Jump to content


Photo

Vfd Sizing


  • Please log in to reply
6 replies to this topic

#1 mbouknight

mbouknight

  • Members
  • Pip
  • 1 posts

Posted 20 July 2010 - 07:11 PM

How do I find the Full Load Amps for a 380 V motor, 3-phase, 200 HP, 1.15 service factor in order to correctly size a VFD?



#2 jOmega

jOmega

    Senior Member

  • Full Member
  • PipPipPipPipPip
  • 254 posts
  • Gender:Male
  • Location:Midwest, USA

Posted 28 July 2010 - 06:30 AM

QUOTE (mbouknight @ Jul 20 2010, 01:11 PM) <{POST_SNAPBACK}>
How do I find the Full Load Amps for a 380 V motor, 3-phase, 200 HP, 1.15 service factor in order to correctly size a VFD?



mbouknight -

Is it the case that the motor is an older design and no longer has its Rating Plate attached to it ?

Advise ..




#3 GreenDriv

GreenDriv

    Junior Member

  • Full Member
  • PipPip
  • 5 posts
  • Gender:Male
  • Location:China
  • Interests:travel, reading, listening to music

Posted 10 February 2011 - 01:37 PM

QUOTE (mbouknight @ Jul 21 2010, 03:11 AM) <{POST_SNAPBACK}>
How do I find the Full Load Amps for a 380 V motor, 3-phase, 200 HP, 1.15 service factor in order to correctly size a VFD?



I think you can refer to the name plate of the motor. When you want to apply the VFD, just remember that the power of VFD should be equal or larger than that of motor. That's enough.
Manufacturing & offering variable speed drives, frequency inverters, closed loop AC drives...

Let's share and communicate!

#4 KJF

KJF

  • Members
  • Pip
  • 1 posts

Posted 01 March 2011 - 03:20 AM

The motor information you give is confusing. 380 V implies that this is a European motor rated for a 380v, 50 Hz system. If so, the nameplate rating would be in kw not horsepower there would be no service factor. European motors are design with a sf of 1.0

If the motor is a 480V, 60 Hz, 200 hp motor with a 1.15 sf and the nameplate FLA is not available, the conservative approach is to size the VFD drive current capacity from the FLA table in NFPA 70 (NEC). When applying this motor in a VFD application, the s.f. must now be assumed to be 1.0 since the additional heating from the VFD waveform will consume all the additional heat capacity the of the 1.15 s.f.

Other factors that may require consideration is the driven load. If this is a constant torque application such as a compressor or positive displacement pump, a higher thermal capacity drive is require since the drive may have to produce near FLA at low voltages. The VFD vendors can make recommendations.
One other caution is that if you need more than a 5:1 speed reduction, you may need to provide additional cooling for the motor. A TEFC motor looses most of the fan effect as the speed drops. As you may expect, this is even more important on constant torque loads.

If this is indeed 380V, 50 Hz motor, let me know.

#5 jOmega

jOmega

    Senior Member

  • Full Member
  • PipPipPipPipPip
  • 254 posts
  • Gender:Male
  • Location:Midwest, USA

Posted 01 March 2011 - 03:51 PM

QUOTE (GreenDriv @ Feb 10 2011, 07:37 AM) <{POST_SNAPBACK}>
I think you can refer to the name plate of the motor. When you want to apply the VFD, just remember that the power of VFD should be equal or larger than that of motor. That's enough.



GreenDriv ..... and when the nameplate (rating plate) is no longer attached to the motor or is missing or isn't able to be viewed .......

what's your advice for determining the motor FLA so that the VFD can be properly selected for the motor...?????


FYI - All VFD's are current rated; ......... power is a relative term.. The amps that a VFD can continuously source to its connected load..... is the guiding factor.

As an example, consider a 460 vac motor rated at 10 HP .... Would you specify a 10 HP VFD (rated at a nominal 14 amps continuous at 40 deg. C.)? Pretty Standard, right ?

So after shipping that VFD off to the customer and proper installation, you start getting phone calls saying the drive is tripping ....

How could that be? You supplied a standard 10 HP VFD for his 10 HP motor application, right ?

Well, when you dig into it a bit further .... turns out that 10 HP motor is an 8-pole machine in a 284T frame, drawing a rated FLA of 16 amps ...(Marathon Elect. Cat. No. 453) ....

So, it looks like that "standard" 10 HP, 14 Amp VFD wasn't a good choice. Probably should have supplied him a 15 HP VFD rated at a nominal 21 amps.

The point is ... that selecting a VFD on the basis of HP alone can... and will... get one into trouble. HP is a relative term. NFPA-70 table is not an absolute either.

ACTUAL MOTOR NAMEPLATE AMPS is the important parameter when selecting a VFD rating.


#6 jOmega

jOmega

    Senior Member

  • Full Member
  • PipPipPipPipPip
  • 254 posts
  • Gender:Male
  • Location:Midwest, USA

Posted 01 March 2011 - 06:57 PM

QUOTE (KJF @ Feb 28 2011, 09:20 PM) <{POST_SNAPBACK}>
The motor information you give is confusing. 380 V implies that this is a European motor rated for a 380v, 50 Hz system. If so, the nameplate rating would be in kw not horsepower there would be no service factor. European motors are design with a sf of 1.0

If the motor is a 480V, 60 Hz, 200 hp motor with a 1.15 sf and the nameplate FLA is not available, the conservative approach is to size the VFD drive current capacity from the FLA table in NFPA 70 (NEC). When applying this motor in a VFD application, the s.f. must now be assumed to be 1.0 since the additional heating from the VFD waveform will consume all the additional heat capacity the of the 1.15 s.f.

Other factors that may require consideration is the driven load. If this is a constant torque application such as a compressor or positive displacement pump, a higher thermal capacity drive is require since the drive may have to produce near FLA at low voltages. The VFD vendors can make recommendations.
One other caution is that if you need more than a 5:1 speed reduction, you may need to provide additional cooling for the motor. A TEFC motor looses most of the fan effect as the speed drops. As you may expect, this is even more important on constant torque loads.

If this is indeed 380V, 50 Hz motor, let me know.





KJF.....

With regard to your comment about S.F. and additional heating as a consequence of the VFD output waveform; that was so back in the 1980s with VVi (six-step) and CSI (current source) schemes the output of which were rich in harmonics.....but the technology of VFDs and motors has improved many times over in the last 30 years.

Today's VFDs contribute very little (negligible) heat to the motor as a consequence of their vastly improved output waveform.

That statement is made as a result of actual test data where a series of motors and VFDs were dynamometer tested on both sine wave power and VFD power and comparing the temperature differences.

As a consequence of the testing and improvement to process and materials that motor manufacturers have done, today they offer motors that are 1.15 S.F. at Class F rise and 1.0 S.F. at Class B rise.

So when considering the VFDs of today's design and technology, the issue of their contributing additional heating into the motor so as to reduce Service Factor capability is..... a non-issue.


I also must take issue with your statements:
"Other factors that may require consideration is the driven load. If this is a constant torque application such as a compressor or positive displacement pump, a higher thermal capacity drive is require since the drive may have to produce near FLA at low voltages. The VFD vendors can make recommendations."

VFDs are designed to operate continuously at their published/nameplated ratings.
That means that a 10 HP - 460vac - 14 Amp - 40 deg. C VFD can operate continuously ...... in a 40 deg. C ambient environment, supplying 14 amperes without any derating as a consequence of what the load is doing .... as long as.... the load doesn't cause the motor to exceed the ratings of the VFD and the environment is within the rating of the VFD.

VFDs are designed to operate at their ratings..... continuously. They can operate at rated output current at 10 v output just as well as at full rated voltage output without eclipsing the thermal rating of the VFD.

It matters not whether the load is a compressor, a conveyor, a fan, a positive displacement pump, an extruder, etc., the thermal capacity of the VFD is NOT affected.

The factors that influence thermal properties of a VFD are : Environmental (ambient temperature, altitude, etc) .... and operating continuously overloaded. (i.e., outside their design parameters)


Kind regards,

#7 Bysikmode

Bysikmode

    Intermediate Member

  • Full Member
  • PipPipPip
  • 11 posts

Posted 20 April 2014 - 01:16 PM

This posts is very informative. Thank you!



 


 





0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users