GENERAL ELECTRICAL DRIVES What is electrical drives? Components of electrical drives Advantages of electrical drives DC drives Vs AC drives Torque equations Load torque profiles Four 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 DC motor: industry workhorse for decades Dominates variable speed applications before PE converters were introduced 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 Motors Advantage: simple torque and speed control without sophisticated electronics Limitations: Regular Maintenance Expensive motor Heavy motor Sparking

General Torque Equation Translational (linear) motion: F : Force (Nm) M : Mass (Kg ) v : velocity (m/s) Rotational motion: T : Torque (Nm) J : Moment of Inertia (Kgm2 ) w : angular velocity ( rad/s )

Torque Equation: Motor drives Te : motor torque (Nm) TL : Load torque (Nm) Acceleration Deceleration Constant speed

…continue Drive accelerates or decelerates depending on whether Te is greater or less than TL During acceleration, motor must supply not only the load torque but also dynamic torque, ( Jdw/dt ). During deceleration, the dynamic torque, ( Jdw/dt ), has a negative sign. Therefore, it assists the motor torque, Te.

Torque Equation: Graphical Te Speed Forward running Forward braking Reverse acc. Reverse running Reverse braking Forward acc.

Load Torque Load torque, TL, is complex, depending on applications. In general: TORQUE TL = k TL = kw TL = kw2 SPEED

4Q OPERATION FB FM RB RM F: FORWARD R: REVERSE M : MOTORING B: BRAKING SPEED w w Te Te FB FM II I TORQUE RB III IV w w RM Te Te

4Q OPERATION: LIFT SYSTEM Positive speed Motor Negative torque Counterweight Cage

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 =Tw = +ve Load (counterweight) : braking P =Tw = -ve

4Q OPERATION: LIFT SYSTEM Speed You are at 10th floor, calling empty cage from gnd floor You are at 10th floor, calling fully-loaded cage from gnd floor FB FM Torque RM RB You are at gnd floor, calling empty cage from 10th floor You are at gnd floor, calling Fully-loaded cage from 10th floor

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

Principle of Operation DC Motors Current in Current out Stator: field windings Rotor: armature windings

Equivalent circuit of DC motor Lf Rf if + ea _ La Ra ia Vt Vf dt di L i R v f = Electromagnetic torque Armature back e.m.f.

Torque-speed characteristics Armature circuit: In steady state, Therefore speed is given by, Three possible methods of speed control: Armature resistance Ra Field flux F Armature voltage Va

Torque-speed characteristics of DC motor No load speed Full load speed Maximum load Torque Torque Separately excited DC motors have good speed regulation.

DC Motor Speed Control By Changing Ra Ra increasing Power loss in Ra Maximum Torque Torque Power loss in Ra Does not maintain maximum torque capability Poor speed regulation

DC Motor Speed Control By Decreasing Flux Flux Decreasing Maximum Torque Torque Trated Slow transient response Does not maintain maximum torque capability

DC Motor Speed Control By Changing Armature voltage Va increasing Maximum Torque Torque Trated 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 Armature voltage control Field flux control Line of Maximum Torque Limitation speed base

Methods of Armature Voltage Control Phase-controlled rectifier (AC–DC) ia T Q1 Q2 Q3 Q4  + Vt  3-phase Or 1-phase supply

Methods of Armature Voltage Control 1. Ward-Leonard Scheme 2. Phase-controlled rectifier (AC–DC) 3. Switch-Mode Converter (Chopper) (DC–DC)

Methods of Armature Voltage Control Phase-controlled rectifier: 4Q Operation + Vt  1 or 3-phase supply 1 or 3-phase supply Q1 Q2 Q3 Q4  T

Phase-controlled rectifier : 4Q Operation AN ALTERNATIVE WAY Phase-controlled rectifier : 4Q Operation F1 F2 R1 R2 + Va - 3-phase supply Q1 Q2 Q3 Q4  T

Converters For DC motor Drives Switch–mode converters: 1Q Converter Q1 Q2 Q3 Q4  T + Vt - T1

Converters For DC motor Drives Switch–mode converters: 2Q Converter + Vt - T1 D1 T2 D2 Q1 Q2 Q3 Q4  T Q1  T1 and D2 Q2  D1 and T2

Converters For DC motor Drives Switch–mode converters: 4Q Converter Q1 Q2 Q3 Q4  T + Vt - T1 D1 T2 D2 D3 D4 T3 T4

Advantages of Switch mode converters Switching at high frequency  Reduces current ripple  Increases control bandwidth Suitable for high performance applications