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Why the reluctance?

Author : Dave Hawley is with ABB

02 February 2017

The synchronous reluctance motor (SynRM) has been around for nearly a century, and yet it is only in recent decades with improvements in rotor design, power converters and control technologies that it has become commercially viable as a high efficiency alternative to the induction motor.

An increasing range of variable-speed drives are available for use with SynRMs, such as ABB’s ACS880

Yet despite its advantages SynRM remains far from ubiquitous in industry. Dave Hawley of ABB discusses some of the reasons why engineers might still be reluctant to make the switch and explores some of the misconceptions surrounding the technology. 

When it comes to efficiency, if you were to put a SynRM head-to-head against an equivalent induction or permanent magnet motor it will easily come out on top. But if they’re so good, why are they not yet dominating the industrial motor landscape?

SynRMs use a specially designed cageless rotor which removes the traditional rotor losses. This reduces overall losses by up to 40 percent and thus produces far less heat than induction motors, helping to prolong operational life-cycle in the bearings and between re-greasing intervals. The stator is similar to that of an induction motor; however in this configuration torque is generated through the principle of magnetic reluctance. A special field is generated inside the motor by the variable-speed drive (VSD) operating it, and this magnetic field is guided through low reluctance paths in the rotor, synchronising the rotor to the field. The field is then rotated and these magnetic “elastic bands” pull the motor around. 

This lower temperature allows manufacturers to generate two distinct designs, one that matches IEC motor frame sizes in an IE4 efficiency package, and one called “high output” where the motor can be shrunk by up to two frame sizes. The change in rotor design also means the motor is lighter by up to 40 percent compared to its equivalent induction motor.

There are already many preconceptions regarding SynRMs in industry, and not all of them are necessarily fair…

MYTH: The SynRM is a specific motor designed for specific purposes

Many potential users perceive SynRMs as special, and designed for unusual applications. Yet they’re actually compatible with almost any application, so long as a VSD is used. The motor can be happily used in constant torque applications where open loop (no encoder feedback) speed accuracies of 0.01 percent can be achieved. SynRMs are particularly well suited for quadratic torque applications, for example pumps and fans. These tend to involve variable loads, meaning that the use of a drive will lead to even greater efficiency savings on top of those already gained by using an IE4 motor. In short, if an application requires variable speed control, then it is highly likely that a SynRM is the best option.

MYTH: It’s a fairly new technology so there may be reliability issues to be ironed out

There is nothing new about magnetic reluctance. It was first observed in the 19th century, with the first operational synchronous reluctance motor arriving in the early 20th century. Commercially viable versions have now been available for decades, during which time the technology has been proven time and again to offer superior efficiency to equivalent induction and permanent magnet motors. With no brush changes required and a much lower operating temperature reducing stress on bearings, the SynRM can potentially have a much longer operational life cycle with less maintenance required compared to other motor types. 

MYTH: They’re more complex, therefore they must be more difficult to maintain and repair

The concept of reluctance is admittedly somewhat complicated, but in practice the motor’s operation still uses the same basic component parts as an induction motor, with a rotor and stator combining to produce torque. To that end, they are just as easy to maintain in the field as an induction motor, and with a cageless rotor and no traditional rotor windings or permanent magnets, it could be argued that they are in fact easier to maintain compared to induction motors. Lower operating temperatures mean daily maintenance intervals are also longer as re-greasing is less frequent.

SynRM offers higher power at the same frame size, or the same power at up to frame sizes smaller

MYTH: SynRMs can’t handle high torque applications

On the contrary, SynRMs can be deployed on any application that can be run with a VSD and are suitable for a range of industries from water and food to mining and cement. The motor also runs much more quietly compared to its induction and permanent magnet counterparts, as there is no field being developed in the rotor. And unlike permanent magnet motors, SynRMs do not generate dangerous voltages in the terminal box when not being energised by the VSD.

MYTH: Few drives are compatible with it

This might have been the case in the past when the technology was newer, but not so much nowadays. Manufacturers have recognised the need for a wider variety SynRM-compatible drives, and are continuing to develop more options for OEMs and end users, including general purpose and industry-specific VSDs that are tailored to operate properly and accurately with the SynRM motor range.

MYTH: SynRMs are only available as motor-drive packages

While a VSD has to be employed to operate a SynRM motor, they do not have to be supplied as a matched pair or sold at the same time. It is perfectly acceptable to take the motor or the drive separately, and a package supplier can provide the relevant information needed to marry up the most appropriate drive later in the supply chain. Suppliers can also provide guaranteed efficiency certification showing how the package will perform when operated.

MYTH: SynRM motors are too expensive

This is arguably the biggest myth of them all. SynRM prices are now comparable to equivalent IE2 induction motors, not to mention offering either the same power from a smaller size (up to two frame sizes smaller), or more power from the same size with a 40 percent reduction in energy losses. By using the same frame sizing, rating and output, this means that installation is very simple as it’s merely a case of swapping like for like. The old IE2 motor can even be kept as a spare. Payback times are highly impressive due to the substantial efficiency gains, typically as little as two years.

Conclusion

Any new technology will always take time to achieve critical mass, and that point has clearly not yet been reached by the SynRM, as the majority of motors used in industry are still induction motors. It is only relatively recently that SynRMs have been able to realistically compete on price, and it takes time to convince engineers of the benefits, particularly when they are being asked to move away from a motor technology they’ve always known and trusted.

With large industrial motor companies like ABB throwing resources behind the SynRM, it is surely a matter of time before engineers see that the myths have been debunked, and embrace the future of motor technology.


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