AC machines ebook

AC Machines Alternate Current Machines 10.12.2012 Contents Electric Motors AC Motors Synchronous AC Motors Asynchronous AC Motors Losses in Motors Power Calculations for Motors Alternate Current Machines 25.11.2011 Electric Motors If an electric current flows through a conductor in a magnetic field, a magnetic force effects the conductor A simple electric motor can be formed if this conductor has a point to rotate around Faraday electric motor and Barlow Wheel are the first experimental representations of the electric motor Alternate Current Machines Electric Motors Faraday Electric Motor There is a free rotating wire which is inserted in a glass full of mercury (or salt water) in Faraday Electric Motor The glass full f mercury has a permanent magnet on center If a current flow through the wire, it starts to rotate This motion is the representation of the magnetic field produced because of the current flows on a wire Alternate Current Machines Electric Motors An electric current passes through the hub of the wheel to a mercury contact on the rim; this is contained in a small trough through which the rim passes Due to health and safety considerations brine (salt water) is sometimes used today in place of mercury The interaction of the current with the magnetic field of a U-magnet causes the wheel to rotate The presence of serrations on the wheel is unnecessary and the apparatus will work with a round metal disk, usually made of copper Alternate Current Machines What is an Electric Motor? An electric motor is a machine that converts electrical energy to mechanical energy • Used is compressors, pumps, air condition fans, electric vehicles, robot mechanisms, cranes, etc • More than the two thirds of the load in industry are the load of electric motors Alternate Current Machines Electric Motors It is the ‘Lorentz Force’ that effects the charge of ‘q’ which has the velocity of ‘V’ in magnetic field ‘B’ The directions of this force, the current and the magnetic field can be seen in the figure |FL|=q V B Sin α Alternate Current Machines Electric Motors The rotating part of an electric motor is called as rotor whereas the fixed part as stator If the rotor is consist of windings, brushes are used to transfer the current The brush is a carbon part which has a contact with the terminals of the coils on the axle of the rotor In some DC motors, permanent magnets are used in rotors and these types are called as Brushless DC Motors (BLDCM) The problem for these types is to sense the position of the rotor Information about the position of the rotor is needed to be sent to the driver of the motor Alternate Current Machines Electric Motors In some AC motors, Aluminum bars are used as the rotor of the motor These type of motors are called as Squirrel Cage type electric motors When the current is changing periodically, the rotor follows the current Alternate Current Machines Electric Motors Alternate Current Machines 10 Three Phase Asynchronous Machine Speed and Slip In an asynchronous motor, the speed of the magnetic field generated by the stator coils and the rotation speed of the rotor is not the same The value of the rotational speed of the rotor is always smaller than the speed of the stator’s magnetic field The reason of the word ‘asynchronous ’ is this The difference of these speed is called as the slip If ‘s’ is negative (rotor’s speed is greater) then the electric machine is running as a generator The slip s is defined as 'the s = [(Ns – Nr)/ Ns ] x 100 difference between synchronous s [ %]: Slip Ns [RPM]: Speed of the magnetic field Nr [RPM]: Rotational speed of the rotor t1 speed and operating speed, at the same frequency, expressed in rpm or in percent or ratio of synchronous speed' Alternate Current Machines 29 Asynchronous Machine Efficiency – Speed – Torque Curves t1 Alternate Current Machines 30 Asynchronous Machine Slip Ring Type Alternate Current Machines 31 Losses in Electric Motors Losses Notation Losses of mechanical Pks frictions Iron loss(hysteresis and PkFe eddy current losses) Ohmic power loss of Pka armature Alternate Current Machines 32 Losses in Electric Motors Losses in Asynchronous Motors Pe Pm Losses Friction and air flow losses Iron loss Loss of conductor (stator - copper) Loss of conductor (rotor - alluminium) Additinal load loss Pfw PXL PR PS Pfe Notation Pfw Pfe PS PR PXL Alternate Current Machines 33 Losses in Electric Motors Losses in Asynchronous Motors Pe Friction and Air Flow They are constant losses during motor run, independent from load and occur in bearings and cooling fan propellers Pm Iron Loss Total affects of losses in cores of coils (hysteresis and eddy current losses) It can be neglected even the rotor composed of coils since the frequency of the induced voltage is low It might be observed as heat in laminated cores when the motor is running It is dependent to the material, thickness and dimensions of the laminated core, the frequency applied to the motor and the square of the voltage applied to the motor It is constant if the frequency and the voltage that the motor is connected not change Pfw PXL PR PS Pfe Alternate Current Machines 34 Losses in Electric Motors Losses in Asynchronous Motors Pe Conductor Loss (Stator) It is heat loss The current flow through the stator coils creats this loss (I2RS ) Pm Conductor Loss (Rotor) It is heat loss The current flow through the stator coils or cage bars creats this loss (I2RR ) Additional Load Loss It is the loss occurs in metal parts of the motor except the laminated cores in rotor and stator because of the leakage because of the load Losses Friction and Air Flow Losses Iron loss Conductor loss (stator) Conductor Loss (rotor) Additional load losses % 0,5 ~ 1,5 % 1,5 ~ 2,5 % 2,5 ~ 4,0 % 1,5 ~ 2,5 % 0,5 ~ 2,5 35 PS Pfe Pfw PXL PR Alternate Current Machines Power Calculations in Electric Motors Colours of wires (TS 6429) i,u R t1 t2 S t3 t4 T t5 Blue- Brown – Black – Gray – Yellow+Green t6 Alternate Current Machines 36 Power Calculations in Electric Motors The nominal power of a DC motor might be expressed as the equation below UDC [V] is the voltage applied to the motor, and IDC [A] is the current flow Pinput [W] is the electrical power, Poutput [W] is the mechanical poweror the nominal power, ωm [RPM] is the rotational speed of the axle of the motor, Tm [Nm] is the torque generated by the motor Ploss [W] is the power loss, η [%] is the efficiency of the motor Pinput =UDC IDC Poutput =ωm Tm Ploss = Pinput − Poutput η=( Poutput / Pinput )x100 Alternate Current Machines 37 Power Calculations in Electric Motors In AC motors, because of the changing current characteristics, there is an important point that , there are three powers called as apperant, true and reactive In AC motors, current is lagging voltage with angle φ This divides the power into two vector parts I [Amper] [VAC] [Hz] Lm Rm Alternate Current Machines 38 Power Calculations in Electric Motors Apparent (imaginary) Power S=IU Reactive (blind) Power Q = U I sinφ [VAR] [VA] ϕ=P/S True (real) Power P = U I cos φ [Watt] Alternate Current Machines 39 Power Calculations in Electric Motors Example: A mono phase asynchronous motor draws 12.3[A] from grid and its power factor is measured as 0.94 What are the powers consumed? Apparent Power= S = U I = 220 x 12,3 = 2706 [kVA] Active Power = P = U I Cos φ =220 x 12,3 x 0,94 = 2,833 [kW] Reactive Power = Q = U I sin φ or = 1,028 [kVAR] Alternate Current Machines 40 Power Calculations in Electric Motors Power Calculations in Three Phase Electric Motors In a balanced three phase circuit: P = √ x U x I x cos φ Q = √ x U x I x sin φ S=√3xUxI P : True Power [Watt] ; Q : Reactive Power [VAR]; S : App Power [VA] U : 380 [V] phase to phase voltage: 380 [V] I : Current drawn from one phase: [A] Alternate Current Machines 41 Power Calculations in Electric Motors Power Calculations in Three Phase Electric Motors The current drawn from the three phase grid by an asynchronous alternate current motor is [A] and the power factor of the motor is measured as 0.85 What are the powers consumed from the grid? P : True Power [Watt] ; Q : Reactive Power [VAR]; S : App.Power [VA] U : 380 [V] Voltage btw Phases: 380 [V] I : Current drawn from one phase: [A] Power in a balanced three phase circuit: P = √ x U x I x cos φ = √ x 380 x x 0,85 = 3916 [W] Q = √ x U x I x sin φ = √ x 380 x x 0,5268 = 2427 [VAR] S = √ x U x I = √ x 380 x = 4607 [VA] Alternate Current Machines 42 References: http://ocw.mit.edu http://en.wikipedia.org http://www.energyefficiencyasia.org www.amidesign.ch Asenkron Elektrik Motorları, Ali Taner, 2011 http://avstop.com/ac/apgeneral/typesofacmotors.html http://www.daviddarling.info/encyclopedia/E/electric_motor.html Alternate Current Machines [...]... induction machines Alternate Current Machines 20 Three Phase Asynchronous Machine Industrial loads or high power loads are needed to be connected to three phase grid whose phases follow each other in 120 degrees instead of mono phase grid Result of this usage is smaller currents Alternate Current Machines 21 Three Phase Asynchronous Machine t1 Alternate Current Machines 22 Three Phase Asynchronous Machine... Synchronous Machines Stators of Synchronous Machines are manufactured using laminated cores which have slots to place the coils on them Synchronous Machines are divided into two groups according to the structure of the rotor that has exiting coil on it If the airgap between the stator and rotor is constant every where, then it is a round rotor (turbo) machine Unless, it is a salient pole synchronous machine... Current Motors (ACM) Direct Current Motors (DCM) Synchronous Induction (Asynchronous Externally Excited Mono Phase Three Phase Series Self-Exited Compound Schunt Alternate Current Machines 11 Alternatif Akım Motorları Alternate Current Motors (ACM) Synchronous ACM Induction (Asynchronous) ACM Slip Ring ACM Squirrel Cage ACM Alternate Current Machines 12 Alternate Current Motors The ACM’s are simplier... Phase Machine i,u 1200 R 1200 S T 1200 1 t1 3 t2 t3 t4 t5 t6 2 t1 Alternate Current Machines 24 Three Phase Asynchronous Machine Rotating Magnetic Field in a Three Phase Machine t2 t1 Time Interval t1 t2 t3 t4 t5 t6 t3 t4 t5 t6 IR IS IT + + 0 0 0 + + 0 - 0 0 + + t1 Alternate Current Machines 25 Three Phase Asynchronous Machine WYE Connection t1 Alternate Current Machines 26 Three Phase Asynchronous Machine... (Fans, washing machines, household devices… etc.) Alternate Current Machines 19 Asynchronous Machines • Three Phase Induction Machine • Magnetic field is generated by three phases • Rotor can be either squirrel cage or composed of coils • Can be started easily • Has great power capacities • There are applications from 1/3 HP to hundreds of HPs: Pumps, compressors, conveyor drums, grinding machines and... Nm, MN = 9550 x Nominal Power [kW] /Nominal Velocity of Rotor [RPM] Alternate Current Machines 14 ACM’s Basic Definitions Torque [Nm] http://www.ebmpapst.se/sv/dat/media/informati on/definitions_for_ec_motors.pdf MA MS MK MN Revolution [RPM] Alternate Current Machines 15 Synchronous Machines “ Synchronous Machine is a machine that runs at a constant speed which is proportional to frequency and number... and more economic than DCM’s An ACM generates more power comparing with a DC motor that has the same weight Maintenance of ACM’s is easier However, their speed control is harder They can be connected to the AC source directly If accuracy in velocity or position control is needed, DCM’s are used But, ACM’s are used more than DCM’s in industry Alternate Current Machines 13 ACM’s Basic Definitions Free Running... Salient pole synchronous machines are used in hydro elecric power plants and for compensating the power factor of the grid ns : velocity of the synchronous machine ns = 120 f / p f : frequency of the source p : number of poles Alternate Current Machines 18 Asynchronous Machine • Mono Phase Induction Machine • Has only one stator coil • Uses only one phase • Rotor of an asynchronous machine can be a squirrel... speed these machines are generally used as generators They are the most common machines used in power plants They can be manufactured to generate electricity up to 2000 [MVA] Cost effectivity due to unit power generated, higher efficiency in greater power generation, less maintenance and control processes made them to be manufactured in greater powers (*see references) Alternate Current Machines 16... http://www.ips.us/industries/fossil-fuelpower/ Alternate Current Machines 17 Synchronous Machines Round rotor synchronous generators are manufactured in small number poles and high synchronous revolution per minute They are used in high velocity steam turbines The length of the rotor is long and radius of the rotor is small in this type of turbines The salient pole synchronous machines are generally have more poles and are ... Alternate Current Machines 11 Alternatif Akım Motorları Alternate Current Motors (ACM) Synchronous ACM Induction (Asynchronous) ACM Slip Ring ACM Squirrel Cage ACM Alternate Current Machines 12 Alternate... manufactured in greater powers (*see references) Alternate Current Machines 16 Synchronous Machines Stators of Synchronous Machines are manufactured using laminated cores which have slots to place... Current Machines 21 Three Phase Asynchronous Machine t1 Alternate Current Machines 22 Three Phase Asynchronous Machine Rotating Magnetic Field: In a three phase AC motor, a rotating field might be achived