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&section=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