What are some types of stepper motors

Some words about stepper motor (advantages, disadvantages and classification)

Some words about stepper motor (advantages, disadvantages and classification)

introduction

A stepper motor is an electromechanical device that converts electrical impulses into discrete mechanical movements . The shaft or spindle of a stepper motor rotates in discrete steps, when electrical command pulses are applied to them in the correct order.

The rotation of the motors has several direct relationships with these applied input pulses. The order of the applied impulses is in directly related to the direction of rotation of the motor shafts .

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

Stepper motor advantages and disadvantages

Stepper motor with cable

advantages

  1. The angle of rotation of the motor is proportional to the input pulse.
  2. The motor has full torque at standstill (when the windings are energized)
  3. Precise positioning and repeatability of the movement, as good stepper motors have an accuracy of 3 - 5% of a step and this error is not cumulative from step to step.
  4. Excellent response to starting / stopping / reversing.
  5. Very reliable as there are no contact brushes in the motor. Therefore, the life of the motor simply depends on the life of the bearing.
  6. The response of the motors to digital input pulses enables control, making the motor easier and cheaper to control.
  7. It is possible to achieve synchronous rotation at very low speed with a load directly coupled to the shaft.
  8. A wide range of speeds can be realized because the speed is proportional to the frequency of the input pulses.

disadvantage

  1. Resonances can occur if not properly controlled.
  2. Not easy to use at extremely high speeds.

Open-loop operation

One of the most important advantages of a stepper motor is its ability to be precisely controlled in an open loop system.

Open loop control means that no feedback on the position is needed. This type of control eliminates the need for expensive sensor and feedback devices, such as e.g. B. optical encoder.

Their position is known simply by following the input step pulses.

Stepper motor types

There are three basic types of stepper motors. You are:

  1. Variable reluctance (VR)
  2. Permanent magnet (PM)
  3. Hybrid (HB)

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 VR stepper motor.

Figure 1 - Cross section of a variable reluctance (VR) motor

This type of engine consists of one multi-toothed soft iron rotor and one wound stator . When the stator windings are excited with direct current, the poles are magnetized. The rotation occurs when the rotor teeth are attracted to the energized stator poles.

Permanent magnet (PM)

The permanent magnet stepper motor, often called " tin can "or" Cylinder box " -Motor is designated, is a typical one and inexpensive motor With low resolution
Step angle from 7.5 ° to 15 ° .

Figure 2 - Principle of a PM or tin can stepper motor

(48 - 24 steps / revolution) PM motors, as the name suggests, have permanent magnets that are added to the motor structure.

The rotor no longer has any teeth like the VR motor. Instead, the rotor is magnetized with alternating north and south poles, which are arranged in a straight line parallel to the rotor shaft.

These magnetized rotor poles provide increased magnetic flux intensity, and therefore the PM motor has improved torque characteristics as compared with the VR type.

Hybrid (HB)

The hybrid stepper motor is more expensive than the PM stepper motorbut offers better performance in terms of step resolution, torque and speed.

Figure 3 - Cross section of a hybrid stepper motor

Typical step angles for the HB stepper motor range from 3.6 ° to 0.9 ° (100 - 400 steps per revolution) .

The hybrid stepper motor unites the best features of PM and VR type stepper motors .

Like the VR motor, the rotor is multi-toothed and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path to help direct the magnetic flux to preferred locations in the air gap. This further increases the cogging, holding and dynamic torque characteristics of the motor compared to both the VR and PM types.

The two most commonly used types of stepper motors are the permanent magnet and the hybrid types.

Make the right decision

If a developer is unsure which type will best suit their application needs, they should evaluate the PM type first as it is typically several times less expensive. If not, then the hybrid engine can be the right choice.

There are also some special stepper motor designs. One is the disc magnet motor. Here the rotor is a disc with rare earth magnets, see figure 4 .

This type of motor has some advantages such as very low inertia and an optimized magnetic flow path with no coupling between the two stator windings.

Figure 4 - Principle of a disc magnet motor developed by Portescap

These properties are essential in some applications.

How the stepper motors are made and how they work (1)

How the stepper motors are made and how they work (2)

Reference: Application Notes for Industrial Circuits - Stepper Motor Basics from PAControl.com

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