Stepper Vs. Servo Motor

The Stepper Vs. Servo motor debate is a very hot topic and you will find excellent arguments for both systems.  In reality, both types of motors work extremely well when the CNC machine is properly designed.

The following comparison assumes the quality, size and features (excluding feedback) are the same.  The characteristics are listed in Alphabetical order as opposed to relevance.

Characteristics Stepper Motors Servo Motors
Availability Stepper motors are readily available from a number of sources ensuring that we can purchase the motors and drivers in a timely basis minimizing back orders. Servo motors are harder to source and not as readily available.
Braking
Capabilities
Stepper motors brake extremely well. Servo motors do not brake as well as stepper motors.
Cost Stepper motors are less expensive than servo motors with the same power rating. Servo motors are more expensive than stepper motors with the same power rating.
Direct Drive
Capability
Stepper motors provide excellent torque at lower speeds (rpm) allowing the motor to be connected directly to a lead screw, ball screw or rack & pinion drive. Servo motors operate better at high rpm and not so well at low rpm.  In order to operate at normal feed rates when cutting a servo motor requires a gear reduction.
Efficiency Stepper motors consume more power and produce more heat and operate at about 70% efficiency. Servo motors are more efficient yielding between 80% and 90% under light loads.
Frame Size Stepper motors come in a variety of NEMA sizes; however, they do not come in as many frame sizes as servo motors in the large size motors. Servo motors are available in a wider range of sizes from smaller to large motors.
Heat Production Stepper motors draw current regardless of load.  The excess power is dissipated as heat. Servo motors draw current proportional to the load resulting in low heat production.
Motor Life Stepper motors only have one wearing part (bearings) resulting in longer motor life. Servo motor brushes should be replaced every 2,000 hours or operation.  Servo motors also have encoders that may need to be replaced periodically.
Noise Stepper motors produce a slight hum.  Noise does not affect the performance but it is noticeable. Servo motors produce very little noise.
Overload Safety Stepper motors are highly unlikely to be damaged by mechanical overload. Servo motors may be damaged by a mechanical overload.
Power Range Stepper motors do not have as broad of power range as servo motors. Servo motors have a broad range of power ranges.
Power to Weight Ratio Stepper motors are less efficient than servo motors resulting in a smaller power to weight ratio. Servo motors have excellent power to weight ratio.
Reliability Stepper motors are extremely reliable in a wide range of environments. Servo motors are less reliable than stepper motors because of the encoder.  The overall reliability or servo motors is also dependent upon the environment and how well the motor is protected.
Repeatability Stepper motors have very good repeatability with little or no tuning required. Servo motors have very good repeatability if setup correctly.  Encoder quality can also affect repeatability.
Reserve
Power & Torque
Stepper motors do not have reserve power. Servo motors can produce up to 200% of the continuous power for short periods of time.
Resolution Stepper motors have a broad range of resolution when using drivers with micro-stepping capabilities.  The resolution is determined by the driver. Servo motors also have a broad range of resolution which is determined by the encoder.
Resonance
and Vibration
Stepper motors vibrate slightly and may have resonance. Servo motors do not have vibration or resonance issues.
Setup Stepper motor setup is extremely easy.  They are practically plug-and-play. Servo motors are more difficult to setup requiring tuning of the closed loop variable circuit (PID) to obtain correct motor function.
Torque at
High Speed
Stepper motors rapidly lose torque at 90% of their maximum rpm. Servo motors maintain their rated torque up to about 90% of their no load rpm.
Torque at
Low Speed
Stepper motors provide excellent torque at low rpm. Servo motors require low friction and gear reduction to maintain torque at low rpm.
Torque to
Inertia Ratio
Stepper motors do not accelerate loads as well as servo motors. Servo motors are very capable of accelerating loads.
Versatility Stepper motors are very versatile in their use for CNC applications.  Because of their simplicity they are found in a very broad range of applications. Servo motors are also very versatile in their use for CNC applications but are not as cost effective or simple to setup and maintain.