EMC Tests of VFDs

Test 1 : Measure the relative EMC voltages for different types of VTdrive ® VFD - Motor connection.

The voltage between the frame of the motor and the frame of the reference VTdrive ® VFD are measured using a 100MHz oscilloscope with a 470 ohm termination resistor.
The VTdrive ® VFD was operated at 33Hz and the the cable and terminations between the VFD and the motor were altered

Output Connection.	EMC Voltage
EMC Screened Cable with EMC Glands to VFD and Motor.	0.82Vpk-pk
EMC Screened Cable with 50mm Pigtail.	3Vpk-pk
EMC Screened Cable with 100mm Pigtail.	7Vpk-pk
EMC Screened Cable with 250mm Pigtail.	15Vpk-pk
EMC Screened Cable with 500mm Pigtail.	30Vpk-pk
Neutral Screened Cable with EMC Glands to VFD and Motor.	4.8Vpk-pk
Unscreened 4 core cable from VFD to Motor.	96Vpk-pk

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Test 2: Compare results for a range of different VTdrive ® VFDs.

The voltage between the frame of the motor and the frame of the reference VTdrive ® VFD are measured using a 100MHz oscilloscope with a 470 ohm termination resistor.
Each VFD was operated at 33Hz and mesurements were made for a correctly terminated screened cable and an unscreened cable.

VFD type	Unscreened Cable	EMC Screened Cable
A	96V pk-pk	0.82V pk-pk
B	40Vpk-pk	1.4V pk-pk
C	162V pk-pk	0.7V pk-pk
D	88V pk-pk	1.2V pk-pk
E	106V pk-pk	3.2V pk-pk (pigtail only means of connection)
F	101V pk-pk	1.6V pk-pk (Output ferrites required)

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EMC Test Conclusions

• To minimise stray noise currents, a screened cable must be used on ALL VFDs.
• The screened EMC cable MUST be correctly terminated at both ends. - clamp or EMC gland.
• Pigtails MUST NOT be used to terminate the screen.
• Good results require the right VFDs and the correct installation practices. - Not all VFDs are equal.