This document discusses electric drive systems. It outlines the basic elements, which include a power source, electric motor, converter, controller, and mechanical load. It describes the torque-speed characteristics of different types of motors and loads. It also covers power supplies for electric drives, including alternating current and direct current sources. Control of electric drives is also mentioned.

1. Chapter -2
Characteristics of
electric motors

2. Outlines
Basic elements of electric drives
Torque-speed characteristics of industrial
driven units
 Torque-speed characteristics of electric motors
 Power supply for electric motors; Control of
electric drives.

3. What is Electrical drive?
• The system which is used for controlling the motion of an electrical
machine.
• The drive which uses the electric motor.
• The electrical drive uses any of the prime movers like
Diesel or a petrol engine
Gas or steam turbines
Steam engines
Hydraulic motors and electrical motors as a primary source of
energy.
• This prime mover supplies the mechanical energy to the drive for
motion control.

4. Parts of Electric Drive System
.
• Restricts the source
and motor current
• Selects motor
modes of operation

5. Advantages of Electrical drive
• The electric drive has very large range of
torque, speed and power.
• Their working is independent of the
environmental condition.
• The electric drives are free from pollution.
• The electric drives operate on all the quadrants
of speed torque plane.
• The drive can easily be started and it does not
require any refueling.
• The efficiency of the drives is high because
fewer losses occur on it

6. Disadvantages of Electrical drive
• 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.
• It has a poor dynamic response.
• The output power obtained from the drive is
low.
• During the breakdown of conductors or short
circuit, the system may get damaged due to
which several problems occur

7. It is used in a large number of industrial and domestic
applications like:-
 Transportation systems
 Rolling mills
 Paper machines
 Textile mills
 Machine tools
 Fans
 Pumps
 Robots and washing, etc.
Applications of Electric drive

8. Power electronics : Deals conversion and control of
electrical power for various applications,
 DC- and AC-regulated power supplies
 Heating and lighting control
 Electrical welding
 Electrochemical processes
 Induction heating
 Active harmonic filtering and static reactive power
generation
 Control of DC and AC machines, and so on.

9. Electrical machine drives: Known as motion control
are particularly a very fascinating and challenging
area in power electronics because of their spectrum of
applications, such as
 Computer peripheral drives
 Machine tool and robotic drives
 Pump and blower drives
 Textile and paper mill drives
 Electric vehicle and locomotive propulsion
 Ship propulsion
 Cement mill and rolling mill drives, and so on.
 With the present trend of global industrial
automation, the application of power electronics and
variable frequency drives is expected to grow
enormously in the future.

10. Basic components of the Electrical Drive
System
 SOURCES: Sources (AC, DC)
 CONVERTERS: Rectifiers, Choppers, Inverters,
Cycloconverters…
 DRIVING/ ACTUATING MOTORS :
Induction Motors, permanent magnet synchronous
Motors, dc Motors…
 CONTROL UNIT / Convertor TRIGGERING
UNIT: field oriented control, brushless dc
control…
 MEASURMENT / FEEDBACK DEVICES:
Current sensor, speed sensor, torque sensor…
 MECHANICAL LOAD

11. A modern electric drive system has five main functional blocks:
A mechanical load, a motor, a converter, a power source, and a controller.
 Power source: Provides a the energy the drive system needs.
 Converter: Interfaces the motor with the power source and provides the motor
with adjustable voltage, current, and /or frequency.
 Controller: supervises the operation of the entire system to enhances overall
system performance and stability.
Often, design engineers do not select the mechanical loads or power sources.
Rather, the mechanical loads are determined by the nature of industrial operation ,
and the power source is determined by what is available at the site. However,
designers usually can select the other three components of the drive systems
(Electric motor, converter, and controller).

12. The basic criteria in selecting
 An electric motor for a given drive application is that it meet the power level
and performance required by the load during steady state and dynamic
operations.
 Certain characteristics of the mechanical loads may require a special type of
motor. For example, in the applications for which:-
 High starting torque is needed, a dc series motor might be a better choice than
an induction motor.
 In constant speed applications, synchronous motors might be more suitable
than induction or dc motors.
 Environmental factors may also determine the motor type. For example, in
food processing, chemical industries, and aviation, where the environment
must clean and free from arcs, dc motors can not be used unless they are
encapsulated. This is because electric discharge that is generated between the
motor’s and brushes and its commutator segments. In those cases, the squirrel
cage induction motor or other brushless machines are probably the better
options.

13.  The cost of the electric motor is another important factor. In general, dc motors
and newer types of brushless motors are the most expensive machines, whereas
squirrel cage induction motors are among the cheapest.
 The function of convertor, as its name implies, is to convert the electric wave-
form of the power source to waveform that the motor can use. For example, if
the power source is an ac type and the motor is a dc machine, the convertor
transforms the ac waveform to dc. In addition, the converter adjusts the voltage
or current to desired values.
 The controller can also be designed to perform a wide range of functions to
improve system stability, efficiency, and performance. In addition, it can be
used to protect the converter, the motor, or both against excessive current or
voltage.

15. MECHANICAL LOADS
Application examples
 Hybrid electric vehicles (HEV) in combination with the
power train
 Electrically operated water pump
 Electrically operated oil pump
 Electrically operated air conditioning system
 Electrically operated Compressor
 Electrically operated elevator
 Electric train
 fly by wire air plan
 Electrically operated over head crane, conveyer, fan,…

16. Torque-speed characteristics of industrial units/ Loads
 Selection of a drive motor and its control scheme depends on the load.
 An adjustable speed control of a fan will certainly differ from that of a winder
in a paper mill, the manufacturing process in the latter case imposing narrow
tolerance bands on speed and torque of the motor.
 Various classifications can be used with respect to loads. In particular, they can
be classified with respect to:
(a) Inertia,
(b) Torque versus speed characteristic, and
(c) Control requirements.
 High-inertia loads, such as electric vehicles, winders, or centrifuges, are more
difficult to accelerate and decelerate than, for instance, a pump or a grinder.
The total mass moment of inertia referred to the motor shaft can be computed
from the kinetic energy of the drive. Consider, for example, a motor with the
rotor inertia of JM that drives a load with the mass moment of inertia of JL
through a transmission with the gear ratio of N.

17. The difference, Td, between the torque, TM, developed in the
motor and the static torque, TL, with which the load resists the
motion is called a dynamic torque. According to Newton’s second
law,
A high mass moment of inertia makes a drive
sluggish, so that a high dynamic torque is required
for fast acceleration or deceleration of the load.
where JT denotes the total mass moment of inertia of
the system referred to the motor shaft.

18. where TL0 and 𝜏 are constants, three basic types, illustrated in Figure 2 can be
distinguished
In most loads, the static torque, 𝑇𝐿, depends on the load speed, 𝜔𝐿. The 𝑇𝐿(𝜔𝐿)
relation, usually called a mechanical characteristic, is an important feature of the load,
because its intersection with the analogous characteristic of the motor, 𝑇𝑀(𝜔𝑀),
determines the steady state operating point of the derive. Expressing the mechanical
characteristic by a general equation
𝑇𝐿 = 𝑇𝐿0 + 𝜏𝜔𝐿
𝑘

19. 1. Constant-torque characteristic, with k≈0,typical for lifts and conveyors and,
generally, for loads whose speed varies in a narrow range only.
2. Progressive-torque characteristic, with k>0, typical for pumps, fans, blowers,
compressors, electric vehicles and, generally, for most loads with a widely varying
speed.
3. Regressive-torque characteristic, with k<0, typical for winders. There with a
constant tension and linear speed of the wound tape, an increase in the coil radius
is accompanied by a decreasing speed and an increasing torque.
Fig 2:

20.  Practical loads are better described by operating areas rather than mechanical
characteristics. An operating area represents a set of all allowable operating points in the
(ωL,TL) plane.
 Taking a pump as an example, its torque versus speed characteristic strongly depends on
the pressure and viscosity of the pumped fluid. Analogously, the mechanical
characteristic of a winder varies with changes in the tape tension and speed. Therefore, a
single mechanical characteristic cannot account for all possible operating points.
 An example operating area of a progressive-torque load is shown in Fig 3a. Clearly, if a
load is driven directly by a motor, the motor operating area in the (ωM,TM) plane is the
same as that of the load. However, if the load is geared to the motor, the operating areas
of the load and motor differ because the gearing acts as a transformer of the mechanical
power.
 The operating area of a motor driving the load in Fig 3a through a frictionless
transmission with a gear ratio of 0.5 is shown in Fig 3b.

21. Fig 3: Example operating areas: (a) load, (b) motor (same speed and torque scales used
in both diagrams).

22. EXAMPLE 1: The coil radius, r, in a textile winder changes from 0.15 m (empty coil) to 0.5
m (full coil). The automatically controlled tension, F, of the wound fabric can be set to any
value between 100N and 500 N, and the linear speed, u, of the fabric is adjustable within the 2
m/s to 4.8 m/s range. Determine the operating area of the winder.
The constant-force, constant-speed operation of the winder. makes the exponent k in torque
equation equal to -1. Indeed, because;
𝜔𝐿 =
𝑢
𝑟
,
𝑇𝐿 = 𝐹𝑟,
𝑇𝐿=
𝐹𝑢
𝜔𝐿
.

23. Assuming that the tension and speed of the fabric can set to any allowable value,
independently from each other, the operating speed of winder is limited to the
1/0.5=2rad/s to 2.4/0.15=16rad/s range. If expressed in r/min, this speed range is
19.1r/min to 152.8r/min. the operating area, shown in figure 4, is bound by two
hyperbolic curves corresponding to the minimum and maximum values of force and
speed.
Fig 4:

24.  In a properly designed drive system, the motor operates safely at every point of its
operating area, that is, neither the voltage, current, nor speed exceeds its allowable
values.
 The gearing may be needed to provide proper matching of the motor to the load.
 A gear ratio less than unity is employed when the load is to run slower than the
motor, with a torque greater than that of the motor.
 Conversely, a high-speed, low-torque load requires a gear ratio greater than unity.

25.  Control requirement depend on the particular application of a drive system.
 In most practical drives, such as those of pumps, fans, blowers, conveyors, or
centrifuges, the main controlled variables is the load speed.
 High control accuracy in such systems is usually not necessary.
 Drives with a directly controlled torque, for instance those of winders or electric
vehicles, are more demanding with regard to the control quality
 Finally, positioning systems, such as precision machine tool or elevator drives, must
be endowed with the highest level of dynamic performance.
 In certain positioning system, control requirements are so strict that induction motors
can not be employed.

27. Fig: (a) Thevinin equivalent circuit of induction motor, (b) Equivalent circuit of dc motor
armature, (c) Equivalent circuit of synchronous motor
Torque-speed characteristics of electric motors
(a) (b) (c)
𝐹𝑜𝑟 induction motor
T3𝜙, mech =
3
𝜔𝑠𝑦𝑛
.
𝑉𝑡ℎ
2
𝑅𝑡ℎ +
𝑅2
𝑠
2
+ 𝑋𝑡ℎ + 𝑋2
2
.
𝑅2
𝑠
𝐹𝑜𝑟 dc motor
T𝑎 =
60
2𝜋
.
𝐸𝑏𝐼𝑎
𝑁
=
60
2𝜋
.
𝑃𝜙𝑍𝐼𝑎
60𝑎
For synchronous motor

28. Electric motors exhibit wide variations of speed-torque characteristics, synchronous or
reluctance motors exhibit a constant speed characteristic (curve I). At steady-state
conditions these motors operate at constant speed regardless of the value of the load
torque.

29.  Motors operate at constant speed regardless of the value of the load torque.
 Curve-II shows a dc shunt or a separately excited motors, where the speed is
slightly reduced when the load torque increases.
 Direct current series motors exhibit the characteristic shown in curve III.;
• The speed is high at light loading conditions and low at heavy loading.
 Induction motors have a somewhat complex speed characteristic similar to
the one given by curve IV;
• During steady state, they operate at the linear portion of the speed –torque
characteristics, which resembles the characteristics of a dc shunt or a
separately excited motor. The maximum developed torque of induction motors
is limited to 𝑇𝑚𝑎𝑥.

30. Fig 6a: IM speed- torque curve
 In electric drive application, electric motor should be selected to match the intended
performance of the loads. For example, in constant-speed applications, the
synchronous motor is probably the best option. Other motors, such as induction or
dc, can also be used in constant-speed applications, provided that feedback circuits
are used to compensate for the change in speed when the load torque changes.

31. 1.4 Electric Motors Load ability
The maximum torque allowed at above synchronous speeds depends on the motor
characteristics and frequency as follows:
𝑇𝑚𝑎𝑥 ≤ 0.6𝑇𝑝
50
𝑓
Nm
Where, 𝑇𝑝= Pull out torque or maximum torque of the motor in Nm
f = actual frequency in the above synchronous range in Hz
0.6 = Factor of safety
Fig 1.7

32. DC motors
Fig 8: DC motor torque-speed characteristic

33. IM Characteristics
V1, Te, I1, P, ω2
Stator voltage V1
Stator current I1
Motor torque Te
Motor power P
Slip frequency ω2
Te α 1/ω1
Te α 1/ω1
2
Te.st
ωm
ωmb
0
Constant
torque drive
Constant power
drive
High speed series
motoring drive
Fig 9: IM characteristics

34. Power supply, convertor and control of
electric drives

35.  Two major types of power sources are used in industrial applications:
Alternating current (AC) and Direct current (DC)
 Alternating current sources are common in industrial installation and
residences. These can either be 1-phase or multiphase systems. 1-
phase power source are common in residences, where the demand for
electric power is limited. Multiphase power sources are used in high
power consumption application.
 The most common type of power source in the United States is the 3-
phase, 60-Hz power source. In Europe, most of the Middle East,
Africa, and Asia, the frequency is 50Hz.
 Extensive industrial installations usually have more than one type of
power source at different voltage and frequencies. Commercial
airplanes, for example may have a 400-Hz ac source in addition to a
270-volt dc source.

36.  The main function of converter is to transform the wave form of a power
source to that required by an electric motor in order to achieve the desired
performance.
 Most converter provide adjustable voltage, current, and/ or frequency to
control the Speed, torque, or power of the motor.
1. DC to AC: the dc waveform of the power source is converted to a single-or
multiphase ac waveform. The output frequency, current, and /or voltage can be
adjusted ac according to the application. This type of converter is suitable for ac
motors, such as induction or synchronous motors.
2. DC to DC: this type is also known as a “chopper”. The constant-input dc
waveform is converted to a dc waveform with variable magnitude. The typical
application of this converter is in dc motor drives.
3. AC to DC: the wave form is converted to dc with adjustable magnitude. The
input could be a single-phase or multiphase source. This type of converter is used
in dc drives.
4. AC to AC: the input waveform is typically ac with fixed magnitude and
frequency. The output is an ac with variable frequency, magnitude, or both. The
conversion can be done directly or through a dc link. The dc link system consists
of two converters connected in cascade; the first is an ac/dc, and the second is a
dc/ac. Typical applications of the dc link converter are ac motors.

37.  A well-designed controller has several functions. The most basic
function is to monitor system variables, compare them with some
desired values, and then readjust the converter output until the system
achieves a desired performance. This feature is used in such
applications as speed or position control. Some derive system may
lack stability due to limitations in the converter or load
characteristics. In such cases, a controller may also be designed to
enhance overall stability.