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Disadvantages of Vector VFD

While vector VFDs offer many advantages, they have a few limitations, too.

First, virtually all of today's vector VFDs require tachometer feedback from the motor. The tach increases the price of the AC motor, and there is cost associated with mounting, wiring, and maintaining it. However, brushless VFDs and many DC drives also require tachometer feedback when speed regulation better than 1% is required.

Second, regeneration is more difficult with vector VFDs and with brushless VFDs than with conventional thyristor DC drives. Snubbers, add-on regen­eration kits, and common-bus configurations have all been used for regenera­tion with vector VFDs, but DC drives have the advantage for low-cost, single-section regeneration.

Third, applications which need dynamic braking (slow-down braking without the VFD operating) will require different solutions when vector VFDs are used. DC and brushless VFDs make use of the fact that they can operate as generators without an active regulator to provide a dynamic braking function. Vector (and other AC) VFDs may require DC injection braking or mechani­cal brakes to provide the functional equivalent of dynamic braking. This is necessary because the regulator on AC VFDs must be active to brake the load.

Finally, a word about motors is appropriate. We noted that "standard" induction motors may be used with vector VFDs. However, the high-performance capabilities of these VFDs can place heavier demands on the motors they power. Special consideration should be given to using motors optimized for constant torque, high-overload duty over a wide speed range. Several manufacturers now offer "vector-duty" motors with tachometer-mounting provisions and blower-cooling. Some companies have gone a step further to design vector-duty motors with lower inertia and special construction features designed exclusively for variable-speed duty.

Vector VFD represents the step forward in performance that many applica­tions have needed to enjoy the advantages of AC technology. It is clear we will see them widely used in new applications. They can also be an alternative to existing mechanical and electrical VFDs.

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