Electrical Power Utilization and TractionUnit IElectrical Drives

IntroductionDriveSpeed RegulatorRotating Machine

Actual Block Diagram

Components of load torqueLoad TorqueFriction Torque Windage Torque ActualLoad TorqueStiction, TsViscous friction, TvCoulomb friction, Tc

Components of friction torqueOverall friction torqueIndividual friction torques

Types of load torque Active load torque:- continues to act in the same direction irrespective of the direction of the drive. e.g. gravitational force or deformation in elastic bodies. Passive load torque:- the sense of the load torque changes with the change in the direction of motion of drive. e. g. torques due to friction, due to shear and deformation of inelastic bodies

Speed-Torque Characteristics in 4 quadrantActual Case: Hoist ApplicationAssume, Speed is positive if the direction of rotation is anticlockwise or in such a way to cause an upward or forward motion of the drive.

For reversible drive positive direction of the speed can be assumed arbitrarily either clockwise or anticlockwise. The motor torque is positive if it produces increase in speed in the positive sense. The load torque is assigned the positive sign when it is directed against the motor torque.

Four Quadrant Operation of hoist

Fully loaded cage > Counter weight > Empty cage

Simplified

Steady state stabilityState of Equilibrium when T = Tl If the drive comes out of the state of equilibrium due to some disturbance, it comes back to steady state for stable equilibrium but for unstable equilibrium the speed of the drive increases uncontrollably or decreases to zero. When the drive coming out of the state of equilibrium preserves it steady state at different speed (lying in small range), it is said to be in neutral range.

capacity of the system which enables it to develop forces of such a nature as to restore equilibrium after any small departure therefore

Steady state stabilityCondition for stability,

Tl = Stable Tl = Unstable

Check out this for stability Results : Stable Stable Stable Unstable UnstableUnstable Indeterminate condition

A Motor equipped with a flywheel is to supply a load torque of 1000 Nm for 10 sec followed by 200 Nm long enough for the flywheel to regain its steady state speed. It is desired to limit the motor torque to 700 Nm. What should be the moment of inertia of the flywheel? Motor inertia 10, no load speed 500 RPM, slip at 500 Nm is 5%.

1861.8 kg.m2

Classes of Motor Duty

The outside surface of 12 HP motor is equivalent to a cylinder of 65 cm and 1 meter length. The motor weighs 400 kg and has a specific heat of 700 J/kg/C. The outer surface is capable of heat dissipation of 12 W/ square meter/C. Find temperature rise and thermal constant of motor when operating at full load and 90 % efficiency

40 C, 3 hours 11 min.

An electric motor has load variation as below:240 Nm for 20 min140 Nm for 10 min300 Nm for 10 min200 Nm for 20 min. If speed is 720 RPM calculate the power rating of motor.

225 Nm 17kW

DC DriveControlled-Rectifier FedArmatureField

Chopper-Fed DriveArmatureField

Characteristics of Separately-Excited MotorsRated speedUse field-current controlUse armature voltage control

Operating ModesIn variable-speed applications, a dc motor may be operating in one or more of the following Modes

MotoringRegenerative brakingDynamic brakingPlugging

Motoring ModeBack emf, Eg is < supply voltage VaBoth Ia and If are positiveDeveloped torque meets load demand

Regenerative Braking ModeMotor acts as a GeneratorEg becomes > supply Voltage Va Ia becomes negativeKinetic energy of the motor is returned to the supply

A 220 V, 200 A, 800 RPM dc separately excited motor has an armature resistance of 0.06. The motor armature is fed from a variable voltage source with an internal resistance of 0.04. Calculate the internal voltage of the variable voltage source when the motor is operating in regenerative braking at 80% of rated motor torque and 600 RPM.

Ans : 140 V

Dynamic BrakingReplace the supply voltage with a resistorPower dissipated in the resistor rather than given back to the source

PluggingReverse the armature terminals while runningVa and Eg act in the same directionIa is reversed, producing braking torque

Four-Quadrant Operation

Induction Motor Characteristics

Power Stages

Three Phase Inverter Switching States

Regenerative Braking

Plugging

AC Dynamic Braking

DC dynamic braking

Summary Table

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