4 Electrical actuation systems

4.1 Electrical systems

4.1.1 Switches

Types of switches: Mechanical switch, Electromechanical switch, Electronical solid-state device. Mechanical switch— Switch bounce.

4.1.2 Mechanical switches

4.1.3 Relays

4.1.4 Solid-state switches

Diode Thyristors, Triacs, Transisters.

4.1.5 Solenoids solenoid can be used to provide electrically operated

actuators.

IGBT – Insulated or Isolated Gate Bipolar Transistor

IGBT combines the positive attributes of BJTs and MOSFETs.BJTs have lower conduction losses in the on-state, especially in devices with larger blocking voltages, but have longer switching times, especially at turn-off while. MOSFETs can be turned on and off much faster, but their on-state conduction losses are larger, especially in devices rated for higher blocking voltages. 

IGBTs have lower on-state voltage drop with high blocking voltage capabilities in addition to fast switching speeds.

(+) Collector

(+) Base

(-) Emitter

NPN IGBT

(-) Base

(-) Collector

(+) Emitter

PNP IGBT

(+) Base

(+) Collector

(-) Emitter

NPN IGBT

(-) Base

(-) Collector

(+) Emitter

PNP IGBT

4.2 Stepping motors

Producing rotation through equal angles, the so-called steps, for each digital pulse supplied to its input.

Solid-state electronics is used to switch the d.c. supply

between the pairs of stator windings.

Terminology: Holding torque Pull-in torque pull-out torque Pull-in rate pull-out rate Slew range.

Driving circuit of step motor

Homework: page 128, problem 1,2,3,5

4.3 Motors

Electromagnetic force (EMF)BILF

1. Where L is the length of conductor in a magnetic field, I is the current of the conductor, and B is flux density of the magnetic field.

dtdΦe /2. is the back e.m.f, Φ is the magnetic flux.

• Basic typesDC Motors: speed and rotational direction control

via voltage= Easy to control torque via current

= low voltage

= linear torque-speed relations

= Quick response

AC Motors: smaller, reliable, and cheaper= speed fixed by AC frequency

= low torque at low speed

= difficult to start

Electric Motors

DC Motors: Principles of Operation A wire carrying current experiences a

force in a magnetic field.

Magnetic Flux Density (Tesla))( BF li

Induced force Current(amp)

Length of wire in the direction of i (m)

BI

I B

sinBIBI

Electromagnetic ForceA wire carrying current in a magnetic

field.

B

i F = ilB

(l = wire length)

Electromagnetic Force

• The force is perpendicular to both the magnetic field and current

A voltage is induced in a wire moved in a magnetic fieldgenerators

B+ + +

v(l = the wire length)

leind )( BvInduced voltage (volt)

Velocity of wire (m/s)

l

- - -

eind =vBl

eind is also called electromotive force (EMF感应电动势 )

eind

Electromagnetic Force

Principle of Electric Motors

• Fundamental principle behind electric motors Current running through coil in magnetic field

experiences forces that cause it to rotate

Fundamental characteristics of DC Motors

N

S

StatorCoils

N

SS

N

Rotor

Stator

S

N

S

N

N

S

End viewTime 0

N

S

StatorCoils

N

S NRotor

Stator

S

N

S

N

N

S

S

End viewTime 0+

Shifting magnetic field in rotor causes rotor to be forced to turn

Nature of commutation Power is applied to armature

windings From V+ Through the +brush Through the commutator

contacts Through the armature (rotor)

winding Through the – brush To V-

Rotation of the armature moves the commutator, switching the armature winding connections

Stator may be permanent or electromagnet

Rotor

V-

V+BrushAssembly

S

S

N

N

N

Stator

Stator

Comutator

V-

V+

Diagram of a Simple DC Motor

Commutator

Armature Armature conductors Field coil Field pole

Commutator Brush Brush wear Brushless

Excitation of motors:

(a) Series: highest starting torque and greatest no-load speed. (b) shunt: lowest starting torque, a much lower no-load speed and has good speed regulation.

(c) compound: high starting torque and good speed regulation.

(d) separate:a special case of the shunt wound motor and

ease to revert the direction.

The speed of such d.c motors can be changed by either changing the armature current or the field current. The variable voltage is often obtained by an electronic circuit.

DC motor wiring topologiesPe

rcen

t of r

ated

Spe

ed

Percent of Rated Torque

120

100

80

60

40

20

0

400300200100

0

Shun

t Fie

ld

Series Field

Shun

t Fie

ld

Series Field

Shunt

Series

Compound

Permanent magnet DC motors

Permanentmagnetpoles

Per

cent

of r

ated

Spe

ed

Percent of Rated Torque

40

20

02001000

120

100

80

60

400300

PermanentMagnet

Permanent Magnet DC Motors Have permanent magnets rather than field windings but with

conventional armatures. Power only to armature. Short response time Linear Torque/Speed characteristics similar to shunt wound motors.

Field magnetic flux is constantCurrent varies linearly with torque.

Self-braking upon disconnection of electrical powerNeed to short + to – supply, May need resistance to dissipate heat.

Magnets lose strength over time and are sensitive to heating.Lower than rated torque.Not suitable for continuous dutyMay have windings built into field magnets to re-magnetize.

Best applications for high torque at low speed intermittent duty.Servos, power seats, windows, and windshield wipers.

4.4 A.C. motorsSingle phase (low power) and Poly phase (high

power)

Induction (Cheaper) and Synchronous motors

Single-phase squirrel-cage induction motor: not self-starting, the rotor rotates at a speed determined by the frequency of the alternating current applied to the stator (synchronous speed), there is difference

between the rotor speed and synchronous speed (slip).

Three-phase induction motor: there is rotating magnetic field

which completes one rotation in one full cycle of the current, self-

starting, the direction of rotation is reversed by interchanging any

two of the line connections.

Synchronous motors: its rotor is a permanent magnet, not

self-starting, used in the case when the precise speed is

required.

Inducing magnetism in the rotor Difference between

angular velocity of rotor and angular velocity of the field magnetism causes squirrel cage bars to cut the field magnetic field inducing current into squirrel cage bars.

This current in turn magnetizes the rotor

Rotor

N SDifference in

rotation of fieldmagnetism and

rotor rotation

Torque/speed curve%

of F

ull-L

oad

Torq

ue

% of Synchronous Speed

Slip (Full load)

100806040200

250

200

150

100

50

0

Breakdown

Torque

Full-Load Torque

Pull-up Torque

Locked rotor

torque

A.C. motor is cheaper, more rugged, reliable and maintenance free.

Speed control of A.C. motor is more complex than with d.c. motors.

4.4.1 Brushless permanent magnet d.c. motors

High performance, reliability and low maintenance.

Current-carrying conductors are fixed and the magnet moves. The current to the stator coils is electronically switched, the switching being controlled by the position of the rotor so that there are always forces acting on the magnet causing it to rotate in same direction.

4.4.2 Speed and position control of D.C motors

PWM modulation：

Position servo system of D.C. motor

4.4.3 A.C servo systemD.C./A.C. Inverter

PWM Variable Frequency Drives AC to DC converter and a DC to AC converter (inverter)

Inverter frequency and voltage output can be varied to allow motor speed to be varied.

Very efficient and cost effective variable speed

L1

L3L2

Co

ntr

ol L

og

ic

M

Rectifier Filter Inverter

480V

650 V

SPWM Modulation

改变脉冲宽度调节平均电压

Speed servo control of synchronous A.C. motor

Direct Vector Control Introduction

Scalar control of ac drives produces good steady state performance but poor dynamic response. This manifests itself in the deviation of air gap flux linkages from their set values. This variation occurs in both magnitude and phase.

Vector control (or field oriented control) offers more precise control of ac motors compared to scalar control. They are therefore used in high performance drives where oscillations in air gap flux linkages are intolerable, e.g. robotic actuators, centrifuges, servos, etc.

Introduction (cont’d)

Why does vector control provide superior dynamic performance of ac motors compared to scalar control ?

In scalar control there is an inherent coupling effect because both torque and flux are functions of voltage or current and frequency. This results in sluggish response and is prone to instability because of 5th order harmonics. Vector control decouples these effects.

Torque Control of DC Motors

There is a close parallel between torque control of a dc motor and vector control of an ac motor. It is therefore useful to review torque control of a dc motor before studying vector control of an ac motor.

Torque Control of DC Motors (cont’d)

A dc motor has a stationary field structure (windings or permanent magnets) and a rotating armature winding supplied by a commutator and brushes. The basic structure and field flux and armature MMF are shown below:

Torque Control of DC Motors (cont’d)

The field flux f (f) produced by field current If is orthogonal to the armature flux a (a) produced by the armature current Ia. The developed torque Te can be written as:

Because the vectors are orthogonal, they are decoupled, i.e. the field current only controls the field flux and the armature current only controls the armature flux.

'e t a fT K I I

Torque Control of DC Motors (cont’d)

DC motor-like performance can be achieved with an induction motor if the motor control is considered in the synchronously rotating reference frame (de-qe) where the sinusoidal variables appear as dc quantities in steady state.

Two control inputs ids and iqs can be used for a vector controlled inverter as shown on the next slide.

With vector control:

ids (induction motor) If (dc motor)

iqs (induction motor) Ia (dc motor)

Thus torque is given by:

where is peak value of sinusoidal space vector.

'e t qs t ds qsrT K i K i i

r r

Torque Control of DC Motors (cont’d)

This dc motor-like performance is only possible if iqs

* only controls iqs and does not affect the flux , i.e. iqs and ids are orthogonal under all operating conditions of the vector-controlled drive.

Thus, vector control should ensure the correct orientation and equality of the command and actual currents.

r

Torque Control of DC Motors (cont’d)

Equivalent Circuit of Induction Motor

The complex de-qe equivalent circuit of an induction motor is shown in the below figure (neglecting rotor leakage inductance).

Equivalent Circuit of Induction Motor (cont’d)

Since the rotor leakage inductance has been neglected, the rotor flux = , the air gap flux.

The stator current vector Is is the sum of the ids and iqs vectors. Thus, the stator current magnitude, is related to ids and iqs by:

r

m

sI

2 2s ds qsI i i

Phasor Diagrams for Induction Motor The steady state phasor (or vector) diagrams

for an induction motor in the de-qe (synchronously rotating) reference frame are shown below:

Phasor Diagrams for Induction Motor (cont’d)

The rotor flux vector is aligned with the de axis and the air gap voltage is aligned with the qe axis. The terminal voltage Vs slightly leads the air gap voltage because of the voltage drop across the stator impedance. iqs contributes real power across the air gap but ids only contributes reactive power across the air gap.

( )r m

mV

Phasor Diagrams for Induction Motor (cont’d)

The first figure shows an increase in the torque component of current iqs and the second figure shows an increase in the flux component of current, ids. Because of the orthogonal orientation of these components, the torque and flux can be controlled independently. However, it is necessary to maintain these vector orientations under all operating conditions.

How can we control the iqs and ids components of the stator current Is independently with the desired orientation ?

Principles of Vector Control

The basic conceptual implementation of vector control is illustrated in the below block diagram:

Note: The inverter is omitted from this diagram.

Principles of Vector Control (cont’d) The motor phase currents, ia, ib and ic are

converted to idss and iqs

s in the stationary reference frame. These are then converted to the synchronously rotating reference frame d-q currents, ids and iqs.

In the controller two inverse transforms are performed:

1) From the synchronous d-q to the

stationary d-q reference frame;

2) From d*-q* to a*, b*, c*.

Principles of Vector Control (cont’d)

There are two approaches to vector control:

1) Direct field oriented current control

- here the rotation angle of the iqse vector

with respect to the stator flux qr’s is being directly determined (e.g. by measuring air gap flux)

2) Indirect field oriented current control

- here the rotor angle is being measured indirectly, such as by measuring slip speed.

Direct Vector Control

In direct vector control the field angle is calculated by using terminal voltages and current or Hall sensors or flux sense windings.

A block diagram of a direct vector control method using a PWM voltage-fed inverter is shown on the next slide.

Direct Vector Control (cont’d)

Direct Vector Control (cont’d)

The principal vector control parameters, ids*

and iqs*, which are dc values in the

synchronously rotating reference frame, are converted to the stationary reference frame (using the vector rotation (VR) block) by using the unit vector cose and sine. These stationary reference frame control parameters ids

s* and iqss* are then changed to

the phase current command signals, ia*, ib*, and ic* which are fed to the PWM inverter.

Direct Vector Control (cont’d)

A flux control loop is used to precisely control the flux. Torque control is achieved through the current iqs

* which is generated from the speed control loop (which includes a bipolar limiter that is not shown). The torque can be negative which will result in a negative phase orientation for iqs in the phasor diagram.

How do we maintain idsand iqs orthogonality? This is explained in the next slide.

Direct Vector Control (cont’d)

Direct Vector Control (cont’d)

Here the de-qe frame is rotating at synchronous speed e with respect to the stationary reference frame ds-qs, and at any point in time, the angular position of the de axis with respect to the ds axis is e (=et).

From this phasor diagram we can write:

and cossdr er sins

qr er

Direct Vector Control (cont’d)Thus,

, , and

The cose and sine signals in correct

phase position are shown below:

cossdr

e

r

sinsqr

e

r

2 2s s

dr qrr

Direct Vector Control (cont’d)

These unit vector signals, when used in the vector rotation block, cause ids to maintain orientation along the de-axis and the iqs orientation along the qe-axis.

Summary of Salient Features of Vector Control

A few of the salient features of vector control are: The frequency e of the drive is not controlled

(as in scalar control). The motor is “self-controlled” by using the unit vector to help control the frequency and phase.

There is no concern about instability because limiting within the safe limit automatically limits operation to the stable region.

sI

4.5 Design method

Transient response will be fast because torque control by iqs does not affect flux.

Vector control allows for speed control in all four quadrants (without additional control elements) since negative torque is directly taken care of in vector control.