The Difference between Stepper Motor and Servo Motor

The Principle of Stepper Motor and Servo Motor:

　　The stepper motor is a mechanism that converts electrical pulses into angular displacement. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in a set direction.

　　Angular displacement can be controlled by the number of pulses and achieves the goal of accurate positioning. Besides by controlling the frequency of the pulse, to control the speed and acceleration of motor rotation, and achieving the goal of speed regulation.

　　The rotor of the servo motor is a permanent magnet. The U/V/W three phases of electricity which is controlled by the drive forms electromagnetic field. The rotor will rotate under the action of the magnetic field. At the same time, the encoder will send a feedback signal to the drive, which will adjust the rotation angle of the rotor according to the difference between feedback value and target value. The precision of the servo motor depends on the accuracy of the encoder.

Specific Difference:

　　(1) Different control modes:

　　The rotation angle of the stepper motor is controlled by the number of pulses, one pulse corresponds to a step angle.

　　The servo motor controls the rotation angle by controlling the length of pulse time.

　　(2) Different work equipment and workflow:

　　The power supply required of the stepper motor is a pulse generator, a stepper motor, a driver; the workflow is that the stepper motor requires two pulses: a signal pulse and direction pulse.

　　The power required by the servo motor is a switch, a servo motor; its working process is that a power connects with switch, and then the servo motor is connected.

　　(3) Low-frequency characteristic:

　　The stepper motor is prone to produce low-frequency vibration phenomena at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally considered that the vibration frequency is half of the no-load jump frequency. The low-frequency phenomenon, which is determined by the working principle of the stepper motor, is harm to the normal operation of the machine. When the step motor works at low rpm, damping technology which includes adding a damper or using subdivision technology on the driver will be used to overcome the low-frequency vibration phenomenon.

　　The AC servo motor runs smoothly and doesn’t vibrate at low speeds. The AC servo system has a resonance function, which can cover the lack of rigidity of the machine, and has a function of frequency analysis(TTT), which can detect the resonance point of the machine and facilitate the system adjustment.

　　(4) Different torque-frequency characteristics:

　　The output torque of the stepper motor decreases with the increase of speed and will decrease sharply at high speed. So the maximum speed is about 300-600r/min.

　　The AC servo motor has constant torque output, in other words, it will provide rated torque at 2000-3000r/min. Beyond rated torque, it produces constant power output.

　　(5) Different overload capacity:

　　Generally, the stepper motor doesn’t have overload capacity, while the AC servo motor has a strong overload capacity.

　　For not having the overload capacity, to overcome the inertia torque, a larger torque motor will be used. But during normal operation, the machine doesn’t require such a large torque, so the torque is wasted.

　　(6) Different speed response performance:

　　From standstill to working speed, the stepper motor requires 200-400ms. The AC servo system has a better acceleration performance, it just takes a few milliseconds.

　　References:

　　“Comparison of Stepper Motor and Servo Motor”, Wang Yong, 2010, Western Coal Chemical Industry.