The permanent magnet electric motor PMAC
The classic structure of a permanent magnet electric motor is very simple and consists of a limited number of components, much fewer than those required for an internal combustion engine.
A permanent magnet electric motor is mainly composed of the following:
- Stator: the fixed part that contains the copper windings that generate the magnetic field to rotate the rotor
- Rotor: the rotating part that includes the motor output shaft end
- Bearings: the supports of the rotor
- Electrical terminals: these are the connectors that link the stator to the electrical power cables
An important aspect of all types of electric motors is their low inertia compared to an internal combustion engine. This is because there are many rotating systems in the latter: the crankshaft, flywheel, gearbox gears and so on. On the other hand, the only rotating part in an electric motor is the rotor; its inertia is almost zero compared to that of a combustion engine. For this reason, an electric motorcycle is much easier to handle than a motorcycle with an internal combustion engine of similar weight.
As we said on the main page of electric motors, the main criterion to divide them is that of the supply currents: DC (Direct Current) motors are powered by direct current, while AC (Alternating Current) motors are powered by alternating current.
DC motors were the first invented; they are cheap and simple to control, but their wear is high, and they have a low power density. AC motors are more difficult to build and control but weigh less and are able to achieve very high efficiencies (>97%) and high torque densities. At the same torque, an AC motor weighs about half of a DC motor and has 30 to 40 times less inertia.
In automotive and motorcycle applications, the synchronous AC is the most used type; the possible versions are:
- R-SM – synchronous reluctance motor
- PMAR-SM – permanent magnet-assisted reluctance synchronous motor
- SPM-SM – surface permanent magnet synchronous motor
- IPM-SM – inner permanent magnet synchronous motor
- WR-SM – wound synchronous rotor motor (protruding pole synchronous motor)
The synchronous AC motor with permanent magnets PMAC (Permanent Magnet AC Motor) is currently the most used type due to its high efficiency and reliability, both in the surface magnet (SPM-SM) and internal magnet (IPM-SM) versions.
Surface Permanent Magnet Electric Motors (SPM-SM)
Surface magnet motors (SPM-SM) are characterized by a rotor composed of ferromagnetic laminated material and a series of pairs of permanent magnets mounted on the outer surface of the rotor. Torque production mainly depends on three factors; the higher they are, the higher the torque the engine can produce.
These factors are:
- The capability to produce a magnetic flux in the stator (Stator Inductance)
- The magnetic flux of permanent magnets
- The electric current flowing in the stator
Once the engine has been built, the first two parameters are fixed, while the last one is the only one we can vary when we ask for more or less torque from our engine by turning the throttle. The positive aspect of these motors is that they are easy to use and control, but they have two main disadvantages:
- At high speeds, centrifugal force tends to detach the magnets from the rotor surface
- A part of the magnetic flux is not used to generate torque
Internal permanent magnet electric motors (IPM-SM)
To maintain high torque density values without having the problems of SPMs, internal permanent magnet (IPM) motors have been developed. The permanent magnets are inserted in special cavities inside the rotor, which keep them in position even for high values of centrifugal force. This constructive feature makes IPM motors suitable for higher rotation speeds than SPMs. In the picture, we can see some schemes of IPM motors.
Internal magnet motors are a recent technology whose optimization is still to be developed; new types and configurations of magnets see the light continuously. In racing applications, IPM-SM motors represent state-of-the-art. Compared to other models, those with internal magnets have higher efficiency, lower weight and higher power and torque density.
The most commonly used magnets in electric motors
The most common types of permanent magnets for EV applications are:
• NdFeB magnets: Sintered Neodymium magnets (known as NdFeB) are the most common type of rare earth magnets. Composed of an alloy of Neodymium, Iron and Boron, these magnets have the highest magnetic characteristics currently available on the market
• SmCo magnets: Sintered Samarium-Cobalt magnets (known as SmCo) date back to the early XNUMXs and were the first example of a rare earth magnet. Composed of an alloy of Samarium and Cobalt, they are generally less powerful than Neodymium magnets. They have two important advantages: excellent thermal behaviour and high resistance to corrosion
• Ferrite magnets: Ferrite magnets are non-conductive ferromagnetic ceramic compounds derived from iron oxides and other metal oxides such as barium and strontium. The first studies on this material date back to the 40s, but practical uses only began in the early 50s.
However, PMAC motors have the disadvantage of being linked to the availability of rare earths to make permanent magnets. Their alternative is the synchronous reluctance motor (SynRM). This solution is characterized not only by the absence of permanent magnets and by its high energy efficiency but also by its reliability and low maintenance requirements.
Introduction to Electric Motorcycles
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