Switched Reluctance Motor (SRM) – Construction, Working Principle, Advantages & Applications

Introduction

The Switched Reluctance Motor (SRM) is not a new concept; however, commercial versions of SRMs have only recently become widely available. These motors are known for their excellent performance, reliability, low cost,
and high efficiency. The concept of SRM has existed since 1838, and its popularity continues to grow due to its
durability, simple construction, and improved motor-drive efficiency.

What is a Switched Reluctance Motor?

A Switched Reluctance Motor (SRM) is an electric motor that operates based on reluctance torque. Unlike conventional brushed DC motors where power is supplied to the rotor, in SRMs the power is supplied exclusively to the stator windings. This motor is also known as a Variable Reluctance Motor (VRM).

SRMs require an electronic switching inverter to energize the windings in a specific sequence. Their control characteristics are similar to DC motors with electronic commutation. Due to their high power-to-weight ratio, these motors are suitable for applications where compactness and performance are important.

Switched Reluctance Motor (SRM)

The mechanical design of an SRM simplifies the transfer of current to the rotating part because there is no rotor winding. The rotor is a simple magnetic structure, which reduces electrical losses. However, the stator requires a switching mechanism to energize different windings in the correct sequence. This design also allows the motor to function as a generator. By altering the load and synchronizing the stator coil excitation with rotor position, an SRM can operate efficiently at very high speeds.

Working Principle

The SRM operates on the principle of variable reluctance. The rotor always tries to align itself with the magnetic field path that produces the least reluctance. Using a power electronic switching circuit, a rotating magnetic field is created in the stator. The reluctance of the magnetic path depends mainly on the air gap between the stator and rotor. By controlling the air-gap magnetic field (through stator excitation), the reluctance changes and the rotor rotates toward the minimum-reluctance position. Reluctance is the magnetic equivalent of electrical resistance. Therefore, controlling reluctance controls the rotor movement.

Construction of Switched Reluctance Motor

A typical SRM consists of six stator poles and four rotor poles, though other configurations exist. The stator is made of silicon steel laminations with projecting poles, each fitted with a field coil.

The stator normally has an even number of poles with windings arranged in phase groups. Opposing poles are connected in series so that their magnetomotive forces (MMF) add up. One phase may consist of a single coil or a group of coils. Each phase winding is connected to the output terminals of a power semiconductor switching circuit that receives a DC input supply. The rotor is made from laminated soft iron with projecting poles but contains no windings or magnets.

A position sensor is usually mounted on the rotor shaft. Its signals control the switching sequence of the power converter, ensuring proper alignment between stator excitation and rotor position. Both stator and rotor structures are designed using soft iron laminations to minimize hysteresis losses.

Advantages of Switched Reluctance Motor

• Simple and robust rotor construction
• Suitable for high-speed operations
• Less complex variable-frequency drives compared to other motor types
• No permanent magnets → lower cost
• No additional ventilation system needed for rotor/stator slots
• High fault tolerance
• Can be driven by simple two-phase or three-phase pulse generators
• Operates even with phase loss
• Direction can be changed by reversing phase sequence
• High torque-to-inertia ratio and self-starting capability

Disadvantages of Switched Reluctance Motor

• Higher noise and acoustic vibration
• Limited torque capability
• Torque ripple at high speeds
• Requires an external rotor position sensor
• Large capacitors needed to reduce harmonics at high speeds
• Higher input current requirements due to absence of permanent magnets

Applications of Switched Reluctance Motors

• Used as alternatives where induction motors are unsuitable
• Textile machinery like rapier looms and towel looms
• Electric vehicles (EVs)
• Oilfield equipment such as beam pumps and vertical pumps
• Mining equipment – conveyors, winches, shearers, drilling machines, coal crushers
• Mechanical press machines such as screw presses