Engr. Zahoor Ahmed Maharvi

Lecture 22-01-14

Electronic Speed Control Method for DC Motors

Engr. Zahoor Ahmed Maharvi

Introduction• Solid state circuits using semiconductor

diodes and thyristors have become very popular for controlling the speed of d.c. motors.

• Solid state circuits are progressively replacing the traditional electric power control circuits based on thyratrons, ignitrons, mercury arc rectifiers, magnetic amplifiers and motor-generator sets etc.

Engr. Zahoor Ahmed Maharvi

Merits of Electronic Control Circuits

• As compared to the electric power control circuits, the electronic control methods have

1. Higher accuracy2. Greater reliability3. Quick response4. Higher efficiency as there are no I2R losses and

moving parts.5. Full 4-quadrant speed control is possible to

meet precise high speed standards.

Engr. Zahoor Ahmed Maharvi

Electronic Speed Control Methods

• All electronic control circuits control the motor speed by adjusting either

1. the voltage applied to the motor armature

2. the field current, or3. both

Engr. Zahoor Ahmed Maharvi

D.C. Motors Power Supply• D.C. motors can be run from d.c. supply if

available or from a.c. supply after it has been converted into d.c. supply with the help of rectifiers which can be either half-wave or full-wave and either controlled (by varying the conduction angles of the thyristors used) or uncontrolled.

Engr. Zahoor Ahmed Maharvi

Motor Speed Control using Thyristor Controlled Rectifier

• The average output voltage of a thyristor-controlled rectifier can be changed by changing its conduction angle and hence the armature voltage of the d.c. motor can be adjusted to control its speed.

Engr. Zahoor Ahmed Maharvi

Motor Speed Control using Thyristor Chopper Circuit

• When run on a d.c. supply, the armature d.c. voltage can be changed with the help of a thyristor chopper circuit which can be made to interrupt d.c. supply at different rates to give different average values of the d.c. voltage.

• If d.c. supply is not available, it can be obtained from the available a.c. supply with the help of uncontrolled rectifiers (using only diodes and not thyristors). The d.c voltages so obtained can be then chopped with the help of a thyristor chopper circuit.

Engr. Zahoor Ahmed Maharvi

Uncontrolled Rectifiers

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Controlled Rectifiers

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Controlled Rectifiers (contd..)

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Thyristor Choppers

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Thyristor Choppers (Contd..)

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Thyristor Speed Control of a Separately Excited D.C. Motor

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Thyristor Speed Control of a D.C. Series Motor

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Full-Wave Speed Control of a Shunt Motor

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Thyristor Speed Control of a Shunt Motor

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Thyristor Speed Control of a D.C. Series Motor

Engr. Zahoor Ahmed Maharvi

?Reference

Chapter # 30Electrical Technology

By B.L.Theraja