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AC Drives for Industrial Motor Speed and Torque Control
AC drives, also called variable frequency drives (VFDs), control motor speed and torque by varying the frequency and voltage supplied to a three-phase induction motor. Within Galco's drives category they are the most common selection for general industrial motor control, distinct from DC drives for legacy DC motor applications and servo drives where precise positioning is required.
Size by motor full load amps (FLA) from the nameplate, not by horsepower. Horsepower ratings on drives assume standard motor efficiency; FLA is the actual current the drive must supply and is the correct sizing baseline. Duty rating is equally important: normal duty (variable torque) ratings suit fans and pumps where load increases with speed; heavy duty (constant torque) ratings are required for conveyors, compressors, and applications with high starting inertia or sustained full-load demand. Using a normal duty rating on a constant torque application will result in nuisance trips or drive failure under load.
Control mode determines low-speed and dynamic torque performance. V/Hz control is sufficient for variable torque loads and simple speed regulation. Sensorless vector control is required where high starting torque, precise speed regulation at low RPM, or dynamic load response is needed. Motors operating on a VFD should be inverter-duty rated per NEMA MG1 Part 31, particularly at 480V and above or with long cable runs, to handle the repetitive voltage spikes the drive's PWM output produces. For installation accessories including line reactors, filters, and conduit fittings, see Galco's drive accessories.
Frequently Asked Questions
Can I connect any AC motor to a VFD?
Standard induction motors can run on VFDs at lower voltages and shorter cable runs, but 480V systems and long cable runs produce high-frequency voltage spikes that degrade standard motor insulation over time. Inverter-duty motors with Class F or higher insulation and reinforced winding construction are the correct specification for new VFD installations. Running a standard motor on a VFD is a reliability risk, not an immediate failure.
What causes bearing failure on motors driven by VFDs?
The PWM switching output of a VFD induces common-mode voltage that capacitively couples to the motor shaft, producing electrical discharge across the bearings - a process that progressively erodes the bearing race, causing characteristic fluting damage. Shaft grounding rings and insulated bearings are the standard mitigation. Bearing current damage is more prevalent on larger frame motors and longer cable runs, and is frequently misdiagnosed as lubrication or mechanical failure.
When should I use a pre-engineered drive package instead of a standalone AC drive?
Standalone drives require panel integration including disconnect, fusing, bypass contactor, and enclosure selection. Pre-engineered drive packages include those components assembled and tested as a unit, reducing engineering time and installation labor. They are worth specifying when the application is straightforward but panel build capacity or time is the constraint.