Always check the drive’s deadband setting, which is used to reduce the reaction to noise. If not carefully adjusted, it can add deadtime and cycling.
An Attractive Option
A VFD can provide much needed power reduction and good operation. Torque and amps developed by an AC motor will determine the size of VFD needed for an application. Always size a VFD based on the load, which is expressed in amps, never on horsepower alone.
As with any instrument or control device, making the most of a VFD requires care and knowledge, including an appreciation of how control with a VFD differs from that with a control valve.
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Making the most of a VFD requires paying careful attention to installation as well as staff skills and training.
Installation considerations. You can’t just mount and wire a VFD any which way. You must follow national electrical codes (NEC) as well as the manufacturer’s recommendations. Pay particular attention to the following six aspects:
1. Wire and cable size, insulation and shielding. Shielded power cable is necessary to prevent interference with other plant instrumentation and controls. Lengths are important; remember at reduced motor voltage and rated current, a larger percentage of the power delivered is dissipated in the power cable.
2. Power distribution. A grounded secondary is preferred. Use the AC line impedance to determine if an isolation transformer or line reactor should be installed.
3. Grounding. Building steel can be used. Ground the shields at a single point to avoid ground loops.
4. Drive installation. For best noise immunity separate the power and signal wiring. Mount the drive in a place that’s accessible; see the NEC for required distances. Consider heat dissipation — the VFD electronics generates heat that must be removed. Remember, the cooler the electronics, the longer the life.
5. Reflective wave issues. PWM waveforms create fast rise-time waveforms on the power cables. The peak value of these waves can be high and cause premature insulation breakdown. Power cable with XLPE insulation is recommended.
6. Electromagnetic interferences. Because of the high frequency components in the PWM waveform, the VFD generates interference. Twisted, shielded cable is recommended for the drive wiring.
A motor and load controlled by a VFD may suffer premature bearing failure and lubricant chemical breakdown due to electrical discharge of high voltages induced in the shaft, so-called electric discharge machining or EDM . The problem is relatively new and is attributed to common mode voltage being induced in the shaft due to common mode voltage between each phase and the neutral. The fast transients generated by the IGBT devices cause high frequencies that are induced in the shaft. The problem can be alleviated by several methods, one of which is to use shaft-grounding bushings.
1. Input supply voltage is landed on the correct terminals.
2. Output wiring going to the motor is landed on the correct terminals.
3. Control wiring is landed on the correct terminals.
Staff skills. Plant personnel don’t need to be rocket scientists to program VFDs — reasonable reading skills coupled with some application knowledge and motor information will suffice.