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Radiated and Conducted Emissions
from Variable Frequency Drives

Copyright © 2011 Francis J. Martino

From the e-mail:

Question:
I am looking for a way to eliminate the EMI and RFI interference from a VFD.
The drive is on an old piece of equipment whose manufacture is not in business
any more. The drive is producing so much electrical noise that our equipment
is overwhelmed. Are there any devices that can be installed on the motor and
controller to stop the interference from this drive?


Answer:
Please see the following link for a description of EMI/RFI generated by VFDs: AC Drives and EMI/RFI Mitigation
Figures 1 through 4 give diagrams that will aid in determining if the proper wiring and grounding methods are used. They will also assist in determining which type of equipment to use.
Figure 2 shows the proper placement of common mode chokes.
Common Mode Chokes will attenuate EMI/RFI which is generated by the output devices of the drives. Place the cores as close to the drive as possible.
Specifications on the cores may be found here: Magnetec Common Mode Chokes
Each core places impedance in the EMI field. Notice there is an amperage rating for each core. If the amperage ratings for the cores are exceeded then the cores will become saturated and lose the ability to attenuate.
The ideal way to select the cores is to measure the EMI field intensity to determine the peak common mode current. Fluke may have a scopemeter that may be useful.
Operating temperature range of the cores are -40 to +120 Celsius. If the cores are running too warm then add more cores two at a time. If you can hold your hand on them then no additional cores need to be added.
The manufacturer suggests four cores as shown in their spec sheet. However, for lower core temperatures and to avoid saturation, start with eight cores.
For a quote on cores please send the following info: Motor volts, HP, motor full load amps, motor load peak amps if different than full load amps. Diameter of the motor cable if all conductors are in one cable. Diameters of each motor cable if the cables are separate and are not bundled in one cable.
All three drive output power conductors must pass through the core as shown in the above mentioned Figure 2. The cores must be selected to allow room for bending that may be in the cables and also space to allow cooling.
Cores are in stock in New Jersey. Your cost will be approximately $250 to $350 each.
Electromagnetic emissions from LS Industrial Systems drives are noted in the attachment. The conducted emissions are 0.15~30MHz. The radiated emissions are 30~1000MHz (1000MHz = 1 GHz).
The common mode chokes will attenuate the conducted emissions. An EMC filter will attenuate the radiated emissions at the C2 and C3 levels. The LS Industrial Systems Model iS7 yields the following emissions: 22kW and below are at the C2 Level, 30kW and above are at the C3 level. Graph of Radiated and Conducted Emissions
If the common mode chokes are not sufficient then replace the drive with a model iS7 drive that has the optional EMC filter. Model iS7



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