Traditional AC Motors: Beyond NEMA Premium Efficiency

Transcription

1 WEBINAR BRIEF / PRESENTED BY Traditional AC Motors: Beyond NEMA Premium Efficiency Sponsored by -1-

2 Beyond NEMA premium efficiency Learn how traditional AC motors are evolving to deliver extreme efficiencies using hybrid technology Virtually every motor that is purchased today is a premium efficiency motor. But with motor efficiency at an all-time high, the new question is how do we take the induction motor, which has faithfully served users since the industrial revolution, and raise it to the next level of efficiency without adding a tremendous amount of cost? If we look to the automotive industry, the answer seems simple: automotive engineers have succeeded by marrying an internal combustion engine (which offers good mileage, good range, and quick refilling capabilities) with an electric motor (which offers the advantages of fuel economy since it doesn t consume any gasoline). Motor manufactures are now trying to capture this same success by combining an induction motor with a permanent magnet synchronous motor. The premise is simple, says Richard Schaefer, former General Product Manager for AC and DC variable speed motors at Baldor Electric Company. Start with a permanent magnet motor, with magnets that are interior to the rotor and then add an induction cage to it. Schaefer says this combination is giving end users the ability to start and run like an induction motor, so the motor can start and run across the line. Most synchronous permanent magnet motors don t run across the line; they can t start like an induction motor, he says. You have to have a drive. Synchronous motors also run at a precise speed. They don t run like an induction motor that has slip. He adds that he likes to call an induction motor a soft motor because it is a very forgiving motor: When you want more torque, the motor slips, and you get more torque; when you have less load, the motor speeds up. In contrast, a synchronous motor such as a permanent magnet motor runs at a very precise speed, unlike the induction motor. I think of synchronous motors as firm motors you are always going to be running at synchronous speed regardless of the load condition that the motor is seeing. Induction motors tend to change speeds as the motor heats up; synchronous motors will not. UNIQUE FEATURES OF HYBRID INDUCTION- SYNCHRONOUS MOTORS One of the advantage of hybrid motor technology using a permanent magnet rotor is the fact that users can virtually eliminate all of the secondary losses in the rotor. With an induction motor, an energy source is required to energize the rotor and start the motor. That s reactive KVA or KVAR, says Schaefer, because that rotor is not inherently magnetized. With a traditional induction motor, users have to have some amount of power to energize the motor itself to get the motor to work. With an interior permanent magnet hybrid design, the magnets are already magnetized, he says, so energy is not required to magnetize that rotor. Another by-product of hybrid induction-synchronous motor technology is that these motors achieve a very high power factor. Because users don t have to energize the rotor, and since the magnets are already present and magnetized, we end up with very high power factor that stays high over a wide load range, says Schaefer. The motor starts just like a traditional induction motor, comes up to speed, and hits its slip speed point. Once it gets to slip speed, the magnets will pull into synchronization with the rotating field. As the magnets pull the motor -2-

3 into synchronization, the motor is no longer slipping, and the induction cage drops out of the circuit. A third benefit to this type of hybrid motor is that they are very cool running, which equates to a much longer motor life than standard induction motors. With a coolerrunning running motor, for example, users can achieve the same amount of torque as larger motors but in a smaller package, explains Schaefer, which increases choice for users who can get either a standard NEMA drop-in replacement, or you can get a power-dense motor. This choice has ramifications for OEMs as well, as a power dense motors have many applications where a great deal of torque is required to fit in a very small space. Finally, this technology is suitable for adjustable frequency power. Today, the drive industry doesn t have specific software optimized for hybrid motors, but simply by running in scaling mode volts per hertz you can get the kind of performance you are looking for out of the motor, adds Schaefer. APPLICATION CONSIDERATIONS With any technology, there are always engineering trade-offs, and this is also true with the hybrid motor design. For example, there are inertia synchronization limitations. The hybrid motor will not achieve the kind of levels that are published in the NEMA standards, so users have to be more cognizant of the amount of inertia that we are trying to synchronize, says Schaefer. Inertia is related to the mass of the application, so users need to have an understanding of what the inertia limits are on the application and make sure that the motor that is being selected can start that level of inertia. Another characteristic of hybrid motors is that transient torques can be observed during initial starting, says Schaefer. Within the first second or two of starting, the motor comes up to speed, and as the magnets are trying to catch the rotating field, the rotor is being held back by the inertia of the system, which leads to transient torque ripple caused by the magnets trying to catch the rotating field. Users should be aware that these hybrid motors can exhibit positive/negative transient torque characteristics early in the starting cycle of the motor. A significant advantage of using the hybrid motor on an adjustable frequency power source (adjustable frequency drive AFD) in addition to the efficiency gains, is that the inertia and transient torque issues are totally eliminated. Additionally by being more efficient, in many cases, a smaller drive can be used as drives should be sized based upon full load motor amps thus reducing the total system cost. -3-

4 Efficiency at high horsepower Cage Slots How line start permanent magnet motors can help you achieve line starting capability with above premium efficiency levels (a) By Robbie McElveen and Mike Melfi, IEEE Senior Members; and Roger Daugherty, IEEE Member Cage Slots The advantages of using permanent magnets (PM) in electric motors to increase efficiency are well documented [1]. In an effort to provide motors with higher than premium efficiency levels, this technology is now making its way into much larger motors. Large PM motors exceeding 1,000 hp have been built and placed into service; however, these motors require the use of a variable frequency drive (VFD) for their operation. Only relatively small PM motors, on the order of 10 hp or less, have been available with direct on line (DOL) starting capability. This article reviews line start interior permanent magnet (LSIPM) technology (including the transients associated with DOL starting of this type of motor), followed by a comparison of LSIPM (b) Figure 1. Example LSIPM Motor Topology (a) symmetric cage, (b) asymmetric cage motors with induction motors, and justify or simply do not want a concluding with key application VFD, there has not been a good guidelines for LSIPM motors. overall solution. OVERVIEW AND MOTOR Line start interior permanent STARTING CONSIDERATIONS magnet (LSIPM) motors are While there are many benefits to recognized to be one of the best operating an electric motor with a candidates for motors that have VFD, there are many fixed speed line starting capability with above applications for which the added premium efficiency levels. Motors costs and losses inherent to a of this type employ an induction drive are not justifiable. For those type starting cage in the rotor body users who want higher efficiency along with the magnets. Example than can be provided with an cross sections of this type of rotor induction motor, but who cannot construction are shown in Figure

5 A LSIPM motor could be called an induction-start, synchronous run motor. A synchronous motor primarily produces torque at synchronous speed. An induction motor produces torque at every speed except synchronous. In this way, a motor with both an induction cage and permanent magnets will produce torque to accelerate from zero speed when started across the line, and then operate as a synchronous motor with no rotor cage losses once fully up to synchronous speed. The LSIPM motor essentially accelerates according to the torquespeed curve of the induction cage (minus some braking torque due to the presence of the magnets) until the rotor approaches near synchronous speed. At this point, the rotor pulls-in to synchronism with the rotating stator field and begins operation as a synchronous motor [2]. Due to the fact that the rotor field (magnets) is on during the asynchronous acceleration portion of this curve, the torque during acceleration contains transients and is not as smooth (see Figure 2). The transient torque is oscillatory and may be both positive and negative in nature. Factors which contribute to the magnitude of these transients are the load torque, the load inertia, characteristics of the induction cage and the strength of the magnets. Figure 2. Steady-State Induction and Synchronous Torque-Speed Characteristics (model results) Starting considerations for a offset in the average current. Unlike LSIPM motor design include transient voltage sag, current magni- motor, the current for a LSIPM the current drawn by an induction tude and transients, rotor heating, motor exhibits a fluctuation in the and average and transient torque RMS current until just before the produced in the air gap (and possibly transmitted to the load). and stable operation is achieved. point of synchronization is reached Transient voltage sag. Depending Transient torque. While the on the stiffness of the power system, voltage sag can be significant periods of negative torque produc- induction motor also has some due the large current drawn during tion, these are limited to a very the starting period. However, this short time period at the beginning of the acceleration. There is is really no different than for an induction motor if the current levels no synchronization event for an are comparable. induction motor, thus there are no transients associated with pulling into synchronism as with the Transient current. As with an induction motor, the initial peaks of LSIPM motor. the current after application of the voltage can be significantly higher Rotor heating. The total cross than the peak value related to the sectional area of the rotor bars RMS current because of a transient used in a LSIPM motor design is -5-

6 generally less than can be provided for an induction motor. The primary purpose of the rotor cage in a LSIPM design is to limit the level of starting current while providing the torque necessary for accelerating the motor from zero speed to the pull-in speed at which the motor will synchronize with the power supply. The large cross-sectional area preferred for achieving high efficiency at rated full load speed in an induction motor is not required. The design of the cage may also be limited by the space taken up by the permanent magnets. Depending on the arrangement and size of the permanent magnets, the cage may be uniform from rotor bar to rotor bar as in Figure 1a or utilize rotor bars of various shapes and orientation as in Figure 1b. The design of the rotor bars must not restrict the desired interaction between the magnetic fields of the stator and the permanent magnets in the rotor which provides the synchronous torque. An additional consideration is the effect of temperature on the magnets. A magnet material must be selected which will not suffer irreversible demagnetization with the operating temperatures expected. Many rareearth permanent magnets are capable of withstanding operating temperatures of 190 C or higher. Figure 3. Speed-Time During Start (model results) Synchronization. Additionally, increased slip and reduced ability the ability for the LSIPM motor to to synchronize a given load and synchronize a given load must be inertia. Once the LSIPM motor analyzed. Earlier, it was shown that synchronizes, the slip is zero. a LSIPM motor can be viewed as a combination of an induction motor The difference in speed at which and a synchronous motor. The difference between synchronous speed induction motor (asynchronous the machine would operate as an and running speed for an induction speed) and the synchronous speed motor is known as slip. For a given is a key attribute for a LSIPM stator flux level, it is a general rule motor design. Figure 3 shows the for induction motor design that increased rotor cage area will result in the synchronization portion of a speed-time characteristic during lower cage resistance and lower slip. LSIPM motor start. It is typical for such a motor to very slightly This has not been found to be true overshoot the synchronous speed for LSIPM motor designs. It is in (in this case 1,800 rpm) before fact a combination of the rotor bar settling to the final speed. Figure area and the magnet flux interaction that results in a given slip. A same time period. Note that the 4 shows the torque during this rotor cage with too much cross average torque is cyclic until after sectional area or a cage which synchronization, at which point impedes the flux paths can result in the average torque remains con- -6-

7 stant at the value of load torque at synchronous speed. The amount of inertia that can be synchronized by a given design for a certain value of load torque is also dependent on the magnet temperature at the time of synchronization. The remanent flux density, or strength of the magnets, decreases with increasing temperature. For most rare earth magnets the decrease is on the order of to %/ C. For a 100 C rise in magnet temperature, the strength of the magnets would be reduced by 3-11% depending on magnet composition. The slip is also dependent on the temperature of the rotor cage during the start. The resistance of the rotor cage is temperature dependent and can change by as much as 50% from cold to hot conditions. COMPARISON OF LSIPM MO- TORS TO INDUCTION MOTORS Figure 5 shows test data during a DOL start of a 50 hp, 1,800 rpm LSIPM motor with a connected inertia of 60 lb-ft2. This data illustrates the two regions of operation (starting and synchronization) previously discussed, and shows the transient nature of torque seen during acceleration of a LSIPM motor. LSIPM motors have some aspects in which they are similar to induction motors and other aspects in Figure 4. Torque-Time During Start (model results) which they are distinctly different. the motor, while in the induction The torque-speed curves of Figure motor case the cage is a necessary component in the production 2 show a distinct difference in terms of the synchronous motor of both starting and steady-state speed being invariant with load torque. As such, the steady-state while the induction motor has efficiency of the induction motor is increasing slip with increasing load. strongly influenced by the slip related losses in the rotor cage. With However, if that induction motor torque-speed curve is thought no steady-state slip, the LSIPM of as the DOL starting transient motor has no slip related losses in performance, then the LSIPM the rotor cage. motor also has a very similar DOL starting behavior. Those similarities Although the starting behavior in can be seen by comparing the time terms of average accelerating torque traces of torque and speed shown in and starting time can be quite similar Figures 5 and 6. between induction and LSIPM motor technologies, there are transient Both motor technologies have a torque pulsations that are noticeably DOL starting behavior that is larger for the LSIPM motor [3]. determined greatly by the design There are pulsating torques during of the rotor squirrel cage. In the DOL starting of induction motors case of the LSIPM motor, the cage [4], but they tend to diminish during is only a factor for the starting of the course of the starting process. -7-

8 Because the torque pulsations of the LSIPM motor during DOL starting include aspects related to the magnet and rotor saliency interacting with the rotating field from the applied stator voltage, they exist throughout the starting event. It is also important to distinguish between air gap torque pulsations and any pulsations transmitted to the coupled load. For this purpose, a simulated start of a 40 hp, 1800 rpm motor with 20 lb-ft2 connected inertia was performed. As shown in Figure 7, during a DOL start, there can be significantly less pulsating torque transmitted to the coupled equipment compared to the developed air gap torque in the motor. The amount of torque transmitted to the load depends on a variety of factors, including the stiffness and damping of the mechanical parts and the relative inertia of the motor and the load. This is worthy of consideration when selecting the coupling to be used as well as when considering the effects the cyclic torque could have on the connected equipment. Since a LSIPM motor can be designed to achieve the same average starting torque and average starting current as an equivalent induction motor, they can achieve similar numbers of starts per hour or per day. However, since the magnet field strength has an influence on the ability to synchronize a given Figure 5. Acceleration of LSIPM Motor (test data) load and since the magnet field strength is a function of temperature, the frequency of starting must also consider magnet characteristics. APPLICATION GUIDELINES In applying LSIPM motors where induction motors have been previously used, several issues should be considered. In the general case where an inverter is used to provide variable frequency / variable speed, the LSIPM motor is simply a dropin replacement for an induction motor. For the following specific cases of DOL starting and fixedspeed operation, some distinctions must be considered. Load Torque and Inertia. A primary consideration is related to the torque / inertia characteristics of the load, as limitations exist in regard to how much load inertia can be successfully started and synchronized at rated load torque. Even if a load is only to be started very infrequently, the inertia which can be accelerated and synchronized is typically significantly less than the maximum standard inertia for induction motor starting given by NEMA MG1, Table 12-7 [5]. It is also possible to use a soft-start coupling in order to increase the inertia which can be synchronized [3]. Transient Torque. Another application consideration is in regard to the oscillatory torques experienced during DOL starting. The coupling selection must account for these torques in order to achieve the intended coupling life. This can be likened to the sort of pulsating torque that -8-

9 Volts line-line filtered RMS Voltage [V] Time [sec] (b) Figure 6. Voltage During DOL Starting at 380 V, 60 Hz, with a coupled load inertia of 100 lb-ft2 (a) time waveform, (b) RMS value of the voltage waveform of (a) (test data) might be seen with loads such as a reciprocating compressor. Coupling selection should account for not only the peak magnitude of the transmitted torque but also the strong possibility of torque reversals during each DOL start. Effects of the oscillatory torques on the connected equipment must also be considered. The frequency of the oscillatory torque in the LSIPM motor varies from rated frequency at zero speed to zero frequency at synchronous speed. It is common that the first natural torsional frequency of the shaft system of the motor and driven equipment falls within that range of frequencies for the oscillatory torque. As a result the oscillatory torque of the LSIPM motor may be in resonance with the natural frequency of the shaft frequency at some point in time during the acceleration of the equipment. Whether this is of significant concern will depend on the rate at which the system is accelerated through the point of resonance and responsiveness of the shaft system to the oscillatory torque. It is important that the LSIPM motor and driven equipment not be operated for any extended period of time near a point of resonance. Load Sharing / Multi-Motor Applications. The slip of an induction motor creates an unavoidable loss component for steady-state operation, but it also provides a natural tendency to load share. If two induction motors are coupled to a common load, the fact that an induction motor will naturally slow down to produce more torque prevents one motor from 380 V, 113 N m fan load, 100 lbm ft Time [sec] (a) taking most or all of the required load. The synchronous operation as provided by a LSIPM motor has no such intrinsic load-sharing behavior. If load sharing is required between two LSIPM motors, one technique -9-

10 is to operate one of the motors on an inverter where the developed motor torque can be controlled so that it can simply match the torque of the line-fed motor. While the slip inherent in an induction motor can provide an intrinsic load-sharing capability, that slip can be a disadvantage in certain applications. When there are several motors running in a single process where speed matching is of paramount importance, the zero slip characteristic of PM motors can be advantageous. Such applications include certain film lines, glass lines, and fiber spinning applications. Whether the power source for the multiple motors is the utility line or a dedicated alternator or an inverter, a series of synchronous motors (such as LSIPM) can provide exact speed matching due to the zero slip characteristic. (a) Hazardous Locations. The two main safety considerations for motors applied in hazardous (classified) locations are temperature and potential sparking sources. Compared to induction motors, LSIPM motors can often be designed to have cooler rotors due to the lack of slip / rotor cage losses during normal operation [6]. While the rotor of an induction motor can often have a higher steady state temperature than the stator, that is not typically the case for a LSIPM motor. The risk of sparking for a LSIPM motor is no greater than for an induction motor. For the case of motor starting, the rotor of a LSIPM motor may have a higher hot spot temperature than (b) Figure 7. (a) Air Gap Torque During DOL Start of a LSIPM Motor, (b) Coupling Torque During DOL Start of a LSIPM Motor (model results) an equivalent induction motor. However, starting is not one of the normal conditions which are considered when applying motors in Division 2 / Zone 2 environments. -10-

11 CONCLUSION LSIPM motor technology in larger horsepower motors is a viable alternative for those wishing to achieve efficiency levels above what is possible with traditional induction motors. However, although LSIPM motors have been in use for more than 30 years, there has been little push for any standardization because of the niche areas in which the motors have been applied. However, as the LSIPM design is now being considered as a higher efficiency alternative to general purpose induction motors the need for some standards has arrived. Although the authors believe this technology will be most advantageous on fixed speed applications with continuous or nearly continuous operation, there are particular performance and dimensional characteristics that need to be considered if a LSIPM motor is to be a drop-in equivalent of an induction motor. The industry and user will benefit from future LSIPM motor standards that include these items. Starting and peak inrush current Starting torque Pull-up or accelerating torque Breakdown torque or pull-out torque Pull-in torque or maximum inertia Physical motor dimensions Test standards must be developed not only to characterize the full load performance of a LSIPM motor, but also the performance of the motor during acceleration. Attributes such as locked rotor torque, starting current, pull-up torque, and maximum inertia synchronization capability must not only be defined, but a method of testing for them must be developed as well. At the present time the Technical Committee of the NEMA Motor and Generator Section is working on a new standard for PM machines. The initial version is centered around LSIPM motors. IEEE has balloted a proposed IEEE 1812 Draft Trial- Use Guide for Testing Permanent Magnet Machines that contains instructions for performing tests to determine the performance characteristics of PM machines. The primary emphasis of the Guide is on PM machines used with VFDs. However, some of the tests are applicable to LSIPM motors. The Guide lacks tests for determining the performance of the LSIPM motor during acceleration. Note: this article was drawn from the IEEE White Paper Line Start Permanent Magnet Motors: Starting, Standards, and Application Guidelines. Click to download the full white paper, which includes a lengthier discussion of starting considerations and standardization challenges. Bio statements Robbie McElveen was formerly a Senior Project Engineer for Baldor Electric Company. Mike Melfi was formerly a Consulting Engineer at Baldor Electric Company. Dr. Roger H. Daugherty was formerly a Consulting Engineer at Baldor Electric Company. Originally published as IEEE Paper No. PCIC-SF35, Line Start Permanent Magnet Motors Starting, Standards, and Application Guidelines, copyright 2014 Institute of Electrical and Electronics Engineers. -11-

12 REFERENCES [1] M.J. Melfi, S. Evon, R. McElveen, Permanent Magnet Motors for Power Density and Energy Savings in Industrial Applications, IEEE PPIC Conference, June 2008 [4] M.J. Melfi, S.D. Umans, Transients During Line-Starting of Squirrel Cage Induction Motors, IEEE PCIC Conference, September 2011 [2] T. J. E. Miller, Synchronization of line-start permanent- magnet ac motors, IEEE Trans. Power App. Syst., vol. PAS-103, no. 7, pp , Jul [5] NEMA Standards Publication No. MG , National Electrical Manufacturers Association, 1300 N 17th Street, Suite 1752, Roslyn, VA [3] M.J. Melfi, S.D. Umans, J.E. Atem, Viability of Highly- Efficient Multi-Horsepower Line-Start Permanent-Magnet Motors, IEEE PCIC Conference, September 2013 [6] R. McElveen, B. Martin, E. Massey, D. Stelzner, Applying Permanent Magnet Motors in an Ex Environment, IEEE PCIC Conference, September