

[ScusaMe]

wayne440:

I don't think he's saying the "supply" is 360v.
I believe he's anticipating that the DC bus in the
VFD will be 360VDC. How he got to that number is a mystery to me.
208vac would rectify to a nominal 1.378 x 208 = 287VDC ...​

So lets see if RobsVette will grace us with some additional—albeit pertinent—information.

Here are the questions, the answers to which will be most helpful in coming up with an answer for you.

[ol]
[li]AC supply voltage for VFD... 208v or 460v ?[/li]
[li]If 208v.... single or three phase ?[/li]
[li]What is motor rating (HP, Amps, Rpm[/li]
[li]What is motor enclosure type - ODP (Open DripProof)? TEFC (Totally Enclosed Fan Cooled ? TENV (Totally Enclosed Non-Ventilated ? etc.[/li]
[li]From motor nameplate... what is the Service Factor (S.F.) ?[/li]
[li]Is the motor Brand New ? or Used ?[/li]
[li]How old is the motor (approximate age...i.e. 1-5 years, 5-10 years, etc ?[/li]
[li]What is the application ... what is motor connected to— and driving ?[/li]
[li]What is ambient temperature in which motor will be installed ?[/li]
[li]What is altitude where motor will be installed ?[/li]
[li]What is the turn-down speed range over which the motor will be operated (i.e., max rpm .... min. rpm) ?[/li]
[/ol]
Remember, the more information you can give us to consider, the easier it is to come up with a reliable answer for you.

Cheers.

p.s. you could, of course, contact the motor manufacture and get your answers from the 'horses mouth' ...

 

[jraef]

I've had lots of experience with this. If you are looking for absolutes, then dream on, there are none. Your assessment of the risk factors is generally correct, they are low. That's the best hope I can offer you, and I think that waross' suggestion of a cost / benefit analysis against prophylactic replacement with an inverter duty motor is very apropos. But here are potential pitfalls that I can think of off the top of my head.
[ol 1]
Watch out for speed / heat issues. In non-inverter duty motors, the cooling system for the motor is designed around full speed. As you slow it down you are reducing the power through the circuit so that helps, but you are also reducing the effectiveness of that cooling system at a faster rate so at some point there is a crossover with the reduced power consumption to where the motor over heats regardless. No way to know for sure, but a generally safe rule is no more than a 4:1 speed turn-down ratio.
[/ol]
[ol 1]
There is another phenomenon regarding bearing damage from induced voltage jumping from the rotor to the frame across the bearing races and causing an EDM effect, pitting and eventually fluting the bearing races. Inverter duty motors now often have shaft grounding capabilities to help prevent this, non-inverter motor never do.
[/ol]
[ol 1]
You say that your motor can be connected 208 or 460V, that's not likely really true. Motor voltages in the US are a 2:1 ratio +- 10%, so that motor is designed for 230/460V. But then with a little more designed-in tolerance, really build it for 220/440V +15 - 10%, which gives you a range of 198 - 506V. But that doesn't really cover the 208V - 10%, it just covers 200V, the accepted "Utilization Voltage" for a 208V system. But recognize that this is 200V minus almost nothing, so a -10% input voltage swing is going to result in a torque loss that the VFD cannot compensate for and cause an overload. Adjust your VFD protection programming accordingly.
[/ol]
[ol 1]
A load reactor would be prudent to help protect the motor even with only 5ft motor lead length. Call it cheap insurance.
[/ol]

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Hi Jeff. In recent years I have been seeing more "triple rated" motors.
200/230/460V or 208/230/460V. I recommend triple rated motors for rural applications where there are large voltage swings and chronic low voltages at times of peak usage. They do make a difference.
Given that 120/208V is a common voltage in supermarkets in North America, I see a lot of triple rated motors in refrigeration equipment.
I am not privy to design details but I imagine that the motors are designed for adequate torque at 208V and low enough flux density that 230V will not saturate the iron.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Maybe I need to update my motor information then. It's been a while (maybe 15 years?) since I've done a deep dive on this issue.

Now if only I can find the time...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Did a little research on the Grainger site.
Some of these single phase motors are rated 115/208-230 volts.

http://www.grainger.com/Grainger/ca.../ecatalog/N-9xr?Ndr=basedimid10071&sst=subset

Some of these three phase motors are rated 208-230/460 Volts.
Baldor lists 208-230 Volt motors also.
I have seen 200-230 Volt ratings in the field but I can't locate any online today.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RobsVette]

Hey guys,

Thanks for all the reponses and help on this issue.

From everyones feedback together, I have come up with what I intend to do. However, to address some of the concers above here is answers to all the questions.

- On the max DC bus voltage of 360VDC that was me being dumb and taking the 208V AC RMS and dividing by squrt of 3 as opposed to squrt of 2. In the future I should proofread a little better.

For all the questions above, here goes

1.AC supply voltage for VFD... 208v or 460v ? - 208VAC IS THE SUPPLY TO VFD
2.If 208v.... single or three phase ? THREE PHASE
3.What is motor rating (HP, Amps, Rpm) 3HP, 3600RPM, DONT RECALL THE AMPS AND DONT HAVE A CALCULATOR OR NEC HANDY
4.What is motor enclosure type - ODP (Open DripProof)? TEFC (Totally Enclosed Fan Cooled ? TENV (Totally Enclosed Non-Ventilated ? etc.) TEFC IF I REMEMBER
5.From motor nameplate... what is the Service Factor (S.F.) ? SERVICE FACTOR IS 1.15
6.Is the motor Brand New ? or Used ? MOTOR IS BRAND NEW.
7.How old is the motor (approximate age...i.e. 1-5 years, 5-10 years, etc ? INSTALLED ABOUT 4 MONTHS AGO
8.What is the application ... what is motor connected to— and driving ? MOTOR IS DRIVING A DOMESTIC HOT WATER PUMP THAT SOMEONE SIZED INCORRECTLY AND I NEED TO SLOW DOWN TO ABOUT 30 HZ
9.What is ambient temperature in which motor will be installed ? IT IS INSTALLED IN A MECHANICAL ROOM SO IT IS AN UNCONDITIONED SPACE. I THINK THE MECHANICAL ROOM TEMP IS ABOUT 95F ON THE BAD DAYS.
10.What is altitude where motor will be installed ? INSTALLATION IS IN NYC SO SEA LEVEL
11.What is the turn-down speed range over which the motor will be operated (i.e., max rpm .... min. rpm) ? THE MOTOR WILL OPERATE AT A SINGLE SPEED ONCE SET, PROBABLY 30HZ BUT POSSIBLY LOWER.

Based on what I have read here I am going to call the manufactorer (dont recall who it is off the top of my head) but with all the information from here I at least have the right questions to ask regarding the cooling system design and a bunch of others.

Thanks again for all the feedback. Please feel free to add anything else based on my answers above.
Rob

 

[waross]

It may be a lot cheaper to just trim the pump impeller.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I would think trimming would not only be cheaper to impliment, but cheaper to operate as well.

I'm curious where that 1.378 factor came from.

 

[jraef]

Ditto on the trimming. VFDs are good for VARYING the speed in a process, but if you are going to operate at a FIXED speed, albeit less than full, then the VFD is going to add unnecessary losses and risks to the system. Alter the pump to match the task, don't do this with a VFD, you will be disappointed.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[RobsVette]

Hey guys, I spoke to baldor Reliance today and they confirmed that the motor I have is Inverter rated and able to be used at varying speeds.

I think based on the concensus feedback from here, I will use the VFD until I figure the proper size for teh motor and then replace the pump with one that fits the application better.

Thanks for all the help.

Rob