DC Machines - Slide Set

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2012/05/0

DC-machines

Wildi

Chapter 5

Introduction (1)

Background – Chapter 4 - DC-generators

Armature

Commutator

FieldBrushes

Brush holders



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Introduction (2)

Applications – Pumps,fans, cars etc.

Three typesShunt

Series

Compound

Only used for torque-speed characteristicsvarying over wide range with highefficiency

Introduction (3)

Maintenance of dc-motor vs. inductionmotors

Direct current from ac – requires rectifiers

- lawnmower motor? Control – more simple than ac-motors

DC-motor can operate as dc-generator



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DC motor operation

B

J

ω, Td

Counter-electromotiveforce

Also called back-emf

Motor action = powerconnected and armatureturns

Moving conductors in magneticfield experience voltage inducedin them = back-emf

Z = Constant (# turns etc.)

n = Speed of rotation

φ = Flux per pole



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Acceleration of the motor Net voltage across armature

= Es – E0

Armature current limited onlyby resistance:

Motor at rest (E0 = 0) and startingcurrent is:

Starting current could be 20 to 30times full-load current of motor(Fuses?)

Example 5.1



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Mechanical power and torque (1)

Counter emf induced in lap woundarmature

Electrical power suppliedto armature

But

Thus


I2R represent heat dissipated in machine

Mechanical power:P = mechanical power developed (W)

E0 = induced voltage in armature (V)

I = Total current supplied to armature (A)



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Mechanical power: P = Tω = T *n*(2π /60)

Thus:

and

T = Torque (N.m.)

Z = # conductors on armature

φ= effective flux per pole (Wb)

I = Armature current (A)

6.28 = 2π = constant+ Example 5.2

Torque vs. speed



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Example 5.2

Speed of rotation

For dc-motor driving load between no-loadand full-load IR voltdrop small comparedto supply voltage

This means cemf is very nearly equal to Es

=

n = speed of rotation (r/min)

Es = armature voltage (V)

Z = # armature conductors



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Speed control Armature speed control – DIY

+ Example 5.3

Ward-Leonard

Modern electronic converters

Rheostat speed control

Field speed control - DIY

Example 5.3



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Shunt motor under load DC-motor under no-load – Ia is low

Load suddenly increased – armaturecurrent too small to provide enough torque

Motor slows down

Back-emf decreases

Current increases

Torque increases

Example 5.4



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Series motor (1)

Identical constructionto shunt motor exceptfor field being connectedin series with the armature

Ia = If Field conductors thicker

and fewer turns

Series motor (2)

Properties different tothat of shunt motor

Shunt motor – field constant (connected to ownsupply)

Series motor – field is function of armaturecurrent (thus function of load)

Low speed – low back-emf – high current – hightorque

Light / no-load will cause excessive speed!



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Series motor speed control Low value shunt resistor in parallel with

field (reduces field current and increasesspeed)

External series resistor – Increases IR-drop across resistor and field thusreducing the armature voltage (reduces

speed)

Example 5.5



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Compound motor (1)

Both series and shunt field present

Cumulative compound motor

Mmf of two fields add

Differential compound motor

Total mmf decreases with increased load

Speed increases as load increases –

instability!

Compound motor (2)



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Selfstudy Reversing direction of rotation

Starting a shunt motor

Face-plate starter

Stopping a motor

Dynamic braking (1)

Dynamic braking – energy dumped into R

Motoring mode Open-circuit armaturegenerating E0

1. 2.

3.



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Dynamic braking (2)

Armature current reverses direction

Reverse torque decelerates motor

Choose R with initial braking current =2*rated motor current

Exponential drop in speed

Plugging (1)

Suddenly reverse armature current byreversing terminals of source

Normal motoring:(R0 = armatureresistance)



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Plugging (2)

Reverse terminals

Voltage acting on armature = (E0 + Es)

Enormous reverse current generated!

Prevent damage by inserting external resistor R(design same as dynamic braking)

Open armature circuit as soon as speed is zero

to prevent rotation in opposite direction

Dynamic braking vs. Plugging



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Dynamic braking and mechanical

time constant (1)

Exponential decrease in speed

T = time for speed to reach 36.8% of initialvalue

T0 = time for speed to decrease to 50% ofinitial value

Relationship between conventional timeconstant and half-time constant:

Dynamic braking and mechanicaltime constant (2)

Mechanical time constant:



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Examples 5.6 and 5.7



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Regenerative braking

Armature reaction and fluxdistortion

No-load flux distribution Flux created by full-loadarmature current

Resultant flux distribution ofmotor running at full-load



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Self study Commutating poles

Poles between main poles

Improve armature reaction

Improve commutation

Compensating windings

Slots cut in main pole faces

Further improves armature reaction

Basics of variable speed control (1)

Constant torque region Constant power region

Base speed (design

speed, V and I)

(P can increase through V) (V and I constant)



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2

Basics of variable speed control (2)

Permanent magnet motors



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Brushless dc-motors

DC Machines - Slide Set

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