EE8601
SOLID STATE DRIVES

UNIT I
DRIVE CHARACTERISTICS
Dr.P.Subha Karuvelam
Professor (CAS)/EEE
Government College of Engineering
Tirunelveli – 627007.

Electric Drives
• Systems employed for motion control are called
as Drives.
• It may employ any of prime movers such as diesel
or petrol engines, gas or steam turbines, steam
engines, hydraulic motors and electric motors, for
supplying mechanical energy for motion control.
• Drives employing electric motors are called as
Electrical Drives.

Parts Of Electric Drives

Parts of Electric Drives
Power Modulator :
• The power modulator regulates the output power of
the source.
• It controls the power from the source to the motor in
such a manner that motor transmits the speed
torque characteristic required by the load.
• During the transient operations like starting,
braking and speed reversing the excessive current
drawn from the source.

POWER MODULATOR(Cont’d)
This excessive current drawn from the source may overload it
or may cause a voltage drop. Hence the power modulator
restricts the source and motor current
The power modulator converts the energy according to the
requirement of the motor
 e.g. if the source is DC and an induction motor is
used then power modulator convert DC into AC.
 It also selects the mode of operation of the motor, i.e.,
motoring or braking.

CONTROL UNIT
Control Unit :
• Consists of linear and digital integrated circuits and
transistors
• The control unit controls the power modulator which
operates at small voltage and power levels. The
control unit also operates the power modulator as
desired.
• It also generates the commands for the protection of
power modulator and motor. An input command
signal which adjusts the operating point of the drive,
from an input to the control unit.

SENSING UNIT
Sensing Unit :
• It senses the certain drive parameter like motor
current and speed.
• It mainly required either for protection or for closed
loop operation.

ADVANTAGE OF ELECTRIC DRIVES
Advantages of Electrical Drive:
• The electrical drives are available in a wide range of
torque, speed and power.
• They are adaptable to almost
conditions such as explosive
any operating
and radioactive
environment, submerged in liquids, vertical
mounting and so on.
• The electrical drive does not pollute the
environment.

ADVANTAGE OF ELECTRIC
DRIVES(Cont’d)
• It can operate in all four quadrants of speed
torque plane.
• They can be started instantly and can
immediately be fully loaded. i.e., there is no need
to refuel or warm up the motor.
• They have flexible control characteristic and can
be employed to automatically control the drive.

ADVANTAGE OF ELECTRIC
DRIVES(Cont’d)
• Because of the following advantages, the mechanical
energy already available from a non-electrical prime
mover is sometimes first converted into electrical
energy by a generator and back to a mechanical energy
of an electrical motor.
• Electrical link thus provides between the non-electrical
prime mover and the load impact to the drive flexible
control characteristic.

Disadvantage Of Electric Drives
Disadvantages of Electrical Drive:
• The power failure completely disabled the whole of
the system.
• The application of the drive is limited because it
cannot use in a place where the power supply is not
available.
• It can cause noise pollution.
• The initial cost of the system is high.

CLASSIFICATION OF ELECTRIC DRIVES
Generally classified into 3 categories:
• Group drive
• Individual Drive
• Multimotor Drive

Group drive
If several group of mechanisms or machines are organized on
one shaft and driven or actuated by one motor, the system is
called a group drive or shaft drive.
Advantage : Most Economical
Disadvantage :
• Any Fault that occurs in the driving motor renders all the
driving equipment idle.
• Efficiency low because of losses occurring in the energy
transmitting mechanisms (Power loss).
• Not safe to operate. Also Noise level at the working spot
is high.

Individual Drive:
• If a single motor is used to drive or actuate a given
mechanism and it does all the jobs connected with
this load , the drive is called individual drive.
• All the operations connected with operating a lathe
may be performed by a single motor.
• Each motor is driven by its own separated motor
with the help of gears , pulleys etc.

Multi Motor Drive:
• Each operation of the mechanism is taken care of by
a separate drive motor.
• The System contains several individual drives each of
which is used to operate its own mechanism.
• Applications: Complicated metal cutting machine
tools, Paper making industries, rolling machines etc

Multi Motor Drive:
Separate motors are provided for actuating different parts
of the driven mechanism.
Advantage :
• Each Machine is driven by a separated motor it can be
run and stopped as desired.
• Machines not required can be shut down and also
replaced with a minimum of dislocation.
• There is a flexibility in the installation of different
machines.

Multi Motor Drive:
• In the case of motor fault, only its connected machine will
stop where as others will continue working undisturbed.
• Absence of belts and line shafts greatly reduces the risk of a
accidents to the operating personnel.
Disadvantage:
• Initial high cost

Dynamics of motor-load system
• When an electric motor rotates, it is usually
connected to a load which has a rotational or
translational motion.
• The speed of the motor may be different from
that of the load. To analyze the relation
among the drives and loads, the concept
of dynamics of electrical drives is introduced.

Dynamics of motor-load combination
Drive
J = Polar moment of inertia of motor load (kg-m2)
m = Instantaneous angular velocity (rad/sec)
T = Instantaneous value of developed motor torque (N-m)
T1 = Instantaneous value of load torque referred to motor
shaft (N-m)

Dynamics of motor-load System
• Now, from the fundamental torque equation
• For the drive with constant inertia
Motor Torque = Load Torque + Dynamic Torque

Dynamics of motor-load System
• Dynamic torque J(dωm/dt) present during the
transient operations.
• During Acceleration
• During Deceleration

COMPONENTS OF LOAD TORQUE
Divided into 3 components
1. Friction Torque -
2. Windage Torque –
3. Torque required to do useful mechanical
work.

COMPONENTS OF LOAD TORQUE
1. Friction Torque
Present at the motor shaft and also in various
parts of the load. TF is the equivalent value of
various friction torques referred to the motor
shaft.
2. Windage Torque (TW):
When motor runs, wind generates a torque
opposing the motion is known as windage
torque.

COMPONENTS OF LOAD TORQUE
3. Torque required to do useful
mechanical work.
• Nature of this torque depends upon
particular application.
• It may be some function of speed, it may
be time invariant or time variant, its
nature may also change with the load’s
mode ofoperation.

Friction Torque
3 Components are
1. Static Friction
Friction at zero speed is
Stiction or static friction
2. Viscous Friction
Varies linearly with speed
B – Viscous friction coefficient
3. Coulomb Friction
Independent of spped

Windage Torque
 Proportional to the square of the speed

COMPONENTS OF LOAD TORQUE
For finite speed,

Common Load Torques
1. Fan and Centrifugal pumps
2. Traction
3. Coiler Drives
4. Diesel Electric Locomotive
5. Excavators
6. Hoist

CONSTANT TORQUE AND CONSTANT
POWER OPERATION

Fan and Centrifugal Loads
Windage torque predominates

Traction Drives
Comprises of
 Windage torque
 Viscous friction
 Coulomb friction
 Stiction

Coiler Drives
Motor will operate at Constant
Power
 Steel Mill
 Paper Mill
 Plastic Mill
 To maintain the tension of the
steel, paper, plastic strip the
motor operated in Constant
power mode

Diesel Electric Locomotive
Act as torque converter
AB – To limit the motor speed within
permissible value
CD – Limit on maximum torque for
1. To limit maximum current
2. To prevent wheel slipage
3. To prevent draw bar fracture

Excavators
When load torque is high, the
motor will simply stop.
To prevent the mechanical damage
to the excavator

Hoist
For low speed hoist, load
torque is mainly due to
gravitational force
Gravity is constant and
independent of speed
For high speed hoist,
viscous friction and
windage plays active role

TORQUE-SPEED SIGN CONVENTON

Classification of load torques
Various load torques can be classified
into broad categories.
• Active load torques
• Passive load torques

Active Load
Load torques which has the potential to drive the
motor under equilibrium conditions are called active
load torques
Retain their sign if the drive rotation is changed
Torque due to gravity, tension, compression and tension
due to elastic body.

Passive loads
Always oppose the motion and change their
direction on the reversal of motion
Friction, Cutting, drilling etc

Steady and Transient state
stability of Electric Drive
• The drive is said to be in equilibrium if
• Stable system:
If the system can able restore the stable speed
after a small disturbance in supply, load or
part of the drive

Steady and Transient state stability of
Electric Drive
• Changes from the state of equilibrium takes
place slowly and the effect of either the
inertia or the inductance is insignificant –
Steady state stability.
Sudden and fast changes from the equilibrium
state so effect of both inertia and inductance
can not be neglected- Dynamic or transient
stability.

Steady and Transient state stability of
Electric Drive
• Criteria for steady state stability:-
• At time ‘t’

Steady State Stability

Steady State Stability

Steady State stability
In new speed,
Hence acceleration will occur and speed of the motor increases.

Steady State stability
In new speed,
Hence deceleration will occur and speed of the motor decreases.

FOUR QUADRANT OPERATION

FOUR QUADRANT OPERATION

Modes of Operation

Speed – Torque Curves with different
speed settings

Modes of Operation
1. Increase in speed in same direction
2. Decrease in speed in same direction
3. Speed Reversal

Increase in speed in same direction

Decrease in speed in same direction

Speed Reversal

Speed Reversal

Choice or selection of drives
Steady State Operating conditions requirements:
Nature of speed torque characteristics, speed regulation, speed
range, efficiency, duty cycle, quadrants of operation, speed
fluctuations if any, ratings etc
Transient operation requirements:
Values of acceleration and deceleration, starting, braking and reversing
performance.
Requirements related to the source:
Types of source and its capacity, magnitude of voltage, voltage
fluctuations, power factor, harmonics and their effect on other loads,
ability to accept regenerative power
Capital and running cost, maintenance needs life.
 Space and weight restriction if any.
Environment and location.
Reliability.