Cable Knowledge
How to Choose Cable for a VFD Motor
Variable-frequency drives produce fast switching waveforms that differ from sinusoidal utility power. The motor cable, drive, motor insulation, grounding and output filtering should be selected as one system.
Electrical Stress
Fast voltage rise times can create reflected waves on long leads and raise motor-terminal voltage. Cable capacitance also influences drive current and permissible length.
Shielding and Grounding
A low-impedance shield path can contain high-frequency common-mode current and reduce emissions. Terminate shields circumferentially using compatible glands or connectors where the system design requires it.
Cable Construction
VFD cable may use XLPE insulation, overall copper tape or braid, symmetrical protective conductors and an industrial sheath. Required construction depends on voltage and drive manufacturer guidance.
Route and Accessories
Separate drive output from sensitive signal cables. Coordinate glands, motor terminal box, filters, reactors and bonding. Avoid assuming ordinary control cable is suitable for motor power.
Motor Lead Length Changes the Problem
A short drive-to-motor run may work without the same mitigation needed on a long run. As length increases, reflected-wave voltage, cable capacitance and common-mode current can require different cable or output filters.
- Use drive manufacturer length limits.
- Terminate shields with low high-frequency impedance.
- Bond motor and drive enclosures effectively.
Information to Confirm
- Drive and motor ratings
- Carrier frequency and output waveform
- Motor lead length
- Cable routing and EMC environment
- Shield and grounding arrangement
- Drive manufacturer's cable limits
Engineering note: VFD cable design must follow the drive and motor manufacturers' requirements and the local installation code.
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Continue reading
- How to Read Cable Markings and Technical Data Sheets
- Cable Factory Testing and Pre-Shipment Inspection
- Cable Installation: Bending Radius, Pulling and Storage
- Cable Drum Length, Packing and International Shipping
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