The two types of linear motors in most prevalent use are Linear Induction Motors (LIMs) and Linear Synchronous Motors (LSMs). While both motor types have appeal for certain applications, there are technical reasons why MagneMotion typically favors the use of the LSM. Some of the technical issues behind this preference are discussed in this section.

For a Linear Synchronous Motor (LSM) to work properly, the control system must accurately track the position of the moving element in order to properly synchronize the moving field current in the stationary frame (stator). If synchronization is lost or interrupted, the motor slips and loss of propulsion can occur. This situation is most likely to be encountered by an LSM when external sensors are used to handle motor synchronization. If a sensor becomes dirty or mis-aligned or if fog, smoke or other line-of-sight obstructions are present, the system will experience problems.

The MagneMotion Synchronization Solution:

MagneMotion's position sensing technique is unique in the industry and is the subject of an international patent application. A transducer attached to the moving element induces a signal into the motor winding (stator) which is picked up by the motor control system. This induced signal enables the control system to determine the exact position of the moving element to within a fraction of a millimeter (fraction of a centimeter for larger systems) and commutate the motor accordingly. Because the sensing is done through the motor stator winding there is no reliance on external sensors. There is no reason why the system should ever experience synchronization problems unless the motor stator winding is physically damaged, in which case no motor will operate properly. MagneMotion also provides this position sensing capability as a stand-alone product.

In addition, the MagneMotion control system has built in diagnostics to identify and flag problem components for rapid repair or replacement. The system architecture has redundant functionality where adjacent modules are continuously polling one another for potential problems and can circumvent and compensate for malfunctioning components. This will prevent the kind of difficulties encountered by the operators of some linear motor-powered systems in identifying and correcting fault conditions.

Linear Induction Motors (LIMs) are significantly different than Linear Synchronous Motors (LSMs) in the way that they produce electro-motive forces or motion. In a LIM, the motor stator creates an Alternating Current (AC) field that induces currents into the reaction plate, which is typically an aluminum fin. This creates eddy currents in the moving element which react with the moving field in the stator to produce thrust. The induced currents in the aluminum plate manifest themselves in the form of heat. In cases of high duty cycles or in locked rotor conditions (where the moving element is not permitted to move) overheating can occur. In several reported cases at amusement parks, LIM fins have cracked from overheating.

The MagneMotion Thermal Management Solution:

The field in a MagneMotion LSM moving element is usually provided by permanent magnets. There are no significant currents induced and very little moving element heating occurs. The MagneMotion LSM uses current control to manage heating in the stator. The motor is designed for a specific application and thermal management for the appropriate power and duty cycle is built in. Where unusual conditions warrant it, finned heat exchangers or water-cooled cold plates can be attached to the stator back iron but this is not typically required. High duty cycle or locked rotor conditions will not overheat a MagneMotion LSM.

There is sometimes a misconception that the magnets used to provide the field in the moving element of an LSM are heavy and are a weight penalty to the system. The impression held by some is that conventional motors with excited copper windings in the moving element or induction motors with a reaction plate made of aluminum or some other material are a lighter weight alternative to magnets. This is not necessarily true.

The Truth About Permanent Magnets:

Pound for pound, high strength permanent magnets have greater energy density than the other moving element materials commonly used in motors. For systems with low-to-medium power ratings, an array of permanent magnets will weigh less than a copper winding every time. At higher power levels tradeoffs are conducted to evaluate the use of copper/iron electro-magnets or superconducting magnets. In many cases permanent magnets are still the best option from a weight and performance standpoint. MagneMotion designs motors based on these considerations.

The weight of a magnet array in an Linear Synchronous Motor (LSM) versus a reaction plate in a Linear Induction Motor (LIM) is usually comparable. It is important to note that because LIMs have no field in the moving element they must produce all of their drive current in the stator (sometimes at very high levels) to produce equivalent power levels to an LSM. This is why LIMs have heavier stator windings and are typically limited to short lengths (to keep motor efficiency at a reasonable level). Another important point to be made is that permanent magnets are passive and do not require external excitation to provide a field in the moving element. Motors with wound moving elements require electrical power to be commutated to the moving frame, which requires a collector (brushes or sliding contacts) and a third rail or catenary line. These additional components, which are not required by a MagneMotion LSM, are a maintenance liability, a weight penalty and a potential safety concern that must be factored into a comparison.

Questions are sometimes raised regarding the reliability and durability of high strength permanent magnets. These questions are usually based more on old perceptions and mis-information than on scientific fact.

The Truth About Magnet Reliability:

Permanent magnets are totally passive devices that require no maintenance and will provide decades of uninterrupted service. Pound for pound they provide better energy density than any other material available to motor designers. When properly applied, permanent magnets are a safe and extremely reliable source of moving field excitation.

Contrary to popular belief, permanent magnets are also very durable. They are monolithic so there are no wearing elements or small parts that can become loose. The US military has performed extensive shock testing of NdFeB magnets to evaluate their use on vehicles, ships and submarines. Permanent magnets have been shown to meet the stringent requirements of military standard MIL-S-901 for shock resistance. The key is proper design of the magnet carrier so that the materials are mechanically loaded in a manner best suited to their physical properties. MagneMotion has extensive experience in this area.

Corrosion is another long-standing concern with magnet materials. Magnet coatings have been perfected that prohibit adverse effects from corrosion. In addition, the magnet manufacturers have made dramatic improvements in the basic corrosion resistance of the alloy itself. Magnets are now available that have ten times the natural corrosion resistance of earlier materials.

There are stories in the news media from time-to-time about potential adverse health effects from prolonged exposure to high magnetic fields. There was an extensive investigation into potential links between living in close proximity to high power electrical transmission lines and the rate of incidence of some types of cancer. The findings of the report discounted any relationship between the two but research continues, and it is certainly advisable to limit exposure to magnet fields to the greatest extent possible.

The MagneMotion Approach To Magnetic Fields:

Two factors have the greatest impact on magnetic field strength - distance and shielding. The strength of the magnetic field generated by a motor is primarily a function of the electrical power applied and magnet energy density. For lower powered systems the field generated is insignificant and falls off rapidly as you move away from the source by even a few inches. For higher powered systems, such as linear motor powered trains, the fields are larger but still dissipate to extremely low levels at a distance of several feet. Fields in the passenger compartments of operating maglev trains are on the order of one Gauss, which is approximately equal to the earth's natural magnetic field strength. Motor shielding, in the form of vehicle structure, is very effective in dissipating field strength. In many cases, back iron is used to focus the field into the gap between the motor moving element and stator for increased efficiency, which further limits the far field effects. Proper management of magnetic fields is a design parameter and MagneMotion uses established practices to develop systems that are safe for operators and passengers.

Another popular misconception is that Linear Motors have poor efficiency. This belief is based on the true fact that a longer portion of motor stator is typically excited than is actually driving the moving element. The inclination is to believe that a ten foot long motor section with a one foot long moving element could only be 10% efficient. This is simply not accurate. There are a number of other factors that determine motor efficiency.

The Facts Regarding LSM Motor Efficiency:

Motor efficiency is impacted by the losses associated with voltage drop due to the resistance in the stator winding. It is true, therefore, that longer stator windings tend to be less efficient. However, the amount of back-EMF (driving force) produced by a motor, which is not a function of stator length, can have an even greater impact on motor efficiency. In fact, the voltage drop due to resistance (I2R loss) is sometimes very small when compared with the motor's back-EMF. MagneMotion typically uses high strength magnets in its motors, which have a large field and produce a high back-EMF (electro-motive force) that is considerably larger than the I2R losses. For small scale systems, Linear Synchronous Motor (LSM) efficiencies of 50% can be expected. For larger scale systems the efficiency can approach 85% or more.

Linear Induction Motors (LIMs) are sometimes evaluated as an alternative to LSMs. It is important to note that because LIMs have no field in the moving element they must produce all of their drive current in the stator (sometimes at very high levels) to produce equivalent power levels to an LSM. This is why LIMs are typically very short in length - to keep the motor efficiency at reasonable levels. LIMs also require tighter air gap tolerances than LSMs for similar reasons.