Special Electric Machines Lecture Notes PDF

Special Electric Machines Lecture 2 Permanent Magnet Synchronous Motors Dr. Maged Ibrahim German International University College of Engineering Department of Electrical Engineering  Lect...

Special Electric Machines Lecture 2 Permanent Magnet Synchronous Motors Dr. Maged Ibrahim German International University College of Engineering Department of Electrical Engineering  Lecture Outline ▪ Permanent magnets in magnetic circuits ▪ Types of permanent magnets ▪ Introduction to permanent magnets synchronous motors ▪ Motor structure ▪ Applications of PM synchronous motors 2  B-H Curves Magnetic materials ▪ Soft magnetic materials like electrical steel have low coercive magnetic field, small hysteresis loss and high saturation flux density. Electrical steels are used in the cores of transformer and motors. ▪ Hard magnetic materials (permanent magnets) have high coercive field and remnant flux density. They are used as a source of magnetic flux in permanent magnet machines. Motor magnets Electrical steel core 3  Permanent Magnets ▪ A hard magnetic material called lodestone was mentioned in ancient Greek as early as ca. 600 B.C. This was a natural magnetic mineral, a form of Magnetite (iron oxide). ▪ In the 11th century in China, it was discovered that quenching red hot iron in the Earth's magnetic field would leave the iron permanently magnetized. This led to the development of the navigational compass. ▪ By the 12th to 13th centuries, magnetic compasses were widely used in navigation. ▪ Currently permanent magnets are widely used in various application with a global market size of about 40 billion $. 4  Permanent Magnets ▪ The magnetic flux density in the magnets (Bm) can be considered to have two components: 1. Intrinsic flux density (Bi), which depends on the permanent alignment of the domains in the direction of the applied field. 2. External flux density that is attributed to the applied external magnetic field H ▪ For a hard magnet with a straight line demagnetization curve, the intrinsic flux density is constant in the second quadrant and this is known as a high-grade PM. If the demagnetization curve is not a ▪ The coercive force required to bring the straight line in the second quadrant, then the magnet magnet’s intrinsic flux density to zero is the intrinsic coercive force, Hci, and that of the becomes easier to demagnetize and it would be normal flux density is the coercivity, Hc. known as low-grade PM. 5  Equivalent Circuit of Permanent Magnets Fa m _ + l l g 0 Rg g S m N g r Rm Rl Rs l ▪ Electric circuit analysis can be employed to calculate the magnet operating point in a magnetic circuit. The magnet can be regarded as a flux source, and it can be represented by a Norton equivalent circuit as a current source of the remnant flux Φr in parallel with the magnet internal reluctance Rm. ▪ The external reluctance of the magnet flux path consists of the air gap reluctance Rg in series with the steel reluctance Rs. The leakage reluctance branch represents the leakage flux Φl that emerges from the magnet but does not cross the air gap. ▪ The magnets can also be exposed to an external magnetic field. This external field is represented by a voltage source of the external mmf (Fa). 6  Equivalent Circuit of Permanent Magnets ▪ The air gap and magnet reluctances can be calculated by, 𝑙𝑔 𝑙𝑚 𝑅𝑔 = 𝑅𝑚 = 𝜇0 𝐴𝑔 𝜇0 𝜇𝑟 𝐴𝑚 ▪ In order to simplify the circuit analytical solution, the leakage flux and steel reluctance can be considered negligible. Thus, the magnet operating flux density Bm can be represented by, Fa 1 m Bm = (B − 0 r H a ) + _ l g Am  r r l g 1+ 0 lm Ag Rg ▪ Am is the magnet area, lm is the magnet length, Br is the magnet r Rm Rl remnant flux density, Ag is the air gap length, Ag is the air gap area, μr is the magnet relative magnetic permeability and Ha is the applied Rs external demagnetizing magnetic field intensity. 7  Graphical Representation of the Magnet Operating Point Air gap Load line line B Lm Ag  0 PC = Lg Am Br No-load operating point Normal load operating point Bm 0r Magnet Ha demagnetization curve H Hm ▪ This magnetic circuit can also be solved graphically by the magnet demagnetization curve and the airgap line. ▪ At no-load, the magnet operating point occurs at the intersection of the demagnetization curve with the air gap line. The slope of the air gap line is know as the permeance coefficient (PC). ▪ When an external demagnetizing field is applied to the magnet, the air gap line is shifted horizontally by the applied magnetic field intensity Ha and the magnet operating flux density is reduced. 8  Demagnetization of Permanent Magnets Air gap line B Load line when a demagnetization field is applied A Br C Recoil line B Ha Demagnetization curve H ▪ Real permanent magnet materials can have a straight demagnetization curve within a certain demagnetization field range. If the applied field exceeds this range, a knee will appear in the demagnetization curve and the magnet will begin to lose its magnetization. ▪ If a magnet is operating at no-load at point A and an external magnetic field Ha is applied, the magnet operating point will shift below demagnetization curve knee to point B, where the demagnetization is irreversible. When the external field is released, the magnet will recoil to a lower no-load flux density at point C. The recoil line is parallel to the original demagnetization curve. 9  Magnetization of Permanent Magnets No load air B gap line Ha Hd Bs Br No-load operating point for Load line when a a fully magnetized magnet magnetizing field is applied Hs H ▪ In order to re-magnetize the magnet, a magnetization field Hs has to shift the magnet operating point beyond the hysteresis loop knee in the first quadrant until saturation is reached. When the magnetizing field is released, the magnet will recoil along the main demagnetization curve to the no-load operating point. ▪ The magnetic field required to magnetize the magnet is much larger than the demagnetization field. 10  Types of Permanent Magnet Materials ▪ Neodymium Iron Boron (NdFeB) have high coercivity and high remnant flux density, but they are expensive and can be demagnetized at high temperatures. ▪ Samarium-cobalt magnets (SmCo) are more stable at high temperature with remnant flux density and coercivity lower than Neodymium magnets. ▪ NdFeB and SmCo are known as rare-earth magnets ▪ Ferrite magnets have low remnant flux density and moderate coercivity. ▪ Alnico magnets have high flux density and low coercivity, so the magnets can be easily demagnetized. 11  Effect of Temperature on Rare-earth Magnets ▪ The remnant flux density and coercivity of rare-earth magnets decrease under high temperatures due to their positive temperature coefficient. ▪ Rare-earth magnets can be demagnetized if the machine is exposed to high temperatures. 12  Effect of Temperature on Ferrite Magnets Temperature increases ▪ The remnant flux density of ferrite magnets decreases under high temperatures, but the coercivity decreases at low temperatures, so the ferrite magnet is susceptible to demagnetization at low temperatures. ▪ The magnets in Ferrite PMSMs can be demagnetized if the machine is exposed to low temperatures. 13  Permanent Magnet Synchronous Motors Permanent magnet synchronous machines (PMSMs) are currently used in various applications such as: ▪ Industrial applications (pumps, fans, compressors, etc.) ▪ Electric/hybrid vehicles ▪ Electric bicycles/scooters ▪ Aerospace motors and generators ▪ Robotics and automation ▪ Wind generators ▪ Domestic applications (air conditioning, washing machines, etc.) ▪ Computers 14  Construction ▪ The PMSM consists of two main parts: 1. Fixed stator with a set of windings (usually three phase windings) 2. Rotating rotor with permanent magnets 15  Stator Structure ▪ The stator of permanent magnet synchronous motors consists of a stack of steel lamination with the stator coils wound into the stator slots. ▪ An insulating material is placed inside the slots to insulate the stator coils from the steel laminations. 16  Steel Laminations ▪ If the machine core is made of a solid steel core, high eddy currents will be generated in the core, due to the changing flux. ▪ The cores of electric machines are made of thin insulated steel laminations in order to limit the eddy current losses. These laminations are stacked and bonded together to form a solid steel core. 17  PMSM Configurations ▪ The main configuration choices for permanent magnet synchronous motors are; 1. Radial flux with inner rotor 2. Radial flux with outer rotor 3. Axial flux ▪ In most motors, flux crosses from the rotor to the stator in the radial direction. ▪ The vast majority of these motors have an inner rotor and outer stator. Inner rotor permanent magnet ▪ In inner rotor PMSMs, the magnet retention must be carefully synchronous motor implemented so that the rotor does not fly apart. ▪ The stator winding process is quite expensive without automatic equipment. 18  Outer Rotor PMSM ▪ The inner rotor configuration is preferred in most applications due to the ease of stator heat removal. ▪ In some applications, this is not as important as the benefits gained from having an outer rotor and inner stator. This machine is known as the outer rotor PMSMs. ▪ The stator of the outer rotor PMSMs is easier to wound Outer rotor PMSM compared to inner rotor machines. ▪ The centrifugal force tends to push the magnets towards the outer rotor core at high speeds. ▪ The outer rotor PMSM can be built with only one bearing. Outer rotor PMSM integrated 20 into fan assembly  Outer Rotor PMSM Direct drive motor for a washing machine TM4 outer rotor motor for an electric bus 21  Axial Flux PMSM ▪ In radial flux PMSMs, the flux flows in the radial direction. In axial flux motors, flux flows in the axial direction, and the windings are oriented along the radial direction. ▪ Axial flux motors can provide torque densities higher than radial flux machines. ▪ This configuration can be used in applications with limited axial space. Radial vs axial flux Single stator – single rotor Double stator and single stator axial flux motor 22  Axial Flux PMSM YASA motor Double rotor and single stator YASA motor PCB stator axial flux motor ▪ Axial flux machines can have may variations such as; single stator – single rotor, single rotor – double stator, double rotor - single stator, Yoke-less Armature Segmented Axial flux (YASA) motor and PCB stator axial flux motor. 23