Download electrical drive

MODULE – 3
Industrial device – Electric drives – advantages – individual drive and group drive factors
affecting choice of motor mechanical characteristics of A.C and D.C motor motors for
particular application like textile mill, steel mill, paper mill, mine, hoists, crane etc. Size
and rating of motor – motor selection of intermittent loads. Electric traction different
systems of traction – comparison – track electrification – different systems – traction
motor characteristics – electric braking – plugging – dynamic and regenerative braking.
ELECTRICAL DRIVE
An electric drive is defined as a form of machine equipment designed to convert electric
energy into mechanical energy and provide electrical control of these processes.
BLOCK DIAGRAM OF AN ELECTRIC DRIVE IS AS SHOWN HERE
Source
Power
Motor
Load
Modulator
Feedback circuit
Control
Sensing
Unit
Unit
Input command
(a) Source : It is either type of electrical power dc or ac supply
(b) Power Modulator : it performs the following functions.
i.
If converts electrical energy received from the source in the form suitable to
the motor ie, if the source is dc and an induction motor is to be employed then
it converts d.c into a variable frequency a.c
ii.
During transient operations, such as starting, braking an speed reversal it
restricts source and motor currents with in permissible limits.
iii.
It selects the mode of operation of motor, ie, motoring or braking
C) Load : is usually a machinery designed to perform a given operation ie, fans machine
tools, domestic appliances, trains, pumps etc.
d) Motor : Motor commonly used in electrical drives are DC motors – shunt, series,
compound and permanent magnet, induction motor – squirrel cage, wound rotor and
linear synchronous motor – cofound field and permanent magnet, Brushless dc motor,
stepper motor and switched reluctance motors.
e) Sensing unit : It is employed for sensing the drive parameters, such as speed, motor
current etc. which may be required either for protection purpose or for closed loop
operations. These signals are fed to control limit.
(f) Control unit : It control the power modulator besides generating commands for the
protection of the motor and power modulator. It usually operates at much lower voltage
and power levels. Input command signal, which adjusts the operation of drive forms an
input o the control unit apart from signals from the sensing unit.
TYPE OF ELECTRIC DRIVES
Electric drives used in industry may be divided into three types
(a) Group drive (b) individual drive (c) Multimotor drive)
GROUP DRIVE.
In this, one motor is used as a drive for two or more than two machines. The motor is
connected to a long shaft, on which belt and pulleys are connected to run other m/cs it is
also called line shaft drive. This type of drive is economical as a single motor of large
capacity costs less than the costs of a number of small motors of the same total capacity.
The use of this type of drive is restricted due to following reasons
1. In case of fault in the motor, all the connected m/cs to this motor will cease to
operate.
2. If at certain instance all the m/cs are not in operation then the motor will be
working at low capacity.
3. It is not possible to install a new m/c at a far away distance.
4. Speed control of different m/cs using belts and pulley is cumbersome.
INDIVIDUAL DRIVE
In this type of electric drive a single electric motor is used to drive one individual m/c.
Through it costs more than group drive but each operator has complete control on his
machine, which enables him to either increase the speed of motor or to stop it while not
in operation. Machines can be located at convenient places. If there is a fault in one
motor this will not effect the production of the industry appreciably.
No a days electric drive is being extensively used in the industry because of its certain in
herent advantages over the other types of drives.
Some of the advantages are
Advantages of Electric drive.
1. Cost is too low. 2) The system is more simple and clean 3) The control is very
easy and smooth 4) Flexible in layout 5) Facility for remote control. 6)
Transmission of power from one part to other can be done with the help of cables
instead of long shafts etc. 7) Maintenance cost is quite low. 8) Can be started at
any time with out delay in time.
DISADVANTAGE
1) Electric drive system is tied only up to the electrified area. 2) The condition
arising under the short circuits, leakage from conductor and breakdown of over
head conductor may lead of fatal accident. 3) Failure in supply for a few minutes
may paralyze the whole system.
ADVANTAGES OF GROUP DRIVE.
1) Initial cost : Initial cost of group drive is less as compared to that of the individual
drive
2) Sequence of operation : Group drive system is useful because all the operations
are stopped simultaneously
3) Space requirement : Less space is squired in group drive as compared to individual
drive
4) Low maintenance cost: small maintenance cost as compared to individual drive.
DISADVANTAGES
1) Power factor : Group drive has low pf.
2) Efficiency : If all the motor are not working together the main motor shall work at
reduced efficiency.
3) Reliability if main motor fails, the whole industry will come to stand still.
4) Flexibility – The system is not flexible
5) Speed: no constant speed
6) Types of machines : Not suitable for driving heavy machines such as cranes, lifts,
hoists etc.
ADVANTAGES OF INDIVIDUAL DRIVES
1. Individual drive give desired operation as each m/c is drive by its own individual
motor
2. Works at good power factor
3. Efficiency of the system is high
4. More reliable
5. May be fitted where ever suitable
6. More useful where constant speed is required
7. Suitable for driving huge machines like canes, lifts, hoist etc.
COMPARISON BETWEEN TWO DRIVE
Points
Individual drive
Group drive
1. Initial cost
More
Less
2. Power factor
Works at good pf.
Low pf.
3. Efficiency
High
Low when worker at light loads.
4. Reliability
More reliable
Not reliable
5. Flexibility
May
be
fitted
where’re Not possible for group drive.
convenient
6. Speed
More useful where constant speed Does not give constant speed
is required
7. Sequence
operation
of Useless
where
sequence
operation is required
of Use full, since all operation are
stopped simultaneously.
FOUR QUADRANT OPERATION OF MOTOR DRIVING A HOIST LOAD
In the first quadrant the load torque acts in a direction opposite to that of rotation. Hence
to drive the loaded hoist up, the developed torque in the motor M must act in the same
direction as the speed of rotation ie, TM should be of +ve sign since the speed is also
positive being an upward motion, the power will also have a positive sign, ie the drive is
said to be motoring. Quadrant I is arbitrarily and conventionally thus designated as
forward motoring quadrant.
The hoisting up of the unloaded cage is represented in the second quadrant. Since the
counter weight is heavier than the empty cage, the speed at which the hoist is moved
upwards may attain a dangerously high value. In order to avoid thin, the motor torque
must act in a direction opposite to that of rotation. ie, motor should switch over to a
braking or generating region. Here TM will have a negative sign and speed still has a
positive sign, being forwards, upwards and counter clockwise, giving power a negative
sign, corresponding to the generating or braking operation.
The third quadrant represents the downward motion of the empty cage. The down ward
journey of the cage opposed by the torque due to the counter weight and friction at the
transmitting parts. So for moving the cage downwards, the motor torque must act in the
same direction as the motion of the cage. The electrical m/c acts as a motor as in the fist
quadrant but in the reverse direction. This quadrant III becomes reverse motoring. The
motor torque has a –ve sign as it causes an increase in speed in the negative sense and the
speed is –ve being a downward motion. Power has a +ve sign. The downward motion of
the loaded cage is in IVth quadrant. The motion can take place under the action of load
itself, with out the use of any motor. But in order to limit the speed of the down ward
motion of the hoist, the electrical m/c must act as a brake. The motor torque has a
positive sign as it causes a decrease in speed in the down ward motion. The speed has a
-ve sign being a down ward journey. The power has –ve sign corresponding to braking
operation of the motor.
THE TRACTION MOTORS
The traction motor incorporates the following features
a) Electrical features
1) High starting torque
2) Series spaced torque characteristics (T α 1/N)
3) Simple speed control.
4) Possibility of dynamic or regenerative braking
5) Better communication
b) Mechanical features
1) Light in weight and small space requirements.
2) Totally enclosed especially if mounted beneath the locomotive
3) Robust and should be able to with stand vibrations
The motor which are used for this purposes are ) series and compound motors operate
satisfaction on dc system.
2) Ac series motor on single phase system
3) Induction motor on 3φ ac system
The speed torque characteristics of dc shut and series motor are given below.
THE SPEED TOQUE CHARACTERISTICS OF DC SHUNT AND SERIES
MOTOR ARE GIVEN BELOW.
T
shunt
N
Shunt
Series
N
Series
Ia
Series
Ia
SPEED TORQUE CHARA. OF AC AND DC SERIES MOTORS.
N
ac
dc
T
ELECTRIC TRACTION
Shnut
T
Traction system can be classified as non – electric and electric traction. Non electric
traction does not use electricity at any stage such as steam engine drive and internal
combustion drive whereas electric traction involves the use of electrical energy at some
stage or the other such as battery electric drive, diesel electric drive and straight electric
drive.
STEAM ENGINE DRIVE HAS THE FOLLOWING ADVANTAGES
1) Simplicity in design 2) Ease of speed control 3) No interference with
communication n/w 4) Low capital cost.
But because of the disadvantages like 1) Low thermal efficiency 2) considerable wear on
the track 3) corrosion of the steel structures due to smoke emitted by the engine 4) air
pollution straight electric systems are preferred.
ELECTRIC TRACTION IS THE MOST EFFICIENT OF ALL OTHER
SYSTEMS BECAUSE OF THE FOLLOWING REASONS.
1. Since electric motion are used as the drive, the system in clean and pollution free.
2. Speed control and braking is simple
3. In fact by using regenerative braking energy can be pumped back into the system
especially during periods of descents.
4. Electric traction is more suitable especially for suburban and urban railway where
frequent starting and stopping and high schedule speeds and required.
5. The coefficient of adhesion in high
6. Over loading of electricmotors is possible for sometimes
7. Centre of gravity of eclectic locomotive in lower than steam locomotive hence
electric locomotives run faster at curved surfaces or routes.
REQUIREMENTS OF AN IDEAL TRACTION SYSTEM
The following are the important requirements of the driving equipment used for traction
purpose
1) The coefficient of adhesion should be light so that high tractive effort at start is
possible and rapid acceleration of the train can be obtained
2) It should be possible to over load the equipment for short periods
3) The wear caused on the brake shoes, wheel tyres and the track should be
minimum.
4) It should be possible to use reganeative braking so that on descents it should be
possible to generate energy and feed back to the supply system.
5) The loco motive or train unit should self contained so that it can run on any route.
6) It should be pollution free.
SYSTEM OF RAILWAY TRACK ELECTRIFICATION
PRESENTLY, FOLLOWING FOUR TYPES OF TRACK ELECTRIFICATION
ARE AVAILABLE.
1. Direct current system – 600V, 750V, 1500V, 3000V.
2. Single phase ac system – 15 – 25kv, 16 2/3 , 25 and 50 hz.
3. Three phase ac system – 3000 – 3500 V at 16 2/3
4. Composite system – involving conversion of single phase ac into 3 phase ac or dc.
DIRECT CURRENT SYSTEM
Direct current at 600 – 750 V is universally employed for tramways in urban areas and
for many suburban railways while 1500 – 3000 V dc is used for main line railways. The
current collection is from third rail upto 750V, where large currents are involved and
from over head wire for 1500V and 3000 V, where small currents are involved since in
majority cases, track rails are used as the return conductor, only one conductor rail is
required. Both of these contact systems are fed from substations which are spaced 3 to 5
km for heavy suburban traffic and 40-50 km for mainlines operating at higher voltages of
1500 V to 300 V. The required voltages are obtained in these substations using rectifiers
or inverters (conversion from ac to dc). These substations are usually automatic and are
remote controlled. The dc supply so obtained is fed via suitable contact system, to the
traction motors which are either dc series motors for electric locomotives or compound
motors for tramways and folly buses.
For heavy suburban service, low voltage dc system is undoubtely superior to 1φ ac
system.
SINGLE PHASE LOW FREQUENCY AC SYSTEM
In this system, ac voltages from 11 to 15 KV at 16 2/3 or 25 Hz are used. If supply is
from a generating station exclusively meant for the traction system, there is no difficulty
in getting the electric supply of 16 2/3 or 25 Hz. If electric supply is taken from the high
voltage transmission lines at 50 Hz then is addition to step down transformer, the
substation is provided with a frequency converter. The frequency converter equipment
consists of a 3φ synchronous motor which drives a 1φ alternator having 25Hz frequency.
The 15KV 16 2/3 or 25 Hz supply is fed to the electric locomotive via a single over head
wire.
A step down transformer carried by the locomotive reduces the 15 KV voltage to 300 –
400 V for feeding the ac series motor. Speed regulation of ac series motors is achieved by
applying variable voltage from the tapped secondary of the above transformer. Low
frequency ac supply is used because apart from improving the commutation properties of
ac motors it increases their efficiency and pf.
Three phase low frequency AC system.
It lines 3 θ induction motors which work on a 3.3kv, 16 2/3 Hz supply. Substation receive
power at a very high voltage from 3 φ transmission lines at the usual industrial frequency
of 50 Hz. This high voltage is stepped down to 3.3Kv by transformer whereas frequency
is reduced from 50 Hz to 16 2/3 Hz by frequency converters installed at the sub station.
This system employs two over head contact wires, the track rail forming the third phase.
Induction motors used in the system are quite simple and robost and give trouble free
operation.
Composite system.
Such a system incorporate good points of two system while ignoring their bad points.
Two such composite system presently in use are :
1) 1 φ to 3 φ system also called kando system
2) 1 φ to dc system
KANDO SYSTEM
In this system single phase 16-KV, 50 HZ supply from the sub station is picked up by the
locomotive through the single over head contact wire. It is then converted into 3φ ac
supply at the same frequency by means of phase converter equipment carried on the
locomotives. The 3φ supply is then fed to the 3 θ induction motors.
SINGLE φ AC TO DC SYSTEM
This system combines the advantages of
high voltage ac distribution at industrial
frequency with the dc series motor traction. It employs overhead 25kv, 50 Hz supply
which is stepped down by the transformer installed in the locomotive itself.The low
voltage ac supply is then converted into dc supply by the rectifier which is also carried on
the locomotive. This dc supply is finally fed to dc series traction motor.
The motors commonly used for particular services :
Domestic uses
Small universal motors of the series type are used in domestic appliances like
vacuum cleaners, refrigerator, washing machines, fans etc.
Grinding and milling machine
Up to 50 HP the motors may be dc shunt or induction with sliprings and arrangements for
pole changing with cage rotors.
PLANNERS
There is cutting stroke and a quick return stroke. Arrangements for revering the speed
have to be incorporated. A dc compound motor may be used.
PUNCHING AND SHEARS
On account of the heavy fluctuation of laod, a flywheel is provided. The motor may be dc
shunt or compound or slip ring induction type.
CRANES AND HOIST WORK
For cranes dc motor of the series of compound wound type are preferred as they have a
high starting torque and the speed control is smooth.
Induction motor are also used for hoisting.
LIFTS
Lift duty involves high acceleration and high retardation. The motor armature must
therefore be light and it should rum at moderate speeds. DC compound, slip ring
induction, induction- repulsion and a-c commutators are used.
TEXTILE INDUSTRY
Motors must be of the totally enclosed type of prevent particles of the material being
manufactured from getting into them. They should also be moisture proof on account of
damp atmosphere inside a textile plant.
Three phase motor are used since their speed is fixed by the supply frequency. DC motor
cannot be used as their speed varies with voltage.
PRINTING MACHINERY
Most presses require a variable speed. Induction motors using rotor resistance may be
used. Where large speed variations are required d-c compound or a-c commutator motors
may be used.
PAPER INDUSTRY
For constant speed, the synchronous motor is used. Constant speed is essential to
maintain uniform thickness of the paper. Where speed is not required to be kept constant,
squirrel cage induction motors or dc motors may be used.
IRON AND STEEL INDUSTRY
Motors in the mill shop are dc shunt with flywheel. Induction motors with speed control
are also used.
MINING WORK
Flame proof motors are essential. Cage motors are used upto 10hp and for large output
slip ring or dc motor.
FACTORS GOVERNING SELECTION OF ELECTRIC MOTORS
The conditions under which an electric motor has to operate and the type of load it has to
handle determine its selection. The various factors that are to be considered in the
selection of an electric motor for a particular service are :
1. Nature of electric supply (Explanation in next page)
2. Type of drive
3. Nature of load
4. Electrical characteristics
a. Operating or running characteristics
b. Staring characteristics
c. Speed control
d. Braking characteristics
5. Mechanical characteristics
a. Type of Enclosures
b. Type of Bearings
c. Type of transmission for drive
d. Noise level.
e. Heating and cooling time constants
6. Service capacity and rating
a. Requirement for continues, intermittent or variable load cycle
b. Pull out torque and overload capacity
7. Appearance
8. Cost
a. Capital or initial cost
b. Running cost – power factor, losses, maintenance and depreciation etc.
SIZE AND RATING
The factors which govern the size and rating of motor for any particular service are its
maximum temperature rise under given load conditions and the maximum torque
required. A motor which is satisfactory from the point of view of maximum temperature
rise usually satisfies the requirement of maximum torque as well. For class A insulation,
maximum permissible temperature size is 400C where as for class – C insulation, it is
500C. This temperature rise depends on whether the motor has to sum continously,
intermittently or on variable load.
DIFFERENT RATINGS FOR ELECTRICAL MOTOR ARE :
1. CONTINOUS RATING
It is based on the maximum load which a motor can deliver for an indefinite
period with out its temperature exceeding the specified limits and also possessing
the ability to take 25% over load for a period of time not exceeding 2 hrs under the
same conditions.
Eg: If a motor is rated continuous 10kw, it means that it is capable of giving an output of
10kw continuously for an indefinite period of time and 12.5 kw for a period of 2 hrs with
out its temperature exceeding he specified limits.
CONTINOUS MAXIMUM RATING
It is the load capacity as given above but with out overload capacity. Hence these motors
are a little bit inferior to the continuous rated motors.
INTERMITTENT RATING
It is based on the output which a motor can deliver for a specified period, say one hour as
½ hour or ¼ hr with out exceeding the temperature rise.
This rating indicates the maximum load of the motor for the specified time followed by a
no load period during which the machine cools down to its original temperature.
DIFFERENT TYPES OF INDUSTRIAL LOADS
The three different types of industrial loads under which electric motors are required to
work are as under.
1) Continuous load 2) Intermittent load and 3) variable or fluctuating load.
The size of the motor depends on two factors. Firstly on the temperature rise which is
turn, will depend on whether the motor is to operate on continuous intermitted or
variable load, seconds it will depend on the maximum torque to be developed by the
motor. Keeping in mind the load torque requirements, the rating of the motor will be
decided by the load contains as described below.
1) CONTINOUS LOAD
In such cases, the calculation of motor size is simpler because the loads like pumps
and fans require a constant power into keep them operating.
INTERMITTENT LOADS
Such loads can be of the following two types.
a) In this type of load, motor is loaded for a short time and then shut of for a
sufficient long time, allowing the motor to cool down to room temperature. In
such cases, a motor with a short time rating is used as in a kitchen mixie.
Load
Load
Temperature
Temperature
Time
b)in this type of load, motor is loaded for a short time and then it is shut off for a short
time. The shut off time is so short that the motor cannot cool dorm to the room
temperature. In such cases, a suitable continuous or short time rated motor is chosen
which, when operating on a given load cycle will not exceed the specified temperature
limit.
Variable loads : In the case of such load, the most accurate method of selecting a suitable
motor is to draw the heating and cooling curves as per the load fluctuation for a number
of motors.
1.NATURE OF ELECTRIC SUPPLY
Electric supply available may be A.C single phase, three phase or D.C. The motor chosen
should suit to the supply available. In case of 3 phase AC supply for small capacity and
slip induction motor for higher capacity can be used. But if speed variation is required
pole changing motor or motors with stepped pulley are to be used. If precise speed
control is required charge motor can be used. Even if DC supply is not available AC can
be rectified to DC and DC motors can be used.
2.TYPE OF DRIVE
The type of drive may be group drive or individual drive. The selection of motor is
dependent on the type of drive chosen
3. NATURE OF LOAD
Loads which are usually met with may be classified according to the speed torque
characteristics as follows.
i)
Constant load torque : Torque is constant at all speeds. Eg: cranes, hoists etc.
ii)
Load torque α speed : - The torque increases linearly with speed and occurs is
case of fluid friction where lubricants is used.
Constant torque
Load
TαN2
Torque
TαN
Tα1/N
Speed
Load – torque characteristics of different type of loads.
iii)Load torque αN2:- The torque increase as square of speed such as in fans, centrifugal
pumps etc.
iv)Load torque α 1/N : The torque varies inversely as the speed and it occurs in case of
grinding boring machines etc.
All electric motor have a dropping speed torque characteristics. But the drop in speed
with increase in torque varies for different type of motor for a given load, the motor
selected should have such a speed torque characteristics so that it interacts the load –
torque characteristics of the load at about 900.
ELECTRICIAL CHARACTERISTICS
i) Starting characteristics
Some motor have high starting torque. Some of them are having moderate starting torque.
Some are not self starting. Eg: Synchronous motor. The starting torque should be capable
of starting and accelerating the motor to rated speed in a reasonable time. When the
motor has to start against full load, this factor is more important. Starting torque is
required to overcome the initial static friction and to accelerate the motor can load to the
full speed. Static friction may be much more than the full load torque, especially when
the machine was idling for a long time. Torque for accelerating depends on load torque.
The time taken to attain the full speed affects the heating of the motor and the control
equipments. DC shut motor has moderate starting torque. DC series motor is having very
high starting torque. But series motor cannot be run with out load. Hence when high
starting torque and a definite no load speed is required, d.c compound motor can be used
AC squirrel cage motors are having moderate starting torque. If we require higher starting
torque slip ring motor can be used. Its starting torque can be increased by connecting a
resistance to its rotor at the time of starting.
Except is case of fractional H.P motors a state in used for starting of electric
motor. A stator is used i) to reduce the starting current. Ii) to obtain the required starting
torque. Different motor have different type of starter. In most cases the technique is to
impress a reduced voltage at the time for tarring and to apply the full voltage when motor
picks up speed.
ii)RUNNING CHARACTERISTICS
Running characteristics such as speed torque characteristics, speed current characteristics,
losses, efficiency and powerfactor are to be considered. Other features such as
temperature rise, insulation strength are taken case of by Indian standards specifications.
DC MOTORS
i)
shunt motor
For a DC motor Nα (V-IaRa)/φ. For a shunt motor φ is almost constant. Hence N α (V – Ia
Ra) since Ia Ra, the armature resistance drop is small compared to V, speed is almost
constant. But depending upon the load current Ia Ra increase and hence there is drop in
speed from no load to full load to full load even though not very appreciable. Shunt
motor can be used for loads which may be suddenly thrown off, with out resulting in
excessive speed increase. Hence shunt motor are used for line shafting, machine lathes,
milling machines.
Conveyors, fans etc. But shunt motor is not suitable for fluctuating loads or parallel
operation.
Speed
Torque in Nm. (Load)
DC SERIES MOTOR
In series motor flux varies with armature current (load) Hence T α φIa α Ia2. Hence
Torque increases as the square of current. Hence the starting torque will be very high. To
provide two times starting torque, the armature current need be increased to 1.414 times,
where as for shunt motor, it should be 2 times. N α V-IaRa Since Ia Ra is small compared
to V, Nα1/φ and is 1/Ia. Hence as the load increase flux increases and speed suddenly
drops. Hence the mechanical characteristics (Torque /speed) of a series motor is as
shown below.
From the characteristics, it may be seen that i) series motor is not having a definite no
load speed. Hence series motor cannot be started with no load. (2) when the motor is
loaded speed drops heavily. This saves the motor from over loading. These two qualities
makes dc series motor the most suitable for electric traction.
Speed
Torque
DC COMPOUND MOTORS
This type of motor has both series field and shunt field winding if series field aids shunt
field, the motor is called cumulative compounded motor. If the series field flux is
opposite to shunt field flux the motor is called differentially compounded motor.
Compound motor have a definite no load speed and can be started on no load. In case of
cumulatively compounded motor speed drops with load and saves the motor from over
loading. Hence these motors can be used for machines which are subjected to sudden
heavy loads, such as rolling mills, shears and punches.
In case of differentially compounded motor series field weakness shunt field and hence
full speed may be equal or more than no load speed.
In heavy overloads, the series field may become stronger and the motors may reverse
direction. This type of motors are rarely used.
A.C. MOTORS
Among A.C motor three phase induction motor is the most commonly used motor it is
simple in design, rugged in construction, reliable is service, low initial cost, easy
operation, low maintenance cost, high efficiency and requires simple control gear for
starting and speed control.
There are mainly two types of induction motors 1) cage induction motors 2) slip sing
induction motor. Both types are having a shunt characteristics, ie speed drops with load.
Squirrel cage motor is having moderate starting torque. But in case of slip ring motor, the
starting torque can be increased by inserting a resistance is its rotor circuit at the time of
starting and cut of circuit when normally running. This rotor resistance can also be used
for speed control. For same rating slip ring motor are larger in size and cost is load high.
Speed torque characteristics of induction motor is as shown here.
T
Stating torque
Maximum
Normal operating region.
Torque
Speed N
Ns
3 phase squirrel cage motors are used for application which require moderate starting
torque and almost constant speed and capacities up to 50kw. Above 50kw, slip ring
motors are insisted by supply company in order to restrict starting current. Slip spring
motors are preferred for high starting torque as in lifts etc. for lower capacities also.
SYNCHRONOUS MOTORS
They are constant speed motors. Speed can be varied by varying the supply frequency.
They are used for constant speed application such as in paper with etc. They are
expensive and require a dc source for excitation and hence maintenance cost also will be
more.
Other A.C motors used for industrial drives are , scharge motor (for wide speed control)
single phase and 3 phase series motors, universal motor, stator fed commutator motor
single phase induction motor repulsion motor etc.
SPEED CONTROL
The speed can be controlled electrically and mechanically. Electrical method is by
controlling the input to the drive motor. Mechanical methods are by means of stepped
pulleys or gears. Some drives may require a continuous speed variation from zero to full
speed, other esquires one or more fixed speed.
DC MOTORS : The speed control of dc motor are simple and easier and hence when
wide range of speed control is essential dc motors are still used.
A.C. MOTORS : speed of induction motor can be varied by changing the frequency, no.
of poles or by injection an voltage to its rotor circuit. In case of slip ring induction motor,
rotor resistance can be varied for speed control. The range of speed control required
determines the choice of the drive motor to a great extent.
MECHANICAL FEATURES
i)
Type of enclosure : Type of enclosure to be used depends on the type of work
and the place of installation. Different types of enclosures are open type,
protected type, drip proof, totally enclosed, pipe ventilated, frame proof etc.
When the motor is open type, cooling will be better, but there are more chances
of entering of foreign material. When totally enclosed cooling will be difficult.
ii)
Bearings : Upto 100 Hp, Ball or roller bearings are used. For larger motors
journal type bearings are used.
iii)
Noise : It is important to keep the noise level with in limits.
SIZE AND RATING OF MOTORS
Size means the kw capacity of the motor and rating means the loading nature specified
for that capacity. The size of motor selected should match with the load.
Depending upon the type of loading such as continuous loading or intermittent loading
size of motor will vary.
COST : Cost is a major consideration in the selection of a drive motor. When we
compromise with quality, initial cost may be less. But efficiency of such motor may be
low or power factor poor and repairs more. So the cost as energy and maintenance will be
more and running cost will be high. Hence the motor selection has to be done for the
lower total cost.
CHARACTERISTICS OF TRACTION MOTORS
A. D. C SERIES MOTORS
The torque eqn of a dc motor is T α φ Ia and in case of series motor flux φ α Ia and
hence T ∞ Ia2. Again N αV-Ia Ra.
Φ
Since Ia Ra is small compared with V, N α 1, and φ α Ia. Hence N α 1/ θa means
φ
N ∞ 1/Ia Hence as load current increase speed drops considerably.
Shunt motor
T
Shunt
N
Series motor
Series
T
Torque- speed chara. Of shunt and series motors.
Armature current
Torque – current chara. Of shunt
and series motors.
The series motor is ideally suited for traction as
i)
It is capable of exerting high starting torque
ii)
It possess high free running speed and
iii)
The speed decreases with increase in torque thereby protecting the motor from
over loads.
iv)
They are best suited for series and parallel operation.
A.C.SERIES MOTORS.
Single phase compensated series motor are used for traction work. They have low pf at
start and therefore the starting torque is low. A.C series motor is not well suited for
suburban work where stops are frequent. It is mainly employed for main line work.
Series field
Series field
Eb
Compensating wdg
Conductively compensated
Inductively compensated.
Eb.
V
I (Xa – Xse)
IXse
IRse
I
If an ordinary series motor is used for AC, due to the alternating flux, there will be heavy
iron loss. There will be heavy sparking at the brushes. To reduce iron loss the entire yoke
is laminated. In order to reduce the sparking a compensating wdg is used either in series
(conductively compensated) or short circuited (inductively compensated)
The effect of the compensative wdg is to reduce the effect of armature reactance. The
compensating edge provides an mmf opposite to that of armature wdg and their by
reduces armature reactance drop. Hence the value of Eb is increased and also power
factor is improved.
ELECTRIC BRAKING
If any electric motor is to be stopped, electric supply to it is disconnected. In this way, the
motor shall take a long time to stop as the motor shall continue to rotate due to inertia. If
the kinetic energy is more it shall take longer time to stop and if it is less, it shall take
smaller time to stop. Sometimes motor is to be stopped at once to avoid some accident or
in other cases motor is to be stopped quickly. So some braking system must be used so
that it stops with in the predetermined short time.
A good braking system must possess the following important features
i)
It should be fast and reliable
ii)
The braking force must be capable of being controlled
iii)
Kinetic energy of the rotating parts of the motor and its driven machine must
be suitably dissipated and suitable means must be provided.
Following are the two types of braking system normally employed.
i)
Mechanical braking : The stored energy of the rotating parts is dissipated in the
form of heat by a brake shoe of brake lining which rubs on a wheel or brake
drum.
ii)
Electric braking or electro dynamic braking : In this type of braking system,
the kinetic energy of the moving part of the system is converted into electrical
energy which in turn is dissipated as heat in a resistance or in certain cases may
be returned to the supply
Advantages of electric braking over mechanical braking
i)
Electric braking is quite fast
ii)
It is quite cheap as far as maintenance part is concerned due to the fact that no
replacement of brake shoes or liming is needed as in the case of mechanical
braking system
iii)
Since electric braking is quire fast, higher speeds can be maintained. This
results in higher capacity of the system
iv)
In some cases of electric braking such as when eh electric train, in moving
down gradient or the heavy weight is lowered by the crane, a part of electric
energy produced driving the braking period, can be fed back to the supply
system.
v)
Heat produced is in no way harmful but heat produced on the lining etc.
During the application of mechanical brakes way result in the failure of brakes
vi)
Electric braking is free from fires and is more smooth than mechanical braking.
DISAD VANTAGES
i)
Electric braking can stop the motor but it cannot hold it stationary
ii)
This cannot be applied to all types of electric motors
iii)
Its initial cost is very high.
THE FOLLOWING TYPES OF ELECTRIC BRAKING ARE EMPLOYED
i)
Plugging or reverse current braking
Reconnection of motor to supply is done in such a way that motor develops a torque
in opposite direction to the movement of the rotor. The system speed decelerates to
zero speed and than it will accelerate in opposite direction. It is necessary to
disconnect the supply of the system as soon as it comes to rest. This method is
applicable to dc, induction and synchronous motors.
In this method of electric braking system, the connection of the supply to the armature
are reversed to that the motor tends to revolve in the opposite direction and in doing
so it comes to rest. As soon as it comes to rest, the supply gets automatically cut off
and the motor does not start revolving in the opposite direction. At that time
mechanical brakes may also be applied. This method of braking can be applied in
both DC as well as AC motors.
a) Plugging as applied to DC shunt motors
The direction of the torque developed in DC shunt motor can be reversed either by
interchanging the field connection or the armature. In shunt motors usually armature
current is reversed. With reversed armature connections, the motor develops a torque
in opposite direction. When speed reduces to zero, motor will accelerate in the
opposite direction. Hence the arrangement is made to disconnect the motor from the
supply as soon as it comes to rest. The circuit shows the running and reversed
connections for shunt motors and drives motors.
Since with reversed connection, V and Eb are in the same direction, voltage across the
armature is almost double of its normal value. In order to this excessive current
through the armature, additional resistance R is connected in series with armature.
This method of braking is wasteful because in addition to wasting kinetic energy of
the moving parts, it draws additional energy from the supply during braking.
Plugging of Induction Motors
This method of braking is applied to an induction motor by transposing any of its two
line leads as shown.
Slipping motors are more suitable for plugging as external resistance can be added to
get the desired braking torque.
It reverse the direction of rotation of the synchronously rotating magnetic field which
produces a torque in the reverse direction, thus applying braking on the motor. Hence
at the first instant after plugging the rotor is running in a direction opposite to that of
the stator field.
RHEOSTATIC BRAKING
In this method of electric braking motor is disconnected from the supply through its
field continues to be energized in the same direction. The motor starts working as a
generator and all the kinetic energy of the equipment to be braked is converted into
electrical energy and is further dissipated in the variable external resistance R
connected across the motor during the braking period. This external resistance must
be less than the critical resistance otherwise there will not be enough current for
generator excitation.
DC and synchronous motors can be braked this way but induction motor require
separate dc source for field excitation.
In this method no power is drawn from the supply during braking.
Rheostat braking of dc motors.
The fig shows connections for dc shunt motor. For applying she static braking
armature is disconnected from the supply and connected to a available external
resistance R while the field remain on the supply. The motor starts working as a
generator whose induced and Eb depends upon its speed. At the start of braking when
speed in high, Eb is large hence Ia is large. As speed decrease, Eb decreases, hence Ia
decreases. Since Tb α φIa, it will be high at high speeds but low at low speeds. By
gradually cutting out R, Ia and hence Tb can be kept constant through out where Ia =
Eb(R+Ra)
Fig. Shows running and braking conditions for a dc series motor. In this case also for
rheostatic braking, the armature is disconnected from the supply and at the same time,
is connected across R. Connection are so made that current keeps flowing through the
series field in the same direction other wise no braking torque would be produced.
The motor starts working as a series generator provided R is less than the critical
resistance.
Rheostatic braking of Induction Motors.
If an induction motor is disconnected from the supply for rheostatic braking, there
would be no magnetic flux and hence, no generated emf and in the rotor and no
braking torque. After disconnection, direct current is passed through the rotor, steady
flux would be set up in the gap which will induce current, in the short circuited rotor.
This current which is promotional to the rotor speed, will produce the required
braking torque where value can be regulated by either controlling dc excitation or
varying the rotor resistance.
Regenerative braking
In this method of braking motor is not disconnected from the supply but is made to
run as a generator by utilizing the KE of the moving train. Electrical energy is fed
back to the supply. The magnetic drag produced on account of generator action offers
the braking torque. It is the most efficient method of braking. Take the case of a shunt
motor. It will run as a generator when ever its Eb becomes grater than V. Now Eb can
exceed V in two ways.
1. By increasing field excitation
2. By increasing motor speed beyond its normal value field current remaining same.
Regenerative braking can be easily applied to dc shunt motors though not down to
very low speeds because it is not possible to increase field current sufficiently. In
dc series motor reversal of current of the field and hence of Eb. Regenerative
braking cannot be used for stopping a motor. The main advantages are :
i)
Reduced energy consumption particularly on main – line railways having long
gradients and mountain railways.
ii)
Reduced wear of brake shoes and wheel types and
iii)
Lower maintenance cost for these items.