Transcript a) How DC motor works

Direct current (DC)motors
Prepared by:131010108011 Guide by:Mihir Patel
Direct current (DC)motors
 Principle of work
 Force and torque generation
 Generation of counter clockwise voltage
 Parts (components) of DC motor (machine)
 Simplified description of DC motor (machine)
 Current commutation
 The types of DC motors
 Mathematical model of DC motor (machine), (with
independent excitation)
 Control of DC motors (machine)
 Speed vs torque characteristics
DC motors- Introduction
• Advantages:
 Leading role until 1960- years.
 Almost ideal Speed vs Torque motor characteristics.
 Possibility of obtaining variable and continuous dc voltage
 Simplicity for control (control paradigm)
 Large range of speed controllabilty
• The lacks:
 Mechanical commutator (inverter/rectifier)
 Large moment of inertia (because of collector)
 Request for often maintaining
 Sensitivity according current overloading , sparking (in commutation)
Direct current (DC)motors
Cross cutting of the DC motor
d-axes
Stator
Фu Фu
Excitation
winding,(coils)
Фa
Фa
Rotor
q-axes
Armature
winding
(coils)
DC motors- Introduction
Back-emf (E) and Force (F) definition
 
E   v x B l


(1)


F  I ( l x B)
(2)
 If armature winding is connected to the supply voltage, the electrical
current appears. Because of existing magnetic field (induction
B), tangential force arise according to equation (2).
 Because of force F (2), torque is generated and push the rotor to
rotate .
 When the rotor rotating in magnetic field, the voltage E, equ1, is
induced in armature winding opposed to the voltage U. This voltage
is called as “back-EMF” (back electromotor force) .This voltage is
proportional to rotor speed v, see equ1).
 If supplied (armature) voltage in steady state (constant rotor speed)
is greater than back-emf, U>E, we are talking about motor
work,(motoring) or motor mode of operation; otherwise, U<E we are
talking about generator work.
a) Description of DC motor components
Construction of DC motor
 The rotor is made of sliced iron (alternated current in the rotor coils)!!.
 Rotor winding consists of one or more solenoid (coils) where each of
them is connected to collector segment (slice), se picture bellow)
 Stator is aimed for a excitation (electromagnetic or permanent magnets)
frame
Covering of
housing
stator
collector
brushes
and girder
shaft
bearing
armature
b) Description of DC motor components
Stator:
 Immobile part, made mainly of massive iron (yoke). It doesn’t be
laminated.
 Main magnetic poles (electromagnets) are fixed on stator and assure
magnetic fields (B) in the air-gap. The amount of magnetic field can be
changed only if electromagnetic excitation is used.
 If permanent magnet is used for excitation, the amount of excitation can
not be changed!
Excitation coils
pole’s shoes
hausing
c) Description of DC motor components
Rotor (Armature):
 Moving part, made off laminated iron (because of alternating current in
armature winding)
 Rotor has slotings with coils in it. Rotor’s winding consits of one or more
coils where each of it is connected to collector.
Fe
laminated
Armature
winding
collector
bearing
shaft
d) Description of DC motor components
Rotor (Armature):
rotor’s winding
collector
brush with girder
e) Description of DC motor components
Rotor (collector):
Armature
winding (coils)
Slices
(segments) on
collector)
brush
brush girder
Collector with brushes (mechanical commutator!!), we wil see that
later!
f) Description of DC motor components
 Collector’s segment is connected to rotor’s coil (as presented in the
picture below).
 Current flow from external source, over brush with girder and over
collector’s slice enter into a coil (at the position of the neutral zone,
where there are no induced voltage in this coil)!
zur Ankerwicklung
To rotor’s coil
Kommutatorfahne
Coil connection to collector’s segment
Kommutatorcollector’s
lamelle
isolator
Isolation
segment
Bürste
brush
Collector’s segment
g) Description of DC motor components
Construction of torque and back-EMF (voltage E)
- Magnetic field (permanent
magnets)
- two brushes
- two collector’s segment
- one coil in magnetic field
a) How DC motor works
Forces under poles “S” and “N” results in equivalent torques on the coil
under “S” pole and “N” pole.
Current direction in the coil under pole S change its direction when the
coil reach the position under N pole. This is the reason why we say that
the current in armature winding, when looking from outside the motor, is
ALTERNATE current!!!
ui
t
collector =
mechanical
rectifier
brush
u
t
Current enter in the motor
from source over collector
and brushes in armature
colector slice
winding . Result is motor
work.
brush
b) How DC motor works
Because magnetic field act on the arm which is changed according to
sinusoidal low, then torque is changed in the same way.
DC motor ANIMATION – motor parts
STATORpermanent magnet or electromagnet);
ROTORarmature winding
DC motor example (cross and longitudinal section)
DC motor – Animation
Slots
with
coils in it
ANIMATION
PICTURE
Red  Magnet or electromagnet with “N” pole
Green  Magnet or electromagnet with “S” pole
Stator may consist from more permanent magnets (multipole DC motor)
Rotor coils are connected to collector (brown colour ), 3 pair of poles
Brushes are dark-gray.
Distance between collector slices is black.
DC motor animation – demonstration of work
DC motor as generator of DC voltage – animation
System “brush-collector” rectifire alternate armature voltage in DC voltage .
DC motor – mathematical model
ia
Ra
ea  ke    m  ce  m
cm  ce
dia
ua  ea  Ra ia  La
dt
mm  km    ia  cm  ia
La
ua
ea  ce  m
km  ke
mm , m
mt , t
Jt
M
Jm
J eq  J m  J t
d m
1

 mm  mt 
dt
J eq
Stacionarno stanje
M m ( M t )
  m 
cm
U a  I a  Ra U a  I a  Ra


ke  
ce
The list of variables and constants
Ua, Ia
Voltage and armature current
R a , La
Resistance and inductance of armature winding
Ea (Ei)
back-emf
c e, k e
constants of back-emf
cm, km
torque constants
Mm, Mt
motor torque and load torque
m, t motor and load speed
Jm, Jt
moment of inertia for motor and load

magnetic field, excitation
How to change the speed of motor?
(1) Changing armature voltage
(3) Changing armature resistor
U a  I a  Ra
  m 
ke  
(2) Changing magnetic field (excitation)
(1) Changing armature voltage (a)
• Historically, first qualitative control solution without considerable losses,
see figure.
• For high power ratings later is used system with asynchronous (induction)
motors
• Next solutions are Induction motor (AM ) which drive machine(generator
G), in order to supply DC motor(M) with separate excitation. Controlling
exciting current of generator G, the armature voltage (motor M voltage) is
directly controlled. DC Motor M has constant excitation uum.
• There is now new solution with power converter in motor armature for 4Q
operation
load
(1) Changing armature voltage (b)
• This solution don’t use rotational machines for voltage change. Voltage is
changed with static converter (in this situation it is simple diode and
autotransformer).
• The alternating voltage from the input side of transformer is changed by
auto-transformer using slider on secondary transformer side. This voltage
is rectifired using diode and forwarded then to the motor .
• It is possible also to change the sign of motor excitation
220V~
0-220V~
M
(1) Changing armature voltage (c)
• New solution with AC/DC converters in u armature.
L1
L2
L3
M
• Two 3-phase converters in antiparalel connection insure 4q operation
with high dynamic performances. The change of current direction is
realized electronically
(1) Changing armature voltage (d)
m
U a1
simplicity of control, speed is proportional
to the supplied voltage
Ua2
U a3
ua  ia Ra
m 
 kua
ke  
Ua4
U a1  U a 2  U a 3  U a 4
const.
Mt
(Speed vs Torque)
(3) Changing armature resistance
• In series with armature coil (winding), resistor is added. The slope of the
characteristic is changed.
m
n
R  Ra
1
R  Ra  Rd 1
2
R  Ra  Rd 2
3
R  Ra  Rd 3
Rd 4  Rd 3  Rd 2  Rd 1
R  Ra  Rd 4
mt
• Rotor’s resistor as starter
• For starting, the maximal resistor
should be used, Rd4, (speed=0) ,
see figure
• After start ,Rd3 resistor is added,
and finally Rd=0 (R=Ra) is added
• High losses, heating, not
economical solution,
(3) Changing armature resistance (a)
• It is used a lot in the past in DC traction drives. There were a lot of losses
in energy conversion. (converted in heat). Steady state points in motoring
and braking were set changing the amount of resistor added to armature
circuit. Example is the tram. No efficient energy balance, great energy
part is converted in the heat.
The types of the excitation systems for DC motors
+
+
+
M
M
Ua
M
Ua
Ua
-
-
Uu
-
a)
Independent excitation
b)
paralel excitation
c)
serial excitation
DC motor – region of speed control
Konst.

ua  ia Ra
1
k
ke  m
m
P  ua I a  kua
Controlled by armature voltage,
magnetic field constant
CONSTANT TORQUE region
Controlled by magnetic field, Armature
voltage constant , CONSTANT POWER
region
I
0

current
n
Flux (mag. field)
0
M
n
torque
0
U
n
armature voltage
0
n
DC motor characteristics-variables of DC motor
Literature
1. http://www.physclips.unsw.edu.au/jw/electricmotors.html#DCmotors
2. http://electronics.howstuffworks.com/motor.htm
3. R.Wolf.”Fundamentals of electrical machines”, str.220-246, Školska
knjiga, Zagreb, 1985. (Osnove električnih strojeva)