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Special Electric Machines Lecture 3 – Permanent Magnet Synchronous Motors Dr. Maged Ibrahim German International University College of Engineering Department of Electrical Engineering ...

Special Electric Machines Lecture 3 – Permanent Magnet Synchronous Motors Dr. Maged Ibrahim German International University College of Engineering Department of Electrical Engineering  Lecture Outline ▪ PMSM advantages and limitations ▪ Operating principle of PMSMs ▪ Rotating magnetic field ▪ Winging configurations 2  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 Yoke Tooth Slot 3  PMSM Advantages ▪ Low maintenance due to the absence of brushes. ▪ No rotor copper losses, which leads to higher efficiency. ▪ Simple rotor construction. ▪ High power density can be achieved by using rare-earth magnets with high remnant flux density. ▪ Lower operating temperature compared to electric machines with windings in the rotor due to the absence of rotor copper losses. Examples of EV motors: (a) 2010 Prius, (b) 2017 Prius, (c) ▪ Low rotor inertia. 2017 Tesla Model 3 and (d) 2016 Chevy volt 4  PMSM Disadvantages Typical PMSM control system ▪ Cannot run directly from a 3-phase power source, as its control requires a power electronic converter. ▪ High cost of high energy rare-earth magnets. ▪ Loss of the flexibility of field flux control. ▪ The magnets can be demagnetized at high or low operating Relative price of rare-earth elements temperatures. ▪ High short circuit currents at high speeds. 5  Operating Principle δ ▪ The torque in permanent magnet synchronous machines (PMSMs) is generated by maintaining a constant angle between the stator and rotor fields (δ), δ which is known as the torque angle. ▪ δ is kept constant in PMSMs by generating a stator field that rotates at the same speed of the rotor magnets. T = K x Bstator x Brotor sin (δ) 6  Production of Torque in Rotating Electrical Machines Axis of stator field Axis of rotor field ▪ The operation of rotating electrical machines can be regarded as an interaction between two magnetic fields, as the torque is produced due to the tendency of the two fields to line up. ▪ In DC machines, the torque angle is maintained constant through the brushes and slip rings. 7  Rotating Magnetic Field ▪ The stator of a PMSM usually contains 3-phase windings whose axes are displaced by 120° ▪ When the stator winding is fed by a balanced three-phase source, a rotating magnetic field is produced at a speed (ns) that depends on the frequency of the applied voltage source (f), and the number of machine poles (p). 120 𝑓 𝑛𝑠 = rpm 𝑃 8  Rotating Magnetic Field Current waveforms in a 3 phase windings and their resulting magnetic field 9  Rotating Magnetic Field ▪ Animations at http://www.ece.umn.edu/users/riaz/animations/abcvec.html 10  MMF Produced by a Single Coil ▪ In order to produce a rotating magnetic field by the stator coils, the stator mmf should have a sinusoidal distribution in in the air gap. ▪ The mmf produced by a single coil has a rectangular distribution with a dominant fundamental component, but it is also rich in harmonics. ▪ The harmonics do not contribute to useful torque and produce only losses. Hence it is important to minimize the harmonics in the stator mmf. ▪ This can be achieved by the different AC machine stator winding configurations such as distributed windings, concentrated windings and fractional windings. 12  Stator Coils A typical coil of an AC machine has two sides, each side is place in a stator slot. The two coil sides are connected at the end of the machine axial length by the end turn. The distance between the coil sides is called the coil span or the coil pitch. The maximum coil span is 180 electrical degrees (Electrical angle = Mechanical angle/number of pole pairs), and it is equal to the Next Lowest Integer (NLI) obtained after dividing the number of slots (Ns) by the number of poles (P). Maximum coil span = NLI (Ns/P) For example, the maximum coil span for a 4-pole 24-slot machine is 6 slots. If the ratio between the number of slots and the number of poles is an integer number, the winding would be known as integral winding. Otherwise, the winding would be fractional winding. 14  Integral-slot Concentric Winding In concentric windings, sinusoidal mmf distribution can be achieved by placing the coils in the slots with different coil pitches. The coil pitches are less than a pole pitch and decrease for coils as they progress from outer to inner slots. The resultant phase mmf is the sum of the individual coil mmfs. If there are infinite slots, the mmf distribution will be triangular, but by varying the number of turns of each coil, a sinusoidal mmf distribution can be achieved. 15  Integral-slot Concentric Winding ▪ The advantages of concentric windings are the ease of manufacturing and its ability to provide a near sinusoid mmf distribution. ▪ The disadvantage is that the effective number of turns is reduced. 16  Integral-slot Distributed Winding In distributed windings, the number of turns is constant for all coils and also the coil pitch is constant for all coils, but each coil is shifted from the previous coil by one slot. The resultant mmf is derived by summing the mmfs of the individual coils. The resultant mmf is closes to a sinusoid waveform. 17  Integral-slot Distributed Winding ▪ The advantages of the distributed windings are the higher effective turns and the better utilization of the slot volume. ▪ Distributed windings have the disadvantage of longer end turns that leads to higher resistive losses. 18  Fractional-slot Winding In integral windings, the ratio between the number of slots and number of poles is always an integer number. Fractional-slot windings has a non-integral number of slots per pole. Double layer winding Single layer winding The fractional windings can be designed with tooth-wound non-overlapping windings that can significantly simply the winding manufacturing process, improve the motor fault Double layer 36 slot – 30 pole motor tolerance and improve the utilization of copper. These type of windings also suffer from high mmf harmonics, high rotor losses, as well as unbalanced radial forces. 19 Single layer 36 slot – 30 pole motor  Examples of Motor Windings 48 slot-8 pole Toyota Prius distributed winding stator 24 slot – 16 pole Hyundai Sonata fractional winding stator 20  Short Pitching and Skewing The mmf harmonics can be reduced by, 1. Short pitching If the coil sides are spatially apart at an angle lower than 180° (coil pitch = ζ), the induced emf in the two coil sides will be phase shifted by an angle ζ. This reduces the mmf harmonics, but also reduces the fundamental mmf component. Short pitching 2. Skewing In order to reduce the mmf harmonics and to reduced the output torque pulsations, the stator windings or rotor magnets can be skewed over the axial length. Straight Skewed magnets magnets Step- skewed rotor Stator skewing 21  Number of Poles ▪ If the number of poles is doubled, the required thickness of the stator yoke is reduced by one half. Therefore, the overall machine diameter can be reduced by increasing the number of poles. ▪ The number of poles should be inversely proportional to the speed of rotation in order to limit the switching losses and iron losses as, Electrical speed = Mechanical speed x number of pole pairs ▪ Increasing the number of poles can increase the time and cost required for magnetizing the magnets. Stator yoke 22