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**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

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**DC DRIVES Vs AC DRIVES DC drives: Advantage in control unit**

Disadvantage in motor AC Drives: Advantage in motor Disadvantage in control unit

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**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

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DC Motors Advantage: simple torque and speed control without sophisticated electronics Limitations: Regular Maintenance Expensive motor Heavy motor Sparking

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**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 )

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**Torque Equation: Motor drives**

Te : motor torque (Nm) TL : Load torque (Nm) Acceleration Deceleration Constant speed

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…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.

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**Torque Equation: Graphical**

Te Speed Forward running Forward braking Reverse acc. Reverse running Reverse braking Forward acc.

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

In general: TORQUE TL = k TL = kw TL = kw2 SPEED

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**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

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**4Q OPERATION: LIFT SYSTEM**

Positive speed Motor Negative torque Counterweight Cage

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**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

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**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

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**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

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**Principle of Operation**

DC Motors Current in Current out Stator: field windings Rotor: armature windings

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**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.

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**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

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**Torque-speed characteristics of DC motor**

No load speed Full load speed Maximum load Torque Torque Separately excited DC motors have good speed regulation.

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**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

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**DC Motor Speed Control By Decreasing Flux Flux Decreasing**

Maximum Torque Torque Trated Slow transient response Does not maintain maximum torque capability

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**DC Motor Speed Control By Changing Armature voltage Va increasing**

Maximum Torque Torque Trated good speed regulation maintain maximum torque capability

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**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

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**Methods of Armature Voltage Control**

Phase-controlled rectifier (AC–DC) ia T Q1 Q2 Q3 Q4 + Vt 3-phase Or 1-phase supply

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**Methods of Armature Voltage Control**

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

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**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

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**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

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**Converters For DC motor Drives**

Switch–mode converters: 1Q Converter Q1 Q2 Q3 Q4 T + Vt - T1

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**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

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**Converters For DC motor Drives**

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

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**Advantages of Switch mode converters**

Switching at high frequency Reduces current ripple Increases control bandwidth Suitable for high performance applications

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