ELE-351Electrical Energy ConversionElectrical Energy Conversion

DC Machines

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OutlineOutline

P i i l f DC hi Principles of DC machinery The equivalent model of a DC motor Characteristics of shunt DC motors Speed control of DC motorsSpeed control of DC motors

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Rotating Loop Between Curved Pole Faces

The magnetic field is always perpendicular to the rotor surface.

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Voltage Induced in the Rotating Loop A source of mechanical power to rotates the loop/rotor. The induced voltage varies with the segment: g g

eind = (v B)l

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Getting DC Voltage (Commutator & Brushes)(Commutator & Brushes)

The two isolated semicircular conductors always connect the end of the loop with positive voltage to theconnect the end of the loop with positive voltage to the positive output terminal, by means of brushes (low friction).

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Induced Torque in the Rotating Loop Flow of current in the loop is required.

F = i (lB) = r F sin

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Characteristics of DC Motors

Great performance for applications that require id i i i dwide variations in speed.

Main types of DC motors: Separately excited DC motor Sh nt DC motor Shunt DC motor Permanent magnet DC motor i Series DC motor Compounded DC motor

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The Equivalent Circuit of a DC Motor Armature and field circuits.

E = k i d = k iEa k ind k ia

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The Magnetization Curve of a DC Motor

Variation of the internal voltage with the field winding current (that produces B).g ( p )

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Separately Excited d Sh t DC M tand Shunt DC Motors

SeparatelyExcitedDCMotor(The field is fed from a separate source)

ShuntDCMotor(The armature and the field are fed from the

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(Thefieldisfedfromaseparatesource) (Thearmatureandthefieldarefedfromthesamesource)

Terminal Characteristics of a Shunt Motor

Plot of the output quantities (torque x speed).

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Response to a load variation (increase): If: E I ( < )

load1load0

If: load, , EA, IA, ind, (1< 0)5:10PM 11

Speed Control of Shunt DC Motors Changing the field resistance (and the field flux) If: RF, IF, , EA, IA, ind, (2 > 1)

The change in IAdominant the change in

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Slope variation changes the control logic at low speeds!The change in dominant the change in IA5:10PM 12

Speed Control of Shunt DC Motors Changing the armature voltage (VA) with IF fixed If: VA, IA, ind, (2> 1)A A ind ( 2 1)

Shunt or Separately excited DC machine

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Dual (IF and VA) Speed Control If: R I and if: R I Since I < I this If: RF, IF, and if: RF , IF , . Since IF < IFbase this

method is good for > base! If: VA, and if VA , . Since VA < VAbase this method is A A A Abase

good for < base!

Risk of overspeeding if the field opensdue to a blown fuse!5:10PM 14

Power & Torque in a DC Machine

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Power & Torque in a DC Machine

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