1. 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

2. DC motors- Introduction
Advantages:
Leading role until 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)

3. Direct current (DC)motors
Cross cutting of the DC motor

4. DC motors- Introduction
Back-emf (E) and Force (F) definition
(2)
(1)
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.

5. 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)
Covering of housing
shaft
bearing
frame
stator
collector
brushes and girder
armature

6. 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

7. 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.
shaft
Fe laminated
Armature winding
collector
bearing

8. d) Description of DC motor components
Rotor (Armature):
brush with girder
collector
rotor’s winding

9. 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!

10. 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)!
To rotor’s coil
Coil connection to collector’s segment
collector’s segment
isolator
brush
Collector’s segment

11. 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

12. 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!!!
collector = mechanical rectifier u i t
brush
Current enter in the motor from source over collector and brushes in armature winding . Result is motor work.
colector slice
brush

13. 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.

14. DC motor ANIMATION – motor parts
STATORpermanent magnet or electromagnet); ROTORarmature winding

15. DC motor example (cross and longitudinal section)

16. DC motor – Animation Slots with coils in it PICTURE 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.

17. DC motor animation – demonstration of work

18. DC motor as generator of DC voltage – animation
System “brush-collector” rectifire alternate armature voltage in DC voltage .

19. DC motor – mathematical model
Stacionarno stanje

20. 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

21. How to change the speed of motor?
(1) Changing armature voltage
(3) Changing armature resistor
(2) Changing magnetic field (excitation)

22. (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

23. (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

24. (1) Changing armature voltage (c)
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

25. (1) Changing armature voltage (d)
simplicity of control, speed is proportional to the supplied voltage
const.
(Speed vs Torque)

26. (3) Changing armature resistance
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,

27. (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.

28. The types of the excitation systems for DC motorsb) c) a)
serial excitation
Independent excitation
paralel excitation

29. DC motor – region of speed control
Konst.
Controlled by armature voltage, magnetic field constant CONSTANT TORQUE region
Controlled by magnetic field, Armature voltage constant , CONSTANT POWER region

30. DC motor characteristics-variables of DC motor
current
Flux (mag. field)
torque
armature voltage
DC motor characteristics-variables of DC motor

31. Literature
R.Wolf.”Fundamentals of electrical machines”, str , Školska knjiga, Zagreb, (Osnove električnih strojeva)