Variable Speed Drive Q & A

Many of you have written in with questions regarding installation methods, drive technologies, operating parameters, and other issues. Since it is often time-prohibitive to respond individually to all of them, I thought I would consolidate and address some of the more common – and commonly applicable – questions that have been submitted.

• “Does the wiring between the VFD and motor require special cabling?
  Can we just use standard 600V wiring such as THHN, THWN, etc.?
  We come across cables that are noted as “VFD rated”. Are these cables required for all VFD installations?”

Drive manufacturers typically recommend shielded, symmetrically constructed cables to reduce the impacts of electromagnetic interference and capacitive coupling. In cases where compliance with European (CE) electromagnetic compatibility is required, these types of cables, properly installed and bonded, are mandatory. Where EU compliance is not a concern, the suitability of the cable used will depend on several factors, including cable length, sensitivity of nearby components and equipment to radiated interference, motor size, and installation methods. Typically, motors rated less than 40 hp, and less than about 100′ of lead length (per phase), can be fed with 600V single conductor cable as long as the conductors are encased in metallic conduit which provides a continuous bond and which is spaced to properly segregate the load conductors from line-side and signal wiring. However, care must be taken to ensure good grounding practices are followed, a properly sized earthing conductor (grounded conductor) is used, and any sensitive equipment nearby is shielded or separated by distance. For motors 30kW/40 hp and up, and/or more than about 100′ away from the drive, using unshielded and/or asymmetrical cable may result in noise problems.

• “It was suggested that we could operate multiple chillers at 95% max rather than 100% and not lose much efficiency. Do you have an opinion?”

The answer depends in part on the design of the chiller equipment and the properties of the electrical system supplying the chillers. If the pumps were sized such that 100% output would require the motor(s) to operate at 100% of base speed, then assuming the chiller pumps were centrifugals a 5% reduction in speed would result in a 14% reduction in power required. This would be offset to some degree by a decrease in motor efficiency due to increased losses in a motor fed by the non-sinusoidal supply of a VFD. The extent of decrease in motor efficiency depends on several site-specific factors, including motor design, and so can’t be generalized. One generalization can be made, however – the closer to the motor’s base speed you operate  a VFD, the less likely you will be to overcome motor efficiency losses and achieve significant savings. In this case, keep in mind also that if the process requires fluctuations in speed and the motors are not required to run at 95% all of the time, additional opportunities for savings exist.

• “How do I sell my supervisors on using training programs and emphasizing proper VFD maintenance to ensure we do not have process interruptions?”

First of all, common sense would seem to dictate that the expenses related to proper VFD operation and maintenance are small compared to the capital costs required for VFD purchase and installation in the first place. Once the potentially high costs of downtime are figured in, the cost/benefit ratio of good training and maintenance drops even further. Looking at equipment or production downtime data will usually quickly reveal that the associated costs far exceed the cost of maintenance. If drive failure does not immediately involve production downtime but rather in facility systems losses, then compare the cost of drive repair or replacement to maintenance and you will likely still come out ahead by taking care of what you already have.

•   “How do servo drives differ from other drives?”

Servo drives are typically used in applications where accuracy of positioning and speed is of critical importance. When driving servo motors and coupled with closed loop feedback devices such as encoders, they are capable of very accurate positioning and rapid adjustment to wide variations in process operating speed. An example would be a bottle labeling application in a high-speed conveying operation. Servo drives differ from “standard” variable speed drives primarily in the firmware and software included with the drive. The servo drive is equipped with control schema to allow precise position control and rapid response.

I hope the answers to these questions have provided some insight into your particular applications. Please feel free to share your feedback in the Comments section of this blog, or contact us at simon.fan@vtdrive.com. And please join me next week for another issue.