Electronic Application Note (E-Note)

Drive Carrier Frequencies have far reaching
HVAC application considerations.
"Hear" the facts and decide how to better
specify and apply Variable Speed Drives.

A few facts about Drive Carrier Frequencies:

  • Higher drive carrier frequencies lower the audible noise emitted from a motor. Example: Drive running a motor at 2 kHz is 3 times as loud as a motor run on sine wave power.
  • Higher carrier frequencies cause higher losses from the power semiconductors in the drive. Example: For every increase of 1 kHz, the thermal losses from the drive increase by almost 2%. Stated a different way, for every 1 kHz carrier frequency increase, the drive maximum output current must be de-rated by 2% (or the heatsink must be larger and more expensive to get rid of the extra heat).
  • Six to eight kHz carrier frequency makes motors run cooler (refer to the IEEE paper for details), since they more accurately simulate a "pure" sine wave. Example: A fully loaded 100 HP motor has a skin temperature of 54.3 degrees Celsius when run at 2 kHz. The same motor has a skin temperature of 50.9 degrees Celsius when run at 4 kHz. Shaft end temperatures also increased by 8 degrees when run on 2 kHz versus 4 kHz carrier frequencies. Remember for every 10 degrees Celsius above the insulation systems maximum allowable temperature, life is cut in half.
  • Sound pressure dBa levels are logarithmic meaning an increase of 3 dB doubles the sound power level. Also pressure level is frequency insensitive, the human ear is not. To compensate for the sensitivity of the human ear, a scale know as a-weighting was developed to multiply the dB levels at more sensitive frequencies. Example: On the a-weighted scale, total mean pressure of a 100 HP motor measured at 3 feet is 75.24 dBa running at 4 kHz and 77.33 at 2 kHz. Human hearing is most sensitive between 500 Hz and 3 kHz. The a-weighted 2,000 center frequency octave value of a motor running at 2 kHz is 75.13 compared to 66.17 at 4 kHz. This sensitive hearing area is critical to apparent noise.
  • The turn-on time of an IGBT determines the slope or dv/dt of the pulse. This fast voltage rise stresses the motor’s insulation system more than slower switching. Why do drive designers switch the IGBT’s so fast? It allows them to make heat sinks as small as possible, and it allows operation at higher carrier frequencies while maintaining stable drive operation. Maximum carrier frequency is a major factor in setting this turn-on time, it is not user selectable. Example: A drive with a maximum carrier frequency of 12 kHz has a turn on time of less than .2 Microseconds and a dv/dt of 3200 volts per Microsecond. By comparison sine wave voltage dv/dt values are .032 volts per Microsecond, about 100,000 times different!

 

Newer Transistors Turn On Faster

The following graph shows the difference in rise times of the IGBT power devices utilized in currently produced drives (VTAC 7) and previous generation (VTAC V) BiPolar Junction Transistors or BJT’s drives.

Voltage Peaks may be different at
Drive and Motor ends of the Cable

Lower dv/dt from the drive allow running a motor at greater cable distances. As these "fast" pulses propagate down the cable, standing waves can cause them to add onto one another causing extreme voltages at the motor terminals. These voltages although only present for a short time can reach 2,500 to 3,000 volts. These "spikes" cause pin hole damage to the 600 volt rated motors and turn to turn shorts. Example: Drive with a rise time of .2 Microseconds should only be applied when it is within 50 feet of a motor. A drive with a slower .5 Microsecond rise time an be applied up to 250 feet without concern for motor insulation breakdown.

 

 

THE DATA:

AMCA certified Sound Room Tests

Hearing is Believing:

The following wave sound recording has 15 second clips beginning with 2 kHz then 4 kHz and finally 8 kHz drive carrier frequencies. These recordings were taken on a fully loaded 100 HP 1800 RPM test motor at full speed.

Note the different tones of the motor noise as well as the level or pressure of the noise.
Click here to download and listen to an Audio File (.wav format) to your PC.(963kb)

Click here to download and save the Audio File (.wav format) to your PC.(750kb)

How Do Drive Manufacturers Rate Their Products?

Data presented is compiled from Manufacturers Product Literature March 1997.

D-152 May 1997