Engineering · Learn · Motion
Motor Control Technologies
AC induction with a VFD, servo, stepper, brushless DC or brushed DC: which motor and drive for which job?
Motor control means starting, stopping, setting the speed, holding the position and limiting the torque of an electric motor. The motor is only half the answer. Each type needs its own drive and its own control method. This page puts the five most common motor technologies side by side, explains how each one works, compares them honestly and helps you shortlist one for your machine.
Numbers on this page are typical values to help you compare. Real values depend on the brand, size, encoder, gearbox and load, so always size from the motor and drive catalogue. The pictures are illustrative drawings, not product photos.
Bars show a relative score: a longer bar means more of it. For cost, complexity and maintenance, more is worse.
Typical best-case accuracy at the motor shaft, indicative only. It depends on the encoder, gearbox, backlash and load. Resolution (the smallest step you can command) is not the same as accuracy (how close you actually get).
A VFD does not position
A VFD sets speed and torque. Stopping at an exact point needs an encoder and a positioning drive, or a different motor type. For indexing and pick-and-place, use a servo or a stepper.
Servo is a system, not a motor
A servo is the motor, the drive and the feedback device working in a closed loop. Most servo motors are permanent magnet synchronous motors (PMSM), close cousins of BLDC motors.
BLDC and PMSM are not the same
A BLDC motor has a trapezoidal back-EMF and is usually driven with simple six-step commutation. A PMSM has a sinusoidal back-EMF and is driven with field-oriented control for smoother torque.
Stepper: resolution is not accuracy
Microstepping makes motion smoother and the commanded step smaller. It does not make the motor more accurate. A standard stepper is accurate to about 5 % of one step, and the error does not add up. But an overloaded open-loop stepper can skip steps without telling you.
Stepper torque falls with speed
Steppers have strong holding torque but lose torque as they speed up. Size them from the torque-speed curve, not the holding torque.
Brushed DC is not dead
Brushes wear and make noise, but a brushed motor with a PWM drive is the simplest and cheapest way to get variable speed from a DC supply. It is common in toys, small tools and vehicle accessories.
Control accuracy figures are typical and indicative. Product names are not mentioned because the five technologies are available from many makers.