Danfoss VLT 6000 (Legacy Product) Gebruikershandleiding

Merk: Danfoss

Model: VLT 6000 (Legacy Product)

Type: Gebruikershandleiding

MN.60.G1.02 - VLT is a registered Danfoss trademark

VLT

6000 HVAC

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IntroductionIntroduction

Introduction

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.

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Causes of Radio Frequency InterferenceCauses of Radio Frequency Interference

Causes of Radio Frequency Interference

(RFI)(RFI)

(RFI)

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.

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

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

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

The most important international standard defining

RFI limits for drives is:

EN 55011/CISPR 11

EN55011 sets three different limits:

Figure 2: Average conducted emission levels

Average levels

EN 55011 2A

EN 55011 1A

EN 55011 1B

150 kHz

500 kHz

5 MHz

30 MHz

Emission,

screened motor cable

dB/uV

VLT is a registered Danfoss trademark

VLT

6000 HVAC

Of these limits only 1A and 1B have been applied as

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.

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 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

When installing a drive, it is important to check the

manufacturers’ guidelines and some general rules of

thumb:

a.a.

Avoid pigtailsAvoid pigtails

Avoid 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.

Figure 3: Motor cable with pigtail

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.

Figure 4: Comparison of transfer impedance for

cable screens and pigtails

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.

0,01

0,1

100

1000

1 10 100 1000 10000 100000

Fre

uenc

in KHz

Screen im

edance 150m

Screen im

edance 50m

Screen im

edance 10m

5cm pigtail

20cm

tail

EMC-cable gland w/ screen grounding

Typical braided copper screen cable

1)1)

2)2)

3)3)

4)4)

5)5)

6)6)

1)1)

2)2)

3)3)

4)4)

5)5)

6)6)

MN.60.G1.02 - VLT is a registered Danfoss trademark

VLT

6000 HVAC

Figure 5: Comparison between good installation

and pigtail installation

b.b.

Use screened motor cable from the drive to theUse screened motor cable from the drive to the

Use screened motor cable from the drive to the

motormotor

motor

, also inside panels., also inside panels.

, also inside panels.

Usually the cable between the panel and the motor is

screened. Unfortunately, output contactors inside

panels are often connected with unscreened cables.

That will give problems with noise being radiated

between the cables.

Figure 6 shows a comparison between a good

installation (curve 1) and a bad one, using

unscreened motor cables inside the panel (curve 2).

In this example great care was taken to separate the

mains cable from the motor cable. It is sometimes

not possible to avoid the use of unscreened motor

cable inside the panels, but as curve 2 shows

compliance with EN 55011 1A is achievable.

Often the mains and motor cables, however, are

placed much too close and this causes transmission

of noise directly between mains and motor cables,

thus by-passing the RFI filter and losing compliance

with EN 55011, as shown in curve 3.

Figure 6: Using unscreened cables inside the

panel.

c.c.

Connect the scrConnect the scr

Connect the scr

een at motor and VLeen at motor and VL

een at motor and VL

To get an effective screening of the electric and

magnetic field from the motor cable it is necessary to

connect the screen at the motor and at the VLT.

Connecting the screen only in one end will screen the

electric field but will have no effect on the magnetic

field. As the noise from the motor cable is mainly

magnetic fields the screen will be very ineffective

when connected only in one end.

Equalising currents will rarely case any problems to

the installation. As a rule of thumb the screen should

therefore always be connected in both ends. If then a

problem should occur, then it is recommended to

connect a 1 µF capacitor at the drive end, rather

than just disconnecting the screen.

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ConclusionConclusion

Conclusion

To ensure optimum performance it is highly important

to select the right drive and filter and to do the

installation work right. A good drive with the

appropriate RFI filter cannot ensure compliance, if

the installation work is not done properly.

Many buildings incorporate equipment that is

sensitive to radio frequency interference. This is not

only the case for airports, telecommunication

facilities and hospitals. Ordinary appartment buildings

and office complexes also have a lot of sensitive

equipment installed. It is therefore especially

important to limit radio frequency interference in

HVAC installations.

Curve 2

The screen effect of the cable

is reduced due to a pig-tail grounding

EN 55011 1B

150 kHz

500 kHz

5 MHz

30 MHz

Average levels

EN 55011 1A

Emission,

screened motor cable

Curve 1

Grounding of screen

through cable clamp

Cable conducted noise

dB/uV

Average levels

EN 55011 1A

EN 55011 1B

150 kHz

500 kHz

5 MHz

30 MHz

on,

screened motor cable

Cable conducted noise

dB/uV

25-35 dB

Curve 1

Screened

motor cable

all the way

15-25 dB

Curve 3

Unscreened Motor cable and

mains cable in close proximity

inside panel

Curve 2

Good distance between

unscreened motor cable

and mains cable

inside panel

Danfoss VLT 6000 (Legacy Product) Gebruikershandleiding

Merk: Danfoss

Model: VLT 6000 (Legacy Product)

Type: Gebruikershandleiding

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English: Danfoss VLT 6000 (Legacy Product) User guide

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