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DIGITAL CONTROLLER DESIGN
Lab Report-02 (15-02-2014)

Arun Rahul S
Sr No: 10578
Open Loop Speed Control of DC Motor

Circuit Diagram
Vdc

F.D

Va eb

eb

ATMEGA 328p
POT

R sense

Controller
ADC

PWM

Gate driver

D

DC motor

Ia sense

Q

Eb sense

Duty cycle adjustment Fig (1).DC motor speed control experimental setup.
The duty cycle is controlled by varying the duty cycle adjustment pot.
Ratings and specification
Vdc

: 24.4 V

PMDC Motor/Generator Rated speed

: 24 V

PMDC Motor/Generator Rated speed

: 1770 RPM

PMDC Motor/Generator rated current : 5A.
MOSFET

: IRF640.

Freewheeling diode

: IRF640 body diode.

R sense

: 0.6Ω

Current sensing

: Opamp LM324 based instrumentation amplifier.

Voltage sensing

: Opamp LM324 based Differential amplifier.

Gate driver

: Complementary transistor with optocoupler isolation.

Controller

: ATMEGA 328P

PMDC motor equivalent circuit ia La

Ra

Va

J
Tem

Permanent magnet eb

w b TL

Fig(2). PMDC motor equivalent circuit diagram.
Dynamic equations for DC motor is given by,
=

+

+

= ф

(1)
(2)

= ф
−

(3)

=

ф=

=

+

(4)
(5)

At steady state
=
=

+
∗

=

At no load condition

(7)
∗

−

(6)

=

(8)
(9)

=0
∗

=

(10)

Current sensing

Fig(3) current sensing circuit
Current scale
I=1.33A then I_sence=1 V
Voltage sensing

Fig(4) voltage sensing circuit
Vin=30V then V_sence=5V

MOSFET gate driver circuit

Fig(5) gate driver circuit.

PMDC motor/generator Speed Vs back emf characteristics
For a PMDC motor back emf is directly proportional to the speed since field is constant. According to equation
=
∗ , Where Km is the back emf constant.
For the experiment PMDC motor/generator set is configured according to circuit diagram and PWM duty cycle is varied from low value of around 15% to 90% using a potentiometer. The ADC senses the