Timely tips for dealing with drive accessories

Beware of VFD-induced motor currents.

A variable frequency drive (VFD) refers to AC drives only and a variable speed drive (VSD) refers to either AC Drives or DC Drives. VFDs vary the speed of an AC motor by varying the frequency to the motor. VSDs referring to DC motors vary the speed by varying the voltage to the motor.

Engineers are often unaware of the currents induced on motor shafts by variable frequency drives (VFDs) and the havoc these currents can wreak on bearings and motors; remember to use shaft grounding effectively.

Don’t make assumptions

Don’t assume that “inverter duty” motors are designed to prevent bearing damage. Most of these motors only protect windings, not bearings.

Bearings need protection

Machines typically quit working due to a failure to protect the motor’s Achilles’ heel – the bearings. Inadequate shaft grounding increases the possibility of electrical bearing damage in VFD-driven motors; electrical discharges can scar the race wall. During each VFD cycle, currents discharge from the motor shaft to the frame via the bearings, leaving small pits in bearings and race walls. Damage eventually leads to noisy bearings, but by the time noise is noticeable, bearing failure is often imminent.

Diminish downtime with grounding rings

Downtime is often so costly that it can wipe out the energy savings obtained with a VFD. In some applications, a momentary production stoppage due to motor failure can cost more than $250,000. Microfiber ring grounding technology offers more sustainability by protecting bearings for the motor’s life; rings are maintenance free, unaffected by dirt or grease, and easily installed on any NEMA (National Electrical Manufacturer Association) or IEC (International Electrotechnical Commission) motor.

Pay attention to drive alerts

Drive failures give warning signs. The best defense is to monitor alarms and faults for abnormalities. Ensure that the drive and its accessories are operating within specified environmental limits to avoid failures. If limits must be surpassed, check with the manufacturer for proper over-sizing information.

Danfoss Output Filters
Danfoss Output Filters

Know your system

If the drive input will be connected to a source that has a large short-circuit current, it may be necessary to limit that current via a reactor or isolation transformer to avoid drive damage.

Consider a filter

Drives generate high-frequency line disturbances; so if electrical noise is a concern, use an input RFI (Radio Frequency Interference) filter. If the run to the motor is long – over 250 ft – it’s also a good idea to include an output reactor to avoid motor dV/dT damage.

Choose cable carefully

Electrical phenomena in a VFD system can affect the drive and motor, as well as the cable that connects them. Be sure to choose VFD cable specifically engineered for this application.

Know your cable specs

A stress control layer or “conductor shield” is critical in a VFD cable because these cables can experience high electric fields and partial discharges. VFD cables should be rated 1,000 V continuous/2,000 V peak and include a stress control layer.

Electrical noise demands a properly grounded, double-shielded cable

To mitigate EMI (Electromagnetic Interference) and RFI effects, unshielded cable is often run in grounded metal conduits or, in less expensive methods, shielded cables are run in PVC conduits or metal trays. Due to significant noise in VFD systems, double-shielded VFD cable should be used. Look for a 100% foil shield along with a braid shield.


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