rectifier chopper

Electrical Drive System

Rectifier & Chopper-‐Controlled DC Drives

EDS

1

Outline

• Introduction • Rectifier• Chopper• References

EDS 2

Controlled rectifier fed dc drives

• Use to get variable dc voltage from an ac source of fixed voltage.• Controlled rectifier fed dc drives also known as Ward-‐Leonard drives.

Types of Controlled Rectifier

Single phase fully controlled rectifier of a dc separately excited motor.


T1 & T3 are given gate signal from 𝛼 𝑡𝑜 𝜋


T2 & T4 are given gate signals from𝜋 + 𝛼 𝑡𝑜 2𝜋


Current does not flow continously-‐ Discontinousconduction


Current flow continously-‐ Continousconduction

Power Electronic Converters for DC Drives� Power electronics converters are used to obtain variable voltage

�Highly efficient� Ideally lossless

� Type of converter used is depending on voltage source :�AC voltage source ⇒ Controlled Rectifiers�Fixed DC voltage source

⇒ DC-‐DC converters (switch mode converters)

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Continuous Conduction

Two quadrant operation

Example

Solution

DC – DC Converter Fed Drives

� To obtain variable DC voltage from fixed DC source� Self-‐commutated devices preferred (MOSFETs, IGBTs, GTOs) over thyristors� Commutated by lower power control signal� Commutation circuit not needed� Can be switched at higher frequency for same rating

� Improved motor performance (less ripple, no discontinuous currents, increased control bandwidth)

� Suitable for high performance applications� Regenerative braking possible up to very low speeds even when fed from fixed DC voltage source

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DC – DC Converter Fed Drives-‐ Step Down Class A Chopper

Motoring• Provides positive output voltage and current• Average power flows from source to load (motor)• Switch (S) operated periodically with period T

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T

Q1Q2

Q3 Q4

ω

V

S

D

Ra

La

Ea

Va

Ia


S is ON (0 ≤ t ≤ ton)

VEdtdiLiR a

aaa =++

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Motoring

Duty Interval( ia ↑ )

•Va = V•Ia flows to motor•|Ia| increases

V

S

D

Ra

La

Ea

Va

Ia

Ra

La

Ea

Va

Ia

V


S if OFF (ton ≤ t ≤ T)

0=++ EdtdiLiR a

aaa

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Motoring

Freewheeling Interval( ia ↓ )

•Va = 0•Ia freewheels through diode DF•|Ia| decreases

Ra

La

Ea

Va

Ia

ID

V

S

D

Ra

La

Ea

Va

Ia

DC – DC Converter Fed-‐ Step Down Class A ChopperMotoring�Duty cycle�Under steady-‐state conditions:Motor side: Chopper side, average armature voltage:Therefore,

�Hence, average armature current:

EDS 21

period chopper where == TTtonδ

EIRVV aaa +==δ

aa R

EVI

−=δ

δ T

Freewheeling Interval( ia ↓ )

Duty Interval( ia ↑ )

EIRV aaa +=

VVa δ=

DC – DC Converter Fed Drives-‐ Step Up Class B Chopper

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T

Q1Q2

Q3 Q4

ω

•Possible for speed above rated speed and down to nearly zero speed•Application:• Battery operated vehicles• Regenerated power stored in battery

V S

D Ra

La

Ea

Va

Ia

Regenerative Braking� Provides positive output voltage and negative average output current

� Average power flows from load (motor) to source

Switch (S) operated periodically with period T


S is ON (0 ≤ t ≤ ton)� Va = 0 (diode blocks V)� ia increases due to E(since E > Va)

� Mechanical energy converted to electrical (i.e. generator)

� Energy stored in La� Any remaining energy dissipated in Ra and S

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Regenerative Braking

Energy Storage Interval( ia ↑ )

EdtdiLiR a

aaa =+

V S

D Ra

La

Ea

Va

Ia

Ra

La

Ea

Va

Ia

S


S if OFF (ton ≤ t ≤ T)• ia flows through diode D and source V• ia decreases in negative direction• Energy stored in La & energy supplied by machine are fed to the source

EDS 24

Regenerative Braking

Duty Interval( ia ↓ )

EVdtdiLiR a

aaa =++

V S

D Ra

La

Ea

Va

Ia

Ra

La

Ea

Va

Ia

V

DC – DC Converter Fed Drives-‐ Step Up Class B ChopperRegenerative Braking� Duty cycle

� Under steady-‐state conditionsGenerator side: Chopper side, average armature voltage:

� Therefore,

� Hence, average armature current:

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periodchopper where == TTtonδ

( ) aaa IREVV −==−δ1

( )a

a RVE

Iδ−−

=1

δ T

Duty Interval( ia ↓ )

Energy Storage Interval( ia ↑ )

aaa IREV −=

( )VVa δ−= 1

Negative because current flows from motor to source

DC – DC Converter Fed Drives-‐ Two-‐quadrant Control• Combination of Class A & B choppers• Forward motoring Q1 -‐ T1 and D2 (Class A)• Forward braking Q2 – T2 and D1 (Class B)

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D2

+Va-

T1D1

T2

D2

+

V

-

T

Q1Q2

Q3 Q4

ω

No Speed Reversal

•Va always +ve ⇒ω always +ve• Ia can be +ve or –ve• Do not fire both switches together⇒ short circuit at supply

DC – DC Converter Fed Drives-‐ Two-‐quadrant Control

� Forward motoring Q1 -‐ T1 and D2 (Class A) �T1 conducting: Va = V (ia ↑) • D2 conducting: Va = 0 (ia ↓)

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T1

T2

D1

+

Va

-

D2

ia

+

V

−

T1

T2

D1

+

Va

-

D2

ia

Average Va Ea

+

V

−

Average Va = δ1V, δ1 = (ton T1 / T ), δ2 = 0

T2 alwaysOFF

T1 choppingON & OFF

•AverageVa positive•AverageVa made larger

than back emf Ea•Ia positive

DC – DC Converter Fed Drives-‐ Two-‐quadrant Control� Forward braking Q2 – T2 and D1 (Class B)

�D1 conducting: Va = V (ia ↓) • T2 conducting: Va = 0 (ia ↑)

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T1

T2

D1

+

Va

-

D2

ia

T1

T2

D1

+

Va

-

D2

ia

Average Va

Ea

•Average Va positive•Average Va made smaller

than back emf Ea•Ia negative (motor acts as

generator)

+

V

−

+

V

−

Average Va =(1 -‐ δ2)V, δ1 = 0, δ2 = (ton T2 / T )

T1 alwaysOFF

T2 choppingON & OFF

DC – DC Converter Fed Drives-‐ Two-‐quadrant Control� For fast transition from motoring (Q1) to braking (Q2) and vice versa, both T1 and T2 are controlled simultaneously, i.e. within a period T:� T1 in ON and T2 is OFF between time 0 < t ≤ ton

� If Ia is positive (Va > E), current flows from supply to motor via T1� If Ia is negative (E > Va), current flows from motor to supply via D1

� T1 is OFF and T2 is ON between ton < t ≤ T� If Ia is positive, current circulates via D2� If Ia is negative, current circulates via T2

� Duty ratio is given by:

� Average armature voltage is:EDS 29

periodchopper where1 == TTt Tonδ

Average Va =δV

DC – DC Converter Fed Drives -‐ Two-‐quadrant Control: Example

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Solution

DC – DC Converter Fed Drives-‐ Four-‐quadrant Control• Operation in all four quadrants• Va and Ia can be controlled in magnitude and polarity• Power flow can be in either direction

• Speed and torque can be reversed

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+ Va -T1

D1

T2D2

D3

D4

T3

T4

T

Q1Q2

Q3 Q4

ω

ia

Note:Polarity of Va and direction of Iaindicated are assumed positive.

DC – DC Converter Fed Drives-‐ Four-‐quadrant Control• When a switch is on (i.e. ‘ON state’) it may or may not conduct current depending on the direction of ia• If a switch conducts current, it is in a conducting state• Converter has two legs (Leg A & Leg B)• Both switches in each leg, are alternately switched• If T1 = ON, T4 = OFF• If T4 = ON, T1 = OFF

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+ Va -T1

D1

T2D2

D3

D4

T3

T4

+

Vdc

-

Leg A

Leg B

ia

DC – DC Converter Fed Drives-‐ Four-‐quadrant Control� Positive Current (Ia > 0)

� Va = Vdcwhen T1 and T2 are ON� Current increases� Q1 operation

� Va = 0 when current freewheels through T2 and D4� Current decreases

� Va = -‐Vdc when D3 and D4conducts current� Current decreases� Energy returned to supply� Q4 operation

EDS 35

+ Va -T1

D1

T2D2

D3

D4

T3

T4

T

Q1Q2

Q3 Q4

ω

+

Vdc

-

ia

T3 and T4 off

DC – DC Converter Fed Drives-‐ Four-‐quadrant Control� Negative Current (Ia > 0)

� Va = -‐Vdc when T3 and T4 are ON� Current increases in negative direction� Q3 operation

� Va = 0 when current freewheels through T4 and D2� Current decreases

� Va = Vdcwhen D1 and D2conducts current� Current decreases� Energy returned to supply

� Q2 operation

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T

Q1Q2

Q3 Q4

ω

+

Vdc

-

ia+ Va -

T1D1

T2D2

D3

D4

T3

T4

T1 and T2 off

DC – DC Converter Fed Drives-‐ Four-‐quadrant Control� For both positive and negative current, output voltage can swing between:� Vdc and -‐Vdc� Vdc and 0

� Four quadrant chopper has two legs, so it requires two switching signals (one for each leg)

� Depending on relationship between the two switching signals, 4-‐quadrant chopper has two switching schemes:� Bipolar switching� Unipolar switching

� Switching scheme determines output voltage swing betweenVdc and -‐Vdc or Vdc and 0.

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DC – DC Converter Fed Drives• Operation of DC motor drive depends on:• Direction of Ia (determined by torque, i.e. motoring or braking)• Polarity of Va and Ea (determined by speed, i.e. forward or reverse)• the duty cycle of the DC-‐DC Converter (either two-‐quadrant or four-‐quadrant)

• Open loop control is achieved by changing the duty cycle manually as and when required

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References

• Dr. Anisa Lecture Slides Handout, Uniten. • Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha Science Int. Ltd., UK, 2001.• Krishnan, R., Electric Motor Drives: Modeling, Analysis and Control, Prentice-‐Hall, New Jersey, 2001.

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rectifier chopper

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