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INTRODUCTION TO ELECTRICAL DRIVES
Drives are employed for systems that require motion control e.g. transportation system, fans,
robots, pumps, machine tools, etc. Prime movers are required in drive systems to provide the
movement or motion and energy that is used to provide the motion can come from varioussources: diesel engines, petrol engines, hydraulic motors, electric motors etc.
Drives that use electric motors as the prime movers are known as electrical drives
There are several advantages of electrical drives:a. Flexible control characteristic This is particularly true when power electronic
converters are employed where the dynamic and steady state characteristics of the motorcan be controlled by controlling the applied voltage or current.
b. Available in wide range of speed, torque and power
c. High efficiency, lower noise, low maintenance requirements and cleaner operation
d. Electric energy is easy to be transported.
A typical conventional electric drive system for variable speed application employing multi-
machine system is shown in Figure 1. The system is obviously bulky, expensive, inflexible andrequire regular maintenance. In the past, induction and synchronous machines were used for
constant speed applications this was mainly because of the unavailability of variable frequencysupply.
Figure 1 Conventional variable speed electrical drive system
With the advancement of power electronics, microprocessors and digital electronics, typicalelectric drive systems nowadays are becoming more compact, efficient, cheaper and versatile
this is shown in Figure 2. The voltage and current applied to the motor can be changed at will
by employing power electronic converters. AC motor is no longer limited to application whereonly AC source is available, however, it can also be used when the power source available is DC
or vice versa
Figure 2 Modern Electric drive system employing power electronic converters
ACmotor
DCgenerator
variableDC DC
motor
variablespeed
Load
fixedspeed
If
Ia
Power
Source
Control
feedback
Power
Processor(Power electronic
Converters)
ControlUnit
Motor Load
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Electric drives is multi-disciplinary field. Various research areas can be sub-divided from
electric drives as shown in Figure 3.
Figure 3 Multi-disciplinary nature of electric drive system
Components of Electrical DrivesThe main components of a modern electrical drive are the motors, power processor, control unit
and electrical source. These are briefly discussed below.
a) Motors
Motors obtain power from electrical sources. They convert energy from electrical tomechanical - therefore can be regarded as energy converters. In braking mode, the flow ofpower is reversed. Depending upon the type of power converters used, it is also possible for
the power to be fed back to the sources rather than dissipated as heat.
There are several types of motors used in electric drives choice of type used depends onapplications, cost, environmental factors and also the type of sources available.. Broadly,they can be classified as either DC or AC motors:
DC motors (wound or permanent magnet)AC motors
Induction motors squirrel cage, wound rotor
Synchronous motors wound field, permanent magnetBrushless DC motor require power electronic converters
Stepper motors require power electronic converters
Synchronous reluctance motors or switched reluctance motor require power electronicconverters
b) Power processor or power modulatorSince the electrical sources are normally uncontrollable, it is therefore necessary to be ableto control the flow of power to the motor this is achieved using power processor or power
modulator. With controllable sources, the motor can be reversed, brake or can be operated
with variable speed. Conventional methods used, for example, variable impedance or relays,to shape the voltage or current that is supplied to the motor these methods however are
inflexible and inefficient. Modern electric drives normally used power electronic converters to
shape the desired voltage or current supplied to the motor. In other words, the characteristic
Machine design Speed sensorless Machine theory
Non-linear control Real-time control DSP application PFC
Speed sensorless Power electronic converters
Utility interface
Renewable energy
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of the motors can be changed at will. Power electronic converters have several advantagesover classical methods of power conversion, such as : More efficient since ideally no losses occur in power electronic converters
Flexible voltage and current can be shaped by simply controlling switching functions of
the power converter Compact smaller, compact and higher ratings solidstate power electronic devices are
continuously being developed the prices are getting cheaper.
Converters are used to convert and possibly regulate (i.e. using closed-loop control) theavailable sources to suit the load i.e. motors. These converters are efficient because the
switches operate in either cut-off or saturation modes
Several conversion are possible:
AC to DC
DC to AC
DC to DC
AC to AC
Dioderectifier
DC-DCconverter
control
Controlledrectifier
control
Inverter(PWM)
control
DC-DCconverter
control
Inverter(six-step)
control
DC-DCConverter
control
ControlledRectifier
control
Inverter
(six-step)
control
DiodeRectifier
control
Inverter(PWM)
MatrixConverter
control
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c) Control Unit
The complexity of the control unit depends on the desired drive performance and the type of
motors used. A controller can be as simple as few op-amps and/or a few digital ICs, or it canbe as complex as the combinations of several ASICs and digital signal processors (DSPs).
The types of the main controllers can be: analog - which is noisy, inflexible. However analog circuit ideally has infinite bandwidth. digital immune to noise, configurable. The bandwidth is obviously smaller than the
analog controllers depends on sampling frequency DSP/microprocessor flexible, lower bandwidth compared to above. DSPs perform faster
operation than microprocessors (multiplication in single cycle). With DSP/microp.,complex estimations and observers can be easily implemented.
d) SourceElectrical sources or power supplies provide the energy to the electrical motors. For high
efficiency operation, the power obtained from the electrical sources need to be regulated
using power electronic convertersPower sources can be of AC or DC in nature and normally are uncontrollable, i.e. their
magnitudes or frequencies are fixed or depend on the sources of energy such as solar orwind. AC source can be either three-phase or single-phase; 3-phase sources are normally for
high power applications
There can be several factors that affect the selection of different configuration of electrical drive
system such as:
a) Torque and speed profile - determine the ratings of converters and the quadrant ofoperation required.
b) Capital and running cost Drive systems will vary in terms of start-up cost and running
cost, e.g. maintenance.c) Space and weight restrictions
d) Environment and location
Comparison between DC and AC drives
Motors : DC require maintenance, heavy, expensive, speed limited by mechanical construction AC less maintenance, light, cheaper, robust, high speed (esp. squirrelcage type)
Control unit: DC drives: Simple control decoupling torque and flux by mechanical commutator the
controller can be implemented using simple analog circuit even for high performancetorque control cheaper.
AC drives, the types of controllers to be used depend on the required drive performance obviously, cost increases with performance. Scalar control drives technique does notrequire fast processor/DSP whereas in FOC or DTC drives, DSPs or fast processors are
normally employed.
Performance:
In DC motors, flux and torque components are always perpendicular to one another
thanks to the mechanical commutator and brushes. The torque is controlled via thearmature current while maintaining the field component constant. Fast torque and
decouple control between flux and torque components can be achieved easily. In AC machines, in particular the induction machines, magnetic coupling between
phases and between stator and rotor windings makes the modeling and torque controldifficult and complex. Control of the steady state operating conditions is accomplished
by controlling the magnitude and the frequency of the applied voltage; which is known
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as the scalar control technique. This is satisfactory in some applications. The transientstates or the dynamics of the machine can only be controlled by applying the vectorcontrol technique whereby the decoupling between the torque and flux components is
achieved through frame transformations. Implementation of this control technique is
complex thus requires fast processors such as Digital Signal Processors (DSPs).
Overview of AC and DC drives
The advancement in electric drive system is very much related to the development in the powersemiconductor devices technology. The introduction of the Silicon-Controlled Rectifier (SCR) in
1957 has initiated the application of solid state devices in power converters. The development ofthe electrical drives systems can be divided into three stages
Before power semiconductor devices were introduced:
AC drives were used for fixed speed operation. Generating an AC voltage with variablefrequency was only possible by using rotary converters, which are bulky and inflexible. Although
it is possible to use variable voltage with fixed frequency sources to control the speed of AC
motors, the efficiency of the drive system will be very poor especially at low speeds. On the otherhand, variable DC supply can be produced using multi-machine configuration and hence could
be used to control the armature voltage of the DC motors. Consequently, DC drives are widelyused for variable speed operation, whereas AC machines were used mainly for fixed speed
applications.
After power semiconductor devices were introduced in 1950sAlthough self turnoff devices (Bipolar Junction Transistor BJT) were available in the
1950s their voltage ratings were too low which make them inappropriate to be used in power
circuit. Silicon-Controlled Rectifier (SCR) was introduced in 1957. The higher ratings of SCRcompared to the solid state transistor at that time, has made it possible for it to be used in
static frequency converters or inverters. Speed control with AC motor can be performed because
variable frequency AC supply can be generated using inverters. However, since the switchingfrequency of an SCR was low which require commutation circuit in order to turn off, square
wave inverters were mainly used in AC drive system. In early 1960s, the improvement in the
fabrication of BJT along with the introduction of pulse width modulation (PWM) controltechnique has significantly contributed to the improvement in the AC motor drives. Transient
torque control to some extend, was nearly achieved to the expense of a very complex algorithmwith numerous approximations. The true high performance torque control similar to DC drives
was still not achievable due to the complex magnetic coupling between phases in the stator androtor of the AC machines. Nevertheless, DC drives were gradually being replaced with AC drives
in medium performance variable speed applications. Applications requiring precise and fast
torque control were still dominated by DC drives.
After semiconductor devices were introduced in 1980s
In 1972, Prof. Blashke published his approach of AC motor control, to what is nowknown as Field Oriented Control (FOC) or vector control. FOC control basically transformed the
control of AC motors to the one similar to DC motor control. In other words, the highperformance torque control can be achieved using AC motors. This is possible through complexframe transformations and algorithm. However not until in the early 80s, where faster
microprocessors were available, the algorithm used for FOC was not practically realizable. In1980s, increasing number of applications utilizing FOC control could be found in industries.Applications which were previously possible only with DC drives were gradually being replaced
with FOC of AC drives. It was predicted that the AC drives will eventually replace the DC drives
in the near future.
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Torque Equations For Rotating Systems
The Newtons Law states that, the net force acting on a body of mass M equals to the rate of
change of its mechanical momentum, which is the product of its mass and its velocity in the
direction of the net force. In the equation form, this is given by
(1)
where F is the net force acting on the body, M is the mass of the body and v is its velocity. This
is illustrated by Figure 4.
Figure 4 Translational motion
With constant mass, (1) can be written as
For rotational motion (which is the case for rotating electrical machines), the force, the mass
and the linear velocity is equivalent to the torque, the moment of inertia and the angular
velocity, respectively. Equation (1) can therefore be written as
(3)
where T is the net torque, J is the moment of inertia and is the angular velocity. The rotational
system which is analogous to the translational system of Figure 4 is shown in Figure 5.
Figure 5 Rotational motion
For most of the cases, J is constant thus reducing (3) to
(4)
In terms of the angular position, , this can be written as
M
x
v
FpFf
, TeTL
J
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(5)
For rotating electrical machines, the net torque is given by
(6)
where Te is the internal electrical torque produced by the motor, Tl is the load torque and/or the
internal friction of the motor. T is the available torque at the shaft and is responsible foraccelerating the inertia of the motor. T is also known as the dynamic torque and it only existsduring the transient (i.e. acceleration and deceleration). In order to accelerate in forward
direction, Te Tl must be positive; which means that the applied electrical torque must be larger
than the load torque. In order to decelerate, the net torque must be negative; the electricaltorque must be made smaller than the load torque and the motor operates in braking mode
more on this later. Note that the speed is always continuous. A discontinuity in speed (i.e. step
change in speed) theoretically will require an infinite torque. This is analogous to the voltage andcurrent across a capacitor in which discontinuity in capacitor voltage is not allowed as it
correspond to an infinite capacitor current.
Equation (4) relates the torque and the mechanical speed (or position) of the machine. For agiven electrical torque profile, with the known moment of inertia and the load torque, the speed
profile of the drive system can be determined. In a torque-controlled drive system, the speed isgoverned by the load. If the load torque comprise of only the frictional torque which is
proportional to the speed, (4) can be written as
(7)
Equation (7) can be easily simulated using SIMULINK as shown in Figure 6. In the simulation, a
square wave torque is applied.
Figure 6 Dynamic simulation of mechanical system
Usually in a cascaded closed-loop control system in which the speed is to be controlled, thereference torque will be generated by the speed controller. In such cases, the torque will be
governed by the speed.
If we multiply (7) with the angular speed, we obtain an equation describing the power balance,
(8)
torque
speed
position
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Where pD = mTe is the driving power, pL= mTl is the load power and is the change in
kinetic energy. Integrating the equation with time and setting the initial speed (0) = 0, we
obtain the following:
(9)
The last term of (9) is the stored kinetic energy of the system. It is analogous to the energy
stored in a capacitor or an inductor . Similar to a capacitor voltage or an
inductor current, an angular velocity must be continuous. An abrupt (discontinuous) change
in will results in an infinite power.
Relation between translational and rotational motionsIn most applications of the drive systems, the translational and rotational motions are related.
An example of a typical system is shown in Figure 7.
Figure 7 Translational and rotational motions
The relation between the torques and the linear forces are given by
Tl = rFl, Tm = rFm .Also,
V = r
If the mass M is constant, we can write
(10)
Equation (10) states that the equivalent moment of inertia of the translational motion referred tothe axis of the pulley is given by Jequ = Mr2
M
FmFl
rr
v
Tm
Tl
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System with gearsIt was found out that machines designed to operate at low speeds are large in size compared tothe ones which are designed to operate at high speeds. In order to avoid the unnecessary large
size machines, high speed operations are normally preferred. However, in some applications,
slow motion with high torque is required. Consequently for such applications, gears whichreduce speed but amplify the torque, are commonly employed. An example of the hoist drive
employing gears is shown in Figure 8.
Figure 8 Hoist drive with gears
The hoist drive system shown in Figure 8 can be represented by an equivalent system similar to
Figure 5. In order to do that, we need to obtain the equivalent moment of inertia and loadtorque. If the mass M3 is considered being moved upwards, with the negligible frictional torque,it can be shown that the torque equation for the equivalent system is given by
(11)
where
Steady state operating speedThe characteristics of the motor and load are normally described based on their torque versus
speed graph or T- characteristics. The T- characteristic of a motor corresponds to the
variation of its torque versus its speed, with all other variables, including the voltage (or current)
and frequency (for AC motor) are kept constant. Typical shape of T- characteristics of different
motors are shown in Figure 9.
The loads on the other hand will have their own T- characteristics. It is the intersection
between the motor and the load T- characteristics that determines the steady state speed. This
can be seen from (6) where at steady state d/dt = 0 and Te = Tl.
J1
J2
M3
J3
1, Tm 2
3
2r3Loss-
free
gear
Synchronous motor Separately excitedDC motor
Induction motor Series DC motor
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The steady state torque-speed characteristic of the motor depends on the applied voltage orcurrent. Hence, by changing the point of intersections between the motor and load torque-speedcurves, different steady-state speeds can be achieved.
Figure 9 Different steady state speeds (Tl = Te) for different motors T- characteristics
It should be noted that the graph in Figure 9 only displayed the steady state characteristics ofthe load and motor. The transient responses before these steady state speeds are reached have
to be dealt with using the dynamic characteristics of the load and motor.
Components of Load Torque, Tl
In general, the load torque Tl can be classified into two types: the passive load torque (frictional
torque) and the active load torque. Frictional toque exists only when there is motion and italways opposes the driving torque. Active load torque on the other hand, is independent of the
direction of motion.
Frictional torque
Moving parts of the motor and load constitute the frictional torque. There are several types of
frictional as described in Figure 4 and explained below:
Coulomb friction exists in bearings, gears, coupling and brakes. It is almostindependent of speed.
Viscous friction exist in lubricated bearings due to the laminar flow of the lubricant. Itis directly proportional to the speed.
Windage friction occurs due the turbulent flow of air or liquid. It is directly proportional
to the square of speed
In practical drive system consisting of load and motor, all components of friction describedabove exist simultaneously. However, in most of the cases, only one or two components are
dominating. For instance, a fan or a propeller will typically have the windage friction
dominating, whereas in paper mill and machine tools, the dominating one could be the viscousfriction.
Torque
speed
Torque-speed characteristic ofthe load, Tl
Different steady-state torque-speedcharacteristics of the motor, Te
1 2 3
Different motor
speeds
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Constant torqueThe direction of constant load torque is independent of speed it retains the direction even
when the direction of rotation reverses or changes, e.g. gravity, tension or compressionundergone by elastic body. This type of torque is capable of driving the motor under equilibriumand is said to be an active torque.
Thermal considerations
The losses in the machines contribute to the temperature increase in the machine. The variousparts of the machine have different temperature limits. Particularly important is the insulation
used for the windings which give rise to the different classes of machines. If the temperaturegoes beyond the allowable temperature, it will cause an immediate breakdown (short circuit inthe winding) or it will deteriorate the quality and hence reduces the lifetime of the insulation
material. Allowable power losses are higher for materials which can withstand higher
temperature which translates to higher costs. The classes of the insulator used for the windingin electrical machines are shown in Table 1.
T
Viscous
Coulomb
Windage
Speed
Torque
Gravitationaltorque
FL
TL
gM
TL = rFL = r g M sin
Te
Figure 10 Frictionaltorque
Figure 11 Constant load torque: gravitational force
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Table 1 Classification of the insulators
Class Max safe temp. oC
V 90
A 105E 120
B 130
F 155
H 180C >180
Three main cause of power losses are:
Conductor losses (i2R)Exist in the windings, cables, brushes, slip rings, commutator, and etc.
Core lossesMainly due to eddy current and hysteresis losses
Friction and windage losses
Mainly due to ball bearings, brushes, ventilation losses
The constructions of the machines are very complex; normally built from various types ofmaterials (heterogeneous) with complex geometrical shapes. To exactly predict the heat flow andhence the temperature distribution is extremely difficult. Based on the assumptions that the
temperature limits of all parts does not exceed the temperature limits under certain operatingconditions, the motors can therefore adequately modeled as homogeneous bodies. Obviously,
this assumption cannot determine the specific internal thermal conditions for the motors.
Figure 12 Homogeneous body
Let us assume that a homogeneous body shown in Figure 12 represents a motor which has athermal capacity C. The input power, which is the losses incurred in the motor, is represented
by p1 whereas the output power, which is the power released as heat by convection, isrepresented by p2. The output power due to radiation is assumed negligible because of the low
operating temperature and back radiation. Under a steady state condition, the input powerequals the output power; this is when the steady state temperature is reached. The equation
describing the power balance is given by
(12)
Thermal capacity, C (Ws/oC)Surface A, (m2)
Surface temperature, T (oC)
Ambient temperature, To
p1
INPUT POWER(losses)
p2
OUTPUT POWER(convection)
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The heat dissipated by convection is given by
p2= A (T To) (13)
where is the coefficient of heat transfer.
If we let T = T To , equation (12) can be written as
or
(14)
where T= C/(A) is the thermal time constant. With T(0)=0 and a step change in the power
input p1 from 0 to ph at t=0, the solution for T is
(15)
At steady state, T() = ph/(A)
During cooling, i.e. when heat is removed at t=0, the temperature of the body decays to the
ambient temperature.
(16)
t
t
Heatingtransient
Coolingtransient
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Figure 13 Heating and cooling transients
The thermal time constant depends on the coefficient of heat transfer which in turn depends
on the velocity of the cooling air. Machines which are self-ventilated will have larger cooling timeconstants compared to their heating (assumed moving) time constants. On the other hand
machines with forced ventilation system will have a cooling and heating time constants of more
or less equal. It should be noted that the thermal time constant of electrical machines aretypically much larger than their mechanical or electrical time constants. It may vary from fewminutes few hours.
If the thermal time constant is large, a temporary overload is therefore possible withoutexceeding the temperature limits. Three typical modes of operation are:
- Continuous duty
- Short time intermittent duty- Periodic intermittent duty
(i) Continuous dutyThe motor is loaded continuously. Obviously the rating of the motor must at least equal the
continuous loading of the machine. Normally, motor with next higher power rating from
commercial available rating is selected.
(ii) Short time intermittent duty
The time of operation is considerably less than the thermal time constant. The motor is allowed
to cool to ambient temperature before the new load cycle is applied. The motor is allowed to beoverloaded provided that the maximum temperature is not exceeded. However, the application of
much higher power than the rated power is subject to the available torque of the machine. ForDC machine this is limited due the sparking between the brushes and the commutator. In
induction machine, this is limited by its pull-out torque.
(iii)Periodic intermittent duty
The load cycle is repeated periodically. The machine is not allowed to cool to ambient when thenext load cycle is applied. The temperature will fluctuate and the mean value will eventually
settle to a steady state value. The machine can be overloaded and amount of overloadingdepends on the duty cycle of the load. The heating and cooling time constant may be different
depending whether the machine is self-cooled or forced-cooled.
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Four-quadrant operation of a drive system
The T plane with motors shaft cross sectional area is shown:
Figure 14 Four-quadrant operation of a drive system
The positive or forward speed is arbitrarily chosen in counterclockwise direction (it can also bechosen as clockwise). The positive torque is in the direction that will produce acceleration in
forward speed, as shown above.
The plane is divided into 4 quadrants , thus 4 modes of operation. The quadrants are marked as
I, II, III and IV
Quadrant IBoth torque and speed are positive the motor rotates in forward direction, which is in the same
direction as the motor torque. The power of the motor is the product of the speed and torque (P
= Te), therefore the power of the motor is positive. Energy is converted from electrical form to
mechanical form, which is used to rotate the motor. The mode of operation is known as forwardmotoring.
Quadrant II
The speed is in forward direction but the motor torque is in opposite direction or negative value.The torque produced by the motor is used to brake the forward rotation of the motor. The
mechanical energy during the braking, is converted to electrical energy thus the flow of energy
is from the mechanical system to the electrical system. The product of the torque and speed isnegative thus the power is negative, implying that the motor operates in braking mode. The
mode of operation is known as forward braking.
Quadrant III
The speed and the torque of the motor are in the same direction but are both negative. The
reverse electrical torque is used to rotate the motor in reverse direction. The power, i.e. theproduct of the torque and speed, is positive implying that the motor operates in motoring mode.
The energy is converted from electrical form to mechanical form. This mode of operation is
known as reverse motoring.
T
III
III IV
TeTe
TeTe
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Quadrant IVThe speed is in reverse direction but the torque is positive. The motor torque is used to brakethe reverse rotation of the motor. The mechanical energy gained during the braking is converted
to electrical form thus power flow from the mechanical system to the electrical system. The
product of the speed and torque is negative implying that the motor operates in braking mode.This mode of operation is known as reverse braking.
Ratings of converters and motors
In order to accelerate to a given reference value, the motor torque has to be larger than the loadtorque. According to (1), the difference between Tl and Te determines how fast the angularacceleration is. For example, the speed and torque responses for a closed-loop speed control DC
drive with two different torque limit setting (10 Nm and 15 Nm) is shown in Figure 7. The higher
the torque during the speed transient, the faster is the speed gets to its reference.
Figure 7 Speed response with different torque limit settings
In most cases, the torque during this transient condition can be up to 3 times the rated torque
of the motor and for servo motor, it can be as high as 8 to 10 times the rated value. Thismomentary high torque is possible due to the large thermal capacity of the motor with suitableinsulators used for the winding. The converter, which conducts the motor current, must be able
to sustain this condition. However since the thermal capacity of a switching device is small, the
current cannot be higher than its rated value even for a short time. Consequently, the currentrating of the converter is normally set to equal the maximum allowable motor current and this
can be as high as the 3 times the motor rated current. The maximum allowable torque during
transient of a drive system is determined by the current rating of the converter used whereasthe continuous torque limit depends on the current rating of the motor. The operating area of a
4-quadrant motor drive is shown in Figure 8. The converter is normally protected from the over-
current condition by the current limiter mechanism within the converter system, which meansthat sustained overloads on the motor has to be protected by an additional thermal protection
mechanism. Above the base speed, b, the toque is limited by the maximum allowable power,
which depends on whether the transient or continuous torque limit is considered. The speed
limit basically depends on the mechanical limitation of the motor.
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Steady-state stability
The motor will operate at the steady-state speed (point where Tl = Te) provided that the speed is
of stable equilibrium. The stable equilibrium speed is investigated using steady-state torque-speed characteristics of the load and motor.
A disturbance in any part of the drive will result in a speed to depart from the steady state
speed. However, if the steady-state speed is of stable equilibrium, the speed will return to thestable equilibrium speed. On the other hand, if the speed is not of the stable equilibrium, the
disturbance will results in the speed to drift away from the equilibrium speed. It can be shown
mathematically that the condition for stable equilibrium is:
(17)
Figure 9 Steady state stability
Te Tl
Motor will decelerate
back to equilibrium
since Tl > Te
Motor will accelerate
away from equilibriumsince Te > Tl
Torque
speed
Torque
speed
Tl Te
Torque
Speed
Power limit fortransient torque
Power limit forcontinuoustorque
Transienttorque limit
Continuoustorque limit
Maximumspeed limit
b- b
Figure 8 Limits for torque, speedand power for drive system
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References
G.K. Dubey, Fundamental of Electrical Drives, Narosa, 1994.
W. Leonhard, Control of Electrical Drives, Springer-Verlag, 2001