ELECTROMECHANICAL DRIVE SYSTEMSDr. Bartłomiej Ufnalski | B.Ufnalski@isep.pw.edu.pl | Electric(al) Drive Division

Science is always wrong. It never solves a problem without creating ten more.

We need science to help us solve all the problems we wouldn’t have if there

were no science.

George Bernard Shaw

The lecture and the laboratory are passed based on two mini-projects and on the bonus points

from exercises (i.e. activity during laboratory+lecture meetings). The final grade is a weighted

average of these marks. Lectures will be combined with hands-on training in a dedicated teaching

laboratory equipped with a Matlab/Simulink/PLECS simulational environment. This means that

lectures are interactive and you can get bonus points during all our meetings for being active.

The typical meeting will include 30 mins of solving problems during lecture-like part, just to create

ten times more problems during 60 mins laboratory-like part. And don’t worry – I’ll be at your

service to help you create these problems.

HANDBOOKS & OTHER (RE)SOURCES OF

INFORMATION/KNOWLEDGE

• Google.com – good for warm-ups

and brain storms – be aware that

most documents indexed by popular

search providers are not reviewed,

thus it is common that highly biased

opinions are published, incl.

statements that solution X is better

than Y without providing any info

on quality indicator taken into

account.

• More advanced filtering is

implemented in Scholar.Google.com

– most indexed documents are of

scientific and of popular scientific

type. Try these keywords: DC drives,

modern AC drives, PID control(lers),

etc.

• E-libraries available at our Main Library: http://www.bg.pw.edu.pl/ezrodla.html

• For the case of this lecture, I can recommend e.g.: IEEExplore www.ieeexplore.ieee.org and ENGnetBASE www.crcnetbase.com.

• Examples of handbooks:

Bose B. K.: „Modern Power Electronics and AC Drives”, Prentice Hall PTR, 2002

[in Polish] Orłowska-Kowalska T.: „Bezczujnikoweukłady napędowe z silnikami indukcyjnymi”, seria„Postępy Napędu Elektrycznego i Energoelektroniki”, -tom 48, Oficyna Wydawnicza Politechniki Wrocławskiej, 2003

[in Polish] Krzemiński Z.: „Cyfrowe sterowaniemaszynami asynchronicznymi” seria „Postępy NapęduElektrycznego i Energoelektroniki” - tom 45,

Wydawnictwo Politechniki Gdańskiej, 2001 (available on-line)

Mohan N.: „Electric Drives: An Integrative Approach”, Mnpere 2003, „Advanced Electric Drives: Analysis, Control and Modeling Using Simulink”, Mnpere 2001, „First Course on Power Electronics”, MNPERE 2009

Syed A. Nasar, Ion Boldea: „Electric Drives”, 2nd edition, 2005

[in Polish] Sieklucki G.: ‚Automatyka napędu”, 2009

ELECTROMECHANICAL DRIVE SYSTEM:

THE DEFINITION

• Electromechanical drive system is a set of devices that

enables to control electromagnetic torque [force] or speed

(velocity) or position [angular or linear] by shaping

electromagnetic torque [force] produced in the rotary

[linear].

• Electromechanical drive system consist of:✓ electric machine (motor, generator)✓ power electronic converter✓ control system (microcontroller)

Tips and tricks:

• The instantaneous speed is the magnitude of the velocity vector. In an electromechanical drive system we control velocity rather than speed because the direction (clockwise or counterclockwise) is usually important. However, it is common to use term „speed” even if it’s not totally consistent with the definition, e.g. speed cannot be negative (according to the definition). Nevertheless, we use negative and positive values to indicate the direction.

CONTROL TASKS: SHAPING MECHANICAL QUANTITIES

(TORQUE [FORCE], SPEED (ANGULAR OR LINEAR), POSITION

(ANGULAR OR LINEAR) AND JERK

• Torque [force] (acceleration is proportional to the resultant torque [force]) – the need for controlling this quantity is present in vast variety of drives. This controller can work as a slave for speed/position controller or can work as a master controller (e.g. screwing with a desired torque on automated assembly line).

• Speed [angular or linear] – the need for controlling this quantity is present in vast variety of variable-speed drives.

• Position – the need for controlling this quantity is present in vast variety of servo-drives, e.g. robotic arms, CNC milling machine.

• Jerk (the derivative of acceleration with respect to time) – its value can be „controlled” by shaping the reference signal (e.g. reference speed signal). This means that usually there is no additional jerk feedback controller present in the system. Jerk can be controlled by feed-forward controller (signal proportional to the change in acceleration is introduced in control scheme). Too high jerk in transportation makes people uncomfortable. Too high jerk is harmful for mechanical power transmission systems (e.g. gears).

• Jerk is the derivative of acceleration with respect to time.

• Acceleration is the derivative of velocity with respect to time.

• Velocity is the derivative of position with respect to time.

• Position is the derivative of… The integral of position control deviation (error) can constitute reference speed – what about units???

Tips and tricks:

• Speed is the first derivative of distance with respect to time (scalar quantity) . Velocity is the first derivative of displacement (or position) with respect to time (vector quantity).

• The derivative of jerk is called jounce or snap (http://sprott.physics.wisc.edu/pubs/paper229.pdf) and was considered in development of e.g. the Hubble Space Telescope's pointing control system.

REFERENCE SIGNAL SHAPING (LIMITING THE JERK)

spe

ed

acc

ele

rati

on

acc

ele

rati

on

spe

ed

jerk

jerk

MATHEMATICAL MODEL OF „THE SIMPLEST” ELECTRIC MOTOR

(MENTIONED SIMPLICITY DOES NOT REFER TO ITS

CONSTRUCTION)

How one can control/change speed of this motor? Topology of control system as a result of

dependencies between state variables and inputs – it’s time for brain storm on whiteboard.

disturbance

input output

output

Tips and tricks:

• State variables of a dynamical system --> search for derivatives (a rule of thumb).

• Outputs --> it depends on what you are willing to control.

• Inputs and disturbances --> external actions (e.g. force acting on an object, voltage imposed to the terminals). In control systems we usually distinguish between the two by a very simple rule: input signal comes from control system (its value can be changed intentionally), whereas disturbance signal comes from source that is unknown/unpredictable to us.

SIMPLIFIED MODEL OF VOLTAGE-SOURCE

CONVERTER

First order inertia as an approximation of converter dynamics.

Tips and tricks:

• Take two first terms of the e^x Taylor (Maclaurin) series.

• Average delay and switching frequency: if the bus pulls out every 30min but you don’t know when this happens then you will have to wait statistically half of this time (15sec).

H-BRIDGE CONVERTER TOPOLOGY

Bi-directional (in terms of energy flow and rotation) converter for brushed DC motor

SEMICONDUCTOR SWITCH IGBT+D. WHY „+D”?

The inductance of output circuit (load circuit) makes diodes indispensable in this

topology. Notice that any current loop has non-zero L (you’ve got at least one turn of

wire).Tips and tricks:

• Observe what happens when you switch off the light using a rocker switch

circuit breaker. See electric spark? It comes from energy stored in magnetic

field of the circuit - you’ve got at least one turn of wire.

FRONT-END CONVERTER TOPOLOGY AND ENERGY

RECOVERY WITH FEEDBACK TO THE GRID

Diode rectifier enables energy flow only from grid. You cannot return energy to the grid! You will

need a break chopper to control DC-link voltage (to dissipate energy in generating mode of the drive),

e.g. during speed reduction (kinetic energy is converted to electric one), elevator going down

(potential energy is converted to electric one).

FRONT-END CONVERTER TOPOLOGY AND ENERGY

RECOVERY WITH FEEDBACK TO THE GRID

Bi-directional (in terms of energy flow) transistor rectifier (fully-controlled rectifier, force-

commuted PWM voltage-source rectifier). Optional break chopper can support the system

during grid failures in generating mode.

ENERGY RECOVERY = MONEY SAVINGS?

Case study 1. WHEEL BALANCER

1kWh=3600kJ=3.6MJ --> 50gr; wheel 1kg*m*m (rough approx.) revolving

at 140rad/s stores ca. 10kJ of kinetic energy

This gives approx. 0.2gr/wheel.

40 cars every day (very optimistic assumption), 300 days per year (incl.

Saturdays) will give savings at the level of 100 PLN/year.

AFE @0.4kW will cost you ca. 400 PLN gross.

Case study 2. TOWING CARRIAGE FOR TOWING TANK IN SHIP MODEL BASIN

50000kg (50t), 12m/s, 1m/s/s --> 3.6MJ=1kWh

Energy trade price: 25gr/kWh (2010)

Let us assume: 15 years of operation, 20 runs a day, 200 days per year

This gives: 15000 PLN

Peak power: 600kW

AFE will cost you ca. 30000 PLN.

Diode rectifier as front-end converter? What about an impact on the grid?

PULSE WIDTH MODULATION (PWM)

• Why power converters (e.g. drive converters) are not designed to work the same way as class A or B audio amplifiers?

• Switching and conduction losses in real semiconductor device.

• Is it impossible or just impractical to build e.g. 10kW drive with a linear (non-PWM) amplifier? Losses and junction cooling discussion.

• Class D amplifiers – switching amplifiers (PWM method also implemented in audio amplifiers)

• Class D amplifiers have efficiency usually higher than 90%.

• Typical drive converters have efficiency at the level of 96%-98% (efficiency depends on the working point).

• PWM concept should be familiar to you from previous lectures (prerequisites). If it happened that you missed them, please visit pages listed on the next slide.

PWM CONCEPT – LET’S PLAY IN VIRTUAL

LABS OR ATTEND WEBMINARS

• Interactive Power Electronics Seminar (iPES) http://www.ipes.ethz.ch/, http://www.ipes.ethz.ch/ipes/PWMsimpel/e_pwmsimpel.html,http://www.ipes.ethz.ch/ipes/Inverter/e_H_Bruecke.html

• Virtual Laboratory of Power Electronics http://www.isep.pw.edu.pl/icg/vlab/index.html

• Wolfram Demonstrations Project http://demonstrations.wolfram.com/PulseWidthModulationPrinciple/

• Motion System Design http://motionsystemdesign.com/engineering-basics/pulse-width-modulation-1000/index.htm

Some videos may also inspire you:

http://www.youtube.com/watch?v=YmPziPfaByw

http://www.youtube.com/watch?v=Lf7JJAAZxEU

PROPORTIONAL-INTEGRAL CONTROLLER (PI

CONTROLLER)

It is assumed that you are familiar with PID control basics (prerequisites), but… let’s discuss some basics:

• continuous-time and discrete-time realizations

• actor as an element with natural constraints (limited/saturated action), e.g. angular position of a vertical steering fin can be limited to 35 degrees port to 35 degrees starboard

• anti-windup (conditional integration)

• derivative action is often deactivated in PWM-based converter systems – why?

• PID controller tuning

DISCRETE-TIME PI CONTROLLER (WITH

ANTI-WINDUP ALGORITHM)

PID (PI, P) CONTROLLER TUNING

PROCEDURES (SELECTED METHODS)

Please see the attached PDFs for more information.

PID (PI, P) CONTROLLER TUNING

PROCEDURES (SELECTED METHODS)

Please see the attached PDFs for more information.

DC MOTOR DRIVE WITH H-BRIDGE

CONVERTER

Now we are ready to „experiment” with simulational model of a DC drive. It is assumed that the group is familiar with Matlab/Simulink environment. Nevertheless, short introduction to Simulink+PLECS tools will take place along with presentation of the attached simulation model of the drive. To make this meeting more smooth, please install on your notebooks Matlab/Simulink (you can connect to license.ee.pw.edu.pl via VPN, more info at www.ee.pw.edu.pl in IT section) and PLECS from Plexim (license server is also started on license.ee.pw.edu.pl).

THANK YOU FOR YOUR ATTENTIONdr. Bartłomiej Ufnalski | B.Ufnalski@isep.pw.edu.pl | Electrical Drive Division

Warsaw University of Technology

Faculty of Electrical Engineering

Institute of Control and Industrial Electronics