17900_introduction to Drives

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Classification of drives

AC,DC (e.g.)

Single excited (e.g.), multi excited(e.g.)

Continuous duty, discrete duty(e.g.)

Components of drives(AC/DC)

Motor

AC,DC, BLDC, PMSM, SRM

Advantages and disadvantages of AC,DC motors

Power converter

AC_AC, AC_DC, DC_AC, DC_DC

Source

Controller

sensors


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Advantages of drives

Flexible control characteristics

Speed and torque can be controlled

Adaptable in most operating conditions

Can handle 4 quadrant operation

Improved efficiency

Available in wide range of speed and torque


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Review of AC and DC drives

In past induction and synchronous motors were used forfixed speed operation

E.g.

Variable speed operations were dominated by DC motor

E.g.

Development of thyristor (1957)

Development of semiconductors lead to thedevelopment of variable speed drives for the inductionmotor


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Advantages of AC motor over DC motor

Cheaper

Reduction in size for same power rating

No maintaince Rugged construction

Ease of handling

Considering all these advantages it was assumed that AC drives will

replace the DC drivesBut they did not.

reasons

Power converter and control circuit of the AC drive is much moreexpensive than DC drive

Therefore the total cost of the AC drive was more than the DCdrive

The technology for the DC drives was well established where as itwas developing for AC drives

Improvement in semiconductor techniques


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Advantages of DC drives

Simple for construction

controller and power converter are not expensive

Matured and reliable technology

Ease of control (variable speed operation)


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Selection of drive for application Operation requirement

Nature of speed torque characteristics

Speed regulation

Speed range Drive duty

Quadrant of operation

Transient operation Continuous acceleration and deceleration

Starting and stopping Source requirement

Type (AC_DC)

Capacity (power rating)

Voltage fluctuation and frequency variation

Capital cost Running and maintaince cost

Location/ environment

reliability


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Dynamics of electrical drives Fundamental torque equation

Motor-load system (rotational motion)

J=inertia of motor load system(kg-m^2) W= angular velocity of the motor shaft (rad/sec) Tm= developed motor torque (N-m)

Tl= load torque (resisting) in N-m

Types of loadsVariable inertia

e.g. industrial robots, mine windersEquation

Fixed inertiae.g. Fan, Compressor motor,

Equation


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Acceleration of motor

Te >> T_load + inertia

E.g drives with large inertia (electric trains ) the developed

torque must be much higher than load torque

Deceleration of motor(braking)

Te < T_load

Quadrants of operation


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Dynamics of drives with multiple loads

Case 1

Multiple loads with rotational motion

Find out the equivalent inertia and torque required

Case 2

Multiple loads with translational motion

Find out the equivalent inertia and torque required


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Dynamics of drives Components of load torques

Frictional torque _ friction due to various motor parts as

well as shaft Windage torque _ torque generated by wind to oppose the

motion or rotation

Load torque required to do useful mechanical work_ may be constant e.g. water pump,

varying based on application, Independent of speed e.g lift,

may depend on the position followed by load e.g. conveyer belt


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Components of frictional torque

It can be resolved into 3 components

Viscous friction

Viscous friction is dependent of the velocity. At zero velocity theviscous friction is zero and the viscous component increases withthe increase of velocity.

Coulomb friction Coulomb friction is independent of velocity and is always present.

This friction component is only dependent on the direction ofmotion, in such way that it is in the direction opposite to thevelocity.

E.g. translational motion-opposition offered by the surface

Standstill torque

Present only at standstill so need not to consider at dynamicanalysis


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Frictional load and its components


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Windage torque

Windage torque is proportional to speed square

The equivalent load torque is the summation of all thetorque components

The term can be written in the form ofviscous coefficient B , therefore the equivalent loadtorque is given as

Equivalent characteristics


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Nature and Classification of load torques

Active and Passive Active_ force and torque that does not change the sign

when the direction of drive rotation has changed E.g. gravity , tension

Passive_ force or torque whose sign depends on the

direction of rotation e.g. friction, windage

Nature of the load torque always depends on the type ofapplication

Torque independent of speed E.g. lift, centrifugal pump, krane

torque is independent of speed

mainly due to gravity, (W_T characteristics)


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Torque dependant on the speed

E.g. fan,

Main opposition is due to windage (W_T characteristics)

traction load

High initial/ staring torque due to heavy trains

Once you gain momentum windage and friction dominates


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Constant power load

It is having hyperbolic speed-torque characteristics and hence theload torque is inversely proportional to speed

The load power constant

E.g. lathes machine, boring machine, certain types of conveyerbelts

C l l i f i l i i


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Calculation of time energy loss in transients

Transients???

E.g starting , braking, speed change and speed reversal

Equilibrium state? When motor attains the final or reference speed

Transient operation is considered to be over when 95%of the reference speed is achieved

Mechanical time constant Electrical time constant

The energy dissipated during transient is E.


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Drive is having following parameters

J= 10 kg-m^2, T= (100-0.1N) n-m , Tload= 0.05N n-m

N is the speed in RPM

Initially drive was working in steady state and now it isreversed with the same torque. Calculate the speed ofreversal..?


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Steady state stability of drive

Concept of steady state stability is developed to readilyevaluate the stability of equilibrium point for steady

state speed torque characteristics Equilibrium point..?

Equilibrium speed is obtained when the motor torque isequal to load torque.(steady state speed)

since during transients we can assume thatdrive is in electrical equilibrium.

If the equilibrium point is restored after the suddendisturbance then the system is in steady state stability

e.g.


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As shown in above speed torque characteristics withthe help of power electronics we can have differentoperating or equilibrium points


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Date post:	14-Apr-2018
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17900_introduction to Drives

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