Almost all gains in motor efficiency above IE4 come from two technologies: the synchronous reluctance motor (SynRM) and the permanent magnet synchronous motor (PMSM). Both leave the classic asynchronous squirrel-cage motor behind, both reach IE5 and in some cases IE6, and both share one thing that is often overlooked: they do not run on the mains, but only with a variable frequency drive. This article explains how they differ, why that difference matters, and when you choose which technology.
Asynchronous vs synchronous: the basics
A classic asynchronous motor (squirrel-cage motor) always runs slightly slower than the stator's rotating field: that difference, the slip, is needed to induce a current in the rotor that generates torque. This induced rotor current causes losses, and it is precisely those losses that form the ceiling of what an asynchronous motor can achieve in efficiency. In practice, IE4 is roughly that ceiling.
A synchronous motor runs exactly in sync with the rotating field, with no slip. No current is induced in the rotor, so the associated losses disappear. This is exactly why both SynRM and PMSM, both synchronous principles, can perform above IE4. The difference between them lies in how the rotor is pulled along in sync.
What is a SynRM (synchronous reluctance)?
A synchronous reluctance motor has a rotor without windings and without magnets: just a cleverly shaped stack of steel laminations with cut-outs (flux barriers). The rotor follows the rotating field because magnetic flux always seeks the path of least resistance (reluctance). The rotating field effectively drags the rotor along the axis of lowest reluctance.
The major advantage: no magnets and no rotor current means a cool, robust rotor and no dependence on rare earth metals. The rotor also fits in the same housing as a standard asynchronous motor, which simplifies like-for-like replacement. ABB markets this technology under the name SynRM, combined with a matching drive.
The drawback: a SynRM has a slightly lower power factor and power density than a PMSM, and delivers its best efficiency only in combination with a well-tuned drive. For most pump, fan and compressor applications, that is more than sufficient.
What is a PMSM (permanent magnet)?
A permanent magnet synchronous motor has magnets in or on the rotor (usually neodymium-iron-boron). These magnets provide a constant rotor field that runs in sync with the stator rotating field. Because the rotor field is supplied by the magnets and does not need to be induced, the PMSM is highly efficient, compact and offers strong torque.
PMSM motors generally achieve the highest power density and the best power factor of all motor types, making them the first choice where space or weight is critical, or where very high part-load efficiency matters. They are also common in servo applications and traction.
The drawbacks: the magnets make the motor more sensitive to high temperatures (demagnetisation risk) and dependent on the price and availability of rare earth metals. Furthermore, a running PMSM can feed voltage back during a mains failure, which requires attention in the design.
SynRM vs PMSM: the comparison
In short: choose SynRM for robustness, a cool rotor, no magnet dependency and simple like-for-like replacement in industrial pump and fan applications. Choose PMSM where maximum power density, the highest efficiency or the smallest build volume are decisive, and where temperature is manageable.
Both technologies reach IE5. With an optimised design, PMSM can grow into IE6. SynRM offers the lowest total cost of ownership in practice for standard industrial applications, while PMSM excels in compact, highly dynamic or weight-critical applications.
How they reach IE5 and IE6
The efficiency classes IE1 through IE6 are defined in IEC 60034-30-1 (IE1 to IE4) and the later IEC TS 60034-30-2 (IE5 and above). Each class step represents roughly 20% fewer losses than the previous one. Where a good asynchronous motor stalls at IE4, SynRM and PMSM reach IE5, and with PMSM, IE6 is achievable in specific versions.
To translate efficiency into euros: at continuous operation, each class step quickly amounts to hundreds of euros per year per motor. Calculate your own situation with our energy savings calculator, which converts the difference between IE classes into kWh, euros and payback time.
Why a variable frequency drive is required
This is the point that most often causes surprises in practice: neither a SynRM nor a PMSM can start directly on the mains. An asynchronous motor starts itself thanks to the slip; a synchronous motor does not, because the rotor cannot catch up from standstill with the stator field suddenly rotating at full speed. Both types therefore always need a variable frequency drive that gradually builds up the frequency from zero.
This is not a disadvantage but often an advantage: the drive provides stepless speed control, soft start and, in pump and fan applications with variable load, extra energy savings on top of the gain from the motor itself. It does mean you must specify motor and drive as a single system. Read more about the choice and EMC aspects in our article on variable frequency drives.
When to choose which?
For a standard pump, fan or compressor in continuous operation, where you want to save energy while staying close to a like-for-like replacement: SynRM. Robust, cool, no magnets, and available from ABB in familiar housings.
For compact applications, high dynamics, servo, traction or where every percent of part-load efficiency counts: PMSM. Where you currently run an IE3 motor and the installation already has or will get a variable frequency drive, the switch to an IE5 SynRM or PMSM is almost always worth calculating.
Undecided between keeping your current asynchronous motor or switching? Our knowledge base covers that trade-off separately in repair or replace and in IE efficiency classes.
Practical recommendations
Always specify motor and variable frequency drive together as a system, not separately. For an IE5/IE6 request, ask for the efficiency at your actual operating point, not just at full load; it is precisely at part load that the technologies diverge. With PMSM, take into account the maximum winding temperature and the regeneration behaviour during a mains failure. And before you replace, have the payback time calculated based on your running hours and electricity price.
Van Bodegraven supplies both ABB SynRM and PMSM solutions, with matching ABB drives, and advises on the right choice for your application. We deliver from ABB EU stock with short lead times.
No rights can be derived from the data mentioned in this article. Please contact us for a specific quotation and current delivery status.
IE5 or IE6 motor for your application?
Provide your power, speed, operating regime and whether a variable frequency drive is already present. We will propose a matched SynRM or PMSM solution with a suitable ABB drive, and calculate the payback time for you.