Choose a proper motor control method for your application

Every motor driven application needs to be evaluated and the starting and control method properly chosen to meet those needs. If variable speed is not required for the process and the power and mechanical systems are robust the direct-on-line (DOL) starting may be the best choice. If the power system and / or the mechanical system are weak and only fixed speed is required for the process then a reduced voltage starter with bypass may be the best choice as long as the reduced starting torque these starters produce is large enough to accelerate the load to full speed in a reasonable time to prevent overheating of the motor rotor, stator and / or the starter.

From a pure torque production perspective, a variable frequency drive (VFD) provides more torque per ampere than any other starting method. I have applied VFDs on centrifugal fan applications where the load inertia was high and the power system was weak and the only way to reliably start the load while not drawing not more than rated current from the motor was with this controller. It took perhaps two minutes to accelerate to full speed but no one cared because the motor was never overloaded. Once the motor was up to speed a bypass contactor was employed to eliminate the losses from the VFD.

VFDs like any medicines have potential side effects. But with careful consideration of these side effects such as harmonics, EMI / RFI, Common-Mode Currents, Reflected Waves, etc. etc. VFD can be successfully applied with minimal impact on other systems.

From an energy savings perspective the only applications that really qualify for potential significant savings are centrifugal fans, pumps and compressors where the application bears out that most of the time they will be operated below the base speed of the motor. The decision to use of VFDs on all other applications should be made on the basis of process and mechanical requirements, not energy savings.

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