A variable frequency drive (VFD) does not deliver a smooth sine wave to the motor. It switches its output on and off at high speed, and those fast voltage edges travel down the motor cable. Without the right cable and installation, the result can be electrical noise in nearby circuits, damaging voltage peaks at the motor, and early bearing wear. This checklist covers the cable and installation points to settle before the drive is commissioned. The drive maker’s manual remains the governing authority and overrides this list where the two differ.
Cable selection
| Item | What to check | Why |
|---|---|---|
| Cable construction | A symmetrical, screened or armoured cable with three phase conductors and a symmetrical earth arrangement, in the form the drive maker recommends | Balanced geometry reduces common-mode current, and the screen contains radiated noise |
| Conductor size | Size for the drive output current and the installation conditions, not for the motor rating alone | The drive output current can differ from the motor nameplate current |
| Insulation | Choose insulation suitable for repeated voltage peaks | Fast edges stress insulation more than a sine wave of the same rating |
| Cable length | Keep the run as short as practical, and stay inside the maximum motor cable length the drive maker specifies | Long leads raise voltage peaks at the motor |
Installation
| Item | What to check | Why |
|---|---|---|
| Screen termination | Terminate the screen all round (360 degrees) at both the drive and the motor, using an EMC gland or a clamp | A pigtail connection loses most of the screening benefit at high frequency |
| Earth bonding | Keep the earth connections short and bond the drive, the motor frame and the cable armour or screen | Gives noise currents a low-impedance return path |
| Segregation | Separate the motor cable from signal, instrument and data cables, and cross them at right angles where they must meet | Limits coupled noise |
| Cable joints | Avoid joins in the motor cable, or use a joint that keeps the screen continuous | A break in the screen creates a noise path |
| Parallel cables | Where several cables are run in parallel per phase, keep them of equal length and matching layout | Unequal sharing overloads one cable |
Mitigation at the drive
Where the lead is long, the motor is old, or the insulation class is uncertain, an output reactor or a dv/dt filter fitted at the drive reduces the voltage peaks that reach the motor. The drive maker can advise on the filter and on the limits for cable length with and without it.
Commissioning checks
- Confirm the screen continuity from drive to motor with a meter before energising.
- Check insulation resistance on the motor cable with the drive disconnected, as the site testing checklist before energising a power cable describes.
- Inspect all glands and screen terminations for a clean, full contact.
- Verify that signal cables near the drive show no noise when the drive runs at full speed.
APAR cable references
APAR makes LV XLPE insulated power cables in armoured and unarmoured forms, and screened instrumentation cables for the signal circuits that run near a drive. For a drive motor cable with a specific screen and earth arrangement, send the drive model and the cable length to the APAR technical team for advice. The cable gland selection guide covers the gland side of the termination.