APPLICATION – MANUAL SPEED CONTROL FOR DC MOTOR

Speed is proportionalto applied voltage, VM0i

Potentiometer

Arm is movable and allows to change R1 and R2 keeping their sum constant:

2 potR R

Battery

Assumption (no load)0i

speed batteryV V

Potentiometer is not affectedby connection to amplifier

Circuit diagram [rpm] 4M M M speedK V K V

[rpm] 4M batteryK V

Model using node analysis

2 1

0O O S

O

V V VI

R R

1 2

100 , 5 , 8

Find ,O O SI A V V V V

R R

22

2 1

s OOV VV

PR R

Resistors values should be high

Why not use very large values?

13V V

1100I A

1

330

100

VR k

A

2

1

2

2

1

3. . :

5

5:

3

O

O

Re g R

I R

or RI

R

Design equations

Use trial and error. PickHighest possible R1, determineR2 and analyze resulting circuit

1

1

527 450

3/O

R k kR I

2Closest R 430k

Analysis of solution

2 1 2

1 2

S O

O

RV R R IV

R R

4.98

OV V

Are there other factors that weshould consider; e.g., is Ioreally constant?, tolerances?

Standard= Off-the-self – available only in certain values

http://www.wiley.com/college/irwin/0470128690/animations/swf/3-23.swf