DC Drives Fundamentals

ABB GroupMarch 17, 2014 | Slide 1

DC drives fundamentalsWiwiet Yuniarto


DC drives fundamentalsGeneral Layout

General layout MV / LV transformerArmature circuit AC fuses Main contactor Commutation chokes Armature converter DC fusesField circuit Field fuses Autotransformer Field contactor Field converter

~~--

Load

MV lineMV / LVtransformer

Main contactor(K1)

AC fuses(F1)

Commutationchokes (L1)

Field fuses(F3)

Armatureconverter

DC fuses

Fieldwinding

Field contactor(K3)

Fieldconverter

Autotransformer(T3)

M


DC drives fundamentals6-pulse thyristor bridge (line commutated)

DC currentAC line current

3 ~ AC network 1 3 5

4 6 2

Uda

Id

iLL 1

uL

~

~

~L 3

L 2N

Controlled voltage sourcedepending on firing angle a

Output voltage can bepositive or negative


L1L3L2a=0

L1L3L2a=0

L1L3L2a=0L12

3~ AC network

DC voltage(controlled)

DC currentAC line current

1 3 5

4 6 2

aUd

Id

iLL 1

uL

~

~

~L 3

L 2

Voltages

Phase voltage (L1, L2, L3)

Phase to phase voltage(L12)

Thyristor 1 and 6 are active

The output is shown in red

DC drives fundamentalsGenerating output voltage


6-pulse thyristor bridge withload

Firing sequence

Thyristor 1 + 6

Thyristor 2 + 1

Thyristor 3 + 2

Thyristor 4 + 3

Thyristor 5 + 4

Thyristor 6 + 5

L1L3L2a=0

3~ AC network 1 3 5

4 6 2

aUd

Id

iLL 1

uL

~

~

~L 3

L2

DC drives fundamentalsHow a thyristor converter works


DC drives fundamentalsMachine is motoring

L1L3L2a=0L12

a = 30

Positive voltage

Firing angle a < 90 Minimum firing angle a is 15

Natural firing angle is theintersection between twophases

In this example the thyristor isfired after 30 (a = 30) fromnatural firing angle


DC drives fundamentalsMachine is generating (regenerative mode)

Negative voltage

Firing angle a > 90 Maximum firing angle a is 150

L1L3L2a=0L12

a = 150


DC drives fundamentalsShoot-through or commutation failure

DC drives can be compromised bycommutation failures causing

Damage fuses

Damage thyristors

Causes of commutation failures

Mains failure

Too large firing angles a Working range has to be limited

Typical firing angles a are between 15and 150

0 30 60 90 120 150 180a

L2 L3 L1

a = 180

Ausgangsgleichspannung

Netzspannung

Zndwinkel

WECHSELRICHTERKIPPEN

t

t

Uda

a

Mains


DC drives fundamentalsShoot-through or commutation failure

Commutation failure begins near firing angles of 180, so typically thefiring angles are limited between 15 and 150

Commutation fault are more likely with 4-Q drives compared to 2-Qdrives. In 2-Q drives the condition will merely cause a loss in outputvoltage. In 4-Q drives, however, a severe overcurrent will occur.Commutation failure will cause very high current flow through motor,DC-breaker (if present), thyristors and fuses. It can cause damage tothe motor, thyristors and fuses.

Commutation failures usually happens while regenerating. The commoncauses are:

Loss of mains or a mains power dip Poor mains quality (too soft mains and thus wide commutation

notches) Excessive armature voltage

Failure or malfunction of a firing pulse circuit


DC drives fundamentalsCurrent in a DC drive

DC currentId

DC current in a branch(120 width)IV2, IV3, IV4

AC current in mains(120 = | Id | and 60 = 0)IL1, IL2, IL3

dI

60 120 240 300 3601800 tw2VI

3VI

4VI

1LI

2LI

3LI


DC drives fundamentalsArmature voltage of 2-quadrant drive

For a 2-Q drive is valid

Firing angle between 15 and 150

Maximum save DC voltage

150 because of commutation (current)and recovery (thyristor)

15 because of safety, due to supplyvoltage jitter

0.9 safety factor for 10 % mains voltagedrop

Voltage source characteristic:Ud

a

Ud ~ cos a

Maximum firing angle

VVU d 470)15(cos40035.19.0 ==a

)15(cos35.19.0 = mainsd UU a


DC drives fundamentalsArmature voltage of 4-quadrant drive

For a 4-Q drive is valid

Firing angle between 15 and 150

Maximum save DC voltage

150 because of commutation (current)and recovery (thyristor)

15 because of safety, due to supplyvoltage jitter

0.9 safety factor for 10 % mains voltagedrop

Positive voltage source characteristic:Ud

a

Ud ~ cos a


Negative voltage source characteristic:Ud

aUd ~ cos a


VVUd 420)150(cos40035.19.0 ==a

)150(cos35.19.0 = mainsd UU a


DC drives fundamentalsContinuous and discontinuous armature current

ContinuousCurrent

DiscontinuousCurrent

Uda

LA

RA

EMK~ n, IF

Uda~ cos a

IA


DC drives fundamentalsQuadrants

The convention for a Cartesiancoordinate system is

The 1st quadrant is on the top right

All other numbers followcounterclockwise

Thus follows:

Quadrant I II III IV

x-coordinate > 0 < 0 < 0 > 0

y-coordinate > 0 > 0 < 0 < 0

II I

III IV

Y

X


DC drives fundamentalsSingle bridge (2-Q)

Typical applications

Extruder

Mixer

Rod and bar mills

IIActive braking

IDriving

IIIDriving

IVBraking

Speed (voltage)

Torque(current)

M

Uda

Id

Forward driving only, opposite speed direction isonly possible if the motor is been turnedexternally!Negative current is not possible!Active braking is not possible!


DC drives fundamentalsDouble bridge (4-Q)


Ski lifts

Test rigs

Winder

For smooth and fast torque reversal

IIActive braking

IDriving

IIIDriving

IVBraking

Speed (voltage)

Torque(current)

M

Uda

Id

Speed in both directions is possible!Negative current is possible!Active braking is possible!


DC drives fundamentalsSingle bridge (2-Q) with field reversal

M

Uda

IdII

Active brakingI

Driving

IIIDriving

IVBraking

Speed

Torque


Mixer

Propulsion

Slow changeover of torque

Less control performance

Useable if P > 500 kW


DC drives fundamentalsMax generating (regenerative) voltage

There is a voltage limitation inquadrants II and IV

The maximum firing angle a is limited to150 since the thyristors need arecovery time of 30

This reduces the motor voltage in a 4-Qdrive

2-Q drives cannot be used for activebraking (positive speed direction), thusthe motor voltage can be higher

Maximumgenerating voltage

IIActive braking

IDriving

IIIDriving

IVBraking

Speed (voltage)

Torque(current)


DC drives fundamentalsMotor acceleration (positive speed direction)

Example: Acceleration in positive direction

Quadrant I is used

Step 1: breakaway torque

Step 2: driving, acceleration at current limit

Step 3: driving (constant speed)

Torque(current)

Speed (voltage)

21

3

IVIII

II I tSpeed(EMF)

1 2 3

tTorque

(current)


DC drives fundamentalsMotor deceleration

Example: Deceleration to zero speed

Quadrants I and II are used


Step 2: active breaking, deceleration at current limit

Step 3: zero speed, current is zero

t

1 2 3

t

21

3

IVIII

II ITorque

(current)

Speed (voltage)

Speed(EMF)

Torque(current)


DC drives fundamentalsMotor acceleration (negative speed direction)

Example: Acceleration in negative direction

Quadrant III is used

Step 1: motor is switched-off

Step 2: driving, acceleration at current limit


2

1

3IVIII

II I t

1 2 3

t

Torque(current)

Speed (voltage)

Speed(EMF)

Torque(current)


DC drives fundamentalsArmature Converter

Mains Commutation

chokes

Thyristor bridge Load

1 3 5

4 6 2

icEMF

aUd

Id

iLXc

uL

~

~

~


DC drives fundamentalsPurpose of commutation chokes

For di / dt limitation during commutation

Prevent interferences between converters connected tothe same line and other upstream connected equipment

Each converter gets its own commutation choke!

When thyristor converters operate, the line voltage isshort-circuited during commutation from one thyristor tothe next. Line reactors are used to reduce thecommutation spikes on the upstream supply.

Commutation chokes lease to a reduction of maximumavailable output voltage, due to its voltage drop

~~

Load


DC drives fundamentalsConfigurations

One commutation choke per drive

uK = 1 % or 4 %

Dedicated transformer

One transformer per drive, typicallyused for large drives

uK = 1 % to 10 %

M M

M


DC drives fundamentalsConfigurations

Autotransformer

Requires an additional commutationchoke

uK = 1 % or 4 %

D7 converters

Maximum two converters pertransformer

uK = 1 % to 10 %

M

M M

Aux. voltage


DC drives fundamentalsFusing of DC drives

Fault in the electronics, application, semiconductors

Wrong tuning of controllers

Wrong parameter settings

Defective Printed Circuit Boards (ageing)

Defective semiconductor (ageing)

Commutation failure

Missing line voltage

Insulation failures

Converter

Wiring

Motor

AC supply

MM



Fuses protect against

Explosion of semiconductors with the risk of fire

Damages of the motor (flash over)

Damages of semiconductors in the converter

Protection philosophy

Size of the system (cost of investment)

Application (2-Q, 4-Q, mainly regenerative)

Acceptable downtime, availability of system

Risk the customer wants to take

Supply voltage conditions (stable networks)

DC fuses (2 of them) should be used for all regenerative (4-Q) drives toprotect the motor in case of a fault during regeneration



Not accordingto standard

fire yesexplosion nomotor yessemiconductor no

M

Recommendati0onfor 4-Q drive

fire yesexplosion yesmotor yessemiconductor yes

M


fire noexplosion nomotor yessemiconductor no

M


fire noexplosion nomotor nosemiconductor noM

Recommendationfor 2-Q drives

fire yesexplosion yesmotor (yes)semiconductor yesM


fire yesexplosion nomotor nosemiconductor noM

Standard fuse

Semiconductor fuse


DC drives fundamentalsFuse dimensioning

Rules

Basic fuse dimensioning is doneaccording to rated current and voltage

Dimensioning based on the A2s-value

Fuse must handle overload conditions

DC fuses must be rated for the samecurrent and voltage as AC fuses ( ACfuses = DC fuses)

DC Drives Fundamentals

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