Brushless PM Motors: ‘Big’ on Power, Efficiency
Better known in relatively small physical sizes and lower power applications, brushless permanent magnet (PM) motors can be made in virtually any size, with no real technological constraints. Large brushless PM motors are not particularly new. They're available from select manufacturers, which now seek to overcome past economic issues that have limited their numbers.
Better known in relatively small physical sizes and lower power applications, brushless permanent magnet (PM) motors can be made in virtually any size, with no real technological constraints. Large brushless PM motors are not particularly new. They’re available from select manufacturers, which now seek to overcome past economic issues that have limited their numbers.
Large brushless PM motors offer numerous benefits, among them high power density and high efficiency without rotor losses. Benefits come at a price, as manufacturing and material costs—including that of high-performance magnets—add up fast. Brushless PM motors also need a variable-frequency drive for control. Yet, cost-benefit figures favor PM motors over alternative technologies (see below) for a growing number of tough applications in industry, marine propulsion, military/defense, and other sectors.
Thorough analysis of a potential application is essential to justify the cost-benefit. Recent developments may lend a helping hand. Widespread availability of drives, along with significant downward price trend for drives and magnet materials suggests more of these motors will be in service in the near future. Greater user awareness about large PM motors would further aid that development.
Drives, magnets spur progress
A notable designer and developer of large brushless PM motors, DRS Technologies Inc. has begun to apply some of its power systems experience in the defense industry to industrial, marine, and transportation applications. Developments in magnets and variable-frequency drives (VFDs)—required to start and operate PM motors—have made it practical to build these motors in large sizes. Associated drives at DRS are medium-voltage (1.5-6.6 kV) units, with pulse-width modulation (PWM) and vector or sensorless vector control.
Edgar S. Thaxton, chief engineer and director of system engineering at the company’s DRS Power Technologies unit, notes dramatic improvement in VFD reliability over the last decade, along with power ratings that can reach 60 MW or more. Increased ratings have come, in part, from newer power-switching devices, such as insulated-gate bipolar transistors (IGBTs) and integrated gate-commutated thyristors (IGCTs). ‘PM motors shine in applications where VFDs are required or where variable-frequency control offers a performance benefit outweighing the VFD cost,’ he says.
As for magnet materials, their corrosion resistance, mechanical properties, and temperature limits also have improved significantly, yielding PM machines capable of handling rugged industrial as well as military applications. Rare-earth permanent magnet cost-per-pound has decreased by a factor of five over the last 10 years, while corresponding magnet strength-per-pound rose by a factor of three, explains Thaxton. This represents a cost-benefit improvement of 15:1 and is significant to ongoing ‘commercial affordability.’
Powertec Industrial Motors Inc. is another company with a history of manufacturing high-performance brushless PM motors (and drives), currently up to 400 hp for standard products. Expertise centers in military/defense, severe industrial, and explosive-atmosphere applications. At one time, the company produced brushless dc motors up through 600 hp ( CE , Dec. 1992, p. 79). These machines, measuring about 25-in. on the OD, used ferrite permanent magnets and were offered in air-cooled and liquid-cooled versions. They’re custom products not routinely manufactured today. A new design for up to 1,000 hp is planned for 2006.
Powertec agrees about the relationship between motors and drives. In the past, large high-power IGBT drives to run brushless motors were not readily available at practical prices. ‘This has changed substantially in the last five years or so with such drives being more readily available and at more affordable costs,’ says Powertec general manager, Ed Lee. The company uses low-voltage drives (up to 600 V) for motor control.
However, Powertec attributes scarcity of large PM motors to factors more market-driven than technology based. ‘While large PM motors are available, they are more expensive than conventional motors, and unless efficiency, smaller size (high power density), and higher dynamic performance are important issues in the application, the buyer will not spend the extra money,’ states Lee. As a result, still very few manufacturers of these motors exist.
Neodymium-iron-boron (Nd-Fe-B) and samarium-cobalt are two rare-earth magnet materials available today for industrial motors. They’re similar in magnetic density, but samarium has better high-temperature characteristics (albeit at higher cost), Lee notes. While Nd-Fe-B magnet pricing has declined substantially in the last five to seven years, it remains more costly than equivalent materials used in ac induction rotor construction. ‘Magnet cost and low manufacturing volume affect the cost premium for brushless PM motors and therefore the probability of choosing them for a given application,’ concludes Lee.
In the view of Siemens AG, Automation & Drives Div., large PM motors are mainly used for high-torque applications; these motors are of the direct-drive (no gear box) design. This relates to customers’ increasing focus on reducing maintenance costs for their assets due to pressures on overall production costs, explains Robert Lehning, product manager for large ac motors at Siemens A&D Large Drives Div. ‘Eliminating gear boxes by using direct-drive technology is one step in that direction,’ says Lehning. ‘Gear boxes need higher maintenance and monitoring attention than motors and have a lower overall availability.’
|DRS Technologies considers features of its higher pole count brushless PM motor design superior to that of the alternative synchronous motor. This includes weight and size reduction.|
He further points to the low-speed design of Siemens’ high-torque motors. ‘In our opinion, large PM motors with rated speeds between 800 and about 5,000 rpm have no real advantage compared to conventional induction machines,’ continues Lehning. PM technology also can be ‘interesting’ for high-speed motors—for example, at 10,000 rpm and more—where efficiency is higher than that of ac induction machines. ‘However, applications in that area are usually very specialized,’ he adds.
Siemens’ Lehning likewise mentions the falling cost of high-performance magnets that make PM motor technology more attractive. However, the overall technology is still costlier compared to induction motors. ‘Therefore, PM motors will not replace conventional induction motors for standard applications in the foreseeable future,’ he says. Lehning also notes improvements in design tools and ‘know-how’ for development of PM motors behind the new visibility of this product line.
Yaskawa Electric concurs that no technical barriers exist to building larger brushless PM motors in the 100s of kW range. The issues are mainly economic. Discussions with customers and quotation on such larger units are happening more frequently, but the product is not typical or ‘off the shelf.’ Yaskawa has been supplying the economics and reliability of internal permanent magnet (IPM) motor technology for higher power needs for several years—as well as using IPM servo motors in its own production machines. (See October 2005 CE , pp. 52-55, for a recent article on IPM servo motors.)
Economic pressure for shorter time-to-market is growing for all product manufacturers. To help its customers realize benefits of digital servo systems in higher power processes and machines, Yaskawa has been extending IPM motor technology to its production facilities in ever higher power ratings, says the company.
Italian company Oemer Motori Elettrici Spa is another manufacturer whose offerings include large brushless PM motors. Among them are torque motors that range up to 300 kW for gearless, direct-drive applications to 500 rpm; liquid-cooled, three-phase servo motors providing ratings to 318 kW at 5,000 rpm; and high-performance units that reach to over 1 MW output at up to 2,600 rpm nominal speed for dynamic industrial applications. Oemer’s product line was on exhibit at the SPS/IPC/Drives show in Germany in November 2005.
PM machines boast 1-2% higher efficiency than ac induction or synchronous motor alternatives at full load—and 10-15% more efficiency at partial load, according to DRS Technologies. Efficiency derives from full-rotor excitation without current and without associated losses at all speeds. Thaxton cites an example of a low-speed marine propulsion motor achieving an amazing 99.3% efficiency!
Motor cooling is simplified since the rotor generates little or no heat. Only the stator needs cooling and, because it’s an ‘outer structure,’ water cooling becomes more attractive. Simpler cooling design also leads to flexible motor geometry. ‘PM machines support a much wider range of aspect ratios than conventional motors. Short, large-diameter and long, narrow machines are feasible, as are both radial (conventional) and axial (pancake) air-gap designs,’ he says.
Compact brushless PM motors reduce size and weight to about, exciter rotor, exciter stator windings and, generally, a rotating rectifier, explains Thaxton.
DRS Technologies has demonstrated prototype PM motors that achieve higher power for a given speed than conventional machines, allowing more flexible load matching and eliminating gears (in a direct-drive motor design). ‘PM motors are at cost parity with conventional machines. As long as the application requires a VFD, little reason exists to choose a conventional machine,’ continues Thaxton.
At Powertec, benefits of large brushless motors are seen as higher efficiency, higher power density (smaller size per power output), and higher dynamic performance—with efficiency difference in the range of 3% for brushless PM over ac induction motors. Lee points out that the differential may not be quite that high if full effort is made to minimize losses in an induction motor (such as in specific energy-efficient designs).
‘Such low slip induction motors will not be practical to run across the line without a drive, however, because of very high starting current and lower starting torque,’ adds Lee. ‘[On the other hand], PM brushless motors cannot be run across the line at all since they must be shaft-position commutated.’
At Siemens, main benefits for applying large brushless PM motors are seen as lower operating costs, higher total system availability, and lower space requirements. Eliminating the gearbox helps in various areas: lower maintenance costs and higher drive system efficiency due to less power loss, as well as lower system complexity.
Control issues, higher pole count
Few control issues exist with large PM motors. PWM-type drives now offer a level of control suited to demanding industrial applications, along with reduced power system harmonics and improved power factor.
Much of brushless PM motors’ superior power density comes from a higher pole number than in conventional motors, according to Thaxton at DRS. ‘In general, PM motors can have three-times more poles than a WFS motor of the same diameter.’ It enables lighter, smaller, and more flexible motor geometry.
However, a higher pole number means a VFD capable of delivering higher electrical frequencies. For a particular speed and PM motor, input requirement of 415 Hz is not unusual. ‘In high-speed, high-rating applications, this can be a design driver for the VFD’s power topology, as well as the control bandwidth,’ he adds.
Other than the need to position-commutate a brushless PM motor, Powertec notes no extra control issues for these motors beyond what exits for induction machines of similar power. About half of today’s available drives accommodate either motor type via software algorithms—commutation control for brushless and slip-based control for induction motors, according to Lee.
‘This is reduced to a simple menu choice.’ He mentions a brushless motor’s ability to act as a PM generator, producing an output without excitation. ‘In applications where motor speeds above ‘base speed’ may occur due to high overhauled speeds, generated voltage can reach levels well above the bus design voltage range,’ states Lee. This affects only a few, unusual applications but must be accommodated in the installation.
Siemens concurs that control issues are not significantly different from that of induction motors. Brushless PM motors operate under VFD control, with only the software algorithm being slightly modified.
DRS Technologies recently has tested under full power a 36.5 MW, 127 rpm ship-propulsion motor for the U.S. Navy that produces more than 2,000,000 lb-ft of torque. Also, Canopy Technologies LLC—a 50/50 joint venture between DRS and Elliott Company Inc. (a leading manufacturer of high-speed rotating equipment)—concluded testing on an 11.4 MW (6,225 rpm) industrial motor (lead photo). These are said to be two of the most powerful PM motors in the world. Large marine propulsion applications are especially attractive for highly efficient PM motors given the rising cost of fuel. DRS also manufactures high-performance PM motors, reportedly achieving over 1 hp/lb in ratings in the 500-1,000 hp range.
Among many applications, Powertec cites the U.S. Navy’s new Advanced Gun System for the Stealth Destroyer program, using numerous large high-dynamic performance brushless PM motors. The elevation motor produces more than 800 lb-ft torque at peak speed of 2,000 rpm (over 300 hp peak), embodied in just a 12.5-inch diameter unit. The Navy mandates application of brushless motors whenever possible for new or replacement purposes, due to smaller size and lighter weight, according to Powertec.
Another application involves azimuth and elevation control plus redundancy requirements for a giant 4-million-lb antenna for the Missile Defense Antenna system. High-dynamic performance and high-power motors were required, delivered by eight 225-hp brushless motors controlling azimuth and four 50-hp motors controlling elevation for a total of 2,000 hp connected.
Powertec’s 400 hp NEMA 3213T frame (16-in. OD) air-cooled motor, shown earlier, illustrates the compactness of these machines. Blower and conduit box look out for proportion to the actual motor. Terminal box seems huge due to NEMA volume requirement to terminate large wires.
The state of brushless PM motor technology represents a conundrum. Some years ago Control Engineering noted that increasingly larger models would be manufactured as demand rises, asking, ‘If they build them, will the users come?’ Powertec’s Ed Lee puts it another way: ‘When more customers want them and will buy them, then more will be offered and in larger sizes.’
A perspective on ac motor types, terminology
There are three principal ac motor (or machine) topologies, of which the brushless permanent magnet (PM) motor is one type. The following comes from a presentation entitled “PM Motor Overview” by Edgar S. Thaxton, chief engineer and director of system engineering for the Power Technologies unit of DRS Technologies Inc.
Machine types in a nutshell
Induction—Rotor magnetic field is induced by current in the stator.
Some of the stator current goes to induce the field and produces no torque (lower power factor);
Rotor spins slightly slower than stator field rotation (slip effect); and
Rotor current leads to rotor losses.
Synchronous—Rotor field is developed by circulating dc current in the rotor windings (acts as electromagnets).
Rotor spins at an integer multiple of stator frequency (synchronous);
Field control offers power-factor control; and
Rotor current leads to rotor losses.
Brushless PM—Rotor field is developed by rotor permanent magnets (PM).
Rotor spins at an integer multiple of stator frequency (synchronous);
Variable-frequency drive (VFD) offers power-factor control; and
No significant rotor losses occur.
“PM machines are a special case of synchronous machines, with permanent magnets rather than electromagnets producing the rotor field,” says Thaxton.
In particular, the term “brushless” or “brushless dc” is loosely defined and often similarly applied in the industrial user community. In case you’ve ever wondered about this, Ed Lee, general manager at Powertec Industrial Motors Inc., lists no fewer than six interchangeable terms to describe this motor type. These are:
DC motor without brushes or commutator;
AC motor with permanent magnet rotor;
Synchronous AC motor with position feedback;
AC brushless servo;
Brushless or brushless servo; and
They’re all the same thing, explains Lee.
In fact, an appropriate definition of brushless dc motor needs to include the associated electronic control or drive. The following is suggested as a good start along the way, as stated in Jan. 1999 Control Engineering, Back to Basics, “Top 20‘must-know’ terms in control & automation,” (p. 100).
Brushless motor—A synchronous three-phase motor that uses electronic commutation for current switching among the phases. Depending on the current waveform and torque characteristics, it is commonly called brushless dc when a trapezoidal current/torque format is used and brushless ac (or just simply brushless) when a sinusoidal current/torque format is applied to the motor windings.