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Faculty of Engineering, Architecture and Science

Department of Electrical and Computer Engineering

Course Number EES 612

Course Title Electrical Machines and Actuators Semester/Year

Instructor

Lab Report #

Lab Title

Lab Date

Lab Section

TA’s Name

Student Name Student ID Signature*

*By signing above you attest that you have contributed to this written lab report and confirm that all work you have contributed to this lab report is your own work. Any suspicion of copying or plagiarism in this work will result in an investigation of Academic Misconduct and may result in a “0” on the work, an “F” in the course, or possibly more severe penalties, as well as a Disciplinary Notice on your academic record under the Student Code of Academic Conduct, which can be found online at: www.ryerson.ca/senate/current/pol60.pdf.

Faculty of Engineering, Architecture and Science Department of Electrical and Computer Engineering

LAB INSTRUCTIONS

EES 612 – Electrical Machines and Actuators

Experiment # 4: DC Motor Control

1. Introduction Pulse-Width Modulation (PWM) is the most widely used method for controlling electric machines of DC or AC type. A few typical applications include: 1) speed control of vehicles, elevators and escalators, hoist and conveyors, and pumps and fans; 2) motion control of disc drive heads, robot arms, surgical tools, and machine tools; 3) current control of electromagnets and other static loads, and 4) illumination control of displays and monitors. The PWM method relies on periodic, intermittent (on-off), connection of the load to the power source. If the mentioned switching process is exercised fast enough, that is, if the switching period is sufficiently small, then an inertial load responds only to the average of the pulsating voltage and not to its fluctuations. Moreover, the average voltage can be controlled by the ratio of the “on time” to the switching period 𝑇. The ratio, 𝑑 = 𝑡𝑜𝑛/𝑇, is known as the “duty ratio” or “duty cycle”. Electronic semiconductor switches are employed due to their ease of control, high on/off speed, and long lifespan. This experiment is concerned with the PWM technique and its enabling solid-state hardware. 2. Pre-lab Assignment 2.1) For the H-bridge converter of Fig. 2.1, draw the missing waveforms in Fig. 2.2 and explain whether the switching process illustrated by Fig. 2.2 corresponds to a half- or to a full-bridge operation. Then derive an expression for the average of the load voltage 𝒗𝑳, as a function of 𝑑 = 𝑡𝑜𝑛/𝑇.

Fig. 2.1. H-Bridge Converter.

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Fig. 2.2. A possible switching scenario for the H-bridge converter.

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2.2) Now draw the missing waveforms of Fig. 2.3 and explain whether they correspond to a half-bridge operation or to a full-bridge operation. Then, derive an expression for the average of the load voltage 𝒗𝑳 as a function of 𝑑 = 𝑡𝑜𝑛/𝑇.

Fig 2.3. Another possible switching scenario for the H-bridge converter.

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3. Lab Work General Safety Note To prevent injury to persons or damage to equipment, the power source must be turned OFF prior to the completion (or change) of any circuit connections. Equipment

• L298 H-Bridge Driver Module (hereinafter, referred in this document to as the “H-Bridge Box”)

• Bench-top power supply • Function generator • 4-channel oscilloscope • 55-𝑚𝐻 inductor • 15-Ω Resistor • Digital multimeter • Tachometer

Experiments 3.1. Half-bridge energization of a resistive load 3.1.1) Connect the power supply to the terminals +12V and COM- on the rear panel of the H-

bridge box. Observe the polarities.

3.1.2) Connect the electrical terminals VA and VB of the H-bridge box to the terminals of the 15-Ω resistor. The resistor will act as the load in this experiment.

3.1.3) Set the toggle switches of the H-bridge box as per Table 3.1.1.

Table 3.1.1. Toggle switch positions for Experiment 3.1.

Toggle Switch Position 1 HB CW/HB CCW HB CW 2 Full/Half Half 3 Brake/Run Brake

3.1.4) Connect the TTL pulse output of the function generator to the BNC 𝒈𝟏of the H-bridge

box.

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3.1.5) Connect the oscilloscope channels to the other BNCs of the H-bridge box, according to Table 3.1.2 below. This is to monitor the signals 𝒈𝟏, 𝒗𝑨, 𝒗𝑩, and 𝒊𝑳, as labeled on the schematic diagram below:

Fig. 3.1.1. H-Bridge Converter.

Table 3.1.2. Signals to be monitored in Experiment 3.1. Signal BNC Oscilloscope Channel

1 𝒈𝟏 g1 Ch1 2 𝒗𝑨 VA Ch2 3 𝒗𝑩 VB Ch3 4 𝒊𝑳 IL Ch4

3.1.6) Turn on the power supply and set its output voltage to 15 V. This results in 𝑉𝐷𝐶 = 15 𝑉

for the H-bridge converter of Fig. 3.1.1.

3.1.7) Set the oscilloscope channels Ch1-Ch4 to the DC coupling mode and the oscilloscope trigger to Source 1. Turn on the function generator and set its frequency range to 10 kHz and its waveform to Square. Monitor the waveform 𝒈𝟏 (i.e., Ch1 of the oscilloscope). Set the switching period and duty ratio to 𝑇 = 0.2 𝑚𝑠 and 𝑑 = 0.5, respectively. Note that these two parameters correspond to a 5-kHz even-symmetrical pulse train for 𝒈𝟏.

3.1.8) Change the switch Brake/Run from “Brake” position to “Run” position and, using the

oscilloscope, monitor and plot on Fig. 3.1.2 the waveforms 𝒈𝟏, 𝒗𝑳 = 𝒗𝑨 − 𝒗𝑩, and 𝒊𝑳, for two periods. To monitor 𝒗𝑳 = 𝒗𝑨 − 𝒗𝑩, utilize the mathematical functionalities of the oscilloscope. Also, for Ch4, switch between DC coupling and AC coupling to monitor the DC offset and the fluctuations of 𝒊𝑳, respectively.

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Fig. 3.1.2. Load voltage and current waveforms for a duty ratio of 50%.

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3.1.9) Maintain the frequency, but change the duty ratio in steps of 0.1, according to Table

3.1.3. Using the multimeter, measure the corresponding DC (average) and AC voltages that drop across the resistor, and complete Table 3.1.3. To change the duty cycle, press down the SYMMETRY button on the function generator and then adjust the CENTER CAL SYMMETRY knob to obtain the desired duty ratio.

Table 3.1.3. Resistor voltage versus duty ratio, for 𝑇 = 1.0 𝑚𝑠. 𝑑

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

DC voltage across

the resistor

AC voltage across

the resistor

3.1.10) Turn off the function generator and the power supply. Change the switch Brake/Run

from “Run” position to “Brake” position.

3.2. Half-bridge energization of a resistive-inductive (RL) load with a low switching frequency 3.2.1) For this experiment, steps 3.1.1 through 3.1.6 (of Experiment 3.1) remain the same.

However, a 55-𝑚𝐻 inductor is added in series with the resistive load. Then, Turn on the function generator and, monitoring its TTL pulse train on Ch1 of the oscilloscope, set the switching period and duty ratio to 𝑇 = 1.0 𝑚𝑠 and 𝑑 = 0.5, respectively. Note that these two parameters correspond to a 1-kHz even-symmetrical pulse train.

3.2.2) Change the Brake/Run switch from Brake position to Run position and, using the oscilloscope, plot on Fig. 3.2.1 the waveforms 𝒈𝟏, 𝒗𝑳 = 𝒗𝑨 − 𝒗𝑩, and 𝒊𝑳, for at least two periods.

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Fig 3.2.1. Load voltage and current waveforms for a duty ratio of 50%, for 𝑇 = 1.0 𝑚𝑠.

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3.2.3) Maintain the frequency, but vary the duty ratio in steps of 0.1. Using the multimeter, measure the DC (average) and AC voltages across the resistor, and complete Table 3.2.1.

Table 3.2.1. Resistor voltage versus duty ratio, for the RL load and 𝑇 = 1.0 𝑚𝑠. 𝑑

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

DC voltage across

the resistor

AC voltage across

the resistor

3.3. Half-bridge energization of a resistive-inductive (RL) load with a high switching frequency

3.3.1) Repeat Experiment 3.2, but for 𝑇 = 0.2 𝑚𝑠 (corresponding to a switching frequency of 5 kHz). Reflect the results on Fig. 3.3.1 and Table 3.3.1.

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Fig 3.3.1. Load voltage and current waveforms for a duty ratio of 50%, for 𝑇 = 0.2 𝑚𝑠.

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Table 3.3.1. Resistor voltage versus duty ratio for the RL load and 𝑇 = 0.2 𝑚𝑠. 𝑑

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

DC voltage across

the resistor

AC voltage across

the resistor

3.3.2) Change the switch Brake/Run from “Run” position to “Brake” position.

3.4. Full-bridge energization of a resistive-inductive (RL) load For this experiment, the test setup is the same as that in Experiment 3.3. Also, monitoring Ch1 of the oscilloscope, set the switching period to 𝑇 = 0.2 𝑚𝑠 (corresponding to a 5-kHz pulse train). However, in contrast to Experiment 3.3, leave the Full/Half toggle switch at “Full” position.

3.4.1) Change the switch Brake/Run from “Brake” position to “Run” position, and monitor

the waveforms 𝒈𝟏, 𝒗𝑳 = 𝒗𝑨 − 𝒗𝑩, and 𝒊𝑳, for at least two periods, for 𝑑 = 0.1, 𝑑 = 0.5, and 𝑑 = 0.9. For the three aforementioned values of duty cycle, plot the corresponding set of waveforms on Fig. 3.4.1, Fig. 3.4.2, and Fig. 3.4.3, respectively.

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Fig. 3.4.1. Load voltage and current waveforms for a duty ratio of 10%.

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Fig. 3.4.2. Load voltage and current waveforms for a duty ratio of 50%.

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Fig. 3.4.3. Load voltage and current waveforms for a duty ratio of 90%.

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3.4.2) Maintain the frequency, but change the duty ratio in steps of 0.1, according to Table 3.4.1. Use the multimeter and measure the corresponding DC (average) and AC voltages that drop across the resistor; complete Table 3.4.1.

Table 3.4.1. Resistor voltage versus duty ratio, for 𝑇 = 0.2 𝑚𝑠. 𝑑

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

DC voltage across

the resistor

AC voltage across

the resistor

3.4.3) Turn off the function generator and the power supply. Change the switch Brake/Run from

“Run” position to “Brake” position.

3.5. Full-bridge energization of a DC motor For this experiment, the test setup is the same as that in Experiment 3.4, with the exception that the RL load is replaced with a DC motor. The DC motor is a part of the H-bridge box, and its shaft has protruded out of the front panel of the H-bridge box. To energize the motor, connect by short wires the electrical terminals VA and VB to the electric terminals MTR+ and MTR-, respectively.

3.5.1) Turn on the power supply and the function generator. Change the switch Brake/Run from “Brake” position to “Run” position, and monitor the waveforms 𝒈𝟏, 𝒗𝑳 = 𝒗𝑨 −𝒗𝑩, and 𝒊𝑳, for at least two periods, for 𝑑 = 0.1, 𝑑 = 0.5, and 𝑑 = 0.9. For the three aforementioned values of duty cycle, plot the corresponding set of waveforms on Fig. 3.5.1, Fig. 3.5.2, and Fig. 3.5.3, respectively.

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Fig. 3.5.1. DC motor armature voltage and current waveforms, for a duty ratio of 10%.

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Fig. 3.5.2. DC motor armature voltage and current waveforms, for a duty ratio of 50%.

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Fig. 3.5.3. DC motor armature voltage and current waveforms, for a duty ratio of 90%.

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3.5.2) Slowly change the duty ratio from its minimum (of approximately zero) to its maximum (of approximately unity) and observe that: a) The motor stops at 𝑑 = 0.5. b) The direction of rotation is different for the values of d larger than 0.5 compared

to those smaller than 0.5. c) The motor runs faster (irrespective of direction) as d differs more from 0.5.

3.5.3) Change the duty ratio in steps of 0.1 and, using the tachometer, measure the

corresponding shaft speeds. Complete Table 3.5.1.

Table 3.5.1. DC motor speed versus duty ratio.

𝑑

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

motor speed in

rpm

3.5.4) With the duty ratio set at 𝑑 = 0.9 (or, alternatively, at 𝑑 = 0.1), corresponding to a fast rotation, change the Brake/Run switch to the “Brake” position and observe the response of the DC motor.

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4. Conclusions and Remarks 4.1) Comment on the AC and DC voltage measurements reported in the Tables 3.1.3, 3.2.1, 3.3.1, and 3.4.1.

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4.2) Compare the DC voltage measurements you reported in Tables 3.3.1 and 3.4.1 with the values that their formulas (that you derived in the Pre-Lab part) predict. Provide reasons for the discrepancies.

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4.3) Comment on the measurements of Table 3.5.1.

Design of experiment and development of manual: A. Yazdani Design and development of equipment: A. Yazdani and J. Koch

Last Updated June 19, 2013—AY

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