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Chapter 8

DC Drives

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DC Drive Fundamentals

• DC drive systems

are divided intothree major 

sections:

 – Operator Control – Drive Controller 

 – DC Motor 

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Drive Controller 

• The drive controller converts an AC sourceinto an adjustable DC voltage which is then

applied to a DC motor armature

•The controller will regulate the speed,direction, torque, and horsepower set by a

reference point

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DC Motor 

• The DC motor converts the adjustable DCvoltage from the drive controller to rotating

mechanical energy

•Typical DC motors used in a drive controlsystem would be shunt wound or permanent

magnet types

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Variable Voltage DC Drive

• A simple speedcontrol circuit makesuse of an SCR 

• The SCR rectifies the

voltage and controlsthe amount of currentdelivered to themotor armature

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Full-wave Speed Control

• The half-wave rectifier 

is seldom used inmotor control because

of excessive heating

and noise

• The full-wave circuit atthe right is more

typical of variable

voltage motor speed

controllers

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Motor SpeedControl

• A complete motor 

speed control

system incorporates

SCR’s, UJT’s, andvarious conditioning

circuits as shown on

the right

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Motor Braking

• Some industrial applications require rotarymachines to be stopped quickly and two

 popular braking methods for DC electric

motors have been developed:

 – Dynamic Braking

 – Regenerative Braking

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Dynamic Braking• Dynamic braking uses a

resistor to absorb the

kinetic energy from thearmature and load

• Solid-state devices arecommonly used in modern

 braking systems• In positioning systems,

dynamic braking isreferred to as indexing  

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Regenerative Braking• In regenerative braking, kinetic power is returned to the power supply

• Typically, SCR bridges are used in regenerative braking systems