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Transcript
ENG H191 Hands-on Lab
Lab 7: Motors
Introduction
Background
Motors are commonly used in the design of machinery. Speed, torque and
electrical requirements are variables that are important to motor selection.
Electrical engineers focus on electrical efficiency where the wire and brush
material, insulation, contact arcs, etc. are all factors. With the widespread use
of digital electronics many engineers concentrate on the design of motor
controllers and position sensors. Industrial engineers must understand the
applications and uses of various actuators in order to select the best item for a
particular job. The power system of any machine must be selected to best suit
its application
Purpose
The purpose of this lab is to familiarize you with the various ways that
electric motors convert electric energy into rotary energy.
Basic Principles
In this lab write-up, we will cover some basic principles behind:
1)
2)
3)
4)
5)
6)
Lab Experience
Electric Motors,
DC Motors,
AC Motors,
Stepper Motors,
Servo Motors, and
Synchronous Motors
The lab experience will encompass:
1) Using a stepper motor,
2) Building a DC Motor,
3) Inspecting an AC Motor,
4) Controlling a servo motor, and
5) Running a small commercial DC motor.
1
Theory
Electric Motors
Electric motors convert electrical energy into rotary motion. This conversion
of energy is not direct. Magnetism is the intermediate stage. Electrical
current produces a magnetic field that attracts or repels another magnetic
field, (possibly a permanent magnet) which causes rotation.
A motor achieves continuous rotation in a manner similar to tying a carrot in
front of a donkey’s nose. The donkey is always trying to get the carrot, but as
soon as it gets closer, the carrot moves farther away. In a motor, the magnetic
fields on the rotor (the rotating element of the motor) and stator (the
stationary part of the motor) are constantly trying to align. But as soon as
they get close, the magnetic field changes to a new orientation, which again
draws the rotor.
There are a variety of motor styles that are controlled in different manners.
The basic categories are DC, AC, stepper, servo and synchronous motors,
each having sub categories. The simplest and cheapest type of a motor is a
DC permanent magnet motor.
DC Motors
A simple DC motor has a wound rotor and a permanent magnet stator. This
means that the rotating portion of the motor (the rotor) has the wound coils,
which produce magnetic fields. The outside of the motor contains a magnet,
which attracts and repels the windings to cause rotation.
The poles of the coils (electromagnets) are changed by the polarity of the
voltage applied to them. The current through the windings is reversed via the
brushes. The brushes complete the electrical connection from the stationary
stator to the rotating rotor coils.
2
AC Motors
AC motors function similarly to the DC motor except that it is not necessary
to reverse the polarity of the windings; AC electricity reverses its own
polarity. The brush arrangement for AC motors is different than that of DC
motors.
Stepper Motors
A stepper motor does not automatically reorient its magnetic field to achieve
continuous rotation. A stepper motor is designed to achieve alignment of its
magnetic fields at fixed intervals through its rotation. A controller tells the
magnetic fields when to shift and 'take a step'. Typical stepper motors have a
resolution of 7.5° per step resulting in 48 discrete angular positions of the
motor shaft. Controlling speed of a stepper motor is a trivial operation.
Simply step the coils at a set frequency and the speed of rotation becomes
fixed. If a 7.5°/step stepper motor needed to run at 1 revolution per second
then it would have to take 48 steps/second.
Stepper motors are excellent for light duty position control. If a stepper
motor was presented with a high torque fighting shaft rotation then it is likely
that the motor will 'miss-step.' When a stepper miss-steps it will typically fall
backward 3 steps instead of moving ahead one. The reason it falls back is
because it is not strong enough to move forward so it aligns itself with an
alternate set of coils. A good stepper motor controller is capable of detecting
a miss-step and indicating a fault. When a fault occurs the controller shuts
down and waits for a user to reset its operation. A fault indicates that the
controller is uncertain where the motor shaft is located.
3
Servo Motors
A servomotor is used for position and or speed control of the motor shaft.
Servomotors typically have a shaft encoder on a DC motor, which senses its
angular position and sends it to a controller, which adjusts the current driving
the motor in order to achieve a desired position or speed. This creates a
closed loop system. Stepper motors are an open loop system since it is told to
take a step, and it is expected to do so but without added sensors there is no
way to know for sure (there is no feedback information). A closed loop
controller sends a signal to drive a motor, senses what happens to it and
adjusts its drive signal based on the desired position or speed of the motor at
any given moment. A servomotor is a general term for several different
control methods. In industry a servomotor would indicate a high precision,
high performance motor with very precise positional sensing and complex
control circuitry.
Another, simple type of servomotor is one used by remote control hobbyists.
An RC servomotor, like a stepper, is designed to move to fixed angular
positions. This type of servomotor is based upon a simple DC motor, which
uses some decoding electronics in order to achieve positional control. A
control signal is an AC wave at a certain frequency. The frequency of the
signal, or signal wave shape (pulse width) is what commands the position of
the servomotor.
Radio Controlled (RC) servos are usually not intended to rotate continuously,
but instead to position themselves at discrete angular locations throughout one
revolution. Remote control cars use servos to control the turn angle of the
front wheels. RC airplanes use servos to control the angle of its wings and
tail flaps. RC helicopters use servos to control the pitch of its blades and
more. The reason RC devices use servos is because transmitters are designed
to send signals in certain frequency ranges. The transmitter signals can be
easily decoded to control servo devices.
Synchronous
Motors
Synchronous motors are a slightly more involved version of an AC motor.
An AC motor alters the strength of the magnetic field of its rotor based upon
the input voltage, the frequency at which the voltage changes commands the
speed of rotation.
Synchronous motors are designed to produce rotational speeds at exact
multiples of their input electrical frequency. A synchronous motor using
110V, 60 Hz AC may be designed to run at 120 Hz or 240 Hz instead of
simply 60 Hz. The positioning of the coils in the motor sets the speed.
4
LAB EXPERIENCE
Make sketches of equipment used in class; include them in your lab write-up.
Use Stepper
Motor



Determine a wiring diagram for the stepper motor provided.
Determine how the switches can be used to control the circuit
Connect the stepper motor to a 5V DC power supply and determine
the sequence of energizing, and de-energizing the coils to produce
rotation.
Questions
1) Show a wiring diagram of your stepper motor and use a table to show
the sequence of switch positions to control your motor for both,
clockwise and counter clockwise motion.
2) What is the resolution of the stepper motor you used? Explain how
many steps can be made within one revolution and how they can be
obtained.
DC Motors
Use the kit provided to build a simple DC motor. Make sure that it works
(have your work checked by a member of the instructional staff). Observe
how it runs and try to reverse it.
Questions
1. Draw the completed DC motor including the electrical connections
and the battery. Label all of the known parts. Indicate how this DC
Motor could be improved.
2. Is the DC motor you built capable of running at a variety of speeds?
How? Why or why not?
5
AC Motors
Control of a
Servo Motor
Inspect a simple AC motor. Do not attempt to run the motor (60 Hz is a
difficult speed to "jump start" the two coil motor). For an AC motor there is
no need to reverse the polarity of the connection, since AC electricity
switches its own polarity in the form of a sine wave. Notice that a different
type of brush configuration is used for an AC motor.
Servomotors adjust the angle of shaft rotation based upon the shape
(frequency and pulse width) of an electrical signal applied to them.
Servomotors are not meant to rotate continuously.
Use the micro controller, and its running software to control the servo.
 Make sure the Handyboard is attached to the serial port of the
computer.
 Run Interactive C (IC) on the PC to:
i. Download operating system: PCode
ii. Download library: servo.lis (or include it into lib_hb.c)
iii. Download code to control the servo motor: Motlab.c:
 Attach the servomotor to digital pin 9 on the Handyboard.
 Observe the servo and Controller (which ports are used?).
Make a sketch of the test setup. Use the potentiometer on the
controller to control the position of the servo’s rotor. Observe
the position on the controller’s LCD screen.
 Connect the oscilloscope across the two outer pins of the
servo’s terminal. Observe the control signal with an
oscilloscope. Determine the frequency of the signal and the
corresponding pulse width for a 0 degree angle.
 Get at least three data points that would allow you to plot Pulse
Width vs. Angle of Rotation.
Questions
Plot Pulse Width vs. Angle of Rotation. Determine a simple equation that
relates the pulse width of the input signal to the output angle. Be sure to
specify units.
6
Small
Commercial
DC motor
Connect the DC motor provided to a 5V DC power supply. See it run.
Estimate the RPM. How could you make an accurate measurement of the
speed of this motor? Decrease the voltage of the power supply so that you
can control the speed of the motor. Also, explain how the direction of
rotation can be reversed. Is the motor direct drive, or gear reduction?
LAB REPORT
Format



Lab report format (INDIVIDUAL or TEAM) will be announced in lab.
Follow the sample lab report format provided.
Nominally 4-5 pages (including figures and tables)
General
Guidelines



Cover page
Description of Experimental Apparatus
Introduction/Background

Description of motors with a sketch showing the wiring with notes about
positive and negative terminals and power supply.

Comments about how to improve both, the stepper motor and the DC motor
that was built.

Analysis of results/Summary.
7