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6 comments
Commentator

Join Date: May 2011
Posts: 80

Harmonics Contain

05/03/2012 11:09 PM

Dear all,

I got a measurement data that our VSD generates 21% of fifth harmonics and 5% of sixth harmonics during full load.

Will it contains the same harmonics composition if we run it in, let say, 10%, 50%, 90% load?

Thanks in advance

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Guru
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#1

Re: Harmonics Contain

05/04/2012 3:09 AM

Why not measure it at these loadings as well (rhetorical question)?

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#2
In reply to #1

Re: Harmonics Contain

05/04/2012 6:46 AM

It's not rhetorical. The one who did the measurement, did it for free only at full load, but they have left the site. I only got the result yesterday. If I were there I would have ask more.

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#3

Re: Harmonics Contain

05/04/2012 8:06 AM

No, the harmonic percentage will be different at different loads. This is simply because that the harmonic currents are created by the DC power supply that your VSD uses to provide the power from which it uses to drive the load.

There will not be a linear relationship, but a change is certain. In general, the closer the load is to the capacity of the drive, the lower the harmonic contribution will be. As the load is reduced, the time it takes to recharge the capacitors of the DC supply becomes shorter and the harmonic content may shift to higher harmonics.

If you VSD is three phase, it is more common to see 11th and 13th harmonic distortion. For really dirt cheap single phase DC supplies the harmonic content tends to be higher in 3rd and then 5th harmonics.

In a plant I used to work in at one time, I put a current clamp on the neutral of a power distribution panel. It measured the primary frequency as 180 Hz. That was some significant (3rd harmonic) distortion. But they claimed it didn't have anything to do with their early motor failures and circuit breakers tripping. After all, I hadn't worked there enough years to know anything......

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#4

Re: Harmonics Contain

05/04/2012 2:10 PM

It should be noted though that the Total DEMAND Distortion, TDD, will go down as speed reduces because the power itself will go down. So although the percentage may be higher or lower, the EFFECTS of it will be lower no matter what. That's why in general, nobody cares what the THD is below full load, they are concerned about the effects, not just the numbers.

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Commentator

Join Date: May 2011
Posts: 80
#5
In reply to #4

Re: Harmonics Contain

05/06/2012 11:44 AM

Thanks to NotUrOrdinaryJoe and JRaef


If I have 6 pulse VSD, I would expect a 5th and 7th harmonics. But if I lower the speed to, let say, 50%, the harmonics can shift to higher than 5th and 7th harmonics, but the TDD of 50% should less than TDD of 100%, eventhough the THD of 50% can be higher than THD of 100%?

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Associate

Join Date: Feb 2012
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#6

Re: Harmonics Contain

05/07/2012 6:16 PM

Hi,

VSDs are equipped with 3% line reactor which is usually sized based on the rated curent of the VSD. Z% reflects the voltage drop across the reactor at the rated current. As the current drops then Z% drops thus reducing the harmonic cancellation effectiveness of the reactor. The reactor acts as low pass filter which attenuates harmonic current and also incurrs voltage drop at harmonic orders.

So if a VSD is running at 85% load then it will draw less fundamental current but the amount of the harmonics will be higher than the case with 100% loading.

lets say your VSD draws 100 A at full load , with 20% 5th order( 20A) . then if the load drops to 80% then your draws 80A but 25-30% 5th order (24A) . So you draw less fundamental current but with higher harmonic content.

As the load drops say to 20%, the THDI% may go up to 120% . Of course this is still a percenatge of the total fundamental current.

In summary, As the load drops the THDI% increases.You need to look at the real Current value (A) rather than the percentage since THDI% can be misleading.

If you are having a harmonic issue then you may contact your nearest Schneider Electric office and they will be able to offer a solution.

cheers

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