Engineering · Motors & Drives
VFD vs Servo vs Stepper Motor
Speed, position or torque: which motor and drive fits the motion you need?
A VFD with an induction motor is made for variable-speed rotation. A servo motor with its drive is made for precise, fast position, speed and torque control. A stepper motor moves in discrete steps and gives simple positioning at low cost. The real difference is feedback: a servo measures what the shaft does, a stepper trusts that it did what it was told, and a VFD controls speed but not position. This page lets you see that in a move simulator, torque-speed curves and a resolution calculator. For the other motor types, see the Motor Control Technologies page.
An illustrative model, not a measurement. The servo follows the move through feedback. The stepper follows until the torque the move needs is more than the motor can give at that speed, and every missed step stays missed because nothing measures it. The VFD runs the motor at a speed set by frequency, so the distance depends on slip and on the ramp, with nothing correcting it. With an encoder a stepper can be made closed loop, and a VFD can run closed-loop vector control, but that is a different system.
Torque is shown as a percentage of each motor’s own continuous rating, with typical curve shapes. A VFD-fed induction motor gives constant torque up to base speed, then constant power. A servo holds its rated torque to rated speed and has a short-time peak of several times that. A stepper has its highest torque at standstill and loses it quickly as speed rises. The real curves are in the motor data sheets.
Microstepping divides each full step into smaller moves. It smooths the motion and gives a finer resolution, but it does not make the position as accurate as the resolution suggests, and the torque per microstep is small. A servo’s resolution is set by its encoder.
| Feature | VFD + induction motor | Servo motor + drive | Stepper motor + driver |
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| VFD + induction motor | Servo motor | Stepper motor |
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Ranges, curves and costs vary between makers and series. The values here are typical, to be confirmed in the data sheets. The simulations are simplified models for understanding, not for sizing. Isolate and lock out before working on a drive.