VFDs and Harmonics

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k.dhana Associate Join Date: Aug 2008 Location: TN, India. Posts: 35 Good Answers: 2	VFDs and Harmonics 12/29/2009 2:57 AM
	Regarding variable frequency drive (VFD), I came across a statement that, "The problem that is created with the application is that a VFD will introduce a nominal 5% harmonic current into a motor in addition to the motor's normal running amperage. The harmonic current will generate 5% more heat within the motor than will be generated with a pure sinusoidal 60 HZ waveform, thus decreasing the motor efficiency by 5%. Therefore, to maintain full load torque, the motor will draw an additional 5% kW input when operated on a VFD". The statement finally tells that a VFD cause 10% increase in current because of 5% harmonic heat loss. Is this correct? Here I need a clarification as I thought that, the heat due to harmonic is a loss on electrical side and it will not to do anything with torque decrement. __________________ ?????????? ????????? ????????,.........??????? ???????????? ???????? ????. ????????????
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electricalexpert65 Guru Join Date: Oct 2009 Posts: 2061 Good Answers: 169	#1 Re: VFD & Harmonics 12/29/2009 8:08 AM
	You see! Any harmonics can be observed to have a phase sequene with reference to the fundamental, which is assumed to be positive. The second harmonics will be negative sequence, third zero, 4th - positive, 5th - negative, 6th - zero and so on. A VFD generates up to infinite order of harmonics. While the positive sequence harmonics would help the fundamental and would create a positive torque, the negative sequence harmonics would create a negative torque, which would try to rotate the rotor in the opposite direction. The net torque, hence would decrease, as has been rightly said in the statement quoted by you. Of course, I am not in full agreement with the statement with regard to the absolute percentage increase in heat produced, I still agree that there will be increased heat in the motor due to skin effect, additional I2R heating due to negative sequence harmonic currents and increased core loss (which is directly proportional to frequency or square of the frequency and hence would be more for harmonics, which are high frequency compos).
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dhayanandhan Guru Join Date: May 2008 Location: CHENNAI, TAMIL NADU, INDIA. Posts: 1851 Good Answers: 65	#2 Re: VFDs and Harmonics 12/30/2009 1:11 PM
	Mr.Dhana, The magnitude of hormonics is not just 5%. In one of the earlier links it has been explained. The harmonics will be 1/n^2 where n = 5, 7, 11 and13. cumulative. n cannot be 9 since 3rd hormonics and multiples of 3 hormincs get cancelled . Hence 5th hormonic will be 1/5^2 = 1/25 = 4% and, 1/7^2= 1/49= 2(approx) 1/13^2= 1/169 and so on. As the order of hormonics increases the impact magnitude of that will be severe and be intensive and hormonics effect will be of nuisance and hence hormonic filters are used. The design of the motor used for vfd application is totally different to take care of high heating etc. Torque considerations will not get affected for a given speed level. dhayanandhan.s, india
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k.dhana Associate Join Date: Aug 2008 Location: TN, India. Posts: 35 Good Answers: 2	#5 In reply to #2 Re: VFDs and Harmonics 01/04/2010 1:25 PM
	Yes Sir, Thanks you for your reply, The Torque considerations should not change with VFD. But your post contradicts with earlier post in this thread as torque may tends to decrease, it says. Can you throw a light on this..? __________________ ?????????? ????????? ????????,.........??????? ???????????? ???????? ????. ????????????
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JOECPQ1 Active Contributor Join Date: Dec 2009 Location: Woodstock GA Posts: 17	#3 Re: VFDs and Harmonics 01/03/2010 7:40 PM
	The amount oh harmonic current created by the VFD is inherent in the design and the base amount created will normally be handled by the design of the motor. However when the quality of the power applied to the drive is not good the transients affecting the power at the drive input will cause erratic timing of the rectifyers and create excess harmonic current. The 5th,7th,11th harmonic current will create excesseve heat on the motor connections and create a degrading or instant failure of the motor. It is essential today to protect the voltage input to the drive to extend the lifespan of the equipment and avoid downtime of the equipment. It should be done y use of true industrial grade protection and it is usually best if it is selected by a power quality professional. This has been proven to extend the equipment life and eliminate costly downtime. __________________ JS CPQ
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Anonymous Poster	#4 Re: VFDs and Harmonics 01/04/2010 10:45 AM
	Just to add to the information on this, is ther a difference between an AC or DC motor with the harmonics created? I ask this question as I had an issue where a gear box was failing and it was indicated the AC motor was the problem. If a DC motor was used this would eliminate my problem. Is there any basis to this statement?
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Anonymous Poster (1); dhayanandhan (1); electricalexpert65 (1); JOECPQ1 (1); k.dhana (1)

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