EGH166 Hands-on Lab

Lab 5: 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)            Electric Motors,

2)            DC Motors,

3)            AC Motors,

4)            Stepper Motors,

5)            Servo Motors, and

6)            Synchronous Motors

Lab Experience

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.

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.

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.

Servo Motors

A servo motor is used for position and or speed control of the motor shaft. Servo motors 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 servo motor is a general term for several different control methods. In industry a servo motor would indicate a high precision, high performance motor with very precise positional sensing and complex control circuitry.

Another, simple type of servo motor is one used by remote control hobbyists. An RC servo motor, like a stepper, is designed to move to fixed angular positions. This type of servo motor 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 servo motor.

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.

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 an undergraduate TA). 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?

AC Motors

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.

Control of a Servo Motor

Servo motors adjust the angle of shaft rotation based upon the shape (frequency and pulse width) of an electrical signal applied to them. Servo motors 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:

·        Attach the servo motor 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.

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 reports must be done as a TEAM

·   Follow given lab report format.

·   Maximum 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.