j.bodak, m.malecha elmech lab1

Faculty of Engineering, Architecture and Science

Department of Electrical and Computer Engineering

Course Number ELE847

Course Title Advanced Electromechanical Systems

Semester/Year FALL 2011

Instructor Venkata Yaramasu

ASSIGNMENT No. 1

Assignment TitleDC Motor Dynamic Performance and

Solid State Rectifiers

Submission Date 4.10.2011

Due Date 4.10.2011

Student Names Jaroslaw Bodak Michal Malecha

Student ID 500481729 500481424

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

EE8410 Power Electronics - F2011 Toronto, 18.10.2011

Code of Academic Conduct, which can be found online at: www.ryerson.ca/senate/current/pol60.pdf.

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http://www.ryerson.ca/senate/current/pol60.pdf

LAB1: DC Motor Dynamic Performance and Solid State Rectifiers

1. Abstract

The purpose of this lab is to study dynamic performance of the separately excited DC motor and characteristics of the diode and thyristor three-phase rectifiers. The range of the lab includes the construction of a Simulink models of the motor and rectifiers as well as investigation of obtained waveforms.

2. Theory

Separately excited DC Motor under investigation is a motor which has two separate circuits. The main terminals convey the ‘power’ or ‘work’ current to the brushes which make sliding contact to the armature winding on the rotor. The smaller pair of terminals connect to the field windings, which surround each stator’s pole. In the steady state all the input power to the field windings is converted into heat – it does not produce mechanical output power. Current through field windings produces constant magnetic field, which is necessary to induce voltage (ea) in the armature. The supply to the field is separate from that for the armature, hence the description ‘separately excited motor’ [3]. Torque in DC motor is produced by the interaction between the axial current-carrying conductors on the rotor and the radial magnetic flux produced by the stator. The flux or excitation is provided by field windings in this case, but it can be also produced by permanent magnets.

Separately excited DC motors have wide range of application, especially in the drives of machine tools, pumps, fans - wherever the high range of motor speed regulation is required.

In the recent years, DC motors are increasingly being supplied with power-electronic drives, which draw power from the common ac grid and convert it to dc power feeding the motor.

Since the mains voltage are standardized (110V, 220-240V, 380-400V, by f=50-60Hz), motors are made with rated voltages which match the range of dc outputs from the converter [3].

In the part B of the Lab1 a three phase diode rectifier has been investigated. The three-phase bridge has 6 diodes total, which conduct in pairs – one in the top half of the bridge and one from a different leg in the bottom half.

Thereby, there are six pulses of the output voltage per mains cycle. The more pulses of output voltage the more smoothness waveforms - the lower ripple voltage. Therefore, in some application even a six-pulse waveform is not smooth enough and some large drive converters must consist of two six-pulses converters with their output in series [3], which give smoother, 12-pulse ripple voltage.

Average output voltage V dat the output of three-phase diode rectifier is constant and given by:

V d=2.34V a (rms)

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Unfortunately, the three-phase diode rectifier is not able to regulate the average output voltage, thus it is not controllable. It means that the diode rectifier is not able to regulate a speed of a motor. A thyristor rectifier holds this ability – it is fully controlled. The topology is similar – only the valves are different. The average output voltage V dat the output of three-phase thyristor rectifier is adjustable by a thyristor firing angle α and is given by:

V d=2.34V a ( rms )cos (α )

This ability is a huge advantage of thyristor rectifier, allowing to control speed of a motor from 0 ÷ ωn.

The simulation models has been created on the base of guidelines contained in [1]and [2]. They are shown in appendix in Fig.7.1 - 7.6.

3. Simulation waveforms

A) The following waveforms showing dynamic performance of the motor during starting has been obtained in accordance with requirements in [1].

A1) Plots obtained at the following assumption: Va=50V, Vf=240V [1]:

Fig.A1. Armature current plotted against time with marked maximum value during starting in open-circuit (no load) and Va=50V, Vf=240V conditions

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Fig.A2. Electromagnetic torque plotted against time in open-circuit and Va=50V, Vf=240V conditions

Fig.A3. Motor speed plotted against time in open-circuit and Va=50V, Vf=240V conditions

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A2) Plots obtained at the following assumption: Va=240V, Vf=240V [1]

Fig.A5. Armature current plotted against time with marked maximum value during starting in open-circuit and Va=240V, Vf=240V conditions

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Fig.A6. Electromagnetic torque plotted against time in open-circuit and Va=240V, Vf=240V conditions

Fig.A7. Motor speed plotted against time in open-circuit and Va=240V, Vf=240V conditions

B) This paragraph contain waveforms of solid-state rectifiers: B1) refers to diode rectifier, whereas B2) refers to thyristor rectifier.

B1) Waveforms of the three-phase diode rectifier:

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Fig.B1.1. AC power supply voltage of a-phase A plotted against time

Fig.B1.2. Positive half of the rectified voltage vd1 plotted against time

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Fig.B1.3. Negative half of the rectified voltage vd2 plotted against time

Fig.B1.4. Rectified output voltage vdc = vd1 ‐ vd2 plotted against time

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Fig.B1.5. Rectified output current

B2) Waveforms of the three-phase thyristor rectifier:

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Fig.B2.1. AC power supply voltage of a-phase plotted against time for α=30°

Fig.B2.2. Positive half of the rectified voltage vd1 plotted against time for α=30°

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Fig.B2.3. Negative half of the rectified voltage vd2 plotted against time for α=30°

Fig.B2.4. Rectifier output voltage vdc = vd1 ‐ vd2 plotted against time for α=30°

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Fig.B2.5. Rectifier output current for α=30°

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Fig.B2.6. AC power supply voltage of a-phase plotted against time for α=90°

Fig.B2.7. Positive half of the rectified voltage vd1 plotted against time for α=90°

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Fig.B2.8. Negative half of the rectified voltage vd2 plotted against time for α=90°

Fig.B2.9. Rectifier output voltage vdc = vd1 ‐ vd2 plotted against time for α=90°

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Fig.B2.10. Rectifier output current for α=90°

4. Calculation results

The maximum starting current (inrush current) Ia,max for rated voltage Va=240V:

I a ,max=V aRa

=240V0.6Ω

=400 A

Ratio of the maximum starting current Iamax to the rated current In:

k=I a , maxI n

= 400 A16.2 A

≈24,7

Inrush current limitation to 30A by adding starting resistor Rst in armature circuit:

I a ,max=V a

Ra+Rst❑⇒

Ra+Rst=V aI a , max

R st=V aI a ,max

−Ra=240V30 A

−0,6Ω=7,4Ω

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Rated torque Te,n:

T e, n=Pnωn

= 5 ∙746

1220 rpm∙2π60

≈29,2Nm

Average value of the 3-phase diode rectifier output voltage:

V d=2.34V a=2.34 ∙110V=257.4V

Average value of the 3-phase diode rectifier output voltage on the base of

obtained waveforms (fig.B1.2., B1.3):

vdc=vd1 ‐ vd2=128V− (−128V )=256V

Derivation of average value of the 3-phase thyristor rectifier output voltage:

V d=3π

∫30°+α

90°+α

√2V absin (ωt+α )dωt

Using trigonometric identity and executing further transformation the following

equiation can be obtained:

V d=3√2πV abcosα=2.34V a cosα

Average value of the 3-phase thyristor rectifier output voltage for:

V d=2.34V a cosα

Average value of the 3-phase thyristor rectifier output voltage for α=30°:

V d=2.34V a cos (30 ° )=222.9V

Average value of the 3-phase thyristor rectifier output voltage for α=90°:

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V d=2.34V a cos ( 90° )=0V

5. Conclusions

The simulation of dynamic performance of separately excited DC motor highlights a very important issue, which is an appropriate motor starting. Direct start results in incredibly high inrush current, which leads to destroying the power converter feeding the machine, before destroying the machine itself. If such attempt would be executed in real world, the power converter certainly will be destroyed and the machine will be exposed to a huge stress – electrical (due to a huge current) and mechanical (due to a huge torque and speed) – that could result in its failure either.

Hence, performing a proper starting process is crucial. There are generally two methods consisting in reduction the inrush current by reduced armature voltage during the starting process. The first method involve adding a starting resistor Rst in the armature circuit. The value of resistance is selected such that inrush current is reduced to the double value of the rated armature current (2Ian). The second method consists in slowly increasing armature voltage from 0V to the rated armature voltage Van. by adjustable dc power supply, for example 3-phase thyristor rectifier. This methods are safe for a machine as well as for a converter and are widely used.

The investigated model of a 3-phase diode rectifier works properly, thus it has been developed correctly. The value of obtained average dc voltage at the output equals to the calculated from formula value. However, the output voltage cannot be adjustable by the diode rectifier – it is constant. The thyristor rectifier gains the ability to adjust the output voltage - it is fully controlled. The average value of rectified output voltage depends on the firing angle α. For α=0° the thyristor rectifier operates like a diode rectifier, whereas increasing of the α results in decreasing output voltage, so that at α=90° - vdc=0. Further α increase leads to an increase of the voltage in the negative values, so that at α=180° the output of thyristor rectifier equals to the minus value of diode rectifier (vdcTR, α=180° = - vdcDR).

The ability of the thyristor rectifier to adjust output voltage is commonly used in a dc motors starting and regulating theirs speed.

6. References

[1] ELE847 Course Manual, Ryerson University, 2011 Edition

[2] ELE847 Lecture, Topic 1 DC Motor Dynamics and Speed Control, Ryerson University, 2011 Edition

[3] Austin Hughes, Electric Motors and Drives. Fundamentals, Types and Applications. Third edition, 2006.

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7. Appendix, Others

Fig.7.1. Simulink model of a separately excited DC motor

Fig.7.2. Subsystem of a separately excited DC motor

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Fig.7.3. Simulink model of a three phase diode rectifier

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Fig.7.4. Subsystem of a three phase diode rectifier

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Fig.7.5. Simulink model of a three phase thyristor rectifier

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Fig.7.6. Subsystem of a three phase thyristor rectifier

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j.bodak, m.malecha elmech lab1

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