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maxon DC motor
? Advantages of coreless DC motors ? The maxon DC motor Programs: RE, A-max, RE-max ? Construction and working principle ? Commutation systems ? Bearing systems
Dr. U

rs Kafader, maxon motor ag, Sachseln, Schweiz

maxon DC motor: Variants
RE-Motor with ? NdFeB magnet ? graphite brushes ? ball bearing

A-max-Motor with ? AlNiCo magnet ? precious metal brushes ? sintered sleeve bearing

2, ? by maxon motor ag, Jan 05

Coreless maxon DC motor: A-max
permanent magnet commutatorplate commutator flange shaft

sintered sleeve bearing

el. connections
3, ? by maxon motor ag, Jan 05

precious metal brushes

housing ( magn. return) winding

Coreless maxon DC motor (RE 30)
self-supported winding brushes

el. connections housing (magn. return) permanent magnet (in the center)

commutator

4, ? by maxon motor ag, Jan 05

Conventional DC motor
el. connections winding iron core

brush system

commutator

permanent magnet (at the periphery)

5, ? by maxon motor ag, Jan 05

housing (magn. return)

Coreless winding systems
corless (DC) – slotless (EC)

Faulhaber Portescap

Mauthe Kodak

maxon
Quelle: Portescap

6, ? by maxon motor ag, Jan 05

Advantage coreless: no cogging
? no soft magnetic teeth to interact with the permanent
magnet

? smooth running even at small speeds ? less vibration and noise ? any rotor position can be controlled in a simple way ? no nonlinear control behaviour
7, ? by maxon motor ag, Jan 05

Advantage coreless: no iron losses
? no iron – no iron losses ? constant magnetization ? high efficiency, up to above 90% ? low no load current, typical < 50 mA ? does not apply to EC motors ? no saturation effects in the iron core ? Even at the highest currents the produced torque remains ?
proportional to the motor current. stronger magnets = stronger motors

8, ? by maxon motor ag, Jan 05

Advantage coreless: small inductance
? less brush fire
– commutation: open and close a contact on an inductive load

? higher live expectancy ? less electromagnetic emissions ? easier to supress interferences:
– capacity between connections – ferrite core at motor cable
9, ? by maxon motor ag, Jan 05

? but fast reaction of the current
– problems in combination with pulsed supply (choke needed)

Advantage coreless: compact design
? more efficient design of the magnetic circuit
(even if the air gap is larger) – more compact magnet in the center – higher ratio of power to volume

? small rotor mass inertia
– hollow cylinder against full cylinder – high dynamics

– typical acceleration times: 5 – 50 ms

10, ? by maxon motor ag, Jan 05

maxon DC motor: Program
year 2000

RE-max

A-max
rolled housing magnet plastic flange

RE
1995

magnet design and production

F

1990

S
1985 design 1980

A

NdFeB
1970

AlNiCo

Ferrite

11, ? by maxon motor ag, Jan 05

maxon DC motor: Programs
permanent magnet motor program
motor example Dn/DM [mNm/min-1] assign. power motor size

Ferrite F motor
2130 GB 1150 3W

AlNiCo A-max, S, A
A-max 19 GB 1150 2.5 W

NdFeB RE, RE-max
RE 13 GB 1250 3W

23.3 cm3

8.2 cm3

4.6 cm3

diameter length cont. torque

30 mm 33 mm 3.3 mNm

19 mm 29 mm 4.4 mNm

13 mm 34.5 mm 3 mNm

12, ? by maxon motor ag, Jan 05

Stator: the magnetic circuit
permanent magnet:
produces magnetic field with north and south poles on opposite sides

air gap:
the larger the air gap, the weaker the magnetic field

housing:
magnetic return path made of steel (iron) guides magnetic field

13, ? by maxon motor ag, Jan 05

Development of permanent magnets
500

max. energy product (kJ/m3)

400

max. energy product ? theoretical limit 960 kJ/m3 ? technically achievable ca. 720 kJ/m3

NdFeB

300

SmCo
200

SmFeN

steel
0

Ferrit

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 year 2010

14, ? by maxon motor ag, Jan 05

100

AlNiCo

Permanent magnets
magnet Nd2Fe14B Sm2Co17 SmCo5 AlNiCo ferrite Curie operation temperature 310° C 825° C 720° C ~850° C 450° C 110-170° C 350° C 250° C 550° C 250-350° C motor design all, EC B [T] 1.2 1.0 only coreless conventional 0.8 0.6 0.4 0.2
15, ? by maxon motor ag, Jan 05

H [kA/m]

900 800 700 600 500 400 300 200 100

Construction of rotors
commutator commutator plate shaft winding connections winding bondage shaft with knurling commutator wire epoxy

16, ? by maxon motor ag, Jan 05

commut. plate winding

Winding: enameled wire
copper core: ? good electrical conductor insulation: ? no short circuits

copper wire insulation lacquer

17, ? by maxon motor ag, Jan 05

lacquer: plastic with solvant ? at enhanced temperature (130-150° C): ? plastic melts and connects neighbouring wires. ? pressing forms the body in narrow tolerances. ? outgassing of solvant: plastic hardens. ? baking of the winding.

maxon winding: standard and knitted
standard maxon winding knitted maxon winding

knitted winding for
? big motors with NdFeB magnet ? RE motors, EC motors ? thick walled windings

18, ? by maxon motor ag, Jan 05

Current flow in maxon winding
1 2 3 4 5 6 7 8 9 1

1 2 3 4 5

5 6 7 8 9 1

19, ? by maxon motor ag, Jan 05

Force and torque production
magnetic return

rhombic current areas
force magnetic field in air gap

force

20, ? by maxon motor ag, Jan 05

Torque and current: torque constant
current direction towards flange

force

magnetic field

forces: force on current leading conductor in a magnetic field

torque: sum of all forces at the distance to the rotating axis
influencing parameters: geometry design field density winding number

force
current direction towards brush

M ? kM ? I

current I

application

21, ? by maxon motor ag, Jan 05

Speed and voltage: speed constant
? winding rotates in air gap
– with inhomogenious magnetic field – induced voltage Uind (back EMF) depending on ? geometry design ? magnetic field density ? winding number application ? speed n

n ? k n ? Uind

? speed constant kn
22, ? by maxon motor ag, Jan 05

– inversely proportional to kM – inversely proportional to generator constant (V/1000 rpm)

Brush cover adjusting at no-load
? adjusting the brush system
– rotating until optimum commutation: commutation picture – for maximum motor life

? no-load current: measure of friction
– the higher the load (friction), the higher the (no-load) current – friction in bearing and commutation – faults: e.g. touching winding, misaligned bearings

? no-load speed: measure of magnet and winding
– too strong a magnet = lower no-load speed
– depends on voltage and magnetic field in the air gap – higher applied voltage = higher speed
23, ? by maxon motor ag, Jan 05

– bad magnet (improper magnetization) = higher no-load speed

Commutation picture

1: ripple 2: modulation because of asymmetrical winding 3: current signal of a revolution

24, ? by maxon motor ag, Jan 05

Commutation process
+
+
1
1 2

_
4
5 5

2
3 3 4 4 5 5 6 6 7 7 1

6
6 7 7 1 1 2 3 3 4 4
25, ? by maxon motor ag, Jan 05

2

_

Torque ripple
5%
rel. torque
1

14%
0.8 0.6

commutator segments 5 6 7 9 11 13

0.4 commut. points 0.2

torque ripple 5% 14 % 2.5 % 60 1.5 % 1% 0.75 %
26, ? by maxon motor ag, Jan 05

10 6 0 14 0 18 22 26

120

180

240

300

360

rotation angle (° )

DC commutation systems
graphite ? graphite brush with 50%
copper ? copper reduces contact and brush resistance ? graphite acts as lubricant ? spring

palladium) contact area ? silver copper commutator ? small contact and brush resistance (50mW) ? CLL for extended service life

27, ? by maxon motor ag, Jan 05

precious metal ? bronze brush body with plated silver (with

DC commutation: rotors
graphite
glas fibre bondage

copper commutator

precious metal

CLL disc

silver commutator

scotch bondage

28, ? by maxon motor ag, Jan 05

2 shaft ends

DC commutation: contact resistance
terminal resistance

graphite
Rmot (I)

precious metal
terminal resistance

Rmot Rwind

IA current Rmot Rwind ~50 mW
29, ? by maxon motor ag, Jan 05

IA current

Precious metal commutation: CLL
the problem
+ _ short-circuit after short circuit commutator arc production commutator wears off

solution
-capacitance between neighbouring commutator segments -energy is deviated into capacitance: no arcs produced

C

C
C

CLL disc

RS

30, ? by maxon motor ag, Jan 05

Precious metal commutation: CLL
voltage
between the commutatorsegments
200 V

without CLL: ? energy is given away very rapidly, ? high voltages, sparks time

10 V

short-circuit

after short-circuit

31, ? by maxon motor ag, Jan 05

with CLL: ? energy is given away slowly ? damped oscillation ? low voltages

Life testing of CLL
Motor 2017.941 I = 50 mA n = 13'000 rpm U = 24 V CLL 10 8 6 4 2 2'500 5'000 7'500 10'000 h Motor 2140.935 I = 250 mA n = 1'500 rpm U = 10 V 10 8 6 4 2 5 10 15 20 x 1000h test terminated
32, ? by maxon motor ag, Jan 05

CLL

DC commutation: pros and cons
graphite ? well suited for high currents and
current peaks ? well suited for start-stop and reversed operation ? bigger motors

precious metal ? well suited for smallest currents ? ? ? ? ?
and voltages well suited for continuous operation smaller motors very low friction and noise low electromagnetic emission favourable price
33, ? by maxon motor ag, Jan 05

? higher friction, higher no-load
currents ? not well suited for small currents ? more audible noise and electromagnetic emission ? more expensive

? not well suited for high currents
and current peaks ? not well suited for start-stop operation

maxon DC motor: service life
service life ? no general statement possible ? average conditions: 1'000 3'000 hours ? under extreme conditions: less than 100 hours ? under favourable conditions: more than 20'000 hours

life influencing factors ? the electric load: higher currents
= higher electric wear (arcing)

? speed: higher speed = higher
mechanical wear

? type of operation: reversed
operation = reduced service life

brushes) enhances service life

? load on shaft (bearings)

34, ? by maxon motor ag, Jan 05

use graphite brushes and ball bearings for extreme operating conditions

? temperature ? humidity with graphite brushes ? CLL (with precious metal

ball bearing
? small friction, rolling balls
– enhanced by axial preload or disbalance

? with lubricant ? suitable for heavy loads
– forces act on balls load onto shaft

35, ? by maxon motor ag, Jan 05

? for larger motors ? higher noise level than sleeve bearings ? more expensive

sintered sleeve bearing
? material
– lubricant between grains, up to 30% of volume – lubricant reservoir depends on bearing size – viscosity and pore size must be tuned

? function
– at high speeds: hydrodynamical lubrication – at low speed: direct contact of shaft and bearing

sintered sleeve bearing

? mechanical
– lower loads than ball bearing lubricant – for smaller motors – lower noise level than ball bearing

shaft
36, ? by maxon motor ag, Jan 05

? lower costs

hydrodynamical lubrication
? small radial load, asymmetrical pressure ? higher speeds ? lubricant circuit
friction

mixed friction
viscous friction ~250 rpm speed
37, ? by maxon motor ag, Jan 05

? no hydrodynamic lubrication at ... - high radial loads - direct contact of shaft and bearing - tilted bearings, wobbling shaft

bearing: maximum axial loads
? dynamic axial load
– maximum permissible force along shaft axis – during operation (dynamic)

? press-fit force (static)
– maximum permissible force along shaft axis – e.g. mounting a pinion – much higher with supported shaft
38, ? by maxon motor ag, Jan 05

– non operating (static)

bearing: maximum radial loads
? maximum forces perpendicular to shaft (Fd )
– depend on distance d to flange bearing – depend on maximum permissible loads of bearings (FA, FB ) – depend on distance l of the bearings

? calculated using leverage
– in catalog mostly given at 5mm from flange – simplified: FA double distance, half the radial load 5 F d ? F5 ? l d

F5

Fd
39, ? by maxon motor ag, Jan 05

d

FB

Examples for bearing information

source: www.faulhaber.de

source: www.portescap.com

40, ? by maxon motor ag, Jan 05

Ball and sleeve bearings: pros and cons
ball bearings ? suitable for high radial and
axial loads ? suited for all operating conditions, in particular for start-stop and reversing operation ? larger motors

sintered sleeve bearings ? suited only for small radial and
? ? ? ?
axial loads well suited for continuous operation at high speeds smaller motors small friction and low noise level favourable price

? higher noise level, particularly
if not preloaded ? preloaded: higher friction, higher losses ? more expensive

41, ? by maxon motor ag, Jan 05

? less suited for start-stop operation


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