SCHOOL OF ENGINEERINGDEPARTMENT OF ELECTRICAL, ELECTRONIC AND COMPUTER ENGINEERINGSTUDY MANUALELECTRIC DRIVES ETE 780## COMPILED byProf. Michael Njoroge Gitau## Date of last revision:## June 2008Copyright reserved
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Lecturer: Prof. Michael Njoroge Gitau Qualifications: B Sc. (Hons) (Nairobi), PhD (Loughborough) Office: Room 14-9 Engineering 1; Main Campus Mail Address: Dept. of Electrical, Electronic and Computer Engineering University of Pretoria Pretoria, 0002 Telephone: (012) 420-2989 (W) Facsimile: (012) 362 5000 (W) E-Mail: mgitau@postino.up.ac.zaConsulting Hours: By appointment Secretary: Me. Heleen Gous Tel: 420-2190 Fax 362 5000 E-mail hgous@postino.up.ac.za Office: Room 14-26 Engineering Tower Block; Main Campus - ^
## 2.1 Prescribed TextbookThe prescribed books will be available from the campus bookstore on a mail-order basis. The campus bookstore can be contacted during office hours at (012) 420-1234 for further arrangements. R. Krishnan, Electric Motor Drives: Modelling, Analysis, and Control, Prentice-Hall, 2001 Recommended Textbooks
**Mohan, Undeland, and Robbins,***Power Electronic Converters, Applications, and Design*, John Wiley and Sons, 3^{rd}Edition, 2003.
**JMD Murphy and FG Turnbull**,*Power Electronics: Control, AC Motors*, Pergamon Press, 1989.
**BK Bose,***Modern Power Electronics: Evolution, Technology, and Applications,*IEEE Press, 1992.
**MH Rashid,***Power Electronics: Circuits, Devices, and Applications,*2^{nd}Edition, Prentice Hall, 1993.
**GK Dubey,***Power Semiconductor Controlled Drives, Prentice Hall,*1989.
**T Kenjo,***Power Electronics for the Microprocessor age, Oxford Science Publications, 1990*
**P Vas,***Vector Control of AC Machines, Clarendon Press Oxford, 1990.*
**B Adkins and RG Harley,***The General Theory of Alternating Current Machines,*Chapman and Hall, London, 1975
**W. Shepherd and LN Hulley,***Power Electronics and Motor Control,*Cambridge University Press, 1987
**N. Mohan,***Advanced Electric Drives: Analysis, Control and Modelling using Simulink,*MPERE, 2001
**N. Mohan,**Electric Drives: An Integrative Approach, MNPERE, 2003
^ Journals and conference proceedings are an invaluable source of the latest developments in the field and students should consult them on a regular basis. The following are among those that are of direct relevance to the course: IEEE Transactions on Industrial Electronics
IEEE Transactions on Industry Applications
Proceedings of IEEE International Symposium in Industrial Electronics
IEE Power Engineering Journal
IEEE Transaction on Energy Conversion
IEE Proceedings Part B
IEEE Transactions Website: http:/www-ieee.org/products/periodicals.html
ABB, SEMIKRON, INTERNATIONAL RECTIFIERS, TEXAS INSTRUMENTS-applications Websites
^ The structure for module Electrical Drives ETE 780 is given below. A pre-requisite to being admitted to this module is evidence of having successfully completed the following undergraduate modules: Electrical machines or electrical drives and power electronics. ## ObjectivesThe objectives of this course are to first ensure that the student has a very good understanding of the fundamentals of electric drives after which the student is introduced to advanced material on modern electric drives. Performance parameters needed to evaluate drive performance -i.e. speed-torque characteristics of different types of drives, drive and converter efficiency, energy efficiency, torque and speed ripple- are also covered. Advantages of adjustable speed drives –including higher copper and energy efficiency, improved drive performance- and effect of non-sinusoidal supplies on drive performance are covered. The students will be exposed to the use computers simulation tools for time domain analysis of modern electric drives ## IntroductionElectric motor drives are used in a very wide power range, from a few watts to many thousand kilowatts, in applications ranging from very precise, high performance position controlled drives in robotics to variable speed drives for adjusting flow rates in pumps. In all drives where the speed and position are controlled, a power electronic converter is needed as an interface between the input power and the motor. Above a few hundred watts power level, there are basically three types of motor drives: DC motor drives, Induction motor drives, and Synchronous motor drives. AC motor drives have replaced DC drives in most applications as they have more advantages to offer. The application or process determines the requirements of the motor drive. For example, a servo-quality drive is needed in robotics, machine tools, paper mill or steel mill drives whereas only an adjustable speed drive is needed in air conditioning system. In servo applications of motor drives, the response time and the accuracy with which the motor follows the speed and position commands are extremely important. These servo systems, using one of these motor drives, require speed and position feedback for precise control. In addition, if an AC motor drive is used, the controller must incorporate sophistication, such as field-oriented control, to make the AC motor meet the servo drive requirements. However, in a large number of applications –e.g. process control, the accuracy and the response time of the motor to follow the speed command is not critical since the processes have large time constants. However, even where speed response is not critical, energy efficiency- both copper and total power demand for a given output power- is always a very important consideration. For a suitable drive to be selected for a specific load or application, complete information about load requirements should first be obtained. A motor having speed-torque and speed current characteristics that suit the load requirements is chosen. A motor will have characteristics compatible to the load if it satisfies the speed and torque requirements of the load without exceeding the current limitation imposed either by the motor rating or the source capacity. Usually the natural speed-torque characteristic is not compatible with the load requirements and a power electronic converter is used to interface the motor and the source. Further, a control strategy that ensures that the drive and load characteristics are well matched is employed. The control strategies most commonly employed with modern electric drives can be grouped into scalar and vector control schemes in the case of AC drives. Power electronics power processors are employed to act as interfaces between the electrical machines and available power sources as already mentioned. The processing involves conversion (DC-AC, AC-DC, DC-DC, and AC-AC) and control using power semiconductor switches. High energy efficiency is very important and the drive should operate with very low reactive power demand and reduce active power demand by operating at the lowest possible shaft speed that ensures proper system operation. However, compared to linear mode power amplification using power semiconductor devices, the switching mode power processing gives higher efficiency with the penalty of harmonic ripples in the load and source sides. In modern power electronic equipment, there are essentially two types of semiconductor elements: the power-semiconductor that can be considered as muscle of the equipment, and the microelectronic chips that act as power of the brain. Both elements are digital in nature, except that one manipulates power up to giga-watts and the other handles only milli-watts. The close co-ordination of the end-of-the-spectrum electronics is offering large size and cost advantages and high level of performance in today’s power electronics apparatus. By using power electronics, we can achieve a high level of productivity in industry and product quality enhancement that cannot be possible by using non-power electronic methods. Today, power electronics is an indispensable tool in any advanced country’s industrial economy. An important aspect of power electronics applications is energy conservation, i.e., more efficient use of electricity. The complete drive system –comprising of the load, motor, converter, source, control unit, and sensing circuits- must be treated as an integrated system. ^ Syllabus theme objective:On completing this part of the course, students should have an in-depth understanding of: What constitutes a general motor drive,
Derivation of models of DC motors.
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| % of semester mark | % of final mark |

Assignment 1: | 6% | 3% |

Assignment 2: | 18% | 9 % |

Assignment 3: | 6% | 3% |

Assignment 4: | 8% | 4% |

Assignment 5: | 22% | 11% |

Test 1 | 20% | 10% |

Test 2 | 20% | 10% |

Examination assignment | - | 50% |

TOTAL | 100% | 100% |

^

Class attendance during mini-blocks is compulsory. Students are also encouraged to participate in discussions in this period and to share knowledge with each other.

Open-book two-hour semester tests will be written during the last day of each mini-block.

Students with semester marks below 40% will not be allowed to attempt the final examination assignment.

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An examination assignment will be sent out during the week of the 13-18 November 2008. Actual date when it will be sent out as well as the submission date will be announced during the second mini-block.

Students are encouraged to discuss course work amongst themselves especially during mini-block weeks. However, each student should submit his/her own work for assignments. Plagiarism, including copying the work of another student and copying from the Internet is absolutely unacceptable. Dishonesty such as plagiarism during tests and final examination can be punished by expulsion from the University.

In addition, please note the following (provided by the University Legal Services Department):

“Under the definitions of misconduct a student is guilty of misconduct if he/she is guilty of any such conduct that infringes copyright of any other form of law of immaterial property and such conduct proves to be detrimental to the university.

The inclusion of the work of other authors (literary works) in dissertations and theses has to be done in accordance with the provisions of the Copyright Act, 98 of 1978. This Act states that the copyright in a literary work (also if made electronically) shall not be infringed by a short quotation therefrom provided that the source shall be mentioned as well as the name of the author. Non-compliance with these provisions will therefore not only be a contravention of the Rules of the University, but also a crime in terms of the South African Law”.

Five assignments will be completed in this course. Hand-in dates are indicated and must be adhered to.

Which are the main building blocks for an adjustable speed drive -ASD? Use a block diagram to explain.

Which types of applications require adjustable speed drives and what are the main advantages that are derived by employing adjustable speed drives?

Compare AC adjustable speed drives with DC adjustable speed drives citing the advantages and disadvantages of each.

What has led to AC adjustable speed drives replacing DC drives in most applications in the last 10 years or so?

Name two types of static power converters that are frequently used in adjustable speed drives.

Name two methods that can be used to provide variable-voltage variable-frequency (VVVF) supply. Why is it important to supply AC machines at a constant voltage to frequency ratio when VVVF supplies are employed?

A 460 V, 60 Hz, 4-pole induction motor develops its rated torque by drawing 10 A at a power factor of 0.866. The other parameters are as follows: R_{s}=1.53 , X_{ls}=2.2 , X_{m}=69 . The motor has a full-load speed of 1750 rpm. Calculate inverter frequency, motor current and line-line voltage during starting for a starting torque of two times rated torque.

A PWM VSI supplies 460 V line-line at a frequency of 60 Hz to an induction motor that develops a rated torque of 50 N-m at 1750 rpm. The motor and inverter efficiencies can be assumed to be constant at 90 and 95%, respectively, while operating at the rated torque of the motor. If the motor is operated at its rated torque and the rated air gap flux, determine the equivalent resistance that can represent the inverter-motor combination at motor frequencies of 60, 45, 30 and 15 Hz. The inverter operates at an amplitude modulation index of 0.8.

A separately-excited DC motor with the following parameters: R_{a}=0.35 , L_{a}=10 mH, J=0.02 kg-m^{2}, B_{fr}=0.015 N.m/rad/sec, 200 V, 1500 rpm, 3 hp. Its armature is connected to a DC-bus of 325 V using a one-quadrant chopper and field circuit is at rated conditions. A switching frequency of 2 kHz is to be employed and V_{ce(sat)}=2.5 V and diode V_{fwd}=1.5 V. Calculate motor efficiency and armature distortion power factor, converter efficiency and distortion power factor, overall efficiency, torque ripple and sketch converter and armature current and voltage waveforms.

When motor drives 10% of rated load at 50% rated speed and

When motor drives 75% rated load at 50% rated speed.

How does the performance at the above two operating conditions compare?

How does DC-bus voltage magnitude affect drive performance?

Obtain the time domain speed response and determine how long the motor takes to attain 50% rated speed if connected directly to a 200 V DC-bus. Assume no-load operation.

1)

Sketch the torque-speed characteristics and the stator current as a function of rotor frequency or slip speed for an induction motor that is fed from a current source.

How do they compare with those for an induction machine operating from a constant voltage and frequency supply?

With reference to an induction motor, explain the difference between the following scalar control schemes employed with AC drives: constant Volts/Hertz, constant slip speed and constant air-gap flux operation. Make use of block diagrams and torque-speed characteristics to do so.

2.

A 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to the stator: R

The motor is controlled using constant slip speed technique. Calculate:

The rated stator, magnetizing, and rotor current, electromagnetic, output shaft and breakdown torque, input power and power factor and efficiency.

The applied stator frequency, stator voltage and current, rotor and magnetising current, efficiency, developed electromagnetic torque, input power and input power factor when the motor operates at 1200 rpm and rated output shaft torque.

The stator frequency, stator, rotor and magnetising current, efficiency, developed electromagnetic torque, input power and input power factor when driving half rated torque at 1200 rpm.

The frequency, stator, rotor and magnetising current, efficiency, developed electromagnetic torque, input power and input power factor at half the rated torque and 600 rpm.

How do the values obtained in the above analyses compare? Explain any differences.

Which parameter(s) of an induction motor determines the maximum torque capability of the motor?

The motor is now operated using constant terminal voltage to frequency control scheme (with voltage boost). Determine:

The applied stator frequency, stator voltage and current, rotor and magnetising current, efficiency, developed electromagnetic torque, input power and input power factor when the motor operates at 1200 rpm and rated output shaft torque.

The motor is now operated using constant terminal voltage to frequency control scheme (without voltage boost). Determine:

The applied stator frequency, stator voltage and current, rotor and magnetising current, efficiency, developed electromagnetic torque, input power and input power factor when the motor operates at 1200 rpm and rated output shaft torque.

The motor is now operated using constant air gap flux control scheme. Determine:

The applied stator frequency, stator voltage and current, rotor and magnetising current, efficiency, developed electromagnetic torque, input power and input power factor when the motor operates at 1200 rpm and rated output shaft torque.

How do these values compare with those of the other control schemes at the same operating conditions?

3.

A 460 V, 60 Hz, 6-pole, 1180 rpm, star-connected squirrel cage induction motor has the following parameters per phase referred to the stator: R

The motor is fed by a six-step inverter (3-phase square-wave inverter), which in turn is fed by a 6-pulse fully controlled rectifier. If the drive is controlled using constant voltage to frequency control scheme- with voltage boost- technique,

Calculate the inverter output voltage and frequency, stator, rotor and magnetising current, efficiency, breakdown torque, input power and power factor when the motor operates at 1000 rpm and drives rated torque.

Calculate the inverter output voltage and frequency, stator, rotor and magnetising current, efficiency, breakdown torque, input power and power factor when the motor operates at 500 rpm and drives rated torque.

Calculate the inverter output voltage and frequency, stator, rotor and magnetising current, efficiency, breakdown torque, input power and power factor when the motor operates at 250 rpm and drives rated torque.

If the motor is now operated at 2000 rpm, calculate the stator voltage and frequency, magnetising, stator and rotor currents, efficiency, input power factor, electromagnetic and breakdown torque.

How do the values obtained in the above analyses compare? Explain any differences.

4.

Why is it important to feed an induction motor or any AC motor for that matter from a nearly sinusoidal if not purely sinusoidal AC source?

Under what conditions will an induction motor operate at speeds above base or rated speed?

Sketch the torque speed characteristics of an induction machine fed from

A constant voltage, constant frequency supply,

A variable-voltage variable-frequency supply, and

A constant-frequency variable-voltage source.

Discuss the merits and demerits of each type of induction motor drive.

When would it be necessary to feed an induction motor drive using from a constant-voltage variable-frequency source?

An induction motor has the following parameters: 21.96 hp, 240 V, 3-phase, 50 Hz, 2-poles, star-connected, R_{s}=0.05 , R_{r}=0.08 , X_{m}=15 , X_{ls}=0.10 , X_{lr}=0.15 . Effective stator to rotor turns ratio is 3. The motor is supplied with its rated and balanced voltages.

Find the q and d axes steady-state voltages and currents and phase currents I_{qrr}, I_{drr}, I_{}, and I_{}when the rotor is locked. Use stator-reference-frames model of the induction motor.

Derive the steady-state equivalent circuit of the motor whose parameters are given in QN 1 above.

An induction motor has the following parameters: 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to the stator: R_{s}=0.5 , R_{r}=0.2 , X_{ls}=X_{lr}=1 , X_{m}=30 , B_{fr}=0.05 Nm/rad/s, J=0.31 kg-m^{2}.

Determine the quadrature and direct components of stator and rotor currents as well as the electromagnetic torque, power factor, line currents and efficiency of the motor when driving rated load at rated conditions and also when driving rated load at 50% rated speed. Make use of the stator, synchronous and rotating reference frames.

The parameters of an induction motor to be used in a variable speed drive are as follows: 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to stator: R_{s}=0.5 , R_{r}=0.2 , X_{ls}=X_{lr}=1 , X_{m}=30 , B_{fr}=0.05 Nm/rad/s, J=0.31 kg-m^{2}.

Making use of the synchronous reference frame, find the errors in the stator flux-linkages magnitude and the electromagnetic torque computations when the stator resistance has risen from its nominal value by 50%. Comments on the results. Operating points to be considered are: _{s}=51.88 rad/sec; _{s1}=8.377 rad/s; V_{ph}=55.16 V _{s}=196.372 rad/sec; _{s1}=8.377 rad/s; V_{ph}=148 V _{s}=268.62 rad/sec; _{s1}=8.377 rad/s; V_{ph}=194.9 V

An induction motor with the following data is to be used with an indirect vector controller: 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to the stator: R_{s}=0.5 , R_{r}=0.2 , X_{ls}=X_{lr}=1 , X_{m}=30 , B_{fr}=0.05 Nm/rad/s, J=0.31 kg-m^{2}.

Find the rated rotor flux linkages, torque commands, the corresponding flux- and torque producing components of the stator current command, the stator-current phasor command, torque-angle command, and the slip-speed command. The drive is assumed to be a torque amplifier.

An induction motor with the following data is to be used in a converter-fed drive system: 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to the stator: R_{s}=0.5 , R_{r}=0.2 , X_{ls}=X_{lr}=1 , X_{m}=30 , B_{fr}=0.05 Nm/rad/s, J=0.31 kg-m^{2}.

Compute the stator-voltage magnitude at rated operating conditions with rated rotor flux linkages and comment on the finding.

An induction motor with the following data is to be used in a closed-loop controlled drive: 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to the stator: R_{s}=0.5 , R_{r}=0.2 , X_{ls}=X_{lr}=1 , X_{m}=30 , B_{fr}=0.05 Nm/rad/s, J=0.31 kg-m^{2}. The induction motor is run in the torque mode and the ratio of flux linkage to its command value was observed to be 1.1. The applicable data available for the operating point are =0.6 and =0.9.

Find the ratio of actual torque to its command value, the slip-speed command and the torque command, if ^{*}_{r}=rated rotor flux linkages.

An induction motor with the following data is to be used in a closed-loop controlled drive: 460 V, 60 Hz, 4-pole, 1720 rpm, star-connected induction motor has the following parameters per phase referred to the stator: R_{s}=0.5 , R_{r}=0.2 , X_{ls}=X_{lr}=1 , X_{m}=30 , B_{fr}=0.05 Nm/rad/s, J=0.31 kg-m^{2}. The inverter and load parameters for a drive utilising an induction motor whose parameters are given above are as follows: I_{f}=12.11 A, f_{c}=2 kHz, B_{fr}=0.05 Nm/rad/s, T_{}=0.002, V_{cm}=12 V, J=0.31 kg-m^{2}, V_{dc}=800 V, H_{c}=0.333 V/A

Determine the speed-controller constants, and verify the validity of the assumptions in design

Sketch the torque speed characteristics of a synchronous motor fed from a variable-voltage variable-frequency supply.

What is the difference between these characteristics and those of an induction motor fed from a similar supply?

A 375-kW, 10-pole, 2300-V, 60 Hz, 3-phase synchronous motor has a magnetizing inductance of 30 mH. A field current of 15 A produces a rated terminal voltage when driven at rated speed at no load. The motor is supplied from an inverter with controllable, near sinusoidal voltage and frequency. The mechanical load is a pump that requires a torque of T_{L}=0.75^{2}Nm. Losses may be ignored. When driving the load at rated speed, the inverter is adjusted to give rated flux linkage and the field current is adjusted to give unity power factor.

Determine the values of the stator flux linkage (RMS), the stator current, and the field current.

Suppose the speed is to be reduced to 50% of rated value, keeping the stator flux linkage and the filed current at the same values as found in (a). Determine the stator voltage, the stator current, and the power factor.

To what value should the field current in (b) have to be reduced to, to achieve unity power factor?

A 3-phase 6600-V, 8-pole, 60 Hz, 1200 kW, star-connected wound field synchronous motor has the following parameters: R_{s}=0.375 , R_{r}=0.15 , X_{ls}=4.5 , X_{m}=22.5 , n=2 and R_{f}=3.75 . When operating at rated power and unity power factor calculate:

The field current and torque angle at full load.

The pullout torque.

The power factor, armature current, and efficiency at half rated torque and rated field current.

The current to get unity power factor at half the rated torque.

If the machine is now controlled from a variable frequency source with constant V/f ratio being maintained up to base speed, calculate

The armature current torque angle and power factor at full-load torque, half the rated speed, and the rated field current.

The armature current and power factor at half the rated speed, half full-load torque, and the rated field current.

The torque and field current for the rated armature current, 1125 rpm, and unity power factor.

A 15 kW, three-phase, 440 V, 60 Hz, 4-pole, Y-connected, permanent magnet synchronous motor has the following parameters: X_{sl}=1.2 , X_{m}=12 negligible R_{s}, rated power factor of 0.8 lagging. The motor is controlled by variable frequency control with a constant terminal voltage to frequency ratio up to base speed and at a constant voltage and variable stator frequency above base speed. Calculate:

The developed torque, mechanical power, magnetizing current, terminal voltage and input power factor when the motor draws rated input current and runs at 80% rated speed.

An application requires that the motor deliver 80% rated power at all speeds above rated speed. What will be the maximum available speed for such an application?

Calculate the torque that leads to unity power factor operation at base speed.

Calculate the speed at which the drive operates at unity power factor and draws 80% rated stator current.

A permanent magnet synchronous motor has the following parameters: R_{sn}=0.164 p.u, L_{dn}=0.45 p.u, L_{qn}=0.6 p.u, V_{sn}=1.5 p.u, I_{sn}=1 p.u Determine:

The maximum speed with and without neglecting stator resistances and

The steady state characteristics in the flux weakening region

Permanent magnet synchronous motor parameters are as follows: R_{s}=1.3, L_{d}=0.00585 H, L_{q}=0.0085 H, _{af}=0.1546 Wb-Turn, B_{t}=0.012 N.m/rad/sec, J=0.007 kg-m^{2}, P=6, f_{c}=2.5 kHz, V_{cm}=10 V, H_{}=0.05V/V, H_{c}=0.8 V/A, V_{dc}=310 V.

Design a symmetric-optimum-based speed-controller, and verify the validity of assumptions made in its derivation. The damping ratio required is 0.707.

^

The 2008 schedule to assist in covering the course material and completing the assignments is given in Table 1.

Table 1: Schedule for ETE 780

Week | Date | Action |

1 | 14-20 July | Study Theme 1 |

2 | 21-27 July | Study Theme 2 |

3 | 28 July-3 August | Study Theme 2 |

4 | 4-10 August | Study Theme 2 |

5 | 11-17 August | First mini-block week |

6 | 18-31 August | Study Theme 2 |

7 | 1-7 September | Study Theme 3 |

8 | 8-14 September | Study Theme 3 |

9 | 15-21 September | Second mini-block week |

10 | 22-28 September | Study Theme 3 |

11 | 29 Sept.-5 Oct. | Study Theme 3 |

12 | 6-12 October | Study Theme 3 |

13 | 13-19 October | Study Theme 4 |

14 | 20-26 October | Study Theme 4 |

15 | 27 Oct.-2 Nov. | Study Theme 4 |

16 | 3-9 November | Study Theme 4 |

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