GENERAL ELECTRICAL DRIVES

What is electrical drives?Components of electrical drivesAdvantages of electrical drives

DC drives Vs AC drivesTorque equationsLoad torque profilesFour quadrant operation

DC DRIVES Vs AC DRIVES

DC drives:

Advantage in control unit

Disadvantage in motor

AC Drives:

Advantage in motor

Disadvantage in control unit

• DC DRIVES: Electric drives that use DC motors as the prime movers

• Dominates variable speed applications before PE converters were introduced

• DC motor: industry workhorse for decades

• Will AC drive replaces DC drive ?

– Predicted 30 years ago

– AC will eventually replace DC – at a slow rate

– DC strong presence – easy control – huge numbers

DC DRIVES

DC Motors

• Limitations:

• Advantage: simple torque and speed control without sophisticated electronics

• Regular Maintenance • Expensive motor

• Heavy motor • Sparking

General Torque EquationTranslational (linear) motion:

dt

dJT

Rotational motion:

dt

dvMF

F : Force (Nm)M : Mass (Kg )v : velocity (m/s)

T : Torque (Nm)J : Moment of Inertia (Kgm2 ) : angular velocity ( rad/s )

Torque Equation: Motor drives

dt

dJTTor

dt

dJTT LeLe

0 Le TT Acceleration

0 Le TT Deceleration

0 Le TT Constant speed

Te : motor torque (Nm) TL : Load torque (Nm)

…continue

Drive accelerates or decelerates depending on whether Te is greater or less than TL

During acceleration, motor must supply not onlythe load torque but also dynamic torque, ( Jd/dt ).

During deceleration, the dynamic torque, ( Jd/dt ), hasa negative sign. Therefore, it assists the motortorque, Te.

Torque Equation: Graphical

Te

Forwardrunning

Speed

Forwardbraking

Reverseacc.

Reverserunning

Reversebraking

Forwardacc.

Load Torque

Load torque, TL, is complex, depending on applications.

SPEED

TORQUE

TL = k2TL = k

TL = k

In general:

4Q OPERATION

SPEED

TORQUE

I

III

II

IV

TeTe

Te Te

FMFB

RMRB

F: FORWARD R: REVERSE M : MOTORING B: BRAKING

4Q OPERATION: LIFT SYSTEM

Counterweight Cage

Motor

Positive speed

Negative torque

4Q OPERATION: LIFT SYSTEM

Convention:

Upward motion of the cage: Positive speed

Weight of the empty cage < Counterweight

Weight of the full-loaded cage > Counterweight

Principle:

What causes the motion?

Motor : motoring P =T= +ve

Load (counterweight) : braking P =T = -ve

4Q OPERATION: LIFT SYSTEM

You are at 10th floor, callingfully-loaded cage from gnd floor

You are at gnd floor, callingempty cage from 10th floor

You are at 10th floor, callingempty cage from gnd floor

You are at gnd floor, callingFully-loaded cage from 10th floor

Torque

Speed

FMFB

RM RB

DC MOTOR DRIVES

Principle of operation

Torque-speed characteristic

Methods of speed control

Armature voltage control

Variable voltage source

Phase-controlled Rectifier

Switch-mode converter (Chopper)

1Q-Converter

2Q-Converter

4Q-Converter

Current in

Current out

Stator: field windings

Rotor: armature windings

Principle of Operation

DC Motors

Equivalent circuit of DC motor

at ikTe Electromagnetic torque

Ea ke Armature back e.m.f.

Lf Rf

if

aa

aat edtdi

LiRv

+

ea

_

LaRa

ia+

Vt

_

+

Vf

_

dtdi

LiRv ffff

Torque-speed characteristics

aaaa EIRV In steady state,

T

ae

T

a

k

VT

k

R 2

Therefore speed is given by,

Three possible methods of speed control:

Armature resistance RaField flux Armature voltage Va

aa

aaa edt

diLiRV

Armature circuit:

Torque-speed characteristics of DC motor

Torque

Speed

MaximumloadTorque

No load speed

Separately excited DC motors have goodspeed regulation.

Full load speed

DC Motor Speed Control

Torque

Speed

MaximumTorque

By Changing Ra

Ra increasing

• Power loss in Ra• Does not maintain maximum torque capability• Poor speed regulation

DC Motor Speed Control

Torque

Speed

MaximumTorque

Flux Decreasing

Trated

• Slow transient response• Does not maintain maximum torque capability

By Decreasing Flux

DC Motor Speed Control

Torque

Speed

MaximumTorque

By Changing Armature voltage

Trated

Va increasing

• good speed regulation• maintain maximum torque capability

Speed control of DC Motors

Below base speed: Armature voltage control (retain maximum torque capability)

Above base speed: Field weakening (i.e. flux reduced) (Trading-off torque capability for speed)

Torque

speed

Line of MaximumTorque Limitation

Armature voltage controlField flux control

base

Methods of Armature Voltage Control

Phase-controlled rectifier (AC–DC)

T

Q1Q2

Q3 Q4

3-phase Or 1-phasesupply

+

Vt

ia

Methods of Armature Voltage Control

1. Ward-Leonard Scheme2. Phase-controlled rectifier (AC–DC)3. Switch-Mode Converter (Chopper) (DC–DC)

Phase-controlled rectifier: 4Q Operation

Q1Q2

Q3 Q4

T

1 or 3-phasesupply

1 or 3-phasesupply

+

Vt

Methods of Armature Voltage Control

Phase-controlled rectifier : 4Q Operation

Q1Q2

Q3 Q4

T

F1

F2

R1

R2+ Va -

3-phasesupply

AN ALTERNATIVE WAY

Switch–mode converters: 1Q Converter

Q1Q2

Q3 Q4

T

+Vt

-

T1

Converters For DC motor Drives

Switch–mode converters: 2Q Converter

+Vt

-

T1D1

T2

D2

Q1Q2

Q3 Q4

T

Q1 T1 and D2

Q2 D1 and T2

Converters For DC motor Drives

Switch–mode converters: 4Q Converter

Q1Q2

Q3 Q4

T+ Vt -

T1D1

T2D2

D3

D4

T3

T4

Converters For DC motor Drives

Switch–mode converters

• Switching at high frequency

Reduces current ripple

Increases control bandwidth

• Suitable for high performance applications

Advantages of Switch mode converters