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Electric motors KON-C2004 Mechatronics Basics Tapio Lantela, Nov 7nd, 2016 Applications Power range http://www.dmaeuropa.com/Clients/Sulzer/News/tabid/3546/itemid/3334/Default.aspx From milliwatts http://small-generator.com/buy/index.php?main_page=product_info&cPath=2&products_id=44&zenid=7am2ohqmufajn06bafie386007 to megawatts Speed range http://www.celeroton.com/en/products/motors.html From a couple hundred rpm to hundreds of thousandsof rpm http://www.ebikes.ca/learn/hub-motors.html Electric motors DC - Brushed - Brushless AC - Synchronous • Permanent magnet motor • Field excitation • Reluctance - Asynchronous (Induction) • Squirrel cage • Wound rotor Physics of a generic electric motor Magnetic flux density B Radius r Stator Current -I Force F Current I Angle ϑ Rotor Force F Torque T Magnetic flux density B Current I Length l Number of turns N Force F = BlI Torque T = 2rF 2rFcos(ϑ) = 2NrBlI = KtI Induced voltage = Blv =2NBlv Blrω =2NBlrω = Keω DC motor working principle Simple two pole example - In practise motors have three or more poles http://www.pcbheaven.com/wikipages/How_DC_Motors_Work/ https://en.wikipedia.org/wiki/Brushed_DC_electric_motor Brushed DC motor construction http://www.electrical-knowhow.com/2012/05/classification-of-electric-motors.html DC motor commutator DC motor field generation Permanent magnet - Permanent magnet rotor (Brushless) - Permanent magnet stator (Brushed) Material saturation limits field magnitude Field coils (Brushed) - Series wound - Parallel (shunt) wound - Separately excited http://www.electrical4u.com/dc-servo-motors-theory-and-working-principle/ Electrical models of a DC motor Equivalent circuit http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed§ion=SystemModeling Mathematical model Mechanical models of an electric motor Physical model Mathematical model Time constants of a motor Winding inductance & resistance Rotor & load inertia Power losses Resistive losses - Proportional to the square of the current (torque) - Resistance depends on temperature (0.4%/K) Core losses - Proportional to the rotating speed Mechanical losses - Bearing friction • proportional to the rotating speed - Damping (air etc.) • proportional to the square of rotating speed Additional losses - Variable frequency drive harmonics etc. Motor equations Input power Pin = UI Output power Pout = Tω Resistive loss in windigs Pres = RI2 Back electromagnetic force Vbemf = Keω Characteristics of a PMDC motor Maximum torque and efficiency are speed dependent Operating limits of a PMDC motor http://www.electrocraft.com/products/pmdc/DPP720/ Field weakening Reduce magnetic field flux density B - Smaller B -> smaller torque constant Kt and BEMF constant Ke - Smaller Kt -> less torque - Smaller Ke -> more angular velocity Field weakening http://ecomodder.com/forum/showthread.php/bsfc-chart-thread-post-em-if-you-got-1466-26.html Operating limits of a motor Temperature - Winding insulation melting temperature - Permanent magnet demagnetization temperature Voltage - Winding insulation breakdown voltage Mechanical strength - Rotor breakdown speed Commutation speed - Drive/controller speed Characteristics of a DC motor Motor data Assigned power rating 1 Nominal voltage 2 No load speed 3 No load current 4 Nominal speed 5 Nominal torque 6 Nominal current 7 Stall torque 8 Starting current 9 Max. efficiency 10 Terminal resistance 11 Terminal inductance 12 Torque constant 13 Speed constant 14 Speed / torque gradient 15 Mechanical time constant 16 Rotor inertia 12 W 12 V 12100 rpm 155 mA 8060 rpm 10.1 mNm 1.25 A 31.3 mNm 3.47 A 63 % 3.46 Ω 0.121 H 9.02 mNm / A-¹ 1060 rpm / V-¹ 406 rpm / mNm-1 9.56 ms 2.25 gcm² Source: Maxon corp. Four quadrant operation Brushed DC motor velocity control The torque is proportional to the current in the winding Current is controlled by voltage - Or usually with pulse width modulation, PWM - If the PWM frequency is high enough, the current stays almost constant (small fluctuations) Brushed DC motor control Motor needs large currents - E.g. microcontroller signal not powerful enough for running the motor - A separate motor drive circuit controls the motor current according to the microcontroller signal One signal for PWM, one for direction M M Brushless DC motor (BLDC) Permanent magnet rotor Stator with windings External commutation - integrated position sensosr, usually hall sensors Almost service free Withstands well short term overloading - heat is transported effectively from the stator into the environment More complex control system BLDC commutation http://www.mpoweruk.com/motorsbrushless.htm BLDC commutation AC motor Two categories - Asynchronous - Synchronous Input usually three phase sinusoidal AC voltage Only bearings need service Control with variable frequency drive http://www.pump-zone.com/topics/motors/ac-motors-part-two-three-phase-operation/page/0/1 AC field generation Three field coils (per pole) - 120 °phase difference http://tdflashzone.net23.net/1_6_Physics-Flashes.html Synchronous AC: Permanent magnet Permanent magnet AC motor is almost the same as BLDC. Synchronous AC: Permanent magnet https://www.ecnmag.com/article/2009/10/comparing-motor-control-techniques Asynchronous AC: Induction Stator field induces current in the bars which causes torque - Requires slip to create induced currents Types - Squirrel cage • Rotor made of conducting short circuited bars in steel frame - Wound rotor http://www.nidec.com/en-IN/technology/motor/basic/00026/?prt=1 AC motor control Rotational speed is determined by the frequency of the input voltage Frequency is controlled with variable frequency drive/inverter (taajuusmuuttaja in Finnish) - Rectification of the three phase voltage to DC voltage - The rectified DC voltage is converted e.g. with pulse width modulation (PWM) to AC voltage with the needed frequency - The AC voltage is fed to one of the three coils of the stator according to the sensors of the control system. Rated values of an induction motor Rated values are for continuous duty - Rated voltage – winding insulation Rated current – ohmic resistive losses Rated field – magnetic saturation of the material Rated power = T*omega For a dimensioning a motor for a variable load, it is possible to calculate an equivalent constant load or simulate the system Motors for servo systems Many designs: DC (brushed or brushless), AC Integrated feedback sensors Ability to tolerate short term overloads Low inductance - Small electric time constant Low rotor inertial mass - Small mechanical time constant http://www.ustudy.in/node/5789 http://www.exlar.com/press_releases/1868 Summary Electric motors can be found anywhere and for any power rating They have excellent efficiency at proper loading conditions - Usually this means a large enough rpm Output power limited by temperature - Can be overloaded for a short while Maximum rotating speed limited by back emf / supply voltage Questions? More info on electric drives - Advanced courses • ELEC-E8407 – Electromechanics • ELEC-E8405 - Electric Drives