VFD Motor cable selection consideration

Summary

VFD Motor cable selection consideration

Question

What should be considered in selecting motor cable for use with 600VAC VFD (variable frequency drive) except the length restriction?

Environment

• Low Voltage AC Drives (non Motion/Servo)
• AC motors

Answer

The motor cable length limitation is mainly for protecting the motor from Reflected Wave damage and capacitive charging current.
The majority of recommendations regarding drive cables are for issues caused by the nature of the drive output. A PWM drive creates AC motor current by sending DC voltage pulses to the motor in a specific pattern. These pulses affect the wire insulation and can be a source of electrical noise. Consider the rise time, amplitude, and frequency of these pulses when choosing a wire/cable type.
Consider these factors when choosing a cable:

• The effects of the drive output once the cable is installed.
• The need for the cable to contain noise caused by the drive output.
• The amount of cable charging current available from the drive.
• Possible voltage drop (and subsequent loss of torque) for long wire runs.

All 600v rated XLPE(polyethylene) cables are adequate to handle the 2 per unit reflected wave transient per our test results.
600v PVC cables will be suspect based on insulation thickness and environment conditions. Applications where moisture is prevalent in the environment should refrain from using THHN (PVC insulation) wire with IGBT based drives.

For 480-600 Vac rated applications, 600 Vac rated cabling from drive to motor will suffice. Normally, based on drive to motor dc bus levels, due to surge impedance, the cable/motor are subjected up to 2 x dc bus levels. This could cause of concern and possibly use much higher voltage rated cables. Typically 600 Vac cables have much higher dielectric strength to withstand these IGBT pulses. In addition note that these are peak voltages and not a constant voltage to the cable/motor. Therefore the 600 Vac VFD rated cable should suffice. Contact cable manufacturing supplier to discuss such application issues.

Cable Recommendations:

• A 3 conductor with ground, PVC jacketed, shielded type Tray Cable with XLPE conductor insulation designed to meet NEC code designation XHHW-2 (wet locations) is recommended. It is superior to loose wires in dry, damp and wet applications and can significantly reduce capacitive coupling and common mode noise.
• A 3 conductor armored cable is also superior to loose wires in dry, damp and wet applications and can significantly reduce capacitive coupling and common mode noise.

For details, refer to Wiring and Grounding Guide, (PWM) AC Drives, Publication Drives-IN001M-EN-P.

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Source: Rockwell Automation’s Technote database.