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Emc Compliant With Unscreened Motor Cables


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#1 niallnz

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Posted 12 August 2009 - 04:06 AM

OK I'll bite, at least one VSD manufacturer is stating that their VSD can meet the EN 61800-3: 1996 and EN 61800- 3/A11: 2002 EMC regulations whilst using up to 300m of unscreened cable on the VSD output.

So has any one got any idea how they are doing this?

I was thinking that they have IGBT's with a slow turn on characteristic which threfore limits the rate of rise of the voltage, but wouldn't this also increase the switching loses of the drive and therefore decrease the efficiency.

Cheers Niallnz

#2 marke

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Posted 12 August 2009 - 09:27 AM

Hello niallnz

If you reduce the rate of rise of voltage on the output terminals of the VFD, you will reduce the higher frequency components of the "noise" caused by the VFD operating.
To be truely effective, you need to have a very slow waveform.

If you look at the emissions testing that is done in a laboratory, there are two major tests, one is radiated noise and the other is conducted noise.
The radiated noise tests essentially cover the range of 30MHz to 1000MHz and conducted noise covers the range of 100KHz to 30MHz.

Slowing down the rate of rise of voltage will reduce the energy in the 30MHz to 1000MHz range and this makes the radiated noise test easier to pass, and it is possible to twist the output cables and lay them well away from all other conductors so that any radiated energy cancels in the far field. (Same principle as a twisted pair signal cable.)

When we consider conducted noise, we must consider what happens in the motor and the earth return path as well.
The VFD generates switching noise by virtue of the way it works. Fast rising and falling edges on the PWM output waveforms are connected to the motor windings via the output cables. The fast edges cause capacitive currents to flow between the windings and the frame of the motor. The capacitance presents a very low impedance at the higher frequencies.
The frame of the motor is connected back to the frame of the VFD either by the screen of a screened cable, or by the earth conductor of an unscreened cable.
The noise generated by the VFD is thus coupled to the frame of the motor by the capacitance within the motor.
The noise must be returned to the DC bus of the VFD to complete the circuit. This is achieved by decoupling capacitors connected between the chassis of the VFD and the DC Bus. Some designers decouple only one bus rail and others decouple both the plus and minus rails. Small capacitors are used to decouple high frequency currents and larger capacitors are used to decouple lower frequencies. It is very important that the decoupling circuit (capacitor and connection bars and/or cables) offer a very low impedance to the high frequency noise currents. Long thin conductors result in a high impedance and inefficient decoupling.
Current that is not adequately decoupled within the VFD will flow through the earth connections to the VFD chassis and find paths back to the input phases to get back to the DC bus via the input rectifiers.

The conducted noise tests are carried out by measuring the high frequency components of the current flowing into the input of the VFD and indicate the level of noise current that is not decoupled within the VFD or finds an alternative earth return path to the VFD.
The EMC conducted noise tests are usually carried out with no connection to the motor frame other than the screen or earth conductor. This eliminates any alternative earth paths for the noise currents and means that the tests are really testing the effectiveness of the decoupling within the VFD. Where the HF impedance on the imput of the VFD is increased by EMC filters, AC reactors etc, the level of noise current flowing in the input to the rectifier is reduced.
Low impedance decoupling back to the DC Bus reduces conducted noise measured and it is not too difficult to design a VFD to pass a laboratory conducted emissions test.

If we now consider a VFD in a practical installation, the motor will virtually never be totally isolated from conductive and capacitive earth paths other than the direct path to the VFD, so we always have alternative earth paths from the motor frame.
If we have a high noise voltage on the frame of the motor, we wil experience high stray currents flowing in the alternative earth paths. (through the steelwork etc) These high stray currents will eventually find the way back to the VFD through the earth system and three phase input. On the way back to the VFD, the High Frequency currents can cause high voltage noise in other circuits.

Consider a voltage source with two equal impedances in series. The voltage at the junction of these two equal impedances will be half of the source voltage. If we reduce the impedance of the path back to ground, we will reduce the voltage at the junction relative to ground.
High frequency currents flow in the surface of a conductor only. If we dramatically increase the surface area of the earth return conductor, we will dramatically reduce the impedance of the earth return conductor and in our circuit above, we will reduce the high frequency voltage at the junction.

Now consider the output of a VFD and look at the transient voltages only. The capacitance in the motor provides a very low impedance to these transients and so for a motor that is connected by an equal impedance for the output to the motor and the motor frame to the VFD frame, the motor frame will be as high as half of the output transient voltage level.
Replace the earth return conductor with a conductor which is 10% of the impedance of the phase conductor impedance, and the voltage on the motor frame will be only 10% of the equal impedance case.
The use of a screened cable reduces the earth return impedance by a factor typically between 20 and 50, so the voltage on motor frame will be reduced by this factor and consequently, the stray noise current amplitudes are also reduced by this level.

While it is possible to pass conducted noise tests with an isolated motor and unscreened cables, a practical installation will have much higher conducted noise within the installation with unscreened cables than the same installation with screened cables.
I have seen stray noise voltages in an installation reduce by a factor of 50 when unscreened cables were replaced with screened cables. The actual difference is a function of the decoupling impedance within the VFD, the impedance ratio between the phase conductor and the earth return path and the frequency spectrum of the output waveform.

Slowing down the rate of rise of voltage on a VFD can be done by an LCR circuit on the output of the VFD and by slowing down the switching times of the switching devices. IGBTs and FETs are easily slowed by increasing the gate resistance between the gate of the IGBT and the Driver circuit.
Slowing down the switching will result in a smoother output transition and will reduce switching noise, ringing and overshoot on the DC bus. Slowing the switching rate will increase the dynamic switching losses in the VFD requiring additional heatsinking and/or output modules. The result is a higher cost and lower efficiency.
Slowing down the rate of switching using an increased gate resistor, slows down the change in current in the IGBT and into a resistive load, this will slow down the rate of change in voltage, but with an inductive load, the moment the current begins to reduce, the inductance of the circuit drives the voltage to the opposite rail and so the rate of change in voltage is not directly controlled by the softer switching of the IGBTs.

The basic concept is simple really if you approach it the right way, and field performance tends to illustrate the description above.

Hope this answers your question, if not post further questions.

Best regards,
Mark.

#3 Kam

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Posted 07 June 2010 - 01:43 PM

Does anyone know if it is required that you use a LISN (Line
Impedance Stabilizing Network) when preforming a conducted Emissions test.
The reason I ask is because we recently supplied a 125 HP, 690 Volt Vacon NXS VFD to an OEM that was having problems passing the conducted emissions test. We have some doubts about the test procedure and the question was raised wether we needed the LISN for the tests.

Thanks
Kam Ghuman

Does anyone know if it is required that you use a LISN (Line
Impedance Stabilizing Network) when preforming a conducted Emissions test.
The reason I ask is because we recently supplied a 125 HP, 690 Volt Vacon NXS VFD to an OEM that was having problems passing the conducted emissions test. We have some doubts about the test procedure and the question was raised wether we needed the LISN for the tests.

Thanks
Kam Ghuman

#4 marke

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Posted 07 June 2010 - 06:36 PM

Hello Kam

For laboratory testing, a Lisn is necessary.

Best regards,
Mark.

#5 Kam

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Posted 07 June 2010 - 11:10 PM

QUOTE (marke @ Jun 7 2010, 11:36 AM) <{POST_SNAPBACK}>
Hello Kam

For laboratory testing, a Lisn is necessary.

Best regards,
Mark.



Thanks Mark,

The reason I asked was that our client hired a CE Certification lab to come in a test the VFD cabinet. The test failed so they started asking us about the filtering in the VFD etc.. We assured them that the Vacon NXS VFDs were EMC Level L: EN61800-3, Category 3.

We also suggested that they insure that the vfd was fully loaded and the carrier frequency be set at factory default per Vacon's recommendations. Vacon also recommended the use of shielded motor cable, termination grounding methods per their user guide.

The client was not able to fully load the VFD and no LISN was used; other than that all recommendation were eventually followed.

Apparently the test still failed. I am trying to assure m customer that the problem is not with the VFD.

Thanks agan,

Kam

#6 marke

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Posted 27 August 2011 - 12:02 PM

For more information on this topic, see http://www.vfd-emc.co.nz
In particular, look at the test results, VFDs tested show stray voltage levels many times higher with unscreened cables than with EMC screened cables.
While it is possible to pass the EN 6100-3 tests, these test really only test that the DC bus decoupling and AC input decoupling is sufficient to decouple the noise coupled to the chasis of the VFD by the internal capacitance between the die of the IGBTs and the heatsinks. This test is totally non indicative of the noise coupled to the frame of the motor.
In a practical installation, it is the stray current caused by the noise capacitively coupled to the frame of the motor thaqt causes the EMC issues and that is not tested in any part of the EMC testingfor CE or CTick marks.

Compliance with EN-6100-3 is not a guarantee of minimum EMC issues.

Best regards,
Mark




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