dc motors ee153.pdf

7/27/2019 dc motors ee153.pdf

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MOTOR converts electrical energy to mechanical

energy Its action is based on a principle that when acurrent-carrying conductor is placed in amagnetic field, it experiences a mechanicalforce, whose direction is given by FlemingsLeft Hand Rule


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Significant Features of DC MotorsDC motors convert electrical energy into mechanical energy

Constant mechanical power output or constant torqueTypes Shunt motors, series motors and compound motorsRapid acceleration or deceleration

Extensively used as a positioning device because its speed as

well as torque can be controlled precisely over a wide range1W to 10,000 hp

Applications in automobiles, robots, VCRs, movie camera,

electric vehicles, in steel and aluminum rolling mills, electrictrains, overhead cranes, control devices, etc.

Biggest advantage over other motors Torque-speed characteristics of dc motors can be varied over a wide

range while retaining high efficiency


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+

VT

_

Rf

Ra

+Eb

_

Iin

IaIf

EQUIVALENT CIRCUIT OF A SHUNTMOTOR

fain III +=

aab RIVE !=

Eb back or counter

emfV input voltageIin input current


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Back emf depends, among other factors, upon the armature speed. High speed!high Eb!small IaLow speed!low Eb!high Ia

Eb acts like a governor, i.e., it makes a motor self-regulating so that itdraws as much current as is just necessary.

CONDITION for MAXIMUM

EFFICIENCY

Thus, gross mechanical power developed by the motor is

maximum when back emf is equal to half the applied


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F

N rpsPulley

r

TORQUE

T=F x r (N-m)Work done by this force in onerevolution= force x distance

= F x 2"r JoulePower developed = F x 2"r x N

= F x r x 2"N

Watt Thus, P=2"NT

Where;P power developed in W N speed in rps T torque in N-m


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Armature Torque

EbIa = 2"NTa where aPZN

Eb

!=

!

"

#$

%

&=

A

PZIT

aa '(2

1

Thus, For shunt motor; Ta#IaFor series motor; Ta#Ia2

aa IT !"


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Shaft Torque

torque available for doing useful work

N2

watts

!

inoutputTsh =

where N in rps Lost Torque = Ta-Tsh

- due to iron and friction losses of the motor


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SPEED OF A DC MOTOR From the equation We get

and we assume $ # field currentFor Shunt motor;

For series motor;

a

PZN

Eb!

=

!

bE

kN =

1

2

1

2

b

b

E

E

N

N=

2

1

1

2

1

2

a

a

b

b

I

I

E

E

N

N!=


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Speed Regulation

Torque and Speed of a DC Motor


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While studying motor characteristics, keep in mind the following relations:


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Three possible methods of speed control: By Variation of Flux or Flux-control Method


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Torque

Speed

MaximumTorque

Flux Decreasing

Trated

Slow transient response Does not maintain maximum torque capability

By Decreasing Flux


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Armature or Rheostatic Control Method(Variation of Ra)


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Torque

Speed

MaximumTorque

By Changing Ra

Ra increasing

Power loss in Ra Does not maintain maximum torque capability Poor speed regulation


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dcmotor 18

Voltage Control Method

(By Controlling Terminal Voltage Vt )This method of speed control is applicable for speeds below

rated or base speed.

%m

VT

T1 T2 T3

T1


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dc motors ee153.pdf