1MN.60.G1.02 - VLT is a registered Danfoss trademark

VLT® 6000 HVAC

■■■■■ IntroductionIntroductionIntroductionIntroductionIntroductionVariable speed drives have been used in industrialapplications for years because of their ability toprovide precise process control. They have alsobecome the standard method of control for heatingventilation and air-conditioning (HVAC) systems dueto their precise control and significant energy savings.

The operational concerns for HVAC systems are quitedifferent from those for industrial applications. In mostHVAC installations there is a large installed base ofsensitive electronic equipment such as computers,outstations and radios. Airports, hospitals andresearch facilities will for example make much heavierdemands on the variable speed drives than theindustrial plants.

This feature note will deal with one aspect of electricalnoise generation in variable speed drives: RadioFrequency Interference (RFI) on the AC power line.We describe the causes and effects of such noise aswell as the considerations that are to be made inconnection with the selection and installation of avariable speed drive.

■■■■■ Causes of Radio Frequency InterferenceCauses of Radio Frequency InterferenceCauses of Radio Frequency InterferenceCauses of Radio Frequency InterferenceCauses of Radio Frequency Interference(RFI)(RFI)(RFI)(RFI)(RFI)

ee

Figure 1: Schematic of Basic Drive design

Most variable speed drives operate by using a bridgerectifier to convert the incoming AC voltage into a DCbus voltage. The inverter bridge of the drive thenconverts the DC bus voltage into the controlledvoltage 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 ACvoltage with controlled amplitude and frequency. Toperform this control most drives incorporatesophisticated control circuitry with micro-processorsof high clock frequencies.

Both the inverter and the control circuitry generateelectrical noise at frequencies higher than 150 kHz.

If the drive is not designed carefully, this noise willbe conducted to the surroundings, causingmalfunction of other electronic equipment,especially if it is not designed with a high level ofimmunity to such high frequency noise.

■■■■■ Measuring Radio Frequency InterferenceMeasuring Radio Frequency InterferenceMeasuring Radio Frequency InterferenceMeasuring Radio Frequency InterferenceMeasuring Radio Frequency InterferenceThe levels of RFI from a drive is dependent on manydifferent factors. The design of the drive is mostimportant, since this determines how low thedistortion can get.

The measuring results for different drives may vary alot, so to get a real picture it is important to knowexactly how the measuring was made. Some of themost important factors are:Impedance between drive chassis and groundType of motor cable used or transfer impedance ofcable screenLength of motor cable

Radiated emission is almost impossible toreproduce. The reason is that even a slight changein the measuring set-up will influence the results alot. Measuring made on site will always beunreliable, since it is impossible to create a cleanenvironment.

■■■■■ Radio frequency interference limitsRadio frequency interference limitsRadio frequency interference limitsRadio frequency interference limitsRadio frequency interference limitsThe most important international standard definingRFI 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

6660565046

150 kHz 500 kHz 5 MHz 30 MHz

Emission,screened motor cable

dB/uV

90

8076

RadioFrequency Interference

Application NoteApplication Note

© Copyright Danfoss, Inc., 2004

VLT is a registered Danfoss trademark2

VLT® 6000 HVAC

Of these limits only 1A and 1B have been applied aslegal requirements.

Each level contains limits to quasi peak and averageconducted emission as well as quasi peak radiatedemission.

Figure 2 shows the limit for average conductedemission. Conducted emission is a cause forconcern. With EN 55011 demands for Quasi Peakand Average were combined in one standard to getmore equal demands and remove the need todetermine which of the two requirements that shouldapply to the equipment. The level is approximately10dB mV lower than the quasi peak levels.

Often the quasi peak limits do not cause any majorproblems in the design of the RFI filter. The averagelimits, however, have caused problems to manymanufacturers.

■■■■■ RFI filtersRFI filtersRFI filtersRFI filtersRFI filtersRFI filters are available in many different designs. Themost economical and best functioning filter will matchthe drive very carefully.

An RFI filter mainly consists of common modereactors and capacitors.

Whether a filter is good at filtering the frequenciesexceeding the limit of the norm will always bedependent on the design. If the filter is not designedfor the drive, the filter components will have to beoversized resulting in increased costs.

■■■■■ Installation considerationsInstallation considerationsInstallation considerationsInstallation considerationsInstallation considerationsWhen installing a drive, it is important to check themanufacturers’ guidelines and some general rules ofthumb:

a.a.a.a.a. Avoid pigtailsAvoid pigtailsAvoid pigtailsAvoid pigtailsAvoid pigtailsA pigtail is as shown in figure 3, where the screen-endis twisted and connected to the PE-terminal or aground screw.

UVWPE

Figure 3: Motor cable with pigtail

This type of ground connection increases thetransfer impedance to ground and increases thenoise levels that can be measured on the mainscable. Figure 4 shows measured values of transferimpedance for different lengths of cable screen andpig-tails. As can be seen even a fairly short pig-tailhas the same transfer impedance as 150m of cablescreen above 10 MHz.

Figure 4: Comparison of transfer impedance forcable screens and pigtails

Figure 5 below shows the impact on compliancewhen a good installation (curve 1)is changed to an installation with a pigtail of 5 cm atthe drive end, while the motor end is left untouched(curve 2). Where the good installation complies withEN 55011 1B, the installation with 5 cm of pig-tailbarely complies with EN 55011 1A. As indicated infigure 4 a longer pig-tail would reduce complianceeven further.

0,01

0,1

1

10

100

1000

1 10 100 1000 10000 100000

Frequency in KHz

Screen impedance 150mScreen impedance 50mScreen impedance 10m5cm pigtail20cm pigtailEMC-cable gland w/ screen grounding

Typical braided copper screen cable

1)1)1)1)1)

2)2)2)2)2)

3)3)3)3)3)

4)4)4)4)4) 5)5)5)5)5)6)6)6)6)6)

1 )1 )1 )1 )1 )2 )2 )2 )2 )2 )3 )3 )3 )3 )3 )4 )4 )4 )4 )4 )5 )5 )5 )5 )5 )6 )6 )6 )6 )6 )

© Copyright Danfoss, Inc., 2004

3MN.60.G1.02 - VLT is a registered Danfoss trademark

VLT® 6000 HVAC

Figure 5: Comparison between good installationand pigtail installation

b.b.b.b.b. Use screened motor cable from the drive to theUse screened motor cable from the drive to theUse screened motor cable from the drive to theUse screened motor cable from the drive to theUse screened motor cable from the drive to themotormotormotormotormotor, also inside panels., also inside panels., also inside panels., also inside panels., also inside panels.Usually the cable between the panel and the motor isscreened. Unfortunately, output contactors insidepanels are often connected with unscreened cables.That will give problems with noise being radiatedbetween the cables.

Figure 6 shows a comparison between a goodinstallation (curve 1) and a bad one, usingunscreened motor cables inside the panel (curve 2).In this example great care was taken to separate themains cable from the motor cable. It is sometimesnot possible to avoid the use of unscreened motorcable inside the panels, but as curve 2 showscompliance with EN 55011 1A is achievable.

Often the mains and motor cables, however, areplaced much too close and this causes transmissionof noise directly between mains and motor cables,thus by-passing the RFI filter and losing compliancewith EN 55011, as shown in curve 3.

Figure 6: Using unscreened cables inside thepanel.

c.c.c.c.c. Connect the scrConnect the scrConnect the scrConnect the scrConnect the screen at motor and VLeen at motor and VLeen at motor and VLeen at motor and VLeen at motor and VLTTTTT:::::To get an effective screening of the electric andmagnetic field from the motor cable it is necessary toconnect the screen at the motor and at the VLT.Connecting the screen only in one end will screen theelectric field but will have no effect on the magneticfield. As the noise from the motor cable is mainlymagnetic fields the screen will be very ineffectivewhen connected only in one end.

Equalising currents will rarely case any problems tothe installation. As a rule of thumb the screen shouldtherefore always be connected in both ends. If then aproblem should occur, then it is recommended toconnect a 1 µF capacitor at the drive end, ratherthan just disconnecting the screen.

■■■■■ ConclusionConclusionConclusionConclusionConclusionTo ensure optimum performance it is highly importantto select the right drive and filter and to do theinstallation work right. A good drive with theappropriate RFI filter cannot ensure compliance, ifthe installation work is not done properly.

Many buildings incorporate equipment that issensitive to radio frequency interference. This is notonly the case for airports, telecommunicationfacilities and hospitals. Ordinary appartment buildingsand office complexes also have a lot of sensitiveequipment installed. It is therefore especiallyimportant to limit radio frequency interference inHVAC installations.

Curve 2The screen effect of the cable

is reduced due to a pig-tail grounding

EN 55011 1B

6660565046

150 kHz 500 kHz 5 MHz 30 MHz

Average levels

EN 55011 1A

Emission,screened motor cable

Curve 1

Grounding of screenthrough cable clamp

Cable conducted noise

dB/uV

Average levels

EN 55011 1A

EN 55011 1B

6660565046

150 kHz 500 kHz 5 MHz 30 MHz

Emission,screened motor cable

Cable conducted noise

dB/uV

25-35 dB

Curve 1Screened

motor cable all the way

15-25 dB

Curve 3Unscreened Motor cable andmains cable in close proximity

inside panel

Curve 2Good distance betweenunscreened motor cable

and mains cableinside panel

© Copyright Danfoss, Inc., 2004

Rev. 2003-09-11MN60G102

*MN60G102*

50 New Town Road 779 Susquehanna AvenuePlainview, NY 11803 Franklin Lakes, NJ 07417Tel: 516-454-9300 Tel: 201-891-9550Fax: 516-454-9307 Fax: 201-891-4298

© Copyright Danfoss, Inc., 2004