Stepper Motor Basics

Stepper Motor Basics

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the

motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

Stepper Motor Advantages and Disadvantages

Advantages

1- The rotation angle of the motor is proportional to the input pulse.

2- The motor has full torque at standstill (if the windings are energized)

3-Precise positioning and repeatability of movement since good stepper motors have an accuracy of

5 % -3 % of a step and this error is non cumulative from one step to the next.

4-Excellent response to starting/ stopping/reversing.

5-Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is

simply dependant on the life of the bearing.

6-The motors response to digital input pulses provides open-loop control, making the motor simpler

and less costly to control.

7- It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the

shaft.

8-A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the

input pulses.

Disadvantages

1. Resonances can occur if not properly controlled.

2. Not easy to operate at extremely high speeds.

Open Loop Operation

One of the most significant advantages of a stepper motor is its ability to be accurately controlled in an open loop system. Open loop control means no

feedback information about position is needed. This type of control eliminates the need for expensive

sensing and feedback devices such as optical encoders. Your position is known simply by keeping track of the

input step pulses.

Stepper Motor Types

There are three basic stepper motor

types. They are :

• Variable-reluctance

• Permanent-magnet

• Hybrid

Variable-reluctance (VR)

This type of stepper motor has been around for a long time. It is probably the easiest to understand from a

structural point of view. Figure 1 shows a cross section of a typical V.R. stepper motor. This type of motor

consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC

current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.

 

Permanent Magnet (PM)

Often referred to as a “tin can” or “canstock” motor the permanent magnet step motor is a low cost and

low resolution type motor with typical step angles of 7.5° to 15°. (48 – 24 steps/revolution) PM motors as the

name implies have permanent magnets added to the motor structure. The rotor no longer has teeth as with

the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight

line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and

because of this the PM motor exhibits improved torque characteristics when compared with the VR type.

Hybrid (HB)

The hybrid stepper motor is more expensive than the PM stepper motor but provides better performance with

respect to step resolution, torque and speed. Typical step angles for the HB stepper motor range from 3.6° to 0.9° (100 – 400 steps per revolution). The hybrid stepper motor combines the best features of both the PM and VR type stepper motors.