Do I need an encoder to run a step motor, right?

Step motors offer the automation industry a cost-effective and simple method to digitally control motion in a wide range of applications — including packaging equipment, 3D printers, material handling and sorting lines, benchtop CNC machines, and more. They serve as critical components of many rotary and linear positioning axes.

No, an encoder is not required to run a step motor in open-loop control. Step motors are the only type of brushless DC motor that accurately and repeatedly position a load using open-loop control. Other motors need some type of position feedback. Open-loop control works well when:

Motion tasks are the same over time.
The load doesn’t change.
The required speeds are relatively low.
Failure to complete the motion task does not result in critical or dangerous machine failure.
If the application doesn’t meet the stated criteria, consider introducing feedback into the system to permit some level of closed-loop control. Adding an encoder to a step motor system offers benefits ranging from basic functions that are essentially open-loop control but with subtle, effective improvements, to fully closed-loop stepper motor control where the step motor operates as part of a servo control system. Contact your step motor and drive supplier for information on the range of feedback and closed-loop control options they offer.

NEMA frame size used for stepper motors and brushless DC motors
Some Knowledge of Shaft Style, Motor Size and Gearing