What is a Permanent Magnet Stepper Motor & Its Working

A permanent magnet stepper motor is a compatible and highly efficient device that has numerous applications. Since the rotor is made of permanent magnets, it does not need any external excitation which makes it very useful in applications such as toys, small motors, etc. Because of its design aspects, the step-angle of each rotation can be easily designed, which makes it useful in delicate applications such as medical instruments and aeronautical structures. Due to its small size, it is highly mobile and easy to use. This article discusses an overview of the permanent magnet stepper motor.

What is a Permanent Magnet Stepper Motor?

Definition: It’s an electromechanical energy conversion device, which converts electrical energy to mechanical energy. In a stepper motor, both the rotor and stator magnetic fields are excited such that interaction of the rotor magnetic field and stator magnetic field produces torque. In a permanent magnet stepper motor, the rotor coils are not excited, instead, we use permanent magnets.

In conventional stepper motors, electromagnets are used, which needs to be externally excited for the creation of a rotor magnetic field. But in this case, we use permanents magnets. This reduces the rotor excitation system and makes the motor more compatible for operation. Due to the absence of rotor excitation, the losses are also reduced.

Permanent Magnet Stepper Motor Construction

It consists of two fundamental parts. The stationary part also called the stator. In stator, stator poles are placed such that, when excited with windings as shown in the diagram, each stator pole forms one magnetic pole. If it is a two-pole machine, then the opposite poles are excited with common winding connected in series, such that, each of the opposite poles from North and South.

Similarly, the other two pairs of poles are excited with series winding in one cycle, such that they too form a pair of poles. The rotor is made of permanent magnets. There are many materials like ceramic which can be used as permanent magnets. The rotor magnets are connected to an external shaft, such that, on rotation, it provides the mechanical output.

Principle of  Stepper Motor

The working principle of the stepper motor is similar to that of a conventional motor. It works on the principle of Lorentz Force law. According to which, whenever a current-carrying conductor is placed in a magnetic field, it experiences a force, due to the interaction of fluxes.

The flux which interacts is stator magnetic flux and rotor magnetic flux. The stator magnetic flux is created due to external excitations and the rotor magnetic flux is created due to permanent magnets. It is also to be noted that, the direction of the motor is governed due to Fleming’s left-hand rule.

Working of Permanent Magnet Stepper Motor

The working permanent magnet stepper motor can be explained in the following modes

Mode 1– In this mode, the A phase of the stator poles are excited together with series winding to create two pairs of magnetic poles. It may be noted that, in this mode, the B phase is not excited at all. When the A phase is excited, it forms the North and South pole. At this moment, the rotor magnetic poles are attracted to the stator magnetic poles.

Mode 2 – In this mode, the B phase of the stator poles are excited together with series winding to create two pairs of magnetic poles. It may be noted that, in this mode, the A phase is not excited at all. When the B phase is excited, it forms the North and South pole. At this moment, the rotor magnetic poles are attracted to the stator magnetic poles. Which makes the rotor rotate in the clockwise direction from Mode 1.

Mode 3 – Again In this mode, the A phase of the stator poles are excited together with series winding to create two pairs of magnetic poles. It may be noted that, in this mode, the B phase is not excited at all. When the A phase is excited, it forms the North and South pole. At this moment, the rotor magnetic poles are attracted to the stator magnetic poles. It makes the rotor rotate in the clockwise direction from mode 2.

Mode 4– Again In this mode, the B phase of the stator poles are excited together with series winding to create two pairs of magnetic poles. It may be noted that, in this mode, the A phase is not excited at all. When the B phase is excited, it forms the North and South pole. At this moment, the rotor magnetic poles are attracted to the stator magnetic poles. Which makes the rotor rotate in the clockwise direction from Mode 3.
In this manner, the rotor makes one complete revolution from mode 1 to mode 4.

Advantages and Disadvantages of Stepper Motor

The advantages of a permanent magnet stepper motor are

• It is compact and small in size, which makes it useful in many applications
• Due to the absence of any external excitation, the losses are less
• Due to the absence of any external excitation, the maintenance is less.
• It can be connected to the external circuit, to control the speed of the motor
• Sensors may be used to locate the rotor windings
• Can be operated in a wide range of speed and torque.
• Precise Control

The disadvantages of a permanent magnet stepper motor are

• Due to limitations in permanent magnet, it cannot be used for high power applications
• Torque produced is limited
• The life of a permanent magnet is limited.

Applications

The applications of a permanent magnet stepper motor are

• Aeronautical industry
• Robotics
• Toys
• Manufacturing
• Control industry
• Mills and printing

Hence we have seen the working principle, constructional aspects, and applications of the permanent magnet stepper motor. It must be noted which magnetic materials are used to improve the performance of these motors and how to control the step angle of the machine?