EMC Tests of VFDs
Test 1 : Measure the relative EMC voltages for different types of VFD - Motor connection.
The voltage between the frame of the motor and the frame of the reference VFD are measured using a 100MHz oscilloscope with a 470 ohm termination resistor.
The 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 |
Test 2: Compare results for a range of different VFDs.
The voltage between the frame of the motor and the frame of the reference 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) |
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.
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