Download electrical drives

Unit 2: Electrical Drives & Control
 Whenever the term electric motor or electrical generator is used, we
tend to think that the speed of rotation of these machines is totally
controlled only by the applied voltage and frequency of the source current.
But the speed of rotation of an electrical machine can be controlled
precisely also by implementing the concept of drive. the motion control is
easily optimized with the help of drive.
The systems which control the motion of the electrical machines, are
known as electrical drives. A typical drive system is assembled with a
electric motor and a sophisticated control system that controls the rotation
of the motor shaft is called as electrical drive system.
In other words, a Drive is a combination of various systems assembled
together for the purpose of motion control.
This electrical drive system is widely used in large number of industrial
and domestic applications like factories, transportation systems, textile
mills, fans, pumps, motors, robots etc.
Drives are employed as prime movers for diesel or petrol engines, gas or
steam turbines, hydraulic motors and electric motors.
Basic Components / Block Diagram of Electric Drive System
The electric drive system has five main functional blocks namely,
1. Power source,
2. Power Modulator(Converter),
3. Motor
4. Mechanical load and
5. Controller (which incorporates sensing unit and control unit).

The power source provides the required energy to the electric drive system.

The converter interfaces the motor with the power source and provides the
motor with adjustable voltage, current and frequency.

The function of converters is to convert the electric waveform of the power
source to a waveform that the motor can use.
For example the available power source is AC and the motor is DC series
motor, then the converter converts ac into dc. In other words a rectifier circuit
is placed in the system.
Block Diagram of Electrical Drive System
 The mechanical loads are determined by the nature of the industrial
operation and the power source is determined by what is available at the
site.
 The controller monitors the operation of the entire system and ensure the
overall system performance and stability.
Advantages of Electrical Drives:
 Do not pollute the environment.
 Unlike other prime movers there is no need to refuel or warm-up the motor.
They can be started instantly and can be loaded immediately.
 They are available with wide range of torque, speed and power.
 Due to Electric braking, electrical drives exhibits flexible control
characteristics.
 Adaptable to any type of operating conditions, no matter how much
vigorous or rough it is.
 They are powered by electrical energy which is atmosphere friendly and
cheap source of power.
Applications of Electrical Drives:
 One of the major application of electric drives is Electric Traction i.e to
transport men and materials from one place to another place. Such as,
i. Electric trains,
ii. Electric buses,
iii. Trolleys,
iv. Battery driven solar-powered vehicles, and etc.
Classification of Electric Drive:
The classification of electrical drives can be done depending upon the
various components of the drive system.
According to the design, the drives can be classified into three types such
as single-motor drive, group motor drive and multi motor drive.
 I. Group drive
 II. Individual Drive
 III. Multi motor Drive
Classification of Electric Drive:
In addition to this drives are further classified,
 Based on supply: AC drives and DC drives
 Based on running speed: Constant speed(single speed) Drives and
Variable speed drives
 Based on number of motors: Single motor drives and Multi motor drives
 Based on control Parameter: Constant torque drives and Constant power
drives
I. 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.
 Noise level at the working spot is high.
 Flexibility.
Applications:
modern industries
II. 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.
Disadvantage:
Power loss occurs.
Applications:
Mainly used in simple metal working, house hold appliances etc.
III. 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.
 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 machine’s.
 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.
Applications:
Used in heavy industries or where multiple motoring units are required
such as railway transport.
Based on Power Supply:
AC Drives and DC Drives:
 The Ac drives work on the Ac signal or Ac voltage such as Singlephase or three-phase Ac voltages.
 The Dc drives work on the DC signal or DC voltage such as Batteries, Dc
power supplies (Rectifier power from Ac source).
 As far as the selection of drive is concerned for best performance it is
entirely depend on the environment and the project engineer to select the
suitable drive for its requirement.
Ac drive advantages:
 It can work on different speeds with flexibility.
 It consumes less power.
 It is free from sparking (As in the case of dc brushes friction and sparking
in dc drives)
 AC motors are smaller, lighter, more commonly available, less expensive
than DC motors and easy to handle
 It can be installed in a number of industrial locations because of spark free
operation.
 AC motors require virtually no maintenance and are preferred for
applications where the motor is mounted in an area not easily reached for
servicing or replacement.
 Whenever the operating environment is wet, corrosive or explosive and
special motor enclosures are required.
DC drive advantages:
 It can produce a higher start torque.
 DC drives are less complex with a single power conversion from AC to DC.
 It can be driven directly from batteries without an extra circuit.
 It is cost effective compare to Ac drive.
 It can rotate in both directions without much extra circuit requirements.
 If Dc shunt Drive is used it would have a constant speed.
 Different Dc drives are available for different uses (Dc-series, Long-shunt,
Short-shunt, Compound. etc….)
Electrical Motor rating:
1.
2.
3.
4.
5.
6.
7.
8.
Now a days, in almost every applications, electric motors are used, and to
control them electrical drives are employed.
But the operating time for all motors are not the same. Some of the motors
runs all the time, and some of the motor's run time is shorter than the rest
period.
Depending on this, concept of motor duty class is introduced and on the
basis of this duty cycles of the motor can be divided in eight categories such
as,
Continuous duty
Short time duty
Intermittent periodic duty
Intermittent periodic duty with starting
Intermittent periodic duty with starting and braking
Continuous duty with intermittent periodic loading
Continuous duty with starting and braking
Continuous duty with periodic speed changes
1.
Continuous Duty: This duty denotes that, the motor is running long enough
AND the electric motor temperature reaches the steady state value.
These motors are used in paper mill drives, compressors, conveyors etc.
2. Short Time Duty: In these motors, the time of operation is very low and the
heating time is much lower than the cooling time. So, the motor cooks off to
ambient temperature before operating again.
These motors are used in crane drives, drives for house hold appliances, valve
drives etc.
3. Intermittent Periodic Duty: Here the motor operates for some time and then
there is rest period. In both cases, the time is insufficient to raise the
temperature to steady state value or cool it off to ambient temperature.
This is seen at press and drilling machine drives.
4. Intermittent Period Duty with Starting: In this type of duty, there is a period
of starting, which cannot be ignored and there is a heat loss at that time. After
that there is running period and rest period which are not adequate to attain the
steady state temperatures.
This motor duty class is widely used in metal cutting and drilling tool drives,
mine hoist etc.
5. Intermittent Periodic Duty with Starting and Braking: In this type of drives,
heat loss during starting and braking cannot be ignored. So, the corresponding
periods are starting period, operating period, braking period and resting period,
but all the periods are too short to attain the respective steady state
temperatures.
These techniques are used in billet mill drive, manipulator drive etc.
6. Continuous Duty with Intermittent Periodic Loading: In this type of motor
duty, everything is same as the periodic duty but here a no load running period
occurs instead of the rest period.
Pressing, cutting are the examples of this system.
7. Continuous Duty with Starting and Braking: It is also a period of starting,
running and braking and there is no resting period.
The blooming mill is an example.
8. Continuous Duty with Periodic Speed Changes: In this type of motor duty,
there are different running periods at different loads and speeds. But there is no
rest period and all the periods are too short to attain the steady state
temperatures.
Electric Motor Failures:
 Electric motors are an essential part of our daily life as many systems,
applications, and services depend on them.
 Motors should have a long service life and require a minimum level of
maintenance to make sure that they perform efficiently.
 It is estimated that 92% of the electric motor failures occur at the start up.
 Usually in large organizations, a motor maintenance program is carried out
in which the causes of motor failures are identified and some necessary
steps are taken to avoid them or lower their impact.
 Whenever a problem occurs, it should be corrected immediately to avoid
further loss.
There are six main causes of electric motor failures:
1. Over-Current
2. Low Resistance
3. Over heating
4. Dirt
5. Moisture
6. Vibration
1.
2.
Over-Current (Electrical Overload): In different operating conditions,
electrical devices will sometimes start to draw more current than their overall
capacity. This unpredictable event will happen very suddenly and will greatly
impact the motor.
To avoid an over-current, there are some devices that need to be installed that
can prevent it from happening. These devices are usually wired in the circuits
and will automatically shut down the extra amount of current flowing in the
circuit.
Low Resistance: Most motor failures occur due to low insulation resistance.
This issue is considered to be the most difficult one to tackle. In the initial
stages of motor installation, the insulation resistance is observed to be more
than one thousand megaohms. After some time, the insulation performance
starts to degrade at an alarming level because the resistance starts to decay
gradually.
After a lot of research, a solution has been found which can prevent low
resistance failures. There are automatic devices that test insulation resistance
from time to time and safeguard rotating equipment is installed that prevents
such failures. It is important that the insulation performance is monitored at
regular intervals.
3. Over Heating: Excessive heat in motors can cause a number of performance
problems. Overheating causes the motor winding insulation weak. For every
ten centigrade rise in temperature, the insulation life is cut in half. It has been
concluded that more than 55% of the insulating failures are caused by over
heating.
Over heating occurs due to a number of factors.
Every electric motor has a design temperature. If a motor is started off at a bad
current value, it starts operating in a much warmer condition than the design
temperature. It is very important that the motors should be matched with their
ideal current values.
Overheating also occurs when an electric motor is forced to operate in a high
temperature environment. This causes the rate at which heat can be conducted
to reduce at an alarming rate.
The area where electric motors are operating must have a proper cooling
system and a ventilation system should be there in case the cooling system
stops working.
4. Dirt: Dirt is one of the major sources that cause damage to the electric motors. It
can damage the motor by blocking the cooling fan which causes its temperature to
raise. It can also affect the insulating value of the winding insulation if it settles
on the motor windings. Proper steps should be taken to prevent the motors from
dirt. Shielding devices are available which are used to avoid dirt.
5. Moisture: Moisture also affects the performance of electric motors. It greatly
contributes in the corrosion of the motor shafts, bearings and rotors. This can
lead to an insulation failure . The motor inventory should be kept dry all time.
6. Vibration: There are a number of possible causes of vibration, such as
misalignment of the motor. Corrosion of parts can also cause the motor to
vibrate. The alignment of the motor should be checked to eliminate this issue.
Power Transistor (Bipolar Junction Transistor i.e. BJT)
 Transistor is a current controlled three terminal device.
 Mainly transistor present in two types i.e. NPN & PNP transistor.
NPN
PNP
Cross-Sectional View of BJT:
Emitter
Collector
NPN
PNP
ECE 442 Power Electronics
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Working of Common-Emitter NPN Transistor:
Reverse bias the CBJ
Forward bias the BEJ
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Working of Common-Emitter NPN Transistor…
 The N type emitter is very heavily doped to provide many free electrons as
majority charge carriers. The lightly doped P type base region is extremely
thin, and the N type collector is very heavily doped to give it a low
resistivity apart from a layer of less heavily doped material near to the base
region. This change in the resistivity of the collector, ensures that a large
potential is present within the collector material close to the base. The
importance of this will become apparent from the following description.
 During normal operation, a potential is applied across the base/emitter
junction so that the base is approximately 0.6V more positive than the
emitter, this makes the base/emitter junction forward biased.
 A much higher potential is applied across the base/collector junction with a
relatively high positive voltage applied to the collector, so that the
base/collector junction is heavily reverse biased. This makes the depletion
layer between base and collector quite wide once power is applied.
 As mentioned above, the collector is made up of mainly heavily doped, low
resistivity material with a thin layer of lightly doped, high resistivity
material next to the base/collector junction. This means that most of the
voltage between collector and base is developed across this thin high
resistivity layer, creating a high voltage gradient near the collector base
junction.
Working of Common-Emitter NPN Transistor…
 When the base emitter junction is forward biased, a small current will flow into
the base. Therefore holes are injected into the P type material. These holes
attract electrons across the forward biased base/emitter junction to combine
with the holes. However, because the emitter region is very heavily doped,
many more electrons cross into the P type base region than are able to combine
with the available holes. This means there is a large concentration of electrons
in the base region and most of these electrons are swept straight through the
very thin base, and into the base/collector depletion layer. Once here, they
come under the influence of the strong electric field across the base/collector
junction. This field is so strong due to the large potential gradient in the
collector material mentioned earlier, that the electrons are swept across the
depletion layer and into the collector material, and so towards the collector
terminal.
 Varying the current flowing into the base, affects the number of electrons
attracted from the emitter. In this way very small changes in base current cause
very large changes in the current flowing from emitter to collector, so current
amplification is taking place.
Characteristics of Transistors
Input Characteristics:
 The Input Characteristic, a graph of base emitter current IBE against base
emitter voltage VBE (IBE/VBE) shows the input Conductance of the
transistor. Plot IB as f(VBE, VCE).
 As conductance I/V is the reciprocal of Resistance (V/I) this curve can be
used to determine the input resistance of the transistor.
 The input conductance is very high, and there is a large increase in current
for a very small increase in VBE. Therefore the input RESISTANCE must be
low.
 As VCE increases, more VBE required to turn the BE on so that IB>0.
 Looks like a PN Junction volt-ampere characteristic.
Input Characteristics:
Output Characteristics:
 The OUTPUT CHARACTERISTIC whose slope gives the value of output
conductance (and by implication output resistance).
 Plot IC as f(VCE, IB).
 A change in collector emitter voltage VCE has almost no effect on collector
current in this region for keeping IB constant.
 Therefore the graph shows that the output resistance of the transistor is high.
 Mainly transistor works in following three regions,
1. Cutoff region (off Switch)
- Both BE and BC reverse biased.
- Here the operating conditions of the transistor are zero input base current
( IB ), zero output collector current ( IC ) and maximum collector voltage
( VCE ) which results in a large depletion layer and no current flowing
through the device. Therefore the transistor is switched “Fully-OFF”.
- No Collector current flows ( IC = 0 )
- VOUT = VCE = VCC = ”1″
- Transistor operates as an “open switch”
Output Characteristics:
2. Active region
- BE Forward biased
- BC Reverse biased
- VBE = Vcut-in
- Vsat < VCE < Vsupply
- IC = β*IB
Output Characteristics:
3. Saturation region (on Switch)
- Both BE and BC forward biased
- Here the transistor will be biased so that the maximum amount of base current is
applied, resulting in maximum collector current resulting in the minimum
collector emitter voltage drop which results in the depletion layer being as small
as possible and maximum current flowing through the transistor. Therefore the
transistor is switched “Fully-ON”.
- Transistor is “fully-ON” ( saturation region )
- Max Collector current flows ( IC = Vcc/RL )
- VCE = 0 ( ideal saturation ) & VOUT = VCE = ”0″
- Transistor operates as a “closed switch”
Output Characteristics:
Output Characteristics:
Transfer Characteristics:
37
Transfer Characteristics:
• The graph of ICE / IBE shown in above Figure is called the TRANSFER
CHARACTERISTIC.
• A BJT is a current operated transistor, So a small base current is controlling
the much larger collector/emitter current.
• The ratio of the two currents, ICE / IBE is constant, provided that the
collector emitter voltage VCE is constant. Therefore, if the base current rises,
so does collector current.
Transistor Operating Point
VB  VBE
IB 
RB
VCE VCC
IC  

RC
RC
VCE  VCC  I C RC
ECE 442 Power Electronics
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DC Load Line
VCC/RC
VCC
ECE 442 Power Electronics
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Transistor Switching Times/ Switching Characteristics:
ECE 442 Power Electronics
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Switching Times









Input voltage rises from 0 to V1
Base current rises to IB1
Collector current begins to rise after the delay time, td
Collector current rises to steady-state value ICS
This “rise time”, tr allows the Miller capacitance to charge to V1
turn on time, ton = td + tr
Input voltage changes from V1 to –V2
Base current changes to –IB2
Base current remains at –IB2 until the Miller capacitance discharges to zero,
storage time, ts
 Base current falls to zero as Miller capacitance charges to –V2, fall time, tf
 turn off time, toff = ts + tf
42
Transistor Applications
 Switching
 Amplification
 Variable Resistor
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MOSFET