Paper ID #26561
Low Cost Experimental Setup for Validating Motor Torque
Dr. Robert Weissbach P.E., Indiana University Purdue University, Indianapolis
Robert Weissbach is currently chair of the department of engineering technology at IUPUI. From 1998 - 2016 he was with Penn State Behrend as a faculty member in Electrical and Computer Engineering Technology. His research interests are in renewable energy, energy storage and engineering education. Mr. Koty Jarrod Miles, Indiana University Purdue University, Indianapolis
My name is Koty Miles. I am a student at IUPUI and before that I was a PLTW student. The major I am seeking is Computer Engineering Technology.
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Low Cost Experimental Setup for Validating Motor Torque
AbstractTypical courses on electric machines discuss the electromagnetic principles that govern the operation of various ac and dc machines. Equations are provided that build on prior circuit analysis concepts. These equations are often used to ultimately determine torque speed characteristics a given machine of, a given type of machine. However, in the workplace graduates are usually trying to figure out what size machine to use for a given application. Typically, the torque and speed of the machine are needed, as well as the size. To measure the torque, additional equipment is needed which can be quite expensive.
In this paper, a low-cost alternative is presented to give sophomore level electrical engineering technology students a visual understanding of the torque provided by a stepper motor. A steel connecting rod is mounted to the shaft of the motor. The rod can be mounted such that
metrology weights can be attached to the rod, allowing the torque applied to the motor to be adjusted.
Introduction
Rotating electrical machines are employed in industry to provide torque at a given speed for conveyor belts, saws, pumps, and a host of other applications. Their understanding is critical for most electrical engineering and electrical engineering technology students pursuing a career in power systems, industrial automation and control, and manufacturing. Introductory courses in electrical machines are often comprised of the following topics:
Magnetic circuits to demonstrate the fundamental electromagnetic principles that govern the relationship between magnetic and electric fields. This introduction to
electromagnetic principles will then be used to explain the operation of all subsequent devices.
Transformer design and operation
Induction motor design and operational characteristics, which includes the traditional torque-speed curve but may also include:
o Variable frequency drives (VFDs) and an introduction to power electronic inverters
o Different motor classes (A, B, C, etc.)
o Testing to determine various motor characteristics, and o Implications of squirrel cage versus wound rotor design Synchronous machine design
If time permits, some courses will explore one or more of the following: dc machines (brushed and/or brushless)
reluctance machines universal machines servomotors
stepper motors linear machines
A variety of textbooks have been employed to teach students the theory of electrical machines [1 – 5]. This list is not comprehensive.
The material in electrical machine courses can be difficult for students to fully comprehend. Two potential reasons exist. First, students do not typically build a rotating electrical machine in the course, as compared to courses in electric circuits where the circuit to be built is constructed from existing parts and connecting wires that students can assemble. Some instructors will have students build a transformer during the course [6], which is a non-rotating machine. Others have had the students build a simple dc motor [7] to demonstrate the application of the basic
electromagnetic principles to yield rotational motion. But a practical rotating electrical machine is complex to construct, given the need for bearings, insulation, balancing for vibration, and design for appropriate cooling.
Second, the laboratory materials required to demonstrate the operation and machine
characteristics can be prohibitively expensive, with some systems costing over one hundred thousand dollars to outfit a lab (see [8, 9] for example). These systems enable students to measure speed, torque, voltage, current and other parameters as necessary so students can better understand the relationship between the variables and the machine operation. But the cost can be sufficiently high to prevent their purchase by college administrators. As compared to general purpose laboratory equipment, this equipment is often used only once or twice each academic year, making a large purchase difficult to justify.
Considering the importance of electric machines to engineering and manufacturing, it is important for students to have foundational knowledge that will allow graduates to properly employ electric machines, especially motors, once they are hired. Foundational knowledge in the electromagnetic principles help students understand how the machines work. But in the workforce, graduates will need to understand how to choose a given motor for a particular application. This means answering questions such as:
Which type of motor is the most appropriate for the given application? At what speed(s) should be motor be operating?
What torque is required for the application?
What environmental issues need to be considered in selecting the motor?
An initial foray into motor selection was briefly discussed as part of a larger discussion involving working with industrial partners [10]. In that project, a capstone design team of Mechanical Engineering Technology (MET) and Electrical Engineering Technology (EET) students designed and constructed a water pumping system with a Programmable Logic Controller (PLC) where students could estimate which size piping and which motor would be best to fill up a tank in a given period of time. However, the cost and complexity of that project may make it unsuitable for replication at other institutions.
Given that graduates employed in engineering and manufacturing positions will need to design solutions for the problems they encounter, motor selection should be an important component of a course in electrical machines. There should be cost-effective opportunities for students to not only analyze electrical machines, but also to employ them and determine if their selection is appropriate.
It is expected that a substantial effort will be required to develop motor selection lecture and laboratory materials. As an initial step, the authors have attempted to develop a simple approach to provide a torque on a rotating machine and attempt to validate whether the machine can handle the rated torque listed on its data sheet.
Laboratory Setup
Figures 1a and 1b show the setup for the system. A stepper motor has been employed due to its low cost (under $20). Stepper motors are used in robotics and printing applications, making them both accessible to students and easy to use if the students have already had some
background using a microcontroller such as an Arduino, Raspberry Pi, Pic, or other embedded system.
Figure 1b
Figure 1. Overall System. Figure 1a shows all parts of the system, while Figure 1b focuses on the wiring of the H-bridge driver
The system in the above figure consists of the following principle subsystems:
A stepper motor, part number 23KM-K005-34U, made by Minebea. It has 760mNm holding torque (107.63 oz-in, per [11]). The actual torque-speed curve can be found from [12]. Although the specific model number is not listed, given the holding torque value provided above, as well as the current rating of 1.5A, part number 23KM-K055U is most likely the part purchased. The distributor specification sheet is provided in Appendix A. An Arduino Uno microcontroller. The Arduino provides the logic sequence (see [13]) to
drive the stepper motor
An L293NE dual H-bridge motor driver to take the logic sequence from the Arduino and provide the power to spin the stepper motor. Its data sheet can be found at [14].
An aluminum bar attached to the center of the shaft to provide a balanced load on each end of the shaft. Holes have been drilled at 1.5” intervals to allow for the addition of metrology weights using hooks or other means. The overall length of the bar is 4.5”, and its weight is 1.6oz, yielding a torque component at its halfway length of 3.6oz-in.
Figure 2. Wiring Schematic Results
A variety of metrology weights were added to the bar at different locations to test whether the motor would spin. The Arduino code to provide the signals to the H-bridge is provided in Appendix B. Given a delay after each set of signals (high or low) is provided to the H-bridge, and given that the stepper motor steps at 1.8 each step, after each sequence of four steps the motor will have traversed 7.2 before the code repeats. Given that there are 360 per motor revolution, the motor speed (neglecting machine cycles to implement the code) can be calculated as follows:
𝑛 𝑟𝑒𝑣/𝑚𝑖𝑛 =
60𝑠𝑒𝑐 min
(𝑇 𝑠𝑒𝑐𝑠𝑡𝑒𝑝 ) (sequence) (4steps 50sequencesrev )
= (0.30/𝑇)𝑟𝑒𝑣/𝑚𝑖𝑛
Where: n = the speed in rpm of the stepper motor T = delay coded in the Arduino for each step
The Arduino was first coded using a delay per step of 10ms to yield a motor rotational speed of 0.5Hz (30rpm), which is well below the motor frequency where the rated torque begins to drop off as indicated in [11]. The results are provided in Table 1. Green colored cells indicate the motor was able to spin, orange colored cells indicate the motor would stutter at startup and then spin freely, while red colored cells indicate that the motor would eventually spin but would stutter the entire time.
When compared to the rated torque of the motor, it is clear that the stepper motor at 30rpm is able to handle torque well above the maximum rated torque listed in [11]. It was not until a
100oz load was applied at 4.5” that the motor was unable to supply the necessary torque to spin. The stepper motor was able to handle four times its rated torque before stalling.
Table 1. Table of Results for Various Weights Attached to the Bar at 30rpm. 30 RPM Weight (oz) Length (in) 1.8 3.5 7.1 7.1 17.6 17.6 1.8 3.5 3.5 7.1 7.1 17.6 2.25 11.7 19.35 27.45 35.55 59.175 82.8 Key:
Spins with no problem
Stutters at startup and then spins with no problem Spins, but stutters the entire time
When the delay was modified to 8ms, yielding a motor speed of 37.5rpm, the motor was able to handle less torque, as shown in Table 2.
Table 2. Table of Results for Various Weights Attached to the Bar at 37.5rpm. 37.5 RPM
Weight (oz)
Length (in) 1.8 3.5 7.1 7.1 17.6 17.6
1.8 3.5 3.5 7.1 7.1 17.6
2.25 11.7 19.35 27.45 35.55 59.175 82.8
Finally, when the delay was modified to 6ms, yielding a motor speed of 50rpm, the motor was able to handle even less torque, as shown in Table 3.
Table 3. Table of Results for Various Weights Attached to the Bar at 50rpm. 50 RPM
Weight (oz)
Length (in) 1.8 3.5 7.1 7.1 17.6 17.6
1.8 3.5 3.5 7.1 7.1 17.6
2.25 11.7 19.35 27.45 35.55 59.175 82.8
At the 50rpm speed, the table in [11] provides information on the torque capability of the motor, and the data of Table 3 may or may not corroborate the torque capability of the motor, since the stuttering is probably not an acceptable initial response when powered. In this case, one could state that the motor was unable to meet its rated torque requirement, but it is also noted that the motor has been subjected to a large number of tests that may have reduced its overall torque capacity. Testing on another motor might be necessary to determine whether results on this motor could be deemed reliable.
Student Questions
A set of student questions have been provided in Appendix C. These questions are broken up into three parts:
Questions about the stepper motor
Questions about the control of the stepper motor A question about the use of a stepper motor in a design
Unfortunately, the assignment was not presented to the students. But these questions would be a first step to gauge students’ understanding of the material.
Conclusion
A simple approach of testing the torque capability of a stepper motor has been presented. Although a lab has not been developed as yet, one possibility is to have the students attempt to validate the torque level at 50rpm on the specification sheet. This would require the students to do the following:
Implement the motor speed control sequencing on the Arduino Construct the H-bridge circuit and spin the motor
Determine the motor torque at 50rpm, given that the specification sheet doesn’t specifically list 50rpm on the logarithmic x-axis
Determine the delay necessary to achieve a 50rpm motor speed Determine the weights necessary to check the torque of the motor Compare with the motor torque that could be accomplished at 30rpm
The design of the system was improved by centering the bar on the motor, versus originally having the weights on only one end of the bar. This resulted in a balanced motor and the bar now has little impact on the torque applied. This would likely make for a better testing system, as only the metrology weights would impact the motor torque. This will be investigated further. References
[1] Hubert, Charles I., Electric Machines: Theory, Operating Applications, and Controls (2nd Edition), ISBN-13: 978-0130612106, Pearson, 2001
[2] Chapman, Stephen J., Electric Machinery Fundamentals (4th Edition), ISBN-13: 978-0072465235, McGraw Hill, 2003
[3] Wildi, Theodore, Electrical Machines, Drives and Power Systems (6th Edition), ISBN-13: 978-0131776913, Pearson, 2005
[4] Gross, Charles A., Electric Machines (Electric Power Engineering Series), ISBN-13: 978-0849385810, CRC Press, 2006
[5] Karady, George G., and Holbert, Keith E., Electrical Energy Conversion and Transport: An Interactive Computer-Based Approach (2nd Edition), ISBN-13: 978-0470936993, Wiley-IEEE Press, 2013
[6] Jewell, Ward T., "Transformer design in the undergraduate power engineering laboratory," in IEEE Transactions on Power Systems, vol. 5, no. 2, pp. 499-505, May 1990
[7] Marshall, J., Reinforcing Induction Motor Principles Via Material Technology
Experiments Paper presented at 2004 Annual Conference, Salt Lake City, Utah.
https://peer.asee.org/13550, June 2004
[8] U.S. Didactic, website http://www.usdidactic.com/ [9] Labvolt, website https://www.labvolt.com/
[10] Weissbach, R. S., Snyder, J. W., Evans, E. R., Jr, & Carucci, J. R. (2017). “Industrial sponsor perspective on leveraging capstone design projects to enhance their business.” American Journal of Engineering Education, 8(1), 13-22.
[11] Website
https://www.eminebea.com/en/product/rotary/steppingmotor/hybrid/standard/23km-k.shtml [12] Website https://www.convertunits.com/from/N-m/to/oz-in
[13] Website http://www.tigoe.com/pcomp/code/circuits/motors/stepper-motors/ [14] Website http://www.ti.com/lit/ds/symlink/l293d.pdf
Appendix A
Appendix B
Arduino Code
/*
Stepper Motor Control
This function provides control signals and delays to spin a stepper motor
The motor speed n = 0.3/Delay , where n is the motor speed in rpm and Delay is the delay of each step in seconds.
*/
int delay1 = 10; // delay between changing each group of logic signals // the setup function runs once when you press reset or power the board void setup() {
// initialize the control signal to the H-bridge as outputs. pinMode(8, OUTPUT); //Switch1
pinMode(9, OUTPUT); //Switch2 pinMode(10, OUTPUT); //Switch3 pinMode(11, OUTPUT); //Switch4 }
// the loop function runs over and over again forever void loop() {
// Sequencing to run the stepper motor digitalWrite(8, HIGH); // Switch1 High digitalWrite(9, LOW); // Switch2 Low digitalWrite(10, HIGH); // Switch3 High digitalWrite(11, LOW); // Switch4 Low delay(delay1);
digitalWrite(8, LOW); // Switch1 Low digitalWrite(9, HIGH); // Switch2 High digitalWrite(10, HIGH); // Switch3 High digitalWrite(11, LOW); // Switch4 Low delay(delay1);
digitalWrite(8, LOW); // Switch1 Low digitalWrite(9, HIGH); // Switch2 High digitalWrite(10, LOW); // Switch3 Low digitalWrite(11, HIGH); // Switch4 High delay(delay1);
digitalWrite(8, HIGH); // Switch1 High digitalWrite(9, LOW); // Switch2 Low
digitalWrite(10, LOW); // Switch3 Low digitalWrite(11, HIGH); // Switch4 High delay(delay1);
Appendix C
Student Questions
ECET 23110 Part 1 – Stepper Motor Questions
1) What was the tested limit of the motor in ounce-inches (oz-in) at 50 RPM?
2) What is the spec limit of the motor in millinewtonmeters (mNm) at 50 RPM? 760 mNm
3) How would you convert mNm to oz-in? What is the spec limit of the motor in oz-in? 0.278 Newtons = 1 ounce 1 m = 0.03937 inches 760 𝑚𝑁𝑚 ∗ 1 𝑜𝑧 0.278 𝑁∗ 0.03937 𝑖𝑛 1 𝑚𝑚 = 107.63 𝑜𝑧𝑖𝑛
Part 2 – Stepper Motor Control Questions
1) Given that the code for the stepper motor turns the motor 1.8 degrees with every instruction and that 4 turning instructions are sent before repeating, how many times must these instructions be sent to the motor to complete a cycle?
1.8 * 4 = 7.2 degrees
360 / 7.2 = 50 times per cycle
2) How many seconds does a single cycle take if there is a 10 ms delay between each turning instruction? What is the frequency?
10 * 4 = 40 ms every time the code repeats 40 * 50 = 2000 ms = 2 seconds per cycle ½ = 0.5 Hz
3) Use the values found to calculate the revolutions per minute (RPM) of the motor. 1 𝑐𝑦𝑐𝑙𝑒
2 𝑠𝑒𝑐𝑜𝑛𝑑𝑠∗ 60𝑠
1 𝑚𝑖𝑛= 30 𝑅𝑃𝑀
4) Find the RPM of the motor with a 6 ms delay 6 * 4 = 24 ms every time the code repeats 24 * 50 = 1200 ms = 1.2 seconds per cycle 1/1.2 = 0.833 Hz
1 𝑐𝑦𝑐𝑙𝑒 1.2 𝑠𝑒𝑐𝑜𝑛𝑑𝑠∗
60𝑠
1 𝑚𝑖𝑛= 50 𝑅𝑃𝑀
Part 3 – Stepper Motor Design
1) Given the stepper motor provided here, assume the stepper motor is being used to transfer a part from one conveyor to another conveyor at the same height. The 8” arm attached to the motor has its 5oz weight evenly distributed along the length of the arm. Assume the part needs to be transferred in a semicircular arc between the two conveyors within 60ms. How much can the part weigh to be moved?
60ms/half-revolution = 120ms/rev = 8.33rev/s = 500rev/min = 500rpm
The torque of the arm is (4”)(5oz) = 20Oz-in
This means that there is 43.73Oz-in of torque the motor can still provide for the part itself. At an 8” length, the part can weigh no more than (43.73Oz-in/8in) = 5.47Oz
