Danfoss VLT 6000 HVAC Application guide

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IntroductionIntroduction
IntroductionIntroduction
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Variable speed drives have been used in industrial applications for years because of their ability to provide precise process control. They have also become the standard method of control for heating ventilation and air-conditioning (HVAC) systems due to their precise control and significant energy savings.
The operational concerns for HVAC systems are quite different from those for industrial applications. In most HVAC installations there is a large installed base of sensitive electronic equipment such as computers, outstations and radios. Airports, hospitals and research facilities will for example make much heavier demands on the variable speed drives than the industrial plants.
This feature note will deal with one aspect of electrical noise generation in variable speed drives: Radio Frequency Interference (RFI) on the AC power line. We describe the causes and effects of such noise as well as the considerations that are to be made in connection with the selection and installation of a variable speed drive.
VLT® 6000 HVAC
Both the inverter and the control circuitry generate electrical noise at frequencies higher than 150 kHz.
If the drive is not designed carefully, this noise will be conducted to the surroundings, causing malfunction of other electronic equipment, especially if it is not designed with a high level of immunity to such high frequency noise.
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Measuring Radio Frequency InterferenceMeasuring Radio Frequency Interference
Measuring Radio Frequency Interference
Measuring Radio Frequency InterferenceMeasuring Radio Frequency Interference
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The levels of RFI from a drive is dependent on many different factors. The design of the drive is most important, since this determines how low the distortion can get.
The measuring results for different drives may vary a lot, so to get a real picture it is important to know exactly how the measuring was made. Some of the most important factors are: Impedance between drive chassis and ground Type of motor cable used or transfer impedance of cable screen Length of motor cable
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Causes of Radio Frequency InterferenceCauses of Radio Frequency Interference
Causes of Radio Frequency Interference
Causes of Radio Frequency InterferenceCauses of Radio Frequency Interference
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(RFI)(RFI)
(RFI)
(RFI)(RFI)
ee
Figure 1: Schematic of Basic Drive design
Most variable speed drives operate by using a bridge rectifier to convert the incoming AC voltage into a DC bus voltage. The inverter bridge of the drive then converts the DC bus voltage into the controlled voltage and frequency that the motor requires.
For the most common types of drives in use today, IGBTs are used to convert the DC voltage into an AC voltage with controlled amplitude and frequency. To perform this control most drives incorporate sophisticated control circuitry with micro-processors of high clock frequencies.
Radiated emission is almost impossible to reproduce. The reason is that even a slight change in the measuring set-up will influence the results a lot. Measuring made on site will always be unreliable, since it is impossible to create a clean environment.
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Radio frequency interference limitsRadio frequency interference limits
Radio frequency interference limits
Radio frequency interference limitsRadio frequency interference limits
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The most important international standard defining RFI limits for drives is: EN 55011/CISPR 11
EN55011 sets three different limits:
Emission, screened motor cable
dB/uV
90 80
76 66 60 56 50 46
150 kHz
500 kHz
5 MHz
Average levels EN 55011 2A
EN 55011 1A
EN 55011 1B
30 MHz
Figure 2: Average conducted emission levels
MN.60.G1.02 - VL T is a r egister ed Danfoss trademark
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Of these limits only 1A and 1B have been applied as
U V W PE
q
legal requirements.
Each level contains limits to quasi peak and average conducted emission as well as quasi peak radiated emission.
Figure 2 shows the limit for average conducted emission. Conducted emission is a cause for concern. With EN 55011 demands for Quasi Peak and Average were combined in one standard to get more equal demands and remove the need to determine which of the two requirements that should apply to the equipment. The level is approximately 10dB mV lower than the quasi peak levels.
VLT® 6000 HVAC
Figure 3: Motor cable with pigtail
Often the quasi peak limits do not cause any major problems in the design of the RFI filter. The average limits, however, have caused problems to many manufacturers.
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RFI filtersRFI filters
RFI filters
RFI filtersRFI filters
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RFI filters are available in many different designs. The most economical and best functioning filter will match the drive very carefully.
An RFI filter mainly consists of common mode reactors and capacitors.
Whether a filter is good at filtering the frequencies exceeding the limit of the norm will always be dependent on the design. If the filter is not designed for the drive, the filter components will have to be oversized resulting in increased costs.
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Installation considerationsInstallation considerations
Installation considerations
Installation considerationsInstallation considerations
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When installing a drive, it is important to check the manufacturers’ guidelines and some general rules of thumb:
This type of ground connection increases the transfer impedance to ground and increases the noise levels that can be measured on the mains cable. Figure 4 shows measured values of transfer impedance for different lengths of cable screen and pig-tails. As can be seen even a fairly short pig-tail has the same transfer impedance as 150m of cable screen above 10 MHz.
1000
100
10
1)1)
1)
1)1)
1
2)2)
2)
2)2)
3)3)
3)
0,1
0,01
3)3)
4)4)
4)
4)4)
5)5)
5)
1 10 100 1000 10000 100000
5)5)
Fre
uency in KHz
6)6)
6)
6)6)
Typical braided copper screen cable
1)1)
1)
1)1)
Screen impedance 150m
2)2)
2)
2)2)
Screen impedance 50m
3)3)
3)
3)3)
Screen impedance 10m
4)4)
4)
4)4)
5cm pigtail
5)5)
5)
5)5)
20cm pigtail
6)6)
6)
6)6)
EMC-cable gland w/ screen grounding
Figure 4: Comparison of transfer impedance for cable screens and pigtails
a.a.
Avoid pigtailsAvoid pigtails
a.
Avoid pigtails
a.a.
Avoid pigtailsAvoid pigtails A pigtail is as shown in figure 3, where the screen-end is twisted and connected to the PE-terminal or a ground screw.
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Figure 5 below shows the impact on compliance when a good installation (curve 1) is changed to an installation with a pigtail of 5 cm at the drive end, while the motor end is left untouched (curve 2). Where the good installation complies with EN 55011 1B, the installation with 5 cm of pig-tail barely complies with EN 55011 1A. As indicated in figure 4 a longer pig-tail would reduce compliance even further.
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