Going magnetic with fixed speed and variable speed couplings

When two identical critical pumps were causing maintenance problems at Fellside CHP, Sellafield in Cumbria, engineers at the plant were keen to investigate alternative coupling technologies. The problem was caused by pressurised water rising from ambient to 350°C that is pumped around a ring main system. This rise in temperature was causing the pump body and pipe work to grow and move significantly. The main result of this excessive movement was manifested in disc coupling degradation. Stainless steel disc packs of the metal membrane type used in this spacer coupling application could not accommodate within their design the large transient misalignments created.

Engineers at the company caught wind of the MagnaDrive magnetic coupling, supplied in UK by Drive & Coupling Solutions and were keen to investigate. A solution was put forward to replace the troublesome disc couplings with the company's magnetic drive coupling that employs rare earth neodymium boron iron magnets in proximity to pure copper to generate an eddy current field strong enough to link these 350kW motor to their respective pump via a 6mm air gap. The installation of the two couplings to the pumps totally removed vibrations, restoring moments and all forces that had previously led to the frequent coupling failure. In addition pump downtime was significantly reduced through lower bearing and seal wear. 

Fast forward eight years, and the couplings are still running, offering the benefits of zero maintenance and zero adjustment, and saving Fellside considerable time and money. And Drive & Coupling Solutions can point to numerous other high profile successes for the tech-nology at the likes Corus Scunthorpe and Bristol Port Company. 

The MagnaDrive coupling is actually a fixed speed version of the MagnaDrive adjustable speed drive, and that's where things really start to get exciting. Simple, reliable and low cost, the MagnaDrive ASD provides the energy savings of speed control without the problems often associated with variable frequency drives. System complexity is reduced, vibration is minimised and alignment problems disappear. Motor systems can be resized for lower cost and more efficient operation. Applications currently benefiting from use of the MagnaDrive ASD include pumps, fans, blowers, centrifuges, and bulk handling equipment. Industries served in-clude water and waste-water treatment, pulp and paper manufac-turing, power generation, oil & gas processing, mining, food processing and HVAC systems.

How does it work

The MagnaDrive Adjustable Speed Drive (ASD) works by transmitting torque from the motor to the load across an air gap. There is no mechanical connection between the driving and driven sides of the equipment. The torque is created by the interaction of powerful rare-earth magnets on one side of the drive with induced magnetic fields on the other side. By varying the air gap spacing, the amount of torque transmitted can be controlled, thus permitting speed control. 

The MagnaDrive ASD consists of three sets of components. A magnet rotor assembly, containing rare-earth magnets, is attached to the load. A copper conductor rotor assembly is attached to the motor. Actuation components control the air gap spacing between the magnet rotors and the conductor rotors. Relative rotation of the copper conductor and magnet rotor assemblies induces a powerful magnetic coupling across the air gap. Varying the air gap spacing between the magnet rotors and the conductor rotors results in controlled output speed. The output speed is adjustable, controllable, and repeatable. The principle of magnetic induction requires relative motion between the magnets and the conductors. This means that the output speed is always less than the input speed. The difference in speed is known as slip. Typically, when the MagnaDrive ASD is operating at full rated motor speed, the slip is between 1% and 3%. 

The output torque of a MagnaDrive ASD is always equal to the input torque. The motor is only required to produce the amount of torque needed by the load. The efficiency of the system is calculated by dividing the output (load) speed by the input (motor) speed. The ability of the ASD to transmit power or to control speed is not affected by minor angular or offset alignment between the motor and load. Vibration due to misalignment is virtually eliminated. Transmission of vibration across the drive is also eliminated due to the air gap configuration.

When installed in a system, the MagnaDrive ASD is controlled from a process signal. The pressure, flow, level or other process control signal is provided to the MagnaDrive ASD actuator. The drive will then modulate the speed of the load to satisfy the control needs.

Performance advantages

In fixed speed configuration, in comparison with conventional couplings, the MagnaDrive coupling eliminates the transmission of any vibration, and so dramatically reduces wear and associated maintenance implications. They also eliminate the need for precision alignment, which can be critical for other coupling technologies. The coupling can also build in an automatic feature to decouple the motor from the load in the event of a load seizure or an overtorque condition. In this case, the two magnet plates on the inside of the coupling move in and uncouple the motor from the load.

How, though, does the MagnaDrive ASD compare with conventional electronic variable frequency drives? The first thing to point out is that MagnaDrive is much more than a speed control device. It offers productivity and equipment life benefits to motor systems unavailable through any other means. All VFDs can cause a current to pass through bearings. At the contact surfaces the process is similar to electric arc welding, resulting in the formation of fluting (corrugation) in the bearing raceways and rollers. Bearings damaged by these circulating shaft currents become noisy and vibration increases because the smooth raceway surfaces are damaged. Unlike VFDs, the MagnaDrive ASD is a mechanical device and therefore does not cause electrical fluting.

Energy efficiency is another major plus point. At average load speeds above 90% of the rated motor speed, MagnaDrive is claimed to be more energy efficient than a VFD when all system power consumption is considered. The MagnaDrive ASD has approximately the same energy efficiency as VFDs when the average load speed is between 80% and 90% of rated motor speed. VFDs may show greater efficiency when the average load speed is below 80% of the rated motor speed, however this occurs when power demands are reduced. Therefore, as long as at least 50% of the duty cycle operates above 80% of the rated motor speed, a MagnaDrive ASD is better than 5% more efficient than a VFD. 

A VFD user also has to consider harmonic distortion that can stray beyond the facility and contaminate the utility power grid. Stray current spikes can damage bearings and increase the likelihood of mechanical equipment failure. In some cases, the VFD is its own worst enemy. The harmonics created by the VFD can contribute to its own early failure. VFD manufacturers address this through the addition of filters, but these add cost to the product, and not all filters are as effective as others. MagnaDrive, by contrast, is a simple mechanical device. It has no impact on the power of the system surrounding the motor. Therefore, there are no harmonic distortion issues associated with the MagnaDrive technology.

Some VFDs require an inverter duty motor that is more expensive than a standard duty motor. Additionally, there can be long lead items, requiring significant advance ordering time, or tying up capital in spare motors. The MagnaDrive does not influence motor type designation. Usually, standard duty motors are less expensive and readily available.