Almost all machinery has some kind of motor behind its operations. Electric motors run by electromagnetism are the popular variations being used. However, these motors have different specifications, and one must be aware of where to use them. Hence, it is important to determine the best motor for your application.
Furthermore, the construction designs of the motor stator and rotor may influence their efficiency, reliability, costs, and operation mechanisms. Here’s a detailed discussion on reluctance motors, which can be AC or DC motors, their characteristics, and how they create rotational energy.
What are Reluctance Motors?
A reluctance motor is a type of motor that operates like an electrical motor with rotors and stators. They polarise the amount of reluctance in their rotor poles to interact with the rotating magnetic field. These motors can operate by syncing the rotor speeds with the stator’s rotating magnetic field (RMF) to create a synchronous design.
Operation of Reluctance Motors
Reluctance motors have an external stationary stator and an inner rotor with a small air gap to separate them. The construction of the motor stator and rotor determines the type of reluctance motor, but the mode of operation stays the same.
The stator has pole-pairs protruding from running current through a wire winding around these protrusions. The rotor is made from ferromagnetic metal and has poles that align with the contours of the stator’s magnetic field.
When the salient pole of a motor rotor aligns with that of the motor stator, the rotor is in its minimum reluctance position. The amount of magnetic resistance is at its lowest and is fully aligned. When the stator poles align with the barriers of the rotor, the motor is at maximum reluctance and not in alignment. The rotor always moves in the position of less reluctance when conserving energy, so there is a reluctance torque produced when the rotor is not aligned.
The torque produced pulls the rotor to the adjacent salient stator pole and causes rotation. Correct timing, control systems equipment, or precise rotor geometry could cause a continuous rotational output.
Specifications of Reluctance Motors
There are specific parameters that are basic for all reluctance motors. They include;
Reluctance motors operate by single-phase or polyphase. Three-phase motors are the popular polyphase designs used for high torque applications. This happens because they do not need additional windings and deliver more current to the motor. Single-phase motors must have starters.
Stator-to-Rotor Pole Ratio
Depending on the type of motor and output, there are specific stator-to-rotor poles. They usually come in pole ratios of 4/3, 6/4, 8/6, and 12/8. 1/1 ratios work best for synchronous reluctance motors to allow for precise synchronism.
The rated output torque, power, and pound force feet display the amount of output power and torque the motor provides when it is steady. The rated power can range from fractional horsepower to hundreds of horsepower to show their versatility.
Torque Ripple and Speed Range
Considering there are certain points where reluctance torque is not produced, some instantaneous variations in torque will occur while the shaft rotates. This causes a torque ripple effect where the shaft will receive torque in a high-reluctance position. The torque ripple could introduce noise and non-desired oscillations into the machine. Reluctance motors, therefore, have ranges of constant torque where there is minimal ripple.
Types of Reluctance Motors
Synchronous Reluctance Motors
Synchronous reluctance motors run by synchronising speed using three-phase rotor windings and rotors that implement salient rotor poled and internal magnetic flux limits. The rotor sometimes has a customised squirrel cage surrounding the salient poles. It, therefore, benefits from the induction effect to self-start. When the motor starts, it moves closer to synchronous speeds through induction to lock into synchronism through reluctance torque from the rotor flux barriers.
Switched Reluctance Motors
These are stepper motors with a unique inverted winding set-up where the field windings are located in the motor stator instead of the rotor. The rotor is made up of poles and notches that electromagnetic poles of the stator act on. While the ferromagnetic components simplify the mechanical design, it complicates the electrical setup. An electrical system is placed to switch the stator poles on and off to lead the rotor poles to the newly aligned positions to cause rotation. The switched reluctance motor causes electronic position sensors that calculate angles between rotor poles and stator windings. Timing systems must also be installed to sync the rotational frequency with the stator poles.
Reluctance motors may appear complicated, but they operate like any other motor. However, these motors’ applications vary despite some common uses, differentiating them from electric motors. Their simple construction without brushes, slip rings, and rotor field windings improve efficiency, reliability, and maintenance costs. Electric motor companies manufacture these motor stators at lower costs which is an elegant choice for most applications.