Mike Howell
EASA Technical Support Specialist

Advancement in power electronics over the last few decades has made it possible to utilize a variety of rotating electric machines that would otherwise not be feasible. One such class of machines is called reluctance machines because of the way they produce an electromagnetic torque. A reluctance machine is an electric machine in which torque is produced by the tendency of its movable part to move to a position where the inductance of the excited winding is maximized. A March 2020 Currents article discussed the switched reluctance motor (SRM) while this article will focus on the synchronous reluctance motor (SynRM). Let’s take a look at some of their similarities and differences.

Reluctance Machines Overview
Switched Reluctance	Synchronous Reluctance
Both stator and rotor have salient poles	The stator has a smooth bore except for slotting
The stator winding is comprised of a set of coils, each of which is wound on one pole.	The stator has a polyphase winding with approximately sine-distributed coils
Excitation is a sequence of current pulses applied to each phase in turn	Excitation is a set of polyphase balanced sinewave currents
Ref: SRM and their control by T.E. Miller

SynRM Machine Basics
Synchronous reluctance motor concepts date back to the late 1800s, but like switched reluctance motors, drive technology was key in making them practical as they are not inherently self-starting machines when direct-on-line. There were a few line-start machines built in the mid- to late-1900s that relied on a hybrid construction, which included a modified induction cage winding on the rotor for starting. But these machines suffered in performance, so they were not attractive long-term.

SynRMs will be much easier to understand than SRMs for those accustomed to the three-phase induction motors and synchronous motors commonly encountered by service centers. Unlike the SRM, the stator and rotor of the SynRM have the same number of poles. But like the SRM, the rotors have no windings and rely on saliency (reluctance varies with rotor position) to produce reluctance torque. A 4-pole SynRM is shown in Figure 1 with the drive-end bracket and rotor end plate removed to show the rotor section lamination section. This type of rotor construction is very common for SynRM designs, though we will discuss a couple of other configurations as well.

The stator windings in modern synchronous reluctance motors are usually no different than those found in three-phase induction motors or conventional wound-pole or permanent magnet synchronous motors. That is, concentric or distributed (two-layer) lap windings are most common. And, like any three-phase synchronous machine, the rotational speed (n) of the synchronous reluctance motor depends on the operating frequency (f) and the number of rotor poles (p). For example, for a 4-pole (p) SynRM with an operating frequency (f) of 60 Hz, the operating speed will be:

Most SynRMs being produced today are three-phase machines as this permits use of standard induction motor stators and standard drive technology using field-oriented (vector) control. They can operate over a wide speed range and are used in a variety of applications including extruders, mixers, crushers, wire drawing machines, pumps, fans and compressors.

Since SynRMs can utilize standard induction motor stators, they are easy to manufacture. Further, with no rotor winding losses, designers can achieve higher torque density than induction machines. Historically, some designs suffered from insufficient saliency and inadequate mechanical robustness in the rotor. Modern design practices using tools like finite element analysis have helped with both issues. Most SynRMs will have smaller air gaps than comparable squirrel cage induction machines (SCIM) due to design challenges around achieving an acceptable power factor. Depending on the OEM, this can affect mechanical tolerances.

SynRM Rotor Configuration
The rotor type shown in Figure 1 is common in production for industrial machines, but there are other topologies used in SynRMs. The three most common types of rotor designs used are shown in Figure 2 with direct (d) and quadrature (q) axes identified. Note that the term “axial” used is describing direction of lamination relative to the d and q axes, not the shaft axis.

The transverse construction rotor with simple saliency shown in Figure 2 is typically used for low cost, small machines where performance is not critical. The axial type construction is very rare due to manufacturing challenges. As previously mentioned, the most common configuration in industrial motors is the transverse arrangement with magnetic barriers. This style can be manufactured in a fashion like standard induction rotors and can be designed for high performance as well. Manufacturers are currently producing SynRMs above 300 kW (400 hp) and most are 4-pole machines. While most manufacturers offer SynRM motor-drive packages, there are off-the-shelf drives capable of operating synchronous reluctance machines; one benefit of this is facilitation of dynamic testing in the service center.

Speed-torque Characteristic

Like the switched reluctance motor, torque produced by a SynRM is a function of its saliency; that is, the difference between the direct axis inductance and the quadrature axis inductance. The torque-speed characteristic of the SynRM is like that of a definite purpose inverter duty induction machine. There will be a constant torque region and a constant power (field weakening) region as shown in Figure 3.

Point (1) is the minimum speed allowed based on temperature considerations. Point (2) is the minimum speed of the constant torque range. For some machines, Point (1) and Point (2) will be identical. Point (3) is the base rating point where nameplate voltage, speed, output power, etc. are defined. Point (4) is the maximum operating speed based on constant horsepower. The region from Point (3) to Point (4) is also called the field weakening or flux weakening region, and torque is inversely proportional to speed. There is sometimes an additional range shown at even higher speeds where the allowable torque is inversely proportional to the square of the speed. The available torque at this extended region can be less for a variety of reasons including control stability and machine characteristics.

Summing It Up
The use of synchronous reluctance machines will likely continue to grow in a variety of applications. This means there will be repair opportunities for service centers. Let’s run through a few tips that are worthwhile to remember. 

SynRM stator windings will typically be no different than those used in typical random wound, three-phase induction motors. However, take care in making any changes to the factory winding data, as changes in winding resistance or inductance may cause issues with starting and/or performance.

Rotors for SynRM machines will be mechanically robust and will have no windings or permanent magnets unless they are some sort of hybrid machine. Shafts, bearings, housings and other mechanical aspects of these machines will be similar to other common rotating machines.

SynRMs may utilize a speed sensor though sensorless control schemes are much more common. Additionally, there are plenty of off-the-shelf drives out there capable of running SynRMs in the service center so dynamic testing is feasible in many cases. 

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Categories: Technical topics, AC motors, Synchronous motors

Tags: Synchronous motors Switched reluctance motors Synchronous reluctance motors

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