Brushless motors
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Brushless Motors. A servosystem is capable of transforming any mathematical function into a mechanical movement it can replace mechanical elements, such as cams and cam shafts, indexing gears, differentials, etc. A servosystem consists of a servomotor with its control unit.

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Brushless motors
Brushless Motors


  • A servosystem is capable of transforming

  • any mathematical function

  • into a mechanical movement

  • it can replace mechanical elements, such as cams

    and cam shafts, indexing gears, differentials, etc.

  • A servosystem consists of a servomotor with its control unit.

  • Servosystems can be used for:

  • Positioning; the position, linear or angular, follows a predetermined position function.

  • Speed control; the motor speed follows a

  • predetermined speed function.

  • Torque control; the torque of the motor follows a

  • predetermined acceleration function.

  • Hybrid control; the system alternates between

  • different kinds of control


24 slots

2 rotor poles


Electro magnetic flux distribution
Electro-Magnetic AC motor, is aFlux Distribution

21 slots

8 rotor poles


Brushless solution pros cons
Brushless Solution AC motor, is aPros&Cons:

  • Velocity (no sparks at the commutator)

  • Efficiency (Torque/Inertia)

  • Weight

  • Dimensions

  • Thermical Dissipation

  • Acoustic Noise

  • Maintenance

  • MTBF

Disadvantages: Cost


Warm AC motor, is a

Cold

Warm

  • Only the stator is warm.

Warm

  • Rotor and commutator are warm

Warm

Heiß

Warm

  • Both rotor and stator are warm

Thermical Behaviour

1. BL-Servo with MP

2. DC-Servo with MP

3. AC-Servo (ASM squirrel cage)


  • Two AC motor, is a brushless motors types exist

  • (they differ in stator windings, permanent magnets lay-down, statoric field wave shape):

  • AC brushless:

  • with sinusoidal field (fcem)

  • DC brushless:

  • with trapezoidal field (fcem)


Permanent magnets pm types
Permanent Magnets (PM) types: AC motor, is a

  • Ferrite: low cost,

  • low Kt, torque loss: 0.2%/K,

  • demagnetization temperature: 150C

  • Samarium Cobalt (Sm2Co17): high cost,

  • high Kt, torque loss: 0.04%/K,

  • demagnetization temperature: >150C

  • (example: Danaher HD series, Rockwell MPG series)

  • Neodimium Iron Boron (NdFeB): medium cost,

  • higher Kt, torque loss: 0.09%/K,

  • demagnetization temperature: >150C

  • (example: Danaher HR series, Rockwell H, Y, 1326, MPL series)


E [kJ/m AC motor, is a3]

400

Nd Fe B

300

Sm Co

200

Steel

Al Ni Co

Ferrite

100

Year

0

1860 1880 1900 1920 1940 1960 1980 2000

Magnetism

  • NdFeB (vs. SmCo)

  • higher energy content

  • worse thermical reversibility

  • lower Curie temperature

  • lower cost

  • corrosion (not present with SmCo)

  • bigger resistance in W,

  • that in turn limits eddy currents


density AC motor, is a

[g/cm^3]

Curie Temp.

[°C]

Br temp. coeff.[% 1/°C]

Sm2 Co17

8,3

825

-0,03 (20°C ÷ 200°C)

Nd Fe B

7,4

315

-0,1 (20°C ÷ 150°C)

SECo5

Campo coercitivo JHc [kA/m]

500 1000 1500 2000 2500

SECo5

qualitàspeciali

(Field that can demagnetize the magnet)

NdFeB

Sm2Co17

50 100 150 200 250 300 350 400

Max Energetic Product (B*H)max [kJ/m^3]

Temperature that causes

para-magnetic behaviour

(i.e. weak magnetisation)

SmCo vs. NdFeB



Servo System Electrical Scheme on: AC or DC brushless, Kt, Cogging Torque (


Ptc resistor
PTC on: AC or DC brushless, Kt, Cogging Torque (Resistor

  • Positive Thermal Coefficient resistor,used as a sensor

  • inside the motor,

  • in order to stop the driver/controller

  • in case of too-high temperature (before to burn the motor)


Servo Motor Shapes on: AC or DC brushless, Kt, Cogging Torque (

and

Air Cooling


Direct drive solution
Direct Drive Solution on: AC or DC brushless, Kt, Cogging Torque (

  • Problem:

  • Motor shaft elastical torsion (i.e. its flexibility):

  • resonance frequency

  • limited band-width

  • low gains in the control loop

  • poor kinematic performances


Direct drive solution cont d
Direct Drive Solution on: AC or DC brushless, Kt, Cogging Torque ((cont’d)

  • Possible Solutions:

  • Digital Filters (only for constant resonance frequencies)

  • High Stiffness Motors (a high inertia would not solve the problem)

  • Torque Motors (Motori Coppia) with:

    • low velocity

    • high stiffness

    • they don’t need gearboxes (i.e. they are direct-drive)


Direct drive solution cont d1
Direct Drive Solution on: AC or DC brushless, Kt, Cogging Torque ((cont’d)

  • Once solved the resonance frequency problem, the control loop gains can be increased and thus a good accuracy in the position sensor becomes mandatory:

  • Resolver:

  • 6 arc min = 0.1 degrees

  • SinCos Encoder:

  • 0.01 arc sec = 2.8E-6 degrees

  • or 1nm for linear encoders (righe ottiche)


Position sensors used in the brushless motors
Position Sensors on: AC or DC brushless, Kt, Cogging Torque (used in theBrushless Motors


Resolver
Resolver on: AC or DC brushless, Kt, Cogging Torque (

  • Characteristics:

  • linearity: 0. 1 - 0.5%

  • resolution: 0. 1 - 0.5°

  • sensitivity: 5 - 10mV/° (Vref =20V)

  • frequency: 20KHz


Resolver cont d
Resolver on: AC or DC brushless, Kt, Cogging Torque ((cont’d)


Resolver cont d1
Resolver on: AC or DC brushless, Kt, Cogging Torque ((cont’d)

  • Pro

    • absolute in one turn

    • low cost

    • robust

  • Cons

    • sinusoidal 20KHz reference voltage

    • non-linear output

    • brushes in some (old) versions

  • It has been the standard position sensor

    on brushless motors


Encoder
Encoder on: AC or DC brushless, Kt, Cogging Torque (


Encoder cont d
Encoder on: AC or DC brushless, Kt, Cogging Torque ((cont’d)


Encoder cont d1
Encoder on: AC or DC brushless, Kt, Cogging Torque ((cont’d)


Encoder cont d2
Encoder on: AC or DC brushless, Kt, Cogging Torque ((cont’d)

  • Encoder types:

    • Absolute

    • Battery Back Up

    • One-Turn Absolute

    • Multi-Turn

    • Incremental

    • SinCos

      (resolver output, encoder design,

      precise as an encoder)

e.g.: Stegmann mounted

on Rockwell MPL motors


  • Degree of Protection on: AC or DC brushless, Kt, Cogging Torque (

  • IPXY (e.g.: IP65) 

  • Digit 1 (X): Solid Objects Protection

  • 0 Non Protected

  • 1 Protected against solid objects > than 50 mm

  • 2 Protected against solid objects > than 12 mm

  • 3 Protected against solid objects > than 2.5 mm

  • 4 Protected against solid objects > than 1 mm

  • 5Dust Protected

  • 6Dust Tight

    • Digit 2 (Y): Water Protection

    • 0 Non Protected

    • 1 Protected against dripping water

    • 2Protected against dripping water

    • when tilted to worse case opening

    • 3 Protected against spraying water

    • 4 Protected against splashing water

    • 5 Protected against waterjets

    • 6 Protected against heavy seas

    • 7 Protected against the effects of immersion

    • 8 Protected against submersion


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