Variable Frequency Drives

Although the variable frequency drive becomes a single point of failure, motor and connected load reliability actually improve in many applications as there are now multiple smaller motors as opposed to one large motor. If one motor fails, it's often possible to continue operation with the remaining motors. In these cases, the remaining motors run at a speed controlled by the variable frequency drive, and operate the overall system at reduced but often still viable capacity. Variable frequency drive shall minimize the audible motor noise through the use of an adjustable carrier frequency. variable frequency drives without this capability must include LC filters on the output of the VFD. Building structure steel is usually the best connection for zero voltage True Earth (TE) potential since girders are connected together in a low impedance grid pattern that have multiple column paths into ground. Ground resistance measurements of 1 to 2 ohms between columns is typical. Ground resistance is affected by soil resistivity which is also a function of moisture content. There have been instances where TE was low impedance until the summer months when the ground water table dries up. Ground rods driven into a plant floor have exhibited 1,000 - 5,000 ohms impedance between them and building structure steel due to stones and dry rocky soil under the concrete floor. However, ground rods in low resistivity soil may be adequate. Variable frequency drives full load amp rating shall meet or exceed the motor rated current. Variable frequency drive manufacturers shall be able to provide full rated output current continuously, 110% of rated current for 60 seconds and 180% of rated current for up to 0.5 second while starting.