Permanent Magnet Couplings and Adjustable Speed Drives Technology reduces your Total Cost of Ownership
Magnetic Couplings
Motor Couplings are used throughout all process industries utilising rotating equipment. While the most basic function of these couplings is to transfer torque between the motor and the driven load; coupling manufacturers have expanded their product scope to address the common issues of misalignment, vibration, and torque pulsations. While the available coupling technologies have been fairly successful at addressing these issues, this expanded scope of operation has, in many cases, increased the maintenance and operating costs of the systems in which they are used. The introduction of Permanent Magnet Couplings (PMC’s) that transmit torque across a gap of air has changed the coupling paradigm. The “Disconnected Connection” offered by PMC’s physically separates the motor from the load resulting in significant reductions in vibration. This reduction in vibration has resulted in large savings in maintenance & operating costs not possible with other technologies.
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Rotating shaft couplings come in many different configurations. Each type offer different constructions that are designed to meet varying process demands. The choice of which type of coupling to use will have significant impact on the performance of processes and on the cost of maintaining and operating process equipment. Field studies have shown that up to 70% of all bearing failures and 80% of all seal failures are caused by excessive vibration. Excessive vibration generally is the result of misalignment between the motor and its driven load.
All too often when a coupling fails in operation it is immediately replaced with another coupling of the same design and configuration. Unfortunately, this approach ignores that fact that the previous coupling may have failed because it was not well suited for the application.
When the long term Total Cost of Ownership (TCO) is considered for the entire system that the coupling is being used in, it often becomes obvious that replacement-in-kind is not the right approach. Choosing the proper coupling for the application will not only extend the life of the coupling itself, it will also increase the life of the bearings, seals, shafts, and other rotating equipment components.
When making a coupling purchase it is important to look at the TCO associated with each of the available options. TCO analysis has to include all costs that accompany each option. Initial purchase price typically is only up to 10% of TCO. Drive system energy efficiency, non-energy system operating costs (such as long term maintenance requirements), drive system life, and the cost of process downtime all need to play a part in the purchase decision. Some of these, such as initial price, are easy to get a handle on. Long-Term coupling maintenance and repair costs however, tend to be a little more difficult to quantify when comparing available options.
Permanent Magnet Couplings.
The introduction of Permanent Magnet Couplings (PMC’s) by MagnaDrive Corporation, distributed in the UK and Ireland by Drive & Couplings Solutions Ltd, has given the process industry another option to consider when reviewing technologies available to transfer torque between a driver and load.
Figure 1 – Permanent Magnet Coupling
PMC‘s consist of two primary components. The first component, a set of copper conductor plates is connected to the motor shaft; the second component of the permanent magnet drive is a rigid assembly of permanent rare earth magnets, which are connected to the load. During operation, relative motion between these parts creates an interwoven eddy current field that transmits torque across the air gap between the magnet and conductor rotors. During operation no physical connection exists between the motor and the load. This “disconnected connection”, exclusive to PMC’s has demonstrated vibration reductions up to 85% when compared to other torque transfer technologies.
Figure 2 – Permanent Magnet Coupling Cross- Section
PMC’s can be used on most rotating equipment applications with the current technology able to handle up to 4 Megawatt loads. The eddy currents being produced in a PMC are created by relative motion, called “slip”, between the conductor rotors and the magnet rotors. In a typical PMC, this slip results in an approximate speed loss of 1% – 2%.
Because PMC’s operate without any physical connection between the driver and its load, these couplings are able to tolerate angular misalignment of 2 degrees with standard designs and up to 4 degrees utilising standard coupling modifications, parallel misalignment of 6.35 mm is allowed and axial shaft movement of 3.175 mm is well tolerated, with no loss in efficiency.
PMC’s also offer the option of characterising the start-up torque curves to meet individual process needs. Field adjustment of a PMC is accurate and repeatable. The transfer of torque between the magnet and conductor rotors is based on physical laws related to the strength of the magnets and the distance between the magnets and the conductors.
PMC’s offer the option of providing over-torque protection. Utilising the physical property that a rapidly moving conductor will repel a stationary magnet, the range includes a torque limiting coupling that magnetically decouples the driver from the load in an over-torque event. No damage is done to the driver, the load, or the coupling and resetting the coupling is as simple as stopping the motor, clearing the load, and then restarting the system.
Because of the disconnected cushioned start the motor does not need to overcome load inertia, the Peak Demand Current and duration of Inrush is reduced significantly. This Cushioned Start results in energy savings (see the graph below) and reduced equipment wear. For many applications a lower Peak Demand Current may contribute to lower electrical power rates as well. Also, the Coupling air gap can be adjusted during installation to operate a pump, fan or blower at less than maximum flow, with sizeable energy savings based on the centrifugal Affinity Laws.
Graph 1 – Energy Saving Coupling Comparison
PMC’s use no bearings or seals in their design and have no wearing components. As a result, no lubrication of any kind is required. The only maintenance recommended by the manufacturer is a periodic check of the coupling’s air gaps to ensure that proper spacing is maintained. Add to this the fact that the magnets themselves have a half-life of over 2000 years and clearly the cost associated with maintaining these couplings will be very low!
Permanent Magnet Adjustable Speed Drives.
Much attention is paid to the energy efficiency of Variable Speed Drive (VSD) technologies. Energy efficiency is only the “tip of the iceberg” when considering the TCO of VSD alternatives. End users of VSD’s need to consider the costs associated with maintaining, repairing, and operating the variable speed control technology they decide to use in their plant. The differences in the costs of replacing parts, repairing vibration caused damage, correcting misalignment, and replacing worn out or obsolete VSD components can be considerably higher than the energy cost differences between the same technologies.
Permanent Magnet Adjustable Speed Drives (PMASD’s) by MagnaDrive Corporation, distributed in the UK and Ireland by Drive & Couplings Solutions Ltd, has given the process industry another option to consider when reviewing adjustable speed applications.
PMASD‘s as with the PMC’s consist of two primary components. A set of copper conductor plates and an assembly of permanent, rare earth magnets. To adjust the speed of the driven load, the amount of torque transmitted from the motor to its load is controlled by changing the distance between the conductor plates and the magnet assembly. During operation and throughout the entire speed range, no physical connection exists between the motor and the load. This “disconnected connection” has demonstrated vibration reductions up to 85%.
Figure 4 – Permanent Magnet Adjustable Speed Drive
PMASD’s can be used on any centrifugal load application with the current technology able to handle up to 3 Megawatt loads. Because the speed of the load is changed while allowing the motor to continuously operate at its nameplate speed, motor heating and the resulting inefficiencies are not an issue. Also, because the permanent magnet drives are mechanical devices, they do not introduce harmonics into the user’s power grid. Filters, transformers, and air conditioning systems are not necessary.
The advantage of PMASD’s is found in the fact that they operate without any physical connection between the driver and the load. By completely isolating these components from each other, PMASD’s eliminate the alignment-related issues experienced by all traditional VSD technologies. The result is increased seal and bearing life in the overall system and a reduction in the cost of operating and maintaining that system over its life.
In summary.
The potential value of permanent magnet technology is clear. The permanent magnet concept for PMC’s and PMASD’s has been proven across all industries. The use of permanent magnet drives can result in significant energy and non-energy operational savings with annual non-energy operating costs as low as 3% of initial installed costs.
Drive & Coupling Solutions Ltd
Windemere, Cumbria
Can be contacted on:
Tel: 01539 442252
Email: paul@driveandcouplingsolutions.co.uk
Web: www.driveandcouplingsolutions.co.uk
