How VFDs save energy.docx

7/27/2019 How VFDs save energy.docx

1/13

http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html

How VFDs save energyAnalysis and examples of power conversion b y v ariab le-

frequency dr ives.

By Edward Tom, Yaskawa America, Inc.

-+

Page 1 of 2

Prev 1 |2NextView on one page

Variable frequency drives (VFD) are becoming more common place and more widelyused in applications. They are capable of varying the output speed of a motorwithout the need for mechanical pulleys, thus reducing the number of mechanical

components and overall maintenance. But the biggest advantage that a VFD has isthe ability to save the user money through its inherit nature to save energy byconsuming only the power thats needed. The main question now is, How does aVFD accomplish this? The simple answer to this question is power conversion.

A VFD is similar to the motor to which its attached, they both convert power to ausable form. In the case of an induction motor, the electrical power supplied to it isconverted to mechanical power through the rotation of the motors rotor and thetorque that it produces through motor slip. A VFD, on the other hand, will convert itsincoming power, a fixed voltage and frequency, to a variable voltage and frequency.This same concept is also the basis to vary the speed of the motor without the needof adjustable pulleys or gearing changes.

ADVERTISEMENT

White Paper: US Electrical Workplace Safety ComplianceFive to 10 times per day in the US, a worker is severely injured or killed in anelectrical arc flash accident. Other electrical incidents also can injure workers -accidental contact with energized parts for example. Electrical incidents can be

devastating to workers & the financial consequences of such events can be verydamaging to the company.Download

Electrical

Electrical power is defined as the following:

Power (P) = 3 x Voltage (V) x Current (I) x Power Factor (PF)

In an ideal VFD, the following would hold true:
http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://adserver.adtechus.com/adlink/5211/857992/0/976/AdId=4199952;BnId=1;itime=684025762;key=articles_2010_06VFDsSaveEnergy_html+motor+drive+energy+efficiency+power+green+vfd+variablefrequency_drives+power_conversion;nodecode=yes;link=http:/www.sereply.com/promo/get.cfm?keycode=j601vhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html


2/13

Powerin = Powerout

But because a VFD has inefficiencies and requires a small amount of powerconsumption to power the brains of the drive, the input power will be slightly greaterthan the output power. For this, we will assume that this extra power draw isnegligible.

With these two equations, we can then define the relationship between the VFDsinput and output:

Vin x Iin x PFin = Vout x Iout x PFout

Taking these equations into account, lets use a 100-hp motor as an example withthe following properties:

Power = 100 hpSpeed = 1,785 rpm

Voltage = 460 VFLA = 115 APower factor = 0.86

A VFD will convert its incoming power, a fixed voltage and frequency, to avariable voltage and frequency.

- Edward Tom , Yaskaw a America, Inc .

Assume that the motor is running at 60 Hz on a VFD, drawing a no-load current of

40 A on the output of the VFD. With this, one would assume that the input currentwould also be the same, 40 A. However, using an ammeter on the drives input, aperson is reading nearly zero amps! How is this possible? Is the drive creating powersomehow? The answer simply is no, the drive is not creating power. The powerfactor causes this discrepancy in current When a motoris running at no load, themotors power factor can be assumed to be zero, not 0.86 86 as stated on thenameplate. The reason the power factor isnt at 0.86 is because this is the motorpower factor at full load. Alternatively, mechanical (friction) and electrical (resistive)losses in the motor prevent the power factor from being zero when running no load,

but well assume these losses to be zero just like we did for the VFD. Therefore, youwould have the following:

Pout = 460V x 40A x 0Pout = 0

Because the output power is zero, the input power also will be zero. With a fixedinput voltage, the two variables would be current and power factor. Because currentis needed for a power factor to exist, both current and power factor are zero, whichmeans the low input current reading is indeed correct.


3/13

This explains why the input current to the VFD is so low when the motor is operatingunder no load conditions. But what about under load? The same concept still applieswhen the motor is under load. For example, assume the same motor is nowoperating at half speed, 30 Hz and producing full motor rated torque and drawing themotors full-load amps (FLA). This means that the electrical power that the motor isdrawing is:

Pout= 3 x 230V x 115A x 0.86 = 39.4kW

Because the VFD is a power converter, this means that the input current is(assuming a 0.89 input power factor from a 3% line impedance):

Iin= (39.4kW) / (3 x 460V x 0.89) = 55.6A

Because the VFD is operating at half speed and under full load, the input current isless than half of the output current.

Power (kW) Voltage (V) Current (A) Frequency (Hz) Power Factor

Input 39.4 460 55.6 60 0.89

Output 39.4 230 115 30 0.86

In this example, the input current is less than half of the output, a result of having ahigher power factor on the input side.

The difference in power factor between the input and output side of the motor is whatmakes it possible to have a higher output current than input current. Assuming the

motor is now running at full load and using the same power factor values, your inputcurrent now becomes:

Iin= (3 x 460V x 115A x 0.86) / (3 x 460V x 0.89) = 111A

which is 4 A lower than the output current.

If Then

PFin < PFout Iin < Iout

PFin = PFout Iin = Iout

PFin > PFout Iin > Iout

Mechanical

The current a VFD draws on the input side also can be related to the mechanicalpower a motor is delivering. The basic relationship for motor power is:

Page 1 of 2

Prev 1 |2NextView on one page
http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=2


4/13

Page 2 of 2

Prev1| 2 Next View on one page

Powermechanical Speed x Torque

This means that if the motor is operating at half the speed and producing full torque,the motor is outputting half of its rated power. Consequently, if the motor is runningat full speed and producing half torque, the motor is also outputting half of its ratedpower.\

Because of motor losses, the power relationship between the electrical power goinginto the motor and the mechanical power is:

PowerElectrical = (Powermechanical) / (EfficiencyMotor)

Revisiting the above example, if the motor is operating at 30 Hz, half the motorsrated speed and producing full torque, then the mechanical power being produced is50 hp. Assuming that the motor is 95% efficient, the electrical power thats requiredis:

PowerElectrical = (50HP x 0.746) / 0.95 = 39.3kW

which means that the current on the input side of the VFD will be approximately 55A. This same current will also hold true even if the motor is operating at full speedand producing half torque.

Table 1. Operating points for examples*

Motor speed (rpm) Motor torque (ft-lb) Motor power (hp) Input power (kW) Input current (A)

0 0 0 0 0

0 100 0 0 0

900 0 0 0 0

900 295 50 39.3 55

1800 0 0 0 0

1800 295 100 78.8 111

*Motor is rated for 100 hp with a rated torque of 295 ft-lb.
http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=1http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=1http://www.plantservices.com/articles/2010/?page=fullhttp://www.plantservices.com/articles/2010/?page=fullhttp://www.plantservices.com/articles/2010/?page=fullhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=1http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html


5/13

Ultimately, a VFD is merely a power conversion device that converts the fixedvoltage and frequency of incoming power to a variable voltage and frequency outputto provide the variable speed capabilities for which it was designed. Keep in mindthe variables associated with electrical power (voltage, current and power factor) andtheir relationships when comparing the VFDs input to its output. This also will holdtrue when using the motors mechanical power (speed and torque) to determine theamount of input power/current to the VFD. Taking all the variables into consideration,

one can be pleasantly surprised to find the input current lower than the outputcurrent

Page 2 of 2

Prev1| 2 Next View on one page

What are the factors that affect VFD rangeability?

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individualsfrom Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question andthe answers each week. If you would like to provide additional answers, please send them toSusan Colwell

at ISA. The eighteenth question is from Muhammad Khalifah in Saudi Arabia:

What are the factors that affect Variable Frequency Drive (VFD) rangeability for flow control?

Greg Shinskey in his study Flow and Pressure Control Using Variable Speed Drives (Control Conference,

Chicago, 1980, pages 161167) found that rangeability of flow control by variable speed pumping exceededthe rangeability of the magnetic flow meter in the test.

In general, the rangeability of VFD can be 40:1 or better if the following guideline is followed:

Guideline to maximize VFD rangeability for flow control

1. Pulse Width Modulation to improve low speed performance reducing torque pulsation

(cogging)

2. Totally enclosed fan cooled (TEFC) motors with constant speed fan or booster fan as

necessary with class F insulation (inverter duty) and 1.15 service factor to preventoverheating

3. Totally enclosed water cooled (TWEC) motors for high temperature fluids to prevent

overheating

4.NEMA frame B motor to prevent steep torque curve

5. Proper pump sizing to prevent operation on flat part of pump curve

6. Use of recycle valve to keep pump discharge pressure well above static head at low

flow (see article Watch out with variable speed pumping)
http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=1http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=1http://www.plantservices.com/articles/2010/?page=fullhttp://www.plantservices.com/articles/2010/?page=fullmailto:[email protected]:[email protected]:[email protected]://www.chemicalprocessing.com/articles/2008/069.htmlhttp://www.chemicalprocessing.com/articles/2008/069.htmlhttp://www.chemicalprocessing.com/articles/2008/069.htmlmailto:[email protected]://www.plantservices.com/articles/2010/?page=fullhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html?page=1http://www.plantservices.com/articles/2010/06VFDsSaveEnergy.htmlhttp://www.plantservices.com/articles/2010/06VFDsSaveEnergy.html


6/13

7. Use of low speed limit to prevent reverse flow for highest possible destination pressure

8. Twelve bit or more signal input cards to improve resolution limit of signal to 0.05% or

better

9. For tachometer control, gear teeth for magnetic pickups and discs with holes or bands

with mirrors on the shafts for optical pickups to provide more pulses per revolution

10.For tachometer control, keep the speed control in the VFD to prevent violation of the

cascade rule where the secondary flow loop should be 5 times faster than the primary(flow) loop as discussed in More Fun with PID Controllers

11.To increase rangeability to 80:1, consider fast cascade control of speed to torque in

VFD to provide closed loop slip control as described in The Control Techniques Drives

and Controls Handbook, IEE Power and Energy Series 35, Cambridge University

Press, 2001

The excerptEssential-Book-Excerpt-VFD-Performancefrom the ISA bookEssentials of ModernMeasurements and Final Elements in the Process Industriesdocuments these and other considerations in

maximizing the performance of variable frequency drives for process control.

Senior Member

Full Member

253 posts

Gender:Male

Location:Midwest, USA

Posted 28 July 2010 - 06:30 AM

QUOTE (mbouknight @ Jul 20 2010, 01:11 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?
http://www.controlglobal.com/articles/2011/more-fun-with-pid-controllers.htmlhttp://www.controlglobal.com/articles/2011/more-fun-with-pid-controllers.htmlhttp://automation.isa.org/wp-content/uploads/2012/04/Essential-Book-Excerpt-VFD-Performance.pdfhttp://automation.isa.org/wp-content/uploads/2012/04/Essential-Book-Excerpt-VFD-Performance.pdfhttp://automation.isa.org/wp-content/uploads/2012/04/Essential-Book-Excerpt-VFD-Performance.pdfhttp://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764http://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764http://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764http://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764http://www.isa.org/Template.cfm?Section=Books3&template=/Ecommerce/ProductDisplay.cfm&ProductID=10764http://automation.isa.org/wp-content/uploads/2012/04/Essential-Book-Excerpt-VFD-Performance.pdfhttp://www.controlglobal.com/articles/2011/more-fun-with-pid-controllers.html


7/13

mbouknight -

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

attached to it ?

Advise ..

j

Back to top

#3 GreenDriv

Junior Member

Full Member

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)

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?
http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/user/9088-greendriv/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/9088-greendriv/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/9088-greendriv/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/9088-greendriv/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=10892http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_body


8/13

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 & offeringvariable speed drives, frequency inverters, closed loop AC drives...

Let's share and communicate!

Back to top

#4 KJF

Members

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.
http://www.inverter-china.com/http://www.inverter-china.com/http://www.inverter-china.com/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11229http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11229http://www.lmpforum.com/forum/user/10285-kjf/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/10285-kjf/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/10285-kjf/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11229http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11229http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.inverter-china.com/


9/13

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 onconstant torque loads.

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

Back to top

#5 jOmega

Senior Member

Full Member

253 posts

Gender:Male


Posted 01 March 2011 - 03:51 PM

QUOTE (GreenDriv @ Feb 10 2011, 07:37 AM)

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.
http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11234http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11234http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11202http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11202http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11234http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11234http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_body


10/13

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 properlyselected 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.

j


11/13

Back to top

#6 jOmega

Senior Member

Full Member

253 posts

Gender:Male


Posted 01 March 2011 - 06:57 PM

QUOTE (KJF @ Feb 28 2011, 09:20 PM)

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



low voltages. The VFD vendors can make recommendations.
http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_bodyhttp://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11235http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11235http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11229http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11229http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11229http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/user/1401-jomega/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/http://www.lmpforum.com/forum/topic/4188-vfd-sizing/index.php?act=findpost&pid=11229http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11235http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#entry11235http://www.lmpforum.com/forum/topic/4188-vfd-sizing/#ipboard_body


12/13

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 andcomparing 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 Factorcapability 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



13/13


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 withoutany 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,

Documents

How VFDs save energy.docx