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7/28/2019 MotorsTraining DC Motors
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© Mahindra Satyam 2009
DC Motors
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© Mahindra Satyam 2009
DC Motors
DC Motors – Fundamental characteristics
Basic function – Types and applications
SeriesShuntCombinationTorque characteristics
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Construction of DC Motor
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Operating principle of AC Motor
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Operation of DC Motor
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Operation of DC Motor
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Operation of DC Motor
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DC Shunt Motor
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DC Series Motor
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Compound Motor
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DC motor wiring topologies
P e r c e n
t o
f r a
t e d S p e e
d
Percent of Rated Torque
120
S e r i e s
C o m p o u n d
S h u n t 100
80
60
40
20
0400300200100 0
S h u n
t F i e l d
Series Field
S h u n
t F i e l d
Series Field
Shunt
Series
Compound
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© Mahindra Satyam 2009
Series Wound DC motors
– Armature and field connected in a series circuit.
– Apply for high torque loads that do not require precise speedregulation. Useful for high breakaway torque loads.
locomotives, hoists, cranes, automobile starters – Starting torque
300% to as high as 800% of full load torque. – Load increase results in both armature and field current increase
Therefore torque increases by the square of a current increase. – Speed regulation
Less precise than in shunt motors – Diminished load reduces current in both armature and field
resulting in a greater increase in speed than in shunt motors.No load results in a very high speed which may destroy the motor.
– Small series motors usually have enough internal friction toprevent high-speed breakdown, but larger motors requireexternal safety apparatus.
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Shunt wound DC motors
– Field coil in parallel (shunt) with the armature.Current through field coil is independant of the armature.
– Result = excellent speed control. – Apply where starting loads are low
fans, blowers, centrifugal pumps, machine tools – Starting torque
125% to 200% full load torque (300 for short periods).
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© Mahindra Satyam 2009
Compound wound DC motors
– Performance is roughly between series-wound and shunt-wound
– Moderately high starting torque – Moderate speed control – Inherently controlled no-load speed
safer than a series motor where load may be disconnected – e.g. cranes
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© Mahindra Satyam 2009
Permanent Magnet DC Motors
Have permanent magnets rather than field windings but with conventionalarmatures. Power only to armature.Short response timeLinear Torque/Speed characteristics similar to shunt wound motors. Fieldmagnetic flux is constant
– Current varies linearly with torque.Self-braking upon disconnection of electrical power
– Need to short + to – supply, May need resistance to dissipate heat.Magnets lose strength over time and are sensitive to heating.
– Lower than rated torque. – Not suitable for continuous duty – May have windings built into field magnets to re-magnetize.
Best applications for high torque at low speed intermittent duty. – Servos, power seats, windows, and windshield wipers.
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© Mahindra Satyam 2009
DC motor control – H-bridge
M
12V
A
A
B
B
Switches control direction“A” switches closed for clockwise “B” switches for counter -clockwise
PWM for speed control“A’s” duty cycle for clockwise speed “B’s” duty cycle for counter -clockwisespeed
Can be configured to brakeBottom “B” and “A” to brake
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© Mahindra Satyam 2009
H-Bridge implementation
InputLogic
PWM
Direction
Brake
Vsupply
M
GroundH-Bridge Circuit
DC Motor
Elements in box are available assingle IC
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DC Motor Speed Control
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Nature of commutation
Power is applied to armaturewindings
From V+Through the +brushThrough the commutator contactsThrough the armature (rotor) windingThrough the – brushTo V-
Rotation of the armature moves thecommutator, switching the armaturewinding connectionsStator may be permanent or electromagnet
Rotor
V-
V+BrushAssembly
S
S
N
N
N
Stator
Stator Comutator
V-
V+