VLT® Frequency Converters

1MC.60.B1.02 - VLT is a registered Danfoss trademark

IntroductionThe human ear has a theoretical audible range from16Hz to 20kHz. For most parts of the adultpopulation, this range is more narrow, typically noiseup to 10-14kHz will be detected. The curve belowshow the sensitivenes of the human. As it can beseen from the curve, the human ear is more sensitiveto noise from 2 kHz to 6 kHz.

In order to generate a variable frequency most PWMor VVC controlled drives have a switching frequencyof 2 to 6 kHz. This switching frequency is oftentransmitted as audible noise by the motor. As can beseen from the above curve, this is within the range,where the human ear is the most sensitive and whereeven low noise levels are often detected. Since it is ahigh frequency noise, most people find it veryanoying, simply because high frequency noises aredifficult to mask and because they can be heard evenat a distance from the source.

The Cause of Acoustic noiseAcoustic noise when operating a motor with a PWMdrive is generated by low frequency distortion. Thisnoise causes ressonance in the motors stator andcooling ribs at the drives switching frequency. Thesemechanical ressonances cause the motor to act as aloud speaker. If the drive operate at low switchingfrequencies this noise is audible to the human ear.

It would be the optimum solution to eliminate this lowfrequency electrical noise in the drives output voltage,but that is unfortunately not possible , without addingpassive components in the output of the drive.

Since the low frequency noise is almost alwaysgenerated in a narrow band around one and twotimes the switching frequency. The second mostobvious way is to change the switching frequency ofout of the sensitive range, by either moving it up ordown. Lowering the switching frequency is not agood solution, since the current and voltage waveform would be completely distorted and the creationof a near sinusoidal voltage waveform would beimpossible. This means that the ability to control themotor would be reduced drastically.

Noise Reduction TechniquesFour different techniques will be compared to give ascomplete a comparison as possible.● High fixed switching frequency● Automatic Switching Frequency Modulation

(ASFM)● Random Switching Frequency● LC output filter

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VLT® Frequency Converters

2 MC.60.B1.02 - VLT is a registered Danfoss trademark

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2 4,5 6 9 12 14 kHz

Inverter

Motor

Distortion

Since the ressonance noise is set off at multiples ofthe switching frequency is prefered to keep theswitching frequency at 4,5 kHz as standard to reduceenergy concumption and avoid too many ressonancefrequencies.

The fact that the switching frequency is highest at lowload means that impact on the efficiency of theelectrical system will be very limited compared to thatof a fixed high switching frequency. In addition, thetotal losses are reduced, since the average lowfrequency distortion power loss in the motor cable ismaintained at low levels, thus assisting in keeping theenergy bill down.

Further, the compliance with requirements for RadioFrequency Interferences is not affected as much, as itwould be if using a fixed high switching frequency.

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Losses Relative to Total Loss

Indexed Output Losses

kHz

Fixed High Switching FrequencyA high fixed switching frequency (12-20 kHz) is thetraditional way of reducing acoustic noise in the mo-tor. This approach however has a lot ofdisadvantages. Some of these are mentioned below:1. Increased Radio Frequency Interference2. Increasing leakage currents, mainly due to

a larger RFI filter.3. Increased power losses, which generates

additional heating in the drive.4. Increased risk of motor insulation damages

Increased Radio Frequency Interference means thatyou have to use a larger and more expensive RFIfilter. This will increase your leakage current, as wellas increase the cost of the drive.

Increasing leakage currents give rise to installationproblems with respect to protection against fire andpersonal hazards, as the Residual Current Devices(RCD) used for this are difficult to use according toregulations.

Increased power losses generate additional heatingthe drive, which will either reduce the lifetime of thedrive or require larger components to be installed.More importantly, however, this means that if thesame drive was operating at a lower switchingfrequency, it would be able to operate the samemotor at lower energy cost or even operate a largermotor. International investigations have shown thatthe motor losses are not influenced by the switchingfrequency. It has further been found, that a switchingfrequency around 4kHz wil give the lowest losses inthe inverter and that even though the voltage distort-ion losses are higher at the low frequencies, the over-all efficiency is highest in the range from 2 to 4,5 kHz.See figures in the next column.

Automatic Switching Frequency Modulation(ASFM)ASFM is an advanced technique implemented in theVLT 6000 HVAC. Using ASFM, the switchingfrequency will be adjusted to the programmedmaximum switching frequency, when ever the load isbelow 60% of full load.

Pump and fan applications have a variable torquecharacteristic, which means, that the load reaches60% at 80% speed. Therefore it is possible tomaintain a high switching frequency at most times,without having to oversize the drive.

Motor

Inverter

Distortion

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2 4,5 6 9 12 14

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VLT® Frequency Converters

3MC.60.B1.02 - VLT is a registered Danfoss trademark

The only obstacle which remains is the increased lea-kage current which is not load dependant, but solelya result of currents generated in the intermediatecircuit of the drive and dependant on the switchingfrequency. The highest levels of leakage current willhowever only be present below 60% load.Therefore ASFM is still superior to high fixed switchingfrequency.

An argument against this technique is of course thataudible noise is still generated between 50% and100% of maximum load. However, in most pump andfan applications, the normal acoustic noise levelgenerated by mechanical vibrations in the installationat full speed are much higher than at 50% speed.The noise generated by the switching frequency willtherefore be disguised by the audible noise of the sy-stem. The formula below shows the increase in noiselevel in a fan system.

55 Log (speed1/speed2) => 55 Log (1500/750) =55 x 0,33 = 16,5 dBA

Danfoss believes that modulating the switchingfrequency relative to the load is a better compromisein HVAC applications, than permantently increasingthe switching frequency.

Random Switching FrequencyRandom switching frequency is also known as “whitenoise”. This approach does also not require deratingof the drive. A continuous alteration of the switchingfrequency within a band around a base switchingfrequency is implemented. The major disadvantage ofthis technique is that the motor sounds as if a bearingis failing. This sound is different from the fixed switch-ing frequency, but not less annoying to most people.

LC output filterAn LC filter can be mounted in the output of thedrive. This filter generate a pure sine wave voltage.Since the low frequency distortion this way iscompletely eliminated, the noise induced to the motoris also completely eliminated. This also means thatthe motor operation in general is improved, such thatin most applications there is no difference for themotor between operating direct on line or operatingon the drive.

In some installation a lot of commutation notchesoccur in the mains voltage, these notches can causethe motor to give a poor torque performance and itmight even damage the motor. Installing a VLT 6000HVAC with an LC output filter, will completelyeliminate these notches from the motor. The VLT6000 HVAC is designed to withstand these notchesand the operation of a VLT 6000 HVAC is notaffected by them.

Unfortunately there are some disadvantages:1. The noise is not removed from the system, just

moved to the LC filter instead.2. A small voltage drop is introduced between the

drive and the motor.3. Increased installation cost, as the LC filter is an

add-on cost.

Automatic Energy OptimiserWhen the voltage/frequency (u/f) ratio is higher thanthe optimum, the motor generates more noise due toover-magnetisation of the stator. Optimising the u/fratio will therefore help reduce the acoustic motornoise.

The VLT 6000 HVAC has a built-in Automatic EnergyOptimiser, which automatically adjusts the u/f ratioaccording to the load on the motor. The primarytarget of this was to reduce the power consumptionused to operate the motor, but a side effect is thatthe acoustic noise is reduced by up to 3 dBA;dependant on the degree of the over-magnetisationof the motor.

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VLT® Frequency Converters

4 MC.60.B1.02 - VLT is a registered Danfoss trademark

Comparison btw Noise Reduction Techniques

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130

Price Noiselevel

dU/dt RFI IGBTlosses

4,5 kHz

ASFM

White Noise

High SwitchingFrequencyLC filter

Cost and BenefitsThe diagram below give an indication of how thedifferent techniques compare.

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It is clear, that LC filter or high switching frequency gi-ves the best result on noise reduction. High switcingfrequency does however cause not only an increasein production costs and there by in the price of thedrive. It also increases the losses in the system,causes higher dU/dt levels and increased radiofrequency interferences (RFI). Only disadvantage ofusing the LC filter is the increased price.

White noise would have been an excelent solution, ifit did not cause the motor to sound as if it haddefective bearings.

ASFM is a better solution than a fixed low switchingfrequency and only adds a little to the system lossesat the same investment.

Motor Design InfluencesIt should be noticed that the noise generated by themotor is dependant on the motors design and theaccuracy of it‘s construction.

Investigations have shown that different motor designsrespond differently to the same levels of over harmoniccurrents. Two motors are being compared here. Formotor number one, the audible noise around theswitching frequency was less audible than around twotimes the switching frequency. For motor number twoon the other hand, the audible noise around two timesthe switching frequency was the least audible. Themain difference between the two motors was that

motor number one had few, but long, cooling ribs,whereas motor number two had many short coolingribs.

Other investigations have indicated that a smaller airgap between the stator and rotor will also helpreduce the motor noise level. These investigations arenot as well documented as the above mentioned de-sign issue, however.

Investigations of multiple motor brands and sizes haslead to the conclusion that there is not one motormanufacturer who has the optimum design to reducenoise levels. The best motor make varies dependanton the motor size. It has therefore not been possibleto isolate all relevant parameters in the motor design.

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