DC Drive Basics, part 1 - ABB · PDF fileMV / LV transformer Main contactor AC fuse...

© ABB Group March 4, 2010 | Slide 1

DC_DRIVE_BASICS_01R0201

DC drives fundamentals, Part 1

E-Learning, DC drives

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Objectives

This training module covers:

General layout of a thyristor based converter

Functionality of a six pulse thyristor bridge

Armature 2 quadrant and 4 quadrant converter

Armature voltage and current

Mains voltage and current

Continuous and discontinuous current

Driving and braking mode

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General Layout with external field excitation

General layout

Power Transformer

Armature circuit

Fuse

Main contactor

Commutation choke

Armature converter

Field circuit

Fuse

Autotransformer

Contactor

Field exciter

~~--

Load

MV lineMV / LV transformer

Main contactor (K1)

AC fuse (F1)

Commutation choke (L1)

Field fuse (F3)

Armature converter

DC fuse

Field winding

Field contactor (K3)

Field converter

Autotransformer (T3)

M


Help



General Layout with on-board field excitation

Internal field excitation

Build-in inside module

Fixed field current depending on module size

Advantages

Compact module

Fast commissioning

Less hardware required (transformer, chokes)~~

--

M Load


Main contactor

AC fuse

Commutation choke

Field fuse

Converter

DC fuse

Field winding

F1

K1

L1


Help



Armature Converter6-pulse thyristor bridge

DC currentAC line current

3∼ AC network 1 3 5

4 6 2

αUd

Id

iLL 1

uL

~

~

~L 3

L 2N


Help



Generating output current

Voltages

Phase voltage

Phase to phase voltage

Thyristor 1 and 6 are active

Output shows a bubble

L1L3L2a=0

L1L3L2a=0

L1L3L2a=0L12

3∼ AC network

DC voltage ( controlled )


1 3 5

4 6 2

αUd

Id

iLL 1

uL

~

~

~L 3

L 2


Help



How works a thyristor converter

6-pulse thyristor bridge with a load

Firing sequence

Thyristor 1 + 6

Thyristor 2 + 1

Thyristor 3 + 2

Thyristor 4 + 3

Thyristor 5 + 4

Thyristor 6 + 5

L1L3L2a=0


4 6 2

αUd

Id

iLL 1

uL

~

~

~L 3

L2


Help



Driving modeMachine works in motor mode

Positive voltage

Firing angle smaller than 90°

Minimum firing angle is 15°

Natural firing point is the intersection point between two phases

In this example the thyristor is fired after 30° from natural firing point

L1L3L2a=0L12

α=30°


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Regenerative modeMachine works in regenerative mode

Negative voltage

Firing angle greater than 90°

Maximum firing angle is 150°

α=150°


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Shoot-throughCommutation failure

DC drives are compromised by shoot-through

Damage fuses

Damage thyristors

Causes of shoot- through

Power failure

Too big firing angles

Working range has to be limited

Typical firing angles are between 15°and 150°

0° 30° 60° 90° 120° 150° 180°

α

L2 L3 L1

α = 180°

Ausgangsgleichspannung

Netzspannung

Zündwinkel

WECHSELRICHTERKIPPENGDCZ142

t

t

balancing voltage

firing angle


Help



Armature Converter

Mains Line chokes Thyristor bridge


Help



Converter current in a DC drive

Average current: Id

DC current in one thyristor arm:IV2, IV3, IV4120° width

AC current in mainsIL1, IL2, IL3

120°: Id

60° : 0

dI

°60 °120 °240 °300 °360°180°0 tω

2VI

3VI

4VI

1LI

2LI

3LI

1

2

3

4

5

6 6

1

1

2

2

3

3

4

4

5

5

6

6

2

3

4


Help



Armature voltage of 2-quadrant drive

2-quadrant drive maximum motor voltage

Firing angle between 15° and 90°

For example:

Voltage source characteristic:Ud

α

Ud ∼ cos α

Maximum firing angle

( ) ( ) VVU A 470%1015cos40035.1max =±⋅°⋅⋅=

( )°⋅⋅= 15cos35.1max mainsd UU


Help



Armature voltage of 4 quadrant drive



For example:

Positive voltage source characteristic:Ud

α

Ud ∼ cos α


Negative voltage source characteristic:Ud

α

Ud ∼ cos α



( ) ( ) VVU A 420%1030cos40035.1max =±⋅°⋅⋅=


Help



DC current and AC current

Calculate AC current with a known DC current

Example with a motor load (2Q):

Commutation chokes, cables, contactors and fuses have to be selected depending on RMS values!

1LIdI

dU82.0321 ⋅=== dLLL IIII

AAII dL 82082,0100082,01 =⋅=⋅=

( ) VVUU dLL 852

9,015cos35,11000

9,0cos35,1 min

max21 =

⋅°⋅=

⋅⋅=− α

MVAAVIUS LLL 21,18208523 121 =⋅=⋅⋅= −

AId 1000= VUd 1000=

maxmaxmax ddd IUP ⋅=

MWAVPd 110001000max =⋅=


Help



Continuous and Discontinuous Armature CurrentPrinciple circuit diagram

Continuous Current

Discontinuous Current


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2-Quadrant converter

Typical applications

Extruder

Mixer

Properties

Only forward driving possible

Braking with positive speed isn’t possible

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

M

Ud

Id

Bridge 1


Help





Ski lifts

Test rigs

Winder

Properties

Smooth and fast torque reversal

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

M

Ud

Id

Bridge 1Bridge 2


Help



2-Quadrant converter and Field Reversal


Mixer, Propulsion

Realize E-Stop functionality

Properties

Slow changeover of torque

Less control performance

Useable if P > 500 kW

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

M

Ud

Id

Bridge 1

Possible bychanging

field current


Help



Usable working range of a DC drive

There is a limitation in quadrant II and IV

Maximum firing angle 150°

Thyristors needs a circuit commutated recovery time

This reduces the motor voltage in a 4 quadrant drive

2 quadrant drives cannot used for braking in positive speed direction

Motor voltage is greater

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

Maximum regenerative voltage


Help



Motor acceleration

Example: Acceleration until 100% motor speed

Quadrant I is used

Step 1: Breakaway torque

Step 2: Acceleration at current limit

Step 3: Driving with constant speed

Torque(current)

Speed / Voltage

21

3

IVIII

II I tSpeed(EMF)

1 2 3

t

Torque(current)


Help



Motor deceleration

Example: Deceleration until standstill

Quadrant I and II are used

Step 1: Driving mode Constant speed

Step 2: Breaking mode Deceleration at current limit

Step 3: Standstill, current is zero

tSpeed(EMF)

1 2 3

t

Torque(current)

Torque(current)

Speed / Voltage

21

3

IVIII

II I


Help



Driving in negative direction

Example: Acceleration in negative direction

Quadrant III is used

Step 1: Motor is switched-off

Step 2: Driving mode Acceleration at current limit


Torque(current)

Speed / Voltage

2

1

3IVIII

II I tSpeed(EMF)

1 2 3

t

Torque(current)


Help



Example for a motor application

Characteristics of a 4-quadrant drive

Acceleration of the machine (1, 2)

Decrease current (3)

Increase current (4)

Torque(current)

Speed / Voltage

7

34

6

5 21

t

t

Speed(EMF)

Torque(current)

1 2 34 5 6 7

Deceleration of the machine (5)

Acceleration in (6)

Driving in reverse direction (7)


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Summary

Key points of this module are:









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Additional information

ThyristorControl element in a line-commutated converter. Delicate against di / dt.

Discontinuous currentCurrent includes gaps because the armature current is too small.

Continuous currentCurrent is continuous because of a big load.

Firing angleFiring angle controls the thyristors. Values between 15°and 150° are typical.

Regenerative modeLine-commutated converter is working as a generator and supply to mains.

CommutationIs the change-over from one thyristor to another.

HarmonicsHarmonics are multiples of the basic oscillation which generates disturbances in network.


1



DC drives fundamentals, Part 1

E-Learning, DC drives

Welcome to the DC Drive Basics training module for the ABB DC drives.

If you need help navigating this module, please click the Help button in the top right-hand corner. To view the presenter notes as text, please click the Notes button in the bottom right corner.

2

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Objectives

This training module covers:








After completing this module, you will know the

• General layout of a thyristor based converter

• Functionality of a six pulse thyristor bridge

• Armature 2 quadrant and 4 quadrant converter

• Armature voltage and current

• Mains voltage and current

• Continuous and discontinuous current

• Driving and braking

3

Help



General Layout with external field excitation

General layout

Power Transformer

Armature circuit

Fuse

Main contactor

Commutation choke

Armature converter

Field circuit

Fuse

Autotransformer

Contactor

Field exciter

• In this chapter the general layout with external field excitation will be explained.

• The picture shows the general layout of a DC drive configuration. In this example, the power supply is a medium voltage line which connects the DC drive via a power transformer to the supply network.

• From this point there is a difference between the armature and the field circuit which will be discussed separately.

• The armature circuit includes an AC fuse, the main contactor or main switch, the commutation choke and the armature converter. In addition, the drives in regenerative mode with weak power supply systems should be protected with DC fuses on the DC side.

• The field circuit includes a fuse, the field contactor, the field converter and an autotransformer which adapts the network voltage to the needed field voltage.

• Note: A fuse cannot be between the field connectors at the motor and the field converter.

4

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General Layout with on-board field excitation

Internal field excitation

Build-in inside module

Fixed field current depending on module size

Advantages

Compact module

Fast commissioning

Less hardware required (transformer, chokes)~~

--

M Load


Main contactor

AC fuse

Commutation choke

Field fuse

Converter

DC fuse

Field winding

F1

K1

L1

• This is the general layout with DCS800 On-Board field excitation. In comparison with the layout from an external field excitation, only a reduced amount of external components are required.

• Inside converter module the internal field excitation is built-in. The maximum field current depends on the size of the module.

• The advantages of this configuration are:

• It is a compact converter module.

• Fast commissioning because all settings can be set with the software tools.

• Less hardware required, transformer, chokes, fuses and a contactor are not needed.

5

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Armature Converter6-pulse thyristor bridge



4 6 2

αUd

Id

iLL 1

uL

~

~

~L 3

L 2N

• In this slide, the armature converter will be looked at.

• Basically, the armature converter is a 6-pulse thyristor bridge. It will be connected to a 3-phase supply because the mains should be loaded symmetrically. The output of this 6-pulse thyristor bridge is in dc current and dc voltage which is controlled by a firing angle and a firing pulse. The output current is a dc current plus a current ripple. The motor is like an inductive load in the circuit which smoothes the dc current.

• Note: The maximum dc voltage is dependent on the incoming AC voltage between two phases!

6

Help



Generating output current

Voltages

Phase voltage

Phase to phase voltage

Thyristor 1 and 6 are active

Output shows a bubble

L1L3L2a=0

L1L3L2a=0

L1L3L2a=0L12

3∼ AC network

DC voltage ( controlled )


1 3 5

4 6 2

αUd

Id

iLL 1

uL

~

~

~L 3

L 2

• Generating DC output current is the task of the 6-pulse bridge. To do this, the thyristors have to be fired in a logical sequence.

• There are 2 voltages present in an AC network. The phase voltage is measured between the neutral point of the transformer and a phase. If the phases are shifted 120°, the graph looks like the picture with the lines marked with L1, L2 and L3. This voltage is not important for the bridge because of a symmetrical load where no neutral conductor is used.

• Important for the DC output voltage is the phase to phase voltage which can be measured between two phases.

• Here is a small example: Thyristor 1 and 6 are active. So the resulting voltage of L1 and L2 is at the output. The output of the converter shows a bubble which can be measured with an oscilloscope.

7

Help



How works a thyristor converter

6-pulse thyristor bridge with a load

Firing sequence

Thyristor 1 + 6

Thyristor 2 + 1

Thyristor 3 + 2

Thyristor 4 + 3

Thyristor 5 + 4

Thyristor 6 + 5

L1L3L2a=0


4 6 2

αUd

Id

iLL 1

uL

~

~

~L 3

L2

• How does a thyristor converter work? This question should be explained on this slide.

• The starting point is a 6-pulse thyristor bridge with a load. The firing sequence of the thyristor bridge is generated from the control board which ensures that two thyristors are fired. Basically, a thyristor from the upper arm and one thyristor from the bottom arm work together.

• The sequence is like the following: Thyristor 6 is fired in the last cycle. In the next step thyristor one is fired and thyristor 6 commutates to thyristor 2. This sequence is continuous.

• Note: The commutation from a thyristor to another thyristor changes from the upper and the under series. In a 50 hertz network, a thyristor is fired every 3.3 milliseconds, alternating from the upper and the under series.

8

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Driving modeMachine works in motor mode

Positive voltage

Firing angle smaller than 90°

Minimum firing angle is 15°

Natural firing point is the intersection point between two phases

In this example the thyristor is fired after 30° from natural firing point

L1L3L2a=0L12

α=30°

• Driving mode means the motor receives energy from the converter which is transformed into mechanical energy.

• To control the output voltage of the DC converter, the firing point of a thyristor has to be set. The firing angle is counted starting with the natural firing point.

• Alpha zero degrees means a diode bridge is in operation. Normally, the firing angle is smaller than 90°in driving mode. The minimum firing angle should be 15° in order to allow for a safety clearance in the event of fluctuation in the mains.

• The maximum output dc voltage is reached if the firing angle has the smallest value.

• The point from which the firing angle is calculated is also significant:

• The natural firing point of a thyristor is the point of intersection between two phases. In principle, the firing angle is a delay between the point of intersection and the firing point.

• With an inductive load at a firing angle of 90°, the dc output voltage is zero.

• In this example, the thyristor is fired 30° from the natural firing point.

9

Help



Regenerative modeMachine works in regenerative mode

Negative voltage

Firing angle greater than 90°

Maximum firing angle is 150°

L1L3L2a=0L12

α=150°

• In this slide, the regenerative mode will be explained.

• In the regenerative mode, mechanical energy is transformed into electrical energy. So the dc machine functions in the regenerative mode as a generator.

• The properties of the regenerative mode are that with an inductive load the firing angle is greater than 90° but limited to 150°. A limitation of regenerative mode is needed to protect the converter against conduction-through. Time is needed to have enough circuit commutation recovery time for the thyristors. It is essential that the thyristor is switched-off after zero-crossing of mains voltage. If there is a re-ignition of a thyristor, the output voltage can be positive again and the converter could be damaged.

10

Help



Shoot-throughCommutation failure

DC drives are compromised by shoot-through

Damage fuses

Damage thyristors

Causes of shoot- through

Power failure

Too big firing angles

Working range has to be limited

Typical firing angles are between 15°and 150°

0° 30° 60° 90° 120° 150° 180°

α

L2 L3 L1

α = 180°

Ausgangsgleichspannung

Netzspannung

Zündwinkel

WECHSELRICHTERKIPPENGDCZ142

t

t

balancing voltage

firing angle

• Shoot-through presents a possible danger for the thyristor converter and the dc motor.

• The DC drives are compromised by shoot-through. This will blow the fuses and the thyristors of the converter because of a short circuit inside the converter. The motor could also be damaged!

• Some causes of shoot-through are power failure and firing angles that are too large.

• If the drive is in regenerative mode and there is a mains power failure which opens the main contactor, the drive is in shoot-through.

• The problem occurs if a thyristor commutates to next thyristor, the output polarity is inverted and load current increases very fast.

• To avoid this unallowed function the working range has to be limited. Typical firing angles are between 15° and 150°. Firing angles greater than 150° could cause a hold-off interval that is too short which eases a reignition of a thyristor.

11

Help



Armature Converter

1 3 5

4 6 2

ik

EαUd

Id

iLXk

uL

~

~

~

Mains Line chokes Thyristor bridge

• The armature converter must be connected with the 3-phase network. In the picture you can see the configuration of the armature converter between mains and motor.

• Switching from one thyristor to the next is a short circuit of mains. To limit the increase of short circuit current "ik", commutation chokes must be connected between mains and a thyristor bridge.

• Basically the 6-pulse thyristor bridge is a controlled voltage or current source.

12

Help



Converter current in a DC drive

Average current: IdDC current in one thyristor arm:IV2, IV3, IV4120° width

AC current in mainsIL1, IL2, IL3

120°: Id60° : 0

dI

°60 °120 °240 °300 °360°180°0 tω

2VI

3VI

4VI

1LI

2LI

3LI

1

2

3

4

5

6 6

1

1

2

2

3

3

4

4

5

5

6

6

2

3

4

• The converter current in a dc drive must be analyzed on the dc and ac side of the supply network. A thyristor bridge using six controlled thyristors produce a constant component dc at the output with a current ripple. If the system is working properly, six bubbles can be measured with an oscilloscope. The average current is the mean value also called the ideal dc current. The picture also shows which thyristors are active.

• As long as a thyristor is active, a dc current flows through it. So each thyristor controls 120° of current width in one period.

• In this picture, only three thyristor currents are shown.

• In an ac circuit, the dc current is represented. But the thyristors from the bottom arm produce a negative current in an ac circuit. In each phase the current is active 120° and inactive 60° in a half period.

• This is responsible for a factor between dc and ac current which is explained in the next slides.

13

Help



Armature voltage of 2-quadrant drive



For example:

Voltage source characteristic:Ud

α

Ud ∼ cos α


( ) ( ) VVU A 470%1015cos40035.1max =±⋅°⋅⋅=


• The armature voltage of a 2 quadrant drive depends on the mains voltage and is limited by the converter type used.

• 2 quadrant drive types have one thyristor bridge with six thyristors. The diagram shows the voltage source characteristic. With the firing angle alpha, the output voltage can be controlled but it is not possible to use the full range from zero to 180 degrees. Typical firing angles are between 15 and 90 degrees.

• For a 2-quadrant drive the maximum motor voltage can be calculated with a maximum firing angle of 15° and the used mains voltage.

• If the mains voltage is 400 volts, the maximum motor voltage can be 470 volts. The limitation for 2-quadrant drives is smaller than with a 4 quadrant drive because the braking mode is not possible in forward direction.

• Note: Because of fluctuations of the mains voltage, it is necessary to calculate 10% in reserve.

14

Help



Armature voltage of 4 quadrant drive



For example:

Positive voltage source characteristic:Ud

α

Ud ∼ cos α


Negative voltage source characteristic:Ud

α

Ud ∼ cos α



( ) ( ) VVU A 420%1030cos40035.1max =±⋅°⋅⋅=

• Armature voltage of a 4 quadrant drive is generated from two thyristor bridges which are anti parallel.

• This configuration allows positive and negative output voltage as well as current.

• For a 4-quadrant drive the maximum motor voltage can be calculated with a maximum firing angle of 30° and the used mains voltage. If the mains voltage is 400 volts the maximum motor voltage can be 420 volts. The limitation comes from the situation that in braking mode 150° is the maximum and a reserve of 30° also has to be used for the driving mode.

• A 4 quadrant drive uses two characteristics which handle the control of one thyristor bridge.

• Note: Only one thyristor bridge can be active. Thyristor bridge 1 on the left hand side reaches the maximum positive output voltage with the smallest firing angle. Thyristor bridge 2 on the left hand side, which is anti parallel, reaches the maximum negative output voltage with the smallest firing angle. Switching from bridge 1 to bridge 2 is automatically calculated by the controller board.

15

Help



DC current and AC current

Calculate AC current with a known DC current

Example with a motor load (2Q):

Commutation chokes, cables, contactors and fuses have to be selected depending on RMS values!

1LIdI

dU82.0321 ⋅=== dLLL IIII

AAII dL 82082,0100082,01 =⋅=⋅=

( ) VVUU dLL 852

9,015cos35,11000

9,0cos35,1 min

max21 =

⋅°⋅=

⋅⋅=− α

MVAAVIUS LLL 21,18208523 121 =⋅=⋅⋅= −

AId 1000= VUd 1000=

maxmaxmax ddd IUP ⋅=

MWAVPd 110001000max =⋅=

• DC current and AC current depend on each other. They are needed to calculate a dc system.

• The ac current can be calculated with a factor of 0.82, if the dc current is known. So the ac current with factor of 0.82 is lower than the dc current.

• Here is a small calculation example with a two quadrant drive.

• The maximum value for the motor current is 1000 amps and 1000 volts for the motor voltage. The dc active power with neglected losses can be calculated by multiplying dc voltage and dc current. In this example the maximum dc active power is 1 megawatt.

• With this information it is easy to calculate the RMS value of an ac current by multiplying the dc current by a factor of 0.82. In this example the ac current is 820 amps. The ac voltage needed in this configuration has to be calculated by using the minimum firing angle and a reserve of 10%. In this example, a minimum ac voltage of 852 volts has to be supplied. The apparent power on the ac side can be calculated with the known ac voltage and current. It is used to select the transformer power.

• Note: Commutation chokes, cables, contactors and fuses have to be selected depending on RMS values.

16

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Continuous and Discontinuous Armature CurrentPrinciple circuit diagram

Continuous Current

Discontinuous Current

• Continuous and discontinuous armature current depends on the load and the size of armature inductance.

• With big loads, the current will be continuous. That means the current is greater than zero the entire time. With small loads the current could include times with zero current.

• The discontinuous current limit is an average dc current where the current bubbles touch the zero line. Typical motors which are supplied by a six pulse thyristor bridge have a discontinuous current limit between 20 and 40%.

• The point where continuous and discontinuous current changes is important for the current controller. There the control performance must be changed which has to be saved in a drive parameter.

17

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Extruder

Mixer

Properties

Only forward driving possible

Braking with positive speed isn’t possible

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

M

Ud

Id

Bridge 1

• Typical applications for 2-quadrant drives are extruder or mixer. These converters can only be used for forward driving. Braking with positive speed is not possible.

• The diagram shows the working range of this converter type. Only one thyristor bridge is installed. That means only quadrant 1 and quadrant 4 can be used because the current cannot change current direction.

18

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Ski lifts

Test rigs

Winder

Properties

Smooth and fast torque reversal

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

M

Ud

Id

Bridge 1Bridge 2

• Typical applications for 4-quadrant converters are ski lifts, test rigs and winders. This converter includes two anti parallel thyristor bridges which allow all 4 quadrants to be used. The motor current can be positive or negative, the same goes for the output voltage. Control behavior of this converter type is the smooth and fast torque reversal for fast braking.

19

Help



2-Quadrant converter and Field Reversal


Mixer, Propulsion

Realize E-Stop functionality

Properties

Slow changeover of torque

Less control performance

Useable if P > 500 kW

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

M

Ud

Id

Bridge 1

Possible bychanging

field current

• Another option of changing the motor direction and of going to quadrants 2 and 3 is to use a field reversal configuration.

• Normally, the field current is fixed and the flux is in one direction. In field reversal, the current direction will be changed which causes a flux direction change. This inverts the motor voltage. The result of this combination is more or less a 4-quadrant drive with reduced components. Motor power is given by negative motor voltage and positive motor current.

• The control behaviors are a slow changeover of the torque, because changes in the field circuit are slower than in an armature circuit. This type of field reversal provides less functionality and is therefore only used if the power is greater than 500 kilowatts. So the costs of a field reversal are cheaper than with a real 4-quadrant armature converter.

20

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Usable working range of a DC drive

There is a limitation in quadrant II and IV

Maximum firing angle 150°

Thyristors needs a circuit commutated recovery time

This reduces the motor voltage in a 4 quadrant drive

2 quadrant drives cannot used for braking in positive speed direction

Motor voltage is greater

IIBraking

IDriving

IIIDriving

IVBraking

Speed / Voltage

Torque(current)

Maximum regenerative voltage

• The usable working range of a DC drive is limited in quadrants 2 and 4 because of the firing angle limitation of 150°. This is needed because of a circuit commutated recovery time. This limitation of the firing angle reduces the motor voltage in a 4-quadrant drive. 2-quadrant drives cannot be used for braking in positive speed direction, so the motor voltage in quadrants 1 and 3 can be higher, compared to classic 4 quadrant drives. This voltage variation is needed when classic 2 quadrant drives are upgraded to 4 quadrant drives.

21

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Motor acceleration

Example: Acceleration until 100% motor speed

Quadrant I is used

Step 1: Breakaway torque

Step 2: Acceleration at current limit

Step 3: Driving with constant speed

Torque(current)

Speed / Voltage

21

3

IVIII

II I tSpeed(EMF)

1 2 3

t

Torque(current)

• Motor acceleration in positive direction is possible in quadrant one.

• The function of an acceleration process of a dc motor should be explained by using an example of an acceleration from zero to 100% motor speed.

• Before a motor is able to turn, a breakaway torque is needed. After that, the torque is at the limit and the motor accelerates up to 100% motor speed. Once the motor has reached the maximum speed, the torque decreases until it reaches a value where it remains constant.

• The motor torque during constant speed includes the mechanical losses as well as the torque from the load.

22

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Motor deceleration

Example: Deceleration until standstill

Quadrant I and II are used


Step 2: Breaking mode Deceleration at current limit

Step 3: Standstill, current is zero

tSpeed(EMF)

1 2 3

t

Torque(current)

Torque(current)

Speed / Voltage

21

3

IVIII

II I

• Motor deceleration is possible in quadrant two. This example should show a typical deceleration from maximum speed to zero. In step one the drive works in quadrant one which means the motor speed and also the torque is positive. Then the machine has to change in breaking mode in step 2, so the torque will be negative and the motor speed will decrease. The dc machine is breaking at the current limitation until the speed is zero. In step 3 the motor is standing still and the current is zero.

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Driving in negative direction

Example: Acceleration in negative direction

Quadrant III is used

Step 1: Motor is switched-off

Step 2: Driving mode Acceleration at current limit


Torque(current)

Speed / Voltage

21

3IVIII

II I tSpeed(EMF)

1 2 3

t

Torque(current)

• Driving in negative direction is also possible with a four quadrant drive.

• The example shows an acceleration of motor speed in negative direction. In step 1 the motor is switched-off. Then in step 2 the motor accelerates in quadrant 3 at the current limitation to 100% motor speed. In step 3 the acceleration process is finished and the dc machine runs in constant speed with the needed torque for losses and the load.

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Example for a motor application

Characteristics of a 4-quadrant drive

Acceleration of the machine (1, 2)

Decrease current (3)

Increase current (4)

Torque(current)

Speed / Voltage

7

34

6

5 2

1

t

t

Speed(EMF)

Torque(current)

1 2 34 5 6 7

Deceleration of the machine (5)

Acceleration in (6)

Driving in reverse direction (7)

• An example for a motor application as a 4-quadrant drive is shown in the pictures.

• The characteristics of a 4-quadrant drive are:

• The machine will be accelerated in step 1 and 2. This is possible in quadrant 1 because the voltage and the current are positive.

• The machine reaches the maximum speed, so the current will be decreased. This is also possible in quadrant 1.

• Now we are switching from quadrant 1 to quadrant 2. So the current will be negative which causes a deceleration of the motor.

• With maximum torque and no voltage, the motor changes the direction of the rotor.

• In step 6 we are switching from quadrant 2 to quadrant 3 which means that the motor voltage will be negative.

• If the machine reaches the minimum speed, the current decreases to zero.

• Braking with reverse bridge is possible in quadrant 4.

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Summary










• General layout of a thyristor based converter

• Functionality of a six pulse thyristor bridge

• Armature 2 quadrant and 4 quadrant converter

• Armature voltage and current

• Mains voltage and current

• Continuous and discontinuous current and

• Driving and braking

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Additional information

ThyristorControl element in a line-commutated converter. Delicate against di / dt.

Discontinuous currentCurrent includes gaps because the armature current is too small.

Continuous currentCurrent is continuous because of a big load.

Firing angleFiring angle controls the thyristors. Values between 15°and 150° are typical.

Regenerative modeLine-commutated converter is working as a generator and supply to mains.

CommutationIs the change-over from one thyristor to another.

HarmonicsHarmonics are multiples of the basic oscillation which generates disturbances in network.

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