DC Motor Drives

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GENERAL ELECTRICAL DRIVES
What is electrical drives? Components of
electrical drives Advantages of electrical drives
DC drives Vs AC drives Torque equations Load
torque profiles Four quadrant operation
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DC DRIVES Vs AC DRIVES

• DC drives
• Advantage in control unit
• Disadvantage in motor
• AC Drives
• Advantage in motor
• Disadvantage in control unit

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DC DRIVES

• DC DRIVES Electric drives that use DC motors as
  the prime movers

• DC motor industry workhorse for decades

• Dominates variable speed applications before PE
  converters were introduced

• Will AC drive replaces DC drive ?

4

• DC Motors

• Advantage simple torque and speed control
  without sophisticated electronics

• Limitations

• Regular Maintenance

• Expensive motor

• Heavy motor

• Sparking

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General Torque Equation
Translational (linear) motion
F Force (Nm) M Mass (Kg ) v velocity (m/s)
Rotational motion
T Torque (Nm) J Moment of Inertia (Kgm2
) w angular velocity ( rad/s )
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Torque Equation Motor drives
Te motor torque (Nm)
TL Load torque (Nm)
Acceleration
Deceleration
Constant speed
7
continue
Drive accelerates or decelerates depending on
whether Te is greater or less than TL
During acceleration, motor must supply not
only the load torque but also dynamic torque, (
Jdw/dt ).
During deceleration, the dynamic torque, ( Jdw/dt
), has a negative sign. Therefore, it assists the
motor torque, Te.
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Torque Equation Graphical
Te
Speed
Forward running
Forward braking
Reverse acc.
Reverse running
Reverse braking
Forward acc.
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Load Torque
Load torque, TL, is complex, depending on
applications.
In general
TORQUE
TL k
TL kw
TL kw2
SPEED
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4Q OPERATION
F FORWARD R REVERSE M MOTORING B BRAKING
SPEED
w
w
Te
Te
FM
FB
I
II
TORQUE
RB
III
IV
w
w
Te
Te
RM
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4Q OPERATION LIFT SYSTEM
Positive speed
Negative torque
Motor
Counterweight
Cage
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4Q OPERATION LIFT SYSTEM
Convention
Upward motion of the cage Positive speed
Weight of the empty cage lt Counterweight
Weight of the full-loaded cage gt Counterweight
Principle
What causes the motion?
Motor motoring P Tw ve
Load (counterweight) braking P Tw -ve
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4Q OPERATION LIFT SYSTEM
Speed
You are at 10th floor, calling fully-loaded cage
from gnd floor
You are at 10th floor, calling empty cage from
gnd floor
FM
FB
Torque
RM
RB
You are at gnd floor, calling empty cage from
10th floor
You are at gnd floor, calling Fully-loaded cage
from 10th floor
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DC MOTOR DRIVES
Principle of operation
Torque-speed characteristic
Methods of speed control
Armature voltage control
Variable voltage source
Phase-controlled Rectifier
Switch-mode converter (Chopper)
1Q-Converter
2Q-Converter
4Q-Converter
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Principle of Operation

• DC Motors

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Equivalent circuit of DC motor
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Torque-speed characteristics
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Torque-speed characteristics of DC motor
Speed
No load speed
Maximum load Torque
Full load speed
Torque
Separately excited DC motors have good speed
regulation.
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DC Motor Speed Control
By Changing Ra
Speed
Ra increasing
Maximum Torque
Torque

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

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DC Motor Speed Control
By Decreasing Flux
Speed
Maximum Torque
Flux Decreasing
Torque
Trated

• Slow transient response
• Does not maintain maximum torque capability

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DC Motor Speed Control
By Changing Armature voltage
Speed
Va increasing
Maximum Torque
Torque
Trated

• good speed regulation
• maintain maximum torque capability

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Speed control of DC Motors
Below base speed Armature voltage control
(retain maximum torque capability)
Above base speed Field weakening (i.e. flux
reduced) (Trading-off torque capability for
speed)
Armature voltage control
Torque
Field flux control
Line of Maximum Torque Limitation
speed
?base
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Methods of Armature Voltage Control
Phase-controlled rectifier (ACDC)
ia
Vt ?
3-phase Or 1-phase supply
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Methods of Armature Voltage Control
1. Ward-Leonard Scheme 2. Phase-controlled
rectifier (ACDC) 3. Switch-Mode Converter
(Chopper) (DCDC)
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Methods of Armature Voltage Control
Phase-controlled rectifier 4Q Operation
Vt ?
1 or 3-phase supply
1 or 3-phase supply
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AN ALTERNATIVE WAY
Phase-controlled rectifier 4Q Operation
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Converters For DC motor Drives
Switchmode converters 1Q Converter
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Converters For DC motor Drives
Switchmode converters 2Q Converter
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Converters For DC motor Drives
Switchmode converters 4Q Converter
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Advantages of Switch mode converters
Switchmode converters

• Switching at high frequency

? Reduces current ripple
? Increases control bandwidth

• Suitable for high performance applications