Ppt 6. Solid State Control - Large Fonts

8/10/2019 Ppt 6. Solid State Control - Large Fonts

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Solid State Control Applied Electricity – EE 2802



Power Switching Devices

Power diode

Power transistor

Silicon controlled rectifier (SCR)

Gate turn-off thyristor (GTO)

MOSFET

Insulated gate bipolar transistor



Motor Drive

A modern variable speed system has

four components:

1. Electric machine

2. Power converter

3. Controller4. Load



Closed Loop Speed Control



Speed Control of DC Motors



Methods

1. Field control2. Armature control

Armature resistance control

Armature voltage control



Field Control

Speed variation is possible only

above the rated speed

Armature Control

Only below the rated speed



Armature & Field Control

By combining armature and field control

-> a wide range of speed control is

possible



Four Quadrant Operation

Operating in both directions of rotation

Producing both motoring and

regeneration



V lt & t



Voltage & urrentRequirement for 4Q

Operation Assuming filed flux is constant, speed is

approximately proportional to applied

voltage

Speed axis becomes the armature

voltage axis

Torque is proportional to armature

current

Torque axis becomes the armature



Operation

Speed Torque Voltage Current

Power

Output

Forward

Motoring

+ + + + +

ForwardRegeneration

+ - + - -

Reverse

Motoring

- - - - +

Reverse

Regeneration

- + - + -



Variable Voltage & Variable

Current DC Output

1. Phase controlled rectifiers

2. Diode bridge rectifiers with choppers



1. Phase controlled rectifiers

Converts AC source voltage directly

into a variable DC voltage

2. Chopper controlled converters

Converts Fixed DC to a variable DC

voltage



1) Phase Controlled Converters

Use Thyristors …….

Thyristor/SCR (Silicon Controlled

Rectifier)

Terminals anode, cathode and gate

To turn ON, a gate current should beinjected when anode is (+)ve comparedto cathode

After turning ON, the thyristor acts like a

diode



4Q Operation



How it works?Example : Single-Phase Controlled Converter

Conduction can be delayed beyond zerocrossing

Delay is introduced in the form oftriggering signals to the gates



Inversion

Increase delay angle > 900

Produce a (-) ve average voltage

Load current is in the same direction

Contribute to a (-) ve power input



2) Chopper Controlled DC

Motor Drives

Used when the source is a constant

voltage DC

- battery or- diode bridge rectified AC

supply

Chopper circuit converts fixed voltage

into a variable voltage



Speed Control of ACInduction Motors



Methods

1. Changing the no. of poles- limited control

- only several steps of speed is

possible

2. Slip control

◦

Varying applied voltage◦ Adding external resistance

3. Frequency control



1. Phase controlled IM drives

1. Stator voltage control

2. Slip energy recovery scheme

2. Frequency controlled IM drives

3. Vector controlled IM drives



1. Phase Controlled IM

Drives1. Stator Voltage Control

Efficiency is very low

Quite similar to 3-phase controlled

bridge converter used in DC motors



2. Slip Energy Recovery Scheme

Efficiency is improved by slip energy

recovery

As speed decreases rotor copper loss

increases -> reduce the efficiency

Here the slip energy is sent back to

the AC supply using an external emf



2. Freq. Controlled IM Drives

Utility supply is having constant

frequency

Need a frequency changer

1. Direct Frequency Changer

2. Indirect Frequency Changer



Direct Frequency Changers

Small range of frequency variation Suitable for low speed large power

applications



Indirect Frequency Changers

Wide range of frequency variation

Consists of

- a rectification (AC to DC) and

- an inversion (DC to AC)

Cannot be used for regeneration



For Regeneration



3. Vector Controlled IM

Drives Previous methods provide good steady

state responses

Poor dynamic responses

- due to deviation in air gap flux

linkage

- in magnitude and phase

Vector control is used to control this

variation



Reference

Electric Motor Drives

Modeling, Analysis, and Control

R. Krishnan

Ppt 6. Solid State Control - Large Fonts

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