Ch2 TRANG BỊ ĐIỆN, ĐIỆN TỬ TRONG MÁY CÔNG NGHIỆP

CHAPTER  An electric motor is an electrical machine that converts electrical energy into mechanical energy  DC motor: ◦ Heat, in the motor windings, due to I2R  AC motor: ◦ Heat, in the motor windings, due to I2R ◦ Eddy current ◦ Hysteresis  Eddy (Foucault) current  Eddy current ◦ Current loops: like shorted transformer secondary turns ◦ Thin isolated laminations: break potential conductive loops into smaller less lossy segments ◦ Material of laminations: silicon added to the alloy to increase electrical resistance which decreases themagnitude of eddy currents  Hysteresis ◦ a lagging behind of magnetic field strength as compared to magnetizing force ◦ If a soft iron nail is temporarily magnetized by a solenoid, the nail does not lose the magnetic field once the solenoid is deenergized due to a small amount of residual magnetization, Br, remains ◦ An alternating current has to expend energy, -Hc the coercive force, in overcoming this residual magnetization before it can magnetize the core back to zero ◦ Hysteresis loss is encountered each time the polarity of the AC reverses  Both rotor and stator cores of AC motors are composed of a stack of insulated laminations  The laminations are coated with insulating varnish before stacking and bolting into the final form  The laminations are made of silicon alloy grain oriented steel  Separately excited  Shunt excited  Series excited  Advantage: ◦ Can be controlled at high performance, both speed (i.e torque), and position due to static magnetic field  Disadvantages: ◦ DC power supply ◦ Brush → losses and unstability   Is stepper motor AC or DC motor? Advantage: ◦ Precision (step) and easy to control  Disadvantage:  Squirrel cage rotor  Wound rotor  Synchronous speed 120 f ω0 = p ◦ f: AC line frequency ◦ p: number of poles per phase  Slip ω0 − ω s= ω0   A three phase motor may be run from a single phase power source However, it will not self-start It may be hand started in either direction, coming up to speed in a few seconds  Permanent-split capacitor motor  Capacitor-start induction motor  Capacitor-run motor induction motor   A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles Non-excited motors ◦ Reluctance motors ◦ Hysteresis motors ◦ Permanent magnet motors  DC-excited motors  Non-excited motors ◦ The rotor is made of a high-retentivity steel ◦ At synchronous speed it rotates in step with the rotating magnetic field of the stator, so it has an almost-constant magnetic field through it ◦ The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it  DC-excited motors ◦ Usually made in larger sizes (larger than about horsepower or kilowatt) ◦ Direct current supplied to the rotor for excitation through slip rings ◦ The direct current may be supplied from a separate DC source or from a DC generator directly connected to the motor shaft The rotating magnetic field is formed from the sum of the magnetic field vectors of the three phases of the stator windings  IS :  armature current IE: field current  In the fractional horsepower range, most synchronous motors are used where precise constant speed is required These machines are commonly used in analog electric clocks, timers and other devices where correct time is required In high-horsepower industrial sizes, the synchronous motor provides two important functions First, it is a highly efficient means of converting AC energy to work Second, it can operate at leading or unity power factor and thereby provide power-factor correction  Large synchronous motors cannot be self-started  ◦ Due to the inertia of the rotor, it cannot instantly follow the rotation of the magnetic field of the stator ◦ Large motors operating on commercial power frequency include a "squirrel cage" induction winding which provides sufficient torque for acceleration and which also serves to damp oscillations in motor speed in operation ◦ Very large motor systems may include a "pony" motor that accelerates the unloaded synchronous machine before load is applied  Small synchronous motors are able to start if the moment of inertia of the rotor and its mechanical load is sufficiently small