An Introduction to Stepper Motors

 

A stepper motor is an electric motor without brushes. It converts digital pulses into mechanical rotations. In the case of a non-overloading stepping motor, the motor speed and stepping location depend on the frequency and number of the pulse signal. They are not affected by the changes in the load.

 

When the stepping driver receives a pulse signal, it will set the direction of rotation at a fixed angle, referred to as “step angle.” This mechanism is based on the step by step movement of a fixed rotation angle. To obtain an accurate position, you need to control the number of pulses and angular displacement. On the other hand, regulating pulse frequency results in controlling the motor rotation speed and acceleration.

 

Why Choose a Stepper Motor?

 

1)  The rotation angle is proportional to the number of pulses.

 
2)  There is a maximum torque when the motor stops.

 
3)  It offers better positional accuracy and repeatability of the movement.

 
4)  Functions flawlessly on start, stop and reverse response.

 
5)  As there is no brush, the motor life depends on the bearings.

 
6)  The digital input pulse determines the response of the motor. The designing process can be relatively simple and affordable as it is possible to use an open-loop control.

 
7)  The load can also be connected directly to the motor shaft at a very low-speed synchronous rotation.

 
8)  As the speed is proportional to pulse frequency, it has a relatively wider range.

 

How to Select the Right Stepper Motor?

 

When it comes to selecting the right stepper motor, you need to consider three factors including step angle (related to the number of phases), static torque, and current of the three elements.

 

1)  Step Angle

 
The angle is dependent on load requirements of precision, the minimum resolution of the load conversion to the motor shaft, and the number of angles including deceleration.

 
Standard stepper motor angles are –

 
  • Five phase motors – 0.36 degrees /0.72 degrees
  • Two or four-phase motors – 0.9 degrees /1.8 degrees
  • Three-phase motors – 1.5 degrees / 3 degrees
 

2)  Static Torque

 
Compared to the dynamic torque, the static torque is easier to determine. You can calculate the working motor load based on the static torque. It is usually of two types, inertia load and friction load. A single inertial load and a single load of friction don’t exist. You need to consider both types of loads if a motor starts directly. In case the motor accelerates, you need to consider the inertia load. However, for a constantly running motor, only friction load needs to be considered. Static torque should be 2X to 3X the friction load. Once static torque is selected, the frame and length of the motor can be determined readily.

 

3)  Current

 
Just like the static torque of the motor, different currents have different characteristics. Usually, a torque-frequency characteristic curve is used to determine the motor current.

 
You can obtain a linear motion by connecting a lead screw/worm gear drive system to a rotary stepper motor. This combination is called a linear stepper motor.

 
Standard AC stepper motor categories by POWER JACK MOTION:

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