Electric motors
KON-C2004 Mechatronics BasicsTapio Lantela, Nov 2nd, 2015
Applications
Power range
From milliwatts
to megawatts
http://small-generator.com/buy/index.php?main_page=product_info&cPath=2&products_id=44&zenid=7am2ohqmufajn06bafie386007
http://www.dmaeuropa.com/Clients/Sulzer/News/tabid/3546/itemid/3334/Default.aspx
Speed range
From a couple hundred rpm
to hundreds of
thousands of rpmhttp://www.ebikes.ca/learn/hub-motors.html
http://www.celeroton.com/en/products/motors.html
Electric motors
DC- Brushed- Brushless
AC- Synchronous
• Permanent magnet motor• Field excitation• Reluctance
- Asynchronous (Induction)• Squirrel cage• Wound rotor
For
ce F
Force F
Magnetic flux density BCurrent IForce F = BlI
Torque T
Radius rTorque T = 2rFcos(ϑ)
Angle ϑ
Current I
Current -IMagnetic flux density B
Force F
Force F
Torque T = 2rF = 2NrBlI = KtI
Stator
Rotor
Induced voltage = Blv = BlrωInduced voltage = 2NBlv =2NBlrω = Keω
Physics of a generic electric motor
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)
Field coils (Brushed)- Series wound- Parallel (shunt) wound- Separately excited
Material saturationlimits field magnitude
http://www.electrical4u.com/dc-servo-motors-theory-and-working-principle/
Electrical models of a DC motor
Equivalent circuit
Mathematical model
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed§ion=SystemModeling
Mechanical models of an electric motor
Physical model
Mathematical model
Motor equations
Input power Pin = UI
Electromagnetic torque T = KtI
Output power Pout = Tω
Resistive loss in windigs Pres = RI2
Back electromagnetic force Vbemf = Keω
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.
Characteristics of a PMDC motor
Maximum torque and efficiency are speed dependent
http://www.electrocraft.com/products/pmdc/DPP720/
Operating limits of a PMDC motor
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
Source: Maxon corp.
Characteristics of a DC motor
Motor dataAssigned power rating 12 W1 Nominal voltage 12 V2 No load speed 12100 rpm3 No load current 155 mA4 Nominal speed 8060 rpm5 Nominal torque 10.1 mNm6 Nominal current 1.25 A7 Stall torque 31.3 mNm8 Starting current 3.47 A9 Max. efficiency 63 %10 Terminal resistance 3.46 Ω11 Terminal inductance 0.121 H12 Torque constant 9.02 mNm / A-¹ 13 Speed constant 1060 rpm / V-¹ 14 Speed / torque gradient 406 rpm / mNm-1 15 Mechanical time constant 9.56 ms16 Rotor inertia 2.25 gcm²
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
Commutating with integrated hall sensorsand external electronics
Almost service
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
AC motor
• Several types• Asynchronous• Synchronous
• Input voltage one or 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 AC motor is almost the same as BLDC.
Induction motor
Conducting short circuited bars embeddes in steel frame
Stator field induces current in the bars which causes torque
http://www.nidec.com/en-IN/technology/motor/basic/00026/?prt=1
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 constanthttp://www.ustudy.in/node/5789
http://www.exlar.com/press_releases/1868
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.
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
Motors can be overloaded for a short while
Output rpm limited by voltage
Questions?
• More info on electric drives• Advanced courses
• ELEC-E8407 - Electromechanics• ELEC-E8405 - Electric Drives