Synchronous Reluctance Motor

Synchronous reluctance motors (SynRM) are electric machines that generate torque through magnetic reluctance and operate on AC power. Unlike other electric machines, their rotors (rotating part) contain no windings; torque is produced based on the design of flux barriers within the rotor. Motor performance can be enhanced by adding permanent magnets to the rotor.

^{Synchronous Reluctance Motor Structure -} Oswos

SynRMs are polyphase alternating current machines. Their stator (stationary part) structure is similar to that of other AC motors, but their rotor designs are more distinct and complex.

To maintain torque at maximum levels, different rotor configurations have been developed, including axial (ALA) and transverse (TLA) laminations.

^{Synchronous Reluctance Motor Components -}

The stator, like in other polyphase AC machines, consists of multiple phase winding groups placed in slots cut along the stator axis.

The rotor features a design known as flux barriers, which distinguish SynRMs from other AC motors and are responsible for torque generation. According to this design, the saliency ratio is defined as the ratio of magnetic flux densities along the d and q axes in the rotor coordinate system. To maximize the torque produced by the motor, the saliency ratio must be increased; therefore, rotor flux barrier designs are implemented in SynRMs.

The primary source of torque in SynRMs is reluctance torque. Reluctance is a magnetic resistance that opposes the flow of magnetic flux in a magnetic circuit and is obtained by dividing the magnetomotive force by the flux. Reluctance torque arises due to differing reluctance values along different axes in a ferromagnetic material. In a motor within a magnetic field, the rotor rotates so that its d-axis, where reluctance is low and inductance is high and magnetic flux flows easily, aligns with the magnetic field to minimize the flux path. This alignment generates reluctance torque.

^{Synchronous Reluctance Motor Flux Barriers -} DergiPark

SynRMs are powered by balanced polyphase (typically three-phase) AC voltage, generating a rotating magnetic field in the air gap between stator and rotor at synchronous speed. The resulting magnetic flux completes its path along the d-axis of the rotor, where reluctance is low and inductance is high, causing the rotor to rotate at synchronous speed.

^{Synchronous Reluctance Motor Operation Demonstration -} ELIN Motoren

As the magnetic flux completes its path along the d-axis, the magnetic field in the windings continuously changes, causing the d-axis to continuously align with the rotating magnetic field, initiating rotation at synchronous speed. The torque produced by the SynRM depends on the inductance values along the d and q axes and is therefore determined by the saliency ratio. To maximize torque, the saliency ratio must be increased, as this parameter is central to machine design and optimization. To achieve maximum torque, the d-axis inductance (Ld) should be as high as possible while the q-axis inductance (Lq) should be reduced to the lowest feasible value.

SynRMs are generally driven by variable frequency inverters. Early designs included short-circuited cage structures on the rotor to assist with motor starting. However, these structures negatively affected the saliency ratio responsible for torque production and are therefore rarely used today. Currently, inverter systems enable control over both motor starting and nominal operation, maintaining constant power and constant torque regions.

^{Synchronous Reluctance Motor Example Control Scheme -} Veichi

Generally, maximum torque per ampere (MTPA) control strategy is employed. Under this strategy, the electrical angle between the current vector and the d-axis is maintained at 45° to produce maximum torque across varying speed and load conditions. Implementation of this control requires knowledge of rotor position and speed. The current, speed, and position data from the motor are processed mathematically to determine the required reference d- and q-axis currents. Based on these calculations, the polyphase AC power supplied to the motor stator is controlled using various semiconductor devices.

1. They require no excitation system, resulting in lower electrical losses.
2. Models without permanent magnets in the rotor have no risk of magnetic demagnetization, making them highly reliable machines.
3. The rotor can be manufactured from robust and low-cost materials.
4. The motor exhibits superior performance with low torque ripple.
5. The motor can be operated using standard PWM inverters.
6. Models without permanent magnets in the rotor can operate at very high temperatures.
7. The motor has a simple and robust construction.
8. It is capable of operating at high speeds.
9. With a high saliency ratio, a power factor of 0.8 can be achieved. Due to the absence of copper losses, the efficiency of reluctance motors is higher than that of some other AC motors.

1. Compared to other AC motors, synchronous reluctance motors are somewhat heavier and have a lower power factor. However, the power factor can be improved by increasing the saliency ratio.
2. The motor cost is higher than that of some other AC motors, particularly due to the complexity of the controller design.
3. The control algorithm is complex.
4. The use of permanent magnets in the rotor reduces motor reliability.
5. When permanent magnets are used in the rotor, the risk of demagnetization at high temperatures increases.

Due to their structural simplicity, robust construction, and suitability for constant-speed operation, SynRMs are used in various applications including electric vehicles, HVAC systems, industrial automation, compressors, pumps, renewable energy generation, and household appliances.

^{Synchronous Reluctance Motor Example Application -} ABB