Motor Part 12

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Electric Motors

• Classification / types• DC Motors• AC Motors• Stepper Motors• Linear motors

• Function• Power conversion - electrical into mechanical• Positional actuation – electrical signal to position

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DC Motors

• DC Motors• Fundamental characteristics

• Basic function

• Types and applications• Series

• Shunt

• Combination

• Torque characteristics

• Modelling

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Fundamental characteristics of DC Motors

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End viewTime 0

End viewTime 0+

Shifting magnetic field in rotor causes rotor to be forced to turn

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Nature of commutation

• Power is applied to armature windings• From V+

• Through the +brush

• Through the commutator contacts

• Through the armature (rotor) winding

• Through the – brush

• To V-

• Rotation of the armature moves the commutator, switching the armature winding connections

• Stator may be permanent or electromagnet

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DC motor wiring topologies

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Series Wound DC motors• Armature and field connected in a series circuit. • Apply for high torque loads that do not require precise speed regulation. 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 to prevent high-speed breakdown, but larger motors

require external 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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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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Permanent magnet DC motors

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Permanent Magnet DC Motors• Have permanent magnets rather than field windings but with conventional armatures.

Power only to armature.• Short response time• Linear Torque/Speed characteristics similar to shunt wound motors. Field magnetic 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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DC motor control – H-bridge

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• 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-clockwise speed

• Can be configured to brake• Bottom “B” and “A” to brake


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H-Bridge implementation

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• Elements in box are available as single IC

Brushless designs

• Commutation is done electronically

• Encoder activated switching

• Hall effect activated switching

• Back EMF driven switching

• PM armature

• Wound/switched fields

• Application

• Few wearing parts (bearings)

• Capable of high speed

• Fractional HP

• Servos

• Low EMC


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Stepper Motors

• Description• Generally a two phase motor• permanent magnet rotor and wound fields• Rotor normally has many poles

• 200 poles = 1.8 degrees per step

• Used primarily for position or velocity control• Typically no position feedback

• Torques are managed so that an intended step is always achieved• Accelerations, decelerations and loads must be managed intelligently

• Two general types of windings• Unipolar• Bi-polar


Winding configurations

• Bi-polar design

• 6 wire

• Unipolar design• 4 wire


AC Motors

• AC Motors• Fundamental characteristics• Types

• Fractional horsepower (single phase)• Integral

• Single phase (Cap start Induction run)

• Three phase

• NEMA Torque characteristics • Modelling

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AC Motors• Relationship between number of poles and motor

synchronous speed

• Squirrel cage motors must operate with some slip .5 to 8% to allow the rotor to be magnetized.

• Actual speed is synchronous speed reduced by the slip.

Poles Synchronous

Speed

(RPM)

2 3600

4 1800

6 1200

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Squirrel Cage Rotor

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Inducing magnetism in the rotor• Difference between

angular velocity of rotor and angular velocity of the field magnetism causes squirrel cage bars to cut the field magnetic field inducing current into squirrel cage bars.

• This current in turn magnetizes the rotor

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Interfacing DC motor with avr


interfacing

• MCU generates signals in form of HIGH (Vcc = 5v) or LOW (zero).

• But this voltage is insufficient to drive a motor. That’s why we need to use a Motor Driver.

• A motor driver always has a battery input Vs (which depends upon the rating of the motor).

• The most commonly used motor driver is the L293D.


L293d connections



Driving motors with L293D

• There are two enable (EN) pins, pin 1 and pin 9. Pin 1 EN enables the motor M1 whereas pin 9 EN enables motor M2.

• Connect motor M1 across OUTPUT1 and OUTPUT2 i.e. across pins 3 and 6.

• Connect motor M2 across OUTPUT3 and OUTPUT4 i.e. across pins 11 and 14.

• The inputs for motor M1 is given across INPUT1 and INPUT2 i.e. across pins 2 and 7.

• The inputs for motor M2 is given across INPUT3 and INPUT4 i.e. across pins 10 and 15.

• Connect GROUND pins 4, 5, 12 and 13 to ground.

• Connect pin 16 to Vcc (=5V) and pin 8 to Vs (battery, 4.5V~36V).


Interfacing stepper motor


• The unipolar stepper motor can be rotated by energizing the stator coils in a sequence.

• The sequence of these voltage signals applied across the motor coils or leads is enough to drive the motor and hence, no driver circuit is required for controlling the direction of the current in the stator coils.


• The two-phase-stepper motor consists of four end wires connected to the coils and two common wires connected to the two end leads to form two phases.

• The common points and end points of the two phases are connected to the ground or Vcc and the microcontroller pins, respectively.

• For rotating the motor, the endpoints of the two phases are to be energized.

• Primarily a voltage is applied to the first end point of the phase1, and further voltage is applied to the first end point of the phase2, and so on


Modes

• The stepper motor can be operated in different modes such as• Wave Drive Stepping Mode,

• Full Drive Stepping Mode and

• Half Drive Stepping Mode.


Wave drive stepping mode


Full drive stepping mode


Half drive stepping mode



Servo motor interfacing


usage

• Servo motors are a type of electromechanical actuators that do not rotate continuously like DC/AC or stepper motors,

• They used to position and hold some object.

• They are used where continuous rotation is not required so they are not used to drive wheels.

• Most common use is to position the rudder of aircrafts and boats etc.


Controlling servo with avr

• You can use the AVR micro controllers PWM feature to control servo motors.

• When a PWM signal is applied to its control pin the, the shaft rotates to a specific angle depending on the duty cycle of the pulse.

• Unlike other motors, Servo motors don’t require any driver


Connection


In the above figure the ON time for pulse is 1ms and off time pulse is 18ms this rotates the shaft to -90 degree. Similarly if the on time of pulse is 1.5ms and the off time of pulse same the servo rotates to 00 and if ON time pulse increases to 2ms it rotates to +900. This gives a complete 180 degree rotation. The motor maintains its position for every corresponding signal.

Engineering

Motor Part 12