Variable Reluctance Stepper Motor – Construction, Working Principle, Advantages & Applications

Introduction

Motors play a vital role in the development of industrial and engineering systems. Their widespread usage has improved power control and automation applications. A stepper motor is a special type of control motor that regulates speed and position without requiring a feedback loop. This is why stepper motors are known as open-loop control motors.

What is a Variable Reluctance Stepper Motor?

The variable reluctance (VR) stepper motor is the most common type of stepper motor. Compared to other types, it has a very simple construction. The rotor is made of soft iron and is not permanently magnetized,
which means there are no attractive forces between the stator and rotor when the motor is not energized.

Due to the absence of magnetism in the rotor, the VR stepper motor does not produce detent torque.
Although it provides lower dynamic torque, it performs well at medium to high speeds. It is also known to produce more audible noise, which makes it suitable for applications where noise is not critical.

Working Principle

The working principle of a VR stepper motor is based on the tendency of its rotor to move toward a position where the reluctance is minimum.

When the stator windings are energized in sequence, they create a rotating magnetic field. This magnetic field has flux lines that pass through the rotor poles. The rotor aligns itself with the energized stator pole to minimize reluctance. As the excitation shifts from one phase to the next, the rotor rotates step-by-step so that its poles remain aligned with the magnetic axis of the excited stator phase.

Construction of Variable Reluctance Stepper Motor

A typical VR stepper motor consists of:

• A stator with multiple projecting poles
• A soft-iron rotor with toothed structure
• Excitation windings placed on stator poles

The stator windings are protected using laminated silicon steel sheets. Generally, three phases are distributed across the stator poles.

For example, a stator may have 12 equally spaced projecting poles, each fitted with its excitation coil. These coils are energized by a DC supply through solid-state switches.

The rotor, on the other hand, is made of slotted soft iron and has no windings. The rotor poles and stator poles are of similar width, but the number of poles differs in each. This difference allows the rotor to start on its own and rotate in both directions.

Relationship Between Stator and Rotor Poles

For a three-phase VR stepper motor, the relationship is: Nr = Ns ± (Ns / q)
where:

• Ns = number of stator poles
• Nr = number of rotor poles
• q = number of phases

Advantages of Variable Reluctance Stepper Motor

• High acceleration capability
• Simple operation and cost-effective design
• Fast dynamic response
• High torque-to-inertia ratio

Disadvantages of Variable Reluctance Stepper Motor

• Lower capacity when driving high inertia loads
• Limited output power