New energy-efficient motor technologies

Increased energy efficiency requirements for drive systems have led to alternative motor technologies taking up the competition with traditional induction motors and AC drive applications.

In the quest to improve the energy efficiency of drive systems, much effort has been spent on improving the efficiency of the electric motor. Through the introduction of a classification system for motor energy efficiency in the IEC 60034-30 standard, users are given a clear and easy framework for comparing motor efficiencies.

With new higher efficiency classes being developed, users will have to get used to considering alternatives to the classic induction motor.

Classification of motor efficiencies

Drive systems consume a major share of the electricity produced and used globally.

In order to put more focus on the efficiency of electric motors and drive systems, the working groups of the IEC have created a framework for the measurement and classification of motor efficiencies with the standards IEC 60034-2-1 ‘Standard methods for determining losses and efficiency from tests’ and IEC 60034-30-1 ‘Efficiency classes of line operated AC motors’. The standard IEC 60034-30-1 (2008) defines the motor-efficiency classes IE1, IE2 and IE3, as well as efficiency limits for each class and motor size. Additionally, in the latest version of IEC60034-30-1 Ed 1.0, a new class IE4 (super premium efficiency) is introduced.

When classifying the efficiency of motors, it is important to take the size of the motor into account. An efficiency of, say, 90% is poor for a large motor but good for a small one — whereas a motor of class IE4 is always just about as good as it gets. Today there are only standards in place for line-fed motors, whereas requirements for defining the extra motor losses, and thus the change of efficiency, for motors connected to variable speed drives (VSDs) are not yet in place.

Upcoming versions of the IEC 60034 family of standards are set to cover this aspect as well, but currently the efficiencies given by motor manufacturers are not suitable for direct comparison of motor efficiencies in VSD applications.

The European Committee for Electrotechnical Standardization (CENELEC) has also created a framework for the comparison of the efficiency of drive systems in the standard EN 50598 - 2 ‘Ecodesign for power drive systems, motor starters, power electronics and their driven applications’. This standard also gives methods for defining the efficiency of a drive-plus-motor system (a power drive system or PDS) in a given application. By the end of 2016, the international version will be released as IEC 61800-9.

Motor technology change ahead

There will probably be a need to define higher efficiency classes than the current IE1 to IE4. Higher efficiency classes, such as IE5, could be expected to require at least 15% lower losses over the previous classes. According to data presented by the IEC, the induction motor is competitive up to energy-efficiency class IE3, but it will be difficult to adapt to IE4 and IE5 requirements in a cost-efficient way, as increasing efficiency in essence is achieved by reducing the I2R losses of the rotor by using copper bars and by increasing surface areas. Reducing the I2R losses of the stator is also possible at the cost of a much larger construction.

Figure 1: Efficiency classes according to IEC600034-30 for line-fed motors.

If the induction motor is hard to adapt to higher efficiency requirements, then what are the alternatives?

The synchronous reluctance motor

The synchronous reluctance motor (SynRM) has been known for almost a century. For reasons related to its lack of direct-on-line starting capability (although hybrid designs with line-start capabilities are available), weak torque behaviour and lack of good solutions for variable frequency control, this motor technology initially did not achieve a wide industrial use. Today it has become interesting again, due to improvements in rotor designs and modern cost-effective and commonly used VSD technology.

Functional principle

A typical SynRM is designed for a three-phase sinusoidal supply voltage with a stator design, which is more or less the same as is used in, and mass-produced for, induction motors.

The main technical difference is in the rotor design, which, instead of a short-circuited squirrel cage, is designed to achieve paths with high and low magnetic reluctance. Reluctance for magnetic flux is analogous to resistance for an electric current.

The path with low magnetic reluctance is normally referred to as the d-axis (direct axis), whereas the corresponding path with high magnetic reluctance is referred to as the q-axis (quadrature axis).

When a magnetic field is applied through the rotor from the motor stator, a torque is created as the rotor tries to align itself in the best flux-conducting angle relative to the stator field. As the magnetic field in the stator is rotating with a rate in proportion to the applied stator frequency, the rotor will follow with a speed synchronised to the speed of the stator field.

Figure 2: Example of SynRM rotor cross-section.

The strength of the torque produced by the SynRM is directly proportional to the ratio of the inductance of the q- and d-axis. Most rotor designs available in the market today are rather simple laminated designs with air gaps in the q-axis direction to increase the reluctance.

In static operation, no current is induced in the rotor, which keeps the rotor losses low, improving the motor efficiency compared to the induction motor. However, in order to magnetise the rotor, a reactive current is needed — which in turn reduces the power factor of the motor compared to the induction motor.

Expect a higher nominal current

The lower power factor of the SynRM increases the nominal current. The decrease in power factor is to some extent compensated, however, by the increase in efficiency — but still the motors available in the market today commonly have a higher nominal current than the corresponding induction motor. The reason for the increased current is also partly due to the fact that the motors are designed for VSD operation and thus designed for a slightly lower nominal voltage than the line voltage.

For the motor user, the higher current of the motor has one major impact — the sizing of the VSD. As VSDs available on the market today have current ratings optimised for IMs, an oversizing of the VSD with one step seems to be typically required, which in turn leads to a slight cost increase for the VSD.

High efficiency

Data provided by motor manufacturers suggests that the efficiency of currently available SynRMs is high due to the low rotor losses. In addition, most motors are not run at full load constantly, and research suggests that the efficiency of SynRMs at partial loads can be even better than the efficiency of a corresponding permanent magnet synchronous motor (PMSM).

In applications that are run constantly, such as pumps and fans, the energy-saving potential of high-efficiency motors is also typically the greatest. In many of these applications, VSDs are already used for reasons related to process control and the energy efficiency of the system. Due the quadratic torque behaviour of centrifugal pumps and fans, these are commonly used at partial load, which underlines the importance of good motor efficiency at partial loads.

VSD support

As already stated, a SynRM cannot normally be started and run directly from the line. Motors with integrated short circuit bars in the rotor, that allow the motor to be started as an induction motor directly from the line, are available on the market.

For most applications, a VSD will always be needed. Until recently, most general-purpose VSDs lacked the control functionality needed to run SynRMs. This is now changing, however, as some VSD suppliers have added support for SynRMs — users interested in applying SynRMs should be in contact with their VSD supplier for more information about the availability of VSD support.

The permanent magnet motor

The first permanent magnet motors were developed in the 19th century. Due to the lack of good magnetic materials and the need for variable frequency control (although hybrid designs with line start capabilities are available), the motor technology did not initially get any large-scale industrial use. With the availability of rare-earth magnetic materials, the torque/power density of permanent magnet motors has improved significantly. Today, permanent magnet motors are used in a wide range of industrial applications — both as servo motors and also increasingly in normal VSD applications. A large portion of the motors used in the machine-building sector are special designs that allow the designer to adapt the motor to the requirements of the driven load, for example, to create gearless solutions.

Functional principle

There are several variants of permanent magnet motors, such as brushless DC motors and servo motors, as well as standard motors designed for VSD applications — and thus suitable for three-phase sinusoidal voltage supply. Such motors are often designed around a standard stator having the same characteristics as the stator of an induction motor.

As with SynRMs, the main difference between the induction motor and PMSM is in the design of the rotor. In PMSMs, the rotor contains strong permanent magnets that create a magnetic field that aligns with the magnetic field of the stator, thus creating torque on the motor shaft. As the magnets of the rotor will follow the magnetic field of the stator, the result is a synchronous motor.

Figure 3: Permanent magnet motor cross-section.

High efficiency at all speeds

As the rotor is magnetised with permanent magnets, there is no actual current flow in the rotor and therefore no I2R losses or slip losses. The power factor remains high over the whole speed range and can, with the help of an adequate drive control, be kept close to unity — which in turn also minimises the I2R losses in the stator windings. A high portion of motors used today are used with partial load or at partial speed — an area where the PM motor efficiency is high compared to the induction motor.

Low-speed, high-pole IMs usually have a rather low power factor and low efficiency due to the less efficient magnetisation of the rotor at lower speeds. Permanent magnet motors do not have this problem and can be designed with a high number of poles without loss of efficiency. In applications where mechanical gears can be saved, both cost and efficiency gains are achieved.

Permanent magnets

The amount of torque that can be produced by a permanent magnet motor is directly proportional to the strength of the magnets. Rare-earth magnetic materials such as neodymium are therefore generally used. In recent years, the price of rare-earth metals has greatly fluctuated, which can have an effect on the cost and availability of this technology.

VSD support is largely there

Modern VSDs normally offer at least basic support for permanent magnet motors. This makes it easier to move to this motor technology, as a wide selection of VSDs are available that support at least the less demanding applications such as pumps and fans. For demanding applications and for special high-pole and direct-drive types of applications, the number of VSDs with support for this kind of use is lower.

Comparing motor efficiencies

In order to compare motor efficiencies, there are several parameters that need to be considered. Most significant is that PMSM and SynRM motors require a variable speed drive, which produces a voltage pattern for the motor that is not fully sinusoidal and thus creates additional motor losses. Most IM motor efficiency data is given for fixed sinusoidal 50/60 Hz supplies — the additional losses caused by the voltage pattern are often not quantified. Today, there is not yet a fully standardised framework in place for definition of motor efficiencies at different speed/torque points, which makes the comparison technically quite demanding.

Application-specific efficiencies

When comparing motor or drive + motor efficiencies, it is also important to consider the behaviour of the load in the actual application for which the motor is to be used. For example, in centrifugal pumps and fans, the power need varies in proportion to the third power of the speed, which means that efficiency at partial load is very strongly linked to the total efficiency of the installation.

PMSM vs SynRM

Laboratory measurements conducted at the University of Beira Interior in Portugal on commercially available 2.2 kW motors indicate that the motor efficiency in quadratic load (pump and fan) applications of both PMSM and SynRM motors reach efficiencies close to 90% in VSD operation — better than the 87% IE3 efficiency limit currently defined for line supply.

As can be seen from Figure 4, the reluctance motor appears to be extremely competitive in the low-speed area.

Figure 4: Efficiency comparison PMM vs SynRM as function of speed with quadratic load curve.

Is change coming?

SynRM and PMSM motors can already compete both in cost and efficiency with the traditional induction motor, but are apparently still sold in rather small quantities. What is still needed for these motors to get into mainstream use? The points below affect how fast these motors will gain ground and how fast and to what extent the transition away from the induction motor will happen:

• Availability: Many users are tied to a certain supplier, making a change of supplier largely if the existing supplier is not capable of supplying new motor technology.
• VSDs: Both SynRMs and PMSMs requiring a VSD in order to operate.
• Efficiency requirements: Energy-efficiency legislation puts emphasis on energy efficiency as a decision-making criterion. These initiatives can be directly motor-related, but there are also very often machine-specific or application-specific requirements.
• Compatibility: The new motor technologies are interesting motors, but backward compatibility — both mechanical and electrical — with existing installations has to be ensured, requiring a higher level of understanding of their behaviour.

References

1. McCoy GA, Energy Efficiency Factsheet, Washington State University, <http://www.energy.wsu.edu/Documents/EEFactsheet-Motors-Dec22.pdf>
2. Estima JO, Marques Cardoso AJ 2013, Super Premium Synchronous Reluctance Motor Evaluation, EEMODS Conference, University of Beira Interior, Covilhã, Portugal.
3. International Electrical Commission, IEC60034-30-1 Efficiency classes of line operated AC motors (IE-code).

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