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
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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)
Cross cutting of the DC motor
Фu
Фa
Excitation winding,(coils)
Armature winding (coils)
Rotor
q-axes
d-axes
Фu
Фa
Stator
Direct current (DC)motors
( )F I l x B
E v x B l
Back-emf (E) and Force (F) definition
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.
(2)(1)
DC motors- Introduction
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)
a) Description of DC motor components
Covering of housing
shaft
bearing
frame stator
collector
brushes and girder
armature
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
b) 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.
shaft
Fe laminated
Armature winding
collector
bearing
c) Description of DC motor components
brush with girdercollector
rotor’s winding
d) Description of DC motor components
Rotor (Armature):
Collector with brushes (mechanical commutator!!), we wil see that later!
Slices (segments) on collector) brush
brush girder
Armature winding (coils)
e) Description of DC motor components
Rotor (collector):
z u r A n k e rw ic k lu n g
B ü rs teI so la t io n
K o m m u ta to r -la m e lle
K o m m u ta to r fa h n e
To rotor’s coil
isolator
collector’s segment
Coil connection to collector’s segment
brush
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)!
Collector’s segment
f) Description of DC motor components
Construction of torque and back-EMF (voltage E)
g) Description of DC motor components
- Magnetic field (permanent magnets)- two brushes- two collector’s segment- one coil in magnetic field
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!!!
a) How DC motor works
Current enter in the motor from source over collector and brushes in armature winding . Result is motor work.
collector = mechanical rectifier
ui
t
u
t
brush
colector slice
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
STATORpermanent magnet or electromagnet); ROTORarmature winding
DC motor example (cross and longitudinal section)
DC motor – Animation
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.
ANIMATION PICTURE
Slots with coils in it
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
aR
aLau
a e me c ,m mm ,t tm
tJ
ai
mJM
a e m e me k c
aa a a a a
diu e R i L
dt
m m a m am k i c i
1mm t
eq
d m mdt J
Stacionarno stanje
m ek k
a a a a a a
e e
U I R U I Rk c
m ec c
( )m tm
m
M Mc
eq m tJ J J
The list of variables and constants
Ua, Ia Voltage and armature current
Ra, La Resistance and inductance of armature winding
Ea (Ei) back-emf
ce, ke 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?
a a am
e
U I Rk
(1) Changing armature voltage (3) Changing armature resistor
(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
• 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
M220V~
0-220V~
(1) Changing armature voltage (b)
M
L1 L2 L3
• New solution with AC/DC converters in u armature.
• 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 (c)
(Speed vs Torque)
simplicity of control, speed is proportional to the supplied voltage
a a am a
e
u i R kuk
const.
(1) Changing armature voltage (d)
• In series with armature coil (winding), resistor is added. The slope of the characteristic is changed.
• 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
(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 thetram. No efficient energy balance, great energy
part is converted in the heat.
MUa
+-
MUa
+
-
MUa
+
-
Uu
Independent excitation
a) b) c)
The types of the excitation systems for DC motors
paralel excitation serial excitation
DC motor – region of speed control
a a aP u I ku
Controlled by armature voltage, magnetic field constant CONSTANT TORQUE region
Controlled by magnetic field, Armature voltage constant , CONSTANT POWER region