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VFD Control Methods
V/f, sensorless vector, closed-loop vector or servo: how does the drive actually control the motor?
Every AC drive turns fixed mains power into variable frequency and voltage. What differs is how smart the control is. V/f control just keeps voltage in step with frequency. Vector control watches motor current and steers torque and flux separately. Adding an encoder makes it far more exact, and a full servo system adds precise position control. This page puts the four methods side by side: how each works, what each can and cannot do, and how to choose.
Figures are typical ranges to help you compare. Real performance depends on the drive model, motor, tuning and load, so use your drive manual and datasheet for design. Pictures are illustrative drawings.
Bars show a relative score: a longer bar means more. For cost, more is worse. Ranges come from drive-maker guides and are typical, not guaranteed.
V/f control keeps the voltage in proportion to the frequency so the motor flux stays about constant. A small boost at low frequency makes up for the voltage lost in the stator resistance. Fans and pumps often use a square-law pattern to save energy.
The inverter makes fast voltage pulses (PWM). Wider pulses at the peak and narrower ones near zero crossing shape a sine wave. At lower frequency the pulse widths shrink because the drive lowers the voltage along with the frequency. The motor inductance smooths the current into a sine.
Illustrative shapes. All four give constant torque up to base speed, then constant power with falling torque above it (field weakening). The big difference is at low speed. A self-cooled motor also needs derating at low speed because its fan slows down.
Uses the ideal affinity law for fans and centrifugal pumps: power falls with the cube of speed. A real system with static head (a pump lifting water, for example) saves less. Drive and motor losses also reduce the saving a little. Constant-torque loads such as conveyors save much less.
Vector control is not just better V/f
V/f sets voltage from frequency and ignores the load. Vector control measures or estimates motor current and rotor flux, then controls torque current and flux current separately, so torque responds to the load.
Sensorless is not encoderless guesswork
Sensorless vector uses a motor model plus measured current and voltage. It needs correct motor data and an auto-tune. It works well on most machines, but the estimate gets poor near zero speed.
Do not use sensorless on cranes and hoists
Zero-speed torque is not reliable without feedback. Hoists should use closed-loop vector or a servo-type drive with a brake sequence.
Closed loop does not mean positioning
An encoder gives the drive speed feedback. Positioning needs a drive with position control and an encoder you can trust. See Induction Motor Position Control for the limits.
Servo is a system, not a setting
Servo control means a servo drive, a permanent-magnet servo motor and a high-resolution encoder, tuned as a set. Some drives can run induction motors in a servo-like mode, but the motor and drive still set the limit.
One drive, many motors
Only V/f can run several motors from one drive. Vector and servo control are tuned to one motor, so they need one drive per motor.
Speed range, speed regulation and starting torque figures are typical values from drive-maker guides. They vary by brand and model, and the servo figures in particular depend on the motor and encoder. Always check the datasheet of the drive you plan to use.