Introduction
A permanent magnet stepper motor is a highly efficient and versatile device widely used in engineering, automation, toys, robotics, and precision systems. Because the rotor is made of permanent magnets, it does not require any external excitation. This makes the motor compact, reliable, and ideal for low-power applications including medical instruments and aerospace systems.
What is a Permanent Magnet Stepper Motor?
A permanent magnet stepper motor converts electrical pulses into controlled mechanical rotation. In this type of motor, both the stator and rotor magnetic fields interact to produce torque. Unlike conventional stepper motors that use electromagnets in the rotor, the PM stepper motor uses permanent magnets, reducing the need for rotor excitation and lowering losses. Because no rotor winding is required, the design becomes more efficient, reduces power consumption, and improves reliability.
Construction of Permanent Magnet Stepper Motor
A PM stepper motor consists mainly of two parts:
- Stator – The stationary part, fitted with windings arranged on multiple projecting poles.
- Rotor – A cylindrical permanent magnet made from materials like ceramic or rare-earth magnets.
When the stator windings are energized in sequence, each energized stator pole produces either a North or South magnetic pole. Opposite poles are typically connected in series to form a magnetic pair. The rotor is mounted on an external shaft, which rotates to produce mechanical output. Since the rotor already contains permanent magnets, torque is generated without requiring any rotor excitation.
Principle of Stepper Motor
A permanent magnet stepper motor operates on the same basic principle as a conventional motor. It follows the Lorentz force law, which states that a conductor carrying current in a magnetic field experiences a force. In a stepper motor, this force results from the interaction of:
- Stator magnetic flux (produced by stator windings)
- Rotor magnetic flux (produced by permanent magnets)
The direction of rotation follows Fleming’s Left-Hand Rule.
Working of Permanent Magnet Stepper Motor
The working sequence of a PM stepper motor can be explained through four excitation modes:
Mode 1
Only phase A of the stator is energized. This produces a pair of magnetic poles. The rotor aligns its magnetic poles with the stator poles, causing the initial movement.
Mode 2
Only phase B is energized while phase A remains off. A new magnetic axis is created, pulling the rotor further in the clockwise direction.
Mode 3
Phase A is energized again (but at the next pair of poles). The rotor continues to align with the new magnetic axis, advancing clockwise.
Mode 4
Phase B is energized again. The rotor moves towards the new magnetic axis. Completing Mode 1 to Mode 4 results in one full step-by-step rotation of the rotor.
Advantages of Permanent Magnet Stepper Motor
- Compact and small in size
- No external rotor excitation → lower losses
- Low maintenance requirements
- Easy to interface with control circuits
- Supports rotor position sensing using sensors
- Wide operating range of speed and torque
- Provides precise and accurate motion control
Disadvantages of Permanent Magnet Stepper Motor
- Not suitable for high-power applications
- Limited torque output
- Permanent magnets may weaken over time
Applications of Permanent Magnet Stepper Motor
- Aeronautical equipment
- Robotics and automation
- Toys and small gadgets
- Manufacturing and process control systems
- Control industry
- Milling and printing machines
