Permanent magnets offer big savings

5 min read

Permanent magnet motors could make a big difference to energy consumption and the bottom line. Paul Fanning reports.

Although permanent magnet motors are by no means new, recent improvements in their quality and magnetic strength means they now offer a viable alternative in a number of applications. Clearly, their key advantage is that the magnetic flux is produced by the magnets themselves without generating any losses. Consequently, at the same torque, the current drawn by magnet motors is significantly lower than for asynchronous motors. This means that the principal source of power loss is eliminated, making for a considerable difference in efficiency. In addition, magnet motors do not suffer from the slip required for operation in asynchronous motors, meaning even less power loss. The ability to replace, or even retrofit, existing systems with these motors is therefore proving increasingly attractive for a range of applications. Dyneo's LSRPM range of synchronous permanent magnet motors is designed for direct replacement with standard induction motors. By reducing rotor losses, the Dyneo system's patented radial magnet rotor technology greatly improves the drive's efficiency and specific output power. According to Philippe Petiolat of Dyneo, this system was previously produced for a single customer, but its success led the company to develop it for general distribution. The LSRPM series makes this technology available in a commonly-used IP 55 IEC mechanism. Equally, LSRPM motors have just as many fields of application and use as those for induction motors, including: pumping, ventilation, compression, conveying, extrusion, process control and generators. The innovative design of the magnet rotor increases efficiency by almost 10% to levels approaching 98%, while the size reduction is around 50%. This type of motor is intended for applications for which electricity consumption is high and continuous. This means that, in a large number of cases, the time to return on investment for replacement of an asynchronous motor by LSRPM will be less than one year, according to the company. Of course, the savings available may well be dependent on the application. Although, says Petiolat: "We offer a tool that allows customers to select the application they want, put in the relevant details and compare the standard AC and PM solutions. The system then calculates the energy consumption, calculates the cost and deducts the savings. With an available power range up to 400 kW in an aluminium frame, the LSRPM is significantly smaller and lighter when compared with a conventional induction motor of the same power rating. Equally, the modularity of the series, with its foot mounted, flange-mounted or face-mounted configuration, plus the numerous associated options, makes it easy to replace any conventional drives already installed. The key advantage of this motor is that the efficiency remains almost constant over the speed range, which allows significant energy savings as soon as the motor is running under rated speed. "It is here you really start to see the difference," says Piotelat. Quite how much difference can be seen in a comparison between the efficiency level of PM, EFF2 and EFF1 (roughly equivalent to IE1 and IE2) motors at full speed (3000rpm) and half speed (1500rpm). At full speed, the PM motor is peaks at just under 98% efficiency, while the EFF1 and EFF2 motors peak at 94%. However, at half speed, the difference is startling, with the EFF1 and EFF2 motors not getting above 86%. "At one company," says Piotelat, "we have undertaken a study that shows that using PM motors in their refrigeration devices 24 hours a day will save them £9400 per year." Another company to have shown the energy-saving benefits of this technology over traditional motors is Baldor, which recently developed a novel permanent magnet-based solution for controlling commercial building cooling tower fans. Although Baldor's Robin Cowley prefaces his comments by pointing out the irony of talking about what is in effect an IE4 technology when in Europe there will not even be a requirement to implement IE2 technology until 2011 ("The EU is way behind the rest of the world – it's criminal," he says), he is nonetheless enthusiastic about the benefits of permanent magnet technology. "The thing about PM motors," he says, "is that they've been around for a long time, but they've been fairly small and expensive because of the rare earth magnets used in them. Now, though, we're going beyond 200MW with them." Traditionally, the heating, ventilating and air conditioning systems of medium to large-scale office and industrial buildings have been driven by standard AC induction motors connected to the fan via a drive shaft and disc coupling arrangement into a right angle gearbox. This approach does offer a number of disadvantages, however. The gearbox runs at high speed and requires regular inspection and maintenance of the lubrication and seals, while misalignments in the power transmission system can cause vibration, wear and noise. Most importantly,, the complex power transmission system introduces significant energy losses. The new technology is based on a permanent magnet motor that packs the high torque required into such a compact space that it can easily be retrofitted into cooling towers – sitting underneath the fan in the space currently required for the gearbox element of conventional power transmission systems. Controlled by a Baldor VS1 drive with a specially-developed speed control algorithm combined with variable speed control that is optimised for the building cooling application, the motor used is a synchronous type from Baldor's RPM family. Thanks to dramatic improvements in the magnetic and thermal properties of permanent magnet materials in recent years, this technology now represents a viable alternative to conventional AC induction motors and delivers a significant energy efficiency advantages – even compared with the latest premium-efficiency types of motor. Baldor's motors employ a permanent magnet internal to the rotor, offering a higher power density than motors with external magnets. Laminated frame technology is used in the design, eliminating the conventional cast iron outer frame of large AC motors and allowing more room for active (ie torque-producing) magnetic material. This produces a highly torque-dense motor, allowing installation in the same space that is currently used for the gearbox. The motor will provide continuous constant torque over its entire range from zero up to base speed, and employs optimum pole construction to maximise both efficiency and power. In this type of application, the interior permanent magnet synchronous motor design uses energy efficient technology that, in combination with the elimination of the gearbox and drive train transmission losses, results in a more efficient system compared to conventional fixed speed designs. Additional significant energy savings can be gained by being able to operate the fan at reduced speeds during non-peak load conditions. The direct drive also reduces noise and eliminates the issue of cooling tower water becoming contaminated from leaky gearboxes. Says Cowley: "By removing the gearbox, we've eliminated a complete component. And, of course, a gearbox only consumes power." To give some idea of what this system can deliver in terms of efficiency, it was trialled prior to its launch and compared with a conventional cooling tower fan drive system at a university building with identical twin cooling towers housing 5.5m fans. One tower was left as originally constructed, while the other was retrofitted with the new Baldor permanent magnet motor and variable speed drive. Independently verified power measurements showed a decrease in input power of approximately 13% for the direct drive arrangement when running at full load – amounting to significant savings over time. According to Cowley, interest in this technology is increasing, with Baldor close to having some of these systems installed in the UK. He says: "Obviously, this is an emerging technology, but in applications where you're looking at a motor with a drive, then permanent magnets make a lot of sense."