The difference between the DC motor, stepper motor and servo motor

DC motor
-DC (Direct Current) Motors are two wire (power & ground), continuous  rotation motors. 
-The speed of DC motors is controlled using pulse width modulation  (PWM), a technique of rapidly pulsing the power on and off. The    percentage of time spent cycling the on/off ratio determines the speed of    the motor, e.g. if the power is cycled at 50% (half on, half off), then the    motor will spin at half the speed of 100% (fully on). Each pulse is so rapid    that the motor appears to be continuously spinning with no stuttering.

Servo Motor:
-Servo motors are generally an assembly of four things: a DC motor, a       gearing set, a control circuit and a position-sensor (usually a     potentiometer).
-The position of servo motors can be controlled more precisely than those of   standard DC motors, and they usually have three wires (power, ground &).   Power to servo motors is constantly applied, with the servo control circuit   regulating the draw to drive the motor.
-PWM is used for the control signal of servo motors. However, unlike DC   motors it’s the duration of the positive pulse that determines the position,   rather than speed, of the servo shaft.

Stepper Motor:
-Stepper motors utilizes multiple toothed electromagnets arranged around   a  central gear to define position.
-Stepper motors require an external control circuit or micro controller (e.g.   a  Raspberry Pi or Arduino) to individually energize each electromagnet   and  make the motor shaft turn. The working accuracy of stepper motor is not as good as servo motor but the life time of stepper motor is usually longer as it is not easily broken.

To summarize the above discussion: There is a table given on the following page.
Choosing a Motor: DC, Stepper, Or Servo.

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