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 girder

collector

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

dm m

dt J

Stacionarno stanje

m ek k

a a a a a a

e e

U I R U I R

k c

m ec c

( )m tm

m

M M

c

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 R

k

(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 Rku

k

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

1a a a

e m m

u i Rk

k

Konst.

n

U

0

n

M

0

n

0

n

I

0

DC motor characteristics-variables of DC motor

armature voltage

current

Flux (mag. field)

torque

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)