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Title: Drive motor characteristics
Description: This notes describes the characteristics of various motors on Dc and Ac for third sem mechanical engineering students.....
Description: This notes describes the characteristics of various motors on Dc and Ac for third sem mechanical engineering students.....
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UNIT-II -DRIVE MOTOR CHARACTERISTICS
Classification of Electrical Drives
Another main classification of electric drive is
DC drive-DC Motors
A)Separately excited DC motor
B)Shunt motor
C)Series motor
D)Compound motor
AC drive-AC Motors
A)Induction motor
B)Synchronous motor
MOTOR CHARACTERISTICS
Electrical characteristics
i) Torque vs
...
armature current and
Mechanical characteristics
(iii) Speed vs
...
Active load torques, Passive load torques
Load torques which has the potential to drive the motor under equilibrium conditions are
called active load torques
...
Components of Load Torques
(i) Friction Torque (TF)-Friction will be present at the motor shaft and also in various
parts of the load
...
(ii) Windage Torque (TW)-When motor runs, wind generates a torque opposing the
motion
...
(iii) Torque required to do useful mechanical work-Nature of this torque depends upon
particular application
...
It may be some
function of speed, it may be time invariant or time variant, its nature may also change
with the load’s mode of operation
...
Similarly
cranes during the hoisting and conveyors handling constant weight of material per unit
time also exhibit this type of characteristics
...
Eg Fans rotary pumps, compressors and ship propellers
...
In motoring, it converts electrical
energy into mechanical energy, which supports its motion
...
Motor can
provide motoring and braking operations for both forward and reverse directions
...
Power developed by a
motor is given by the product of speed and torque
...
DC MOTORS
D
...
machines are the electro mechanical energy converters which work from
a d
...
source and generate mechanical power or convert mechanical power into
a d
...
power
...
Working principle of a DC motor
...
The principle of working of a DC motor is that "whenever a current carrying conductor is
placed in a magnetic field, it experiences a mechanical force"
...
Now to go into the details of the operating principle of
DC motor its important that we have a clear understanding of Fleming’s left hand ruleto
determine the direction of force acting on the armature conductors of DC motor
...
M
...
Provide supply to the Ohmic IaRa drop
...
(1)
Where
Eb = Back E
...
F
IaRa = Armature Current X Armature Resistance
This is called “Voltage Equation of the DC Motor”
...
M
...
c voltage and is called back e
...
f
Where Eg is the Generated Emf
TYPES OF DC MOTORS
Separately excited dc motor
Self excited DC motor
DC motor are of 3 types they are
1
...
DC SERIES MOTOR
3
...
The armature and field winding voltage loop equations are
V=Eb+IaRa
VF=IFRF
SEPARATELY EXCITED DC MOTOR
self excited DC motor
In case of self excited DC motor, the field winding is connected either in series or in
parallel or partly in series, partly in parallel to the armature winding, and on
...
The resistance of shunt field winding (Rsh) is always higher than that is armature
winding
...
The cross-sectional area of the wire used for field winding is smaller than that of the wire
used for armature winding
...
The field winding should have less
no
...
Rse is very small
DC COMPOUND MOTOR
The DC compound motor is a combination of the series motor and the shunt motor
...
The combination of series and shunt winding allows the motor to have the torque
characteristics of the series motor and the regulated speed characteristics of the shunt
motor
...
In cumulative compound motor the flux produced by both the windings is in the same
direction
...
DC COMPUND MOTOR-LONG SHUNT,SHORT SHUNT
The compound wound DC motor can further be subdivided into 2 major types on the
basis of its field winding connection with respect to the armature winding and they are
Long Shunt Compound Wound DC Motor Short Shunt Compound Wound DC Motor
Long Shunt Compound Wound DC Motor -In long shunt compound wound DC motor: •
The shunt field winding is connected in parallel across the series combination of both the
armature and series field coil • Current Equation I 𝑡𝑜𝑡𝑎𝑙 = 𝐼𝑠ℎ + 𝐼𝑠𝑒 •
Voltage Equation E= 𝐸 𝑏 + 𝐼 (𝑅 𝑎 + 𝑅 𝑠𝑒)
...
series field coil is exposed to the entire supply
current, before being split up into armature and shunt field current •
Current equation 𝐼𝑡𝑜𝑡𝑎𝑙 = 𝐼 𝑎 + 𝐼𝑠ℎ • Voltage equation: E=𝐸 𝑏 + 𝐼 𝑎 𝑅 𝑎 + 𝐼𝑠𝑒 𝑅 𝑠𝑒 ∴
𝐼𝑠𝑒 = 𝐼𝑡𝑜𝑡𝑎𝑙 E=𝐸 𝑏 + 𝐼 𝑎 𝑅 𝑎 + 𝐼𝑡𝑜𝑡𝑎𝑙 𝑅 𝑠𝑒
TORQUE EQUATION
Torque is the twisting force that tends to cause rotation
...
Mathematically torque can be written asT= F * r
and it has units of Newton-meters
...
When armature conductors of a DC motor carry current in the presence of stator field
flux, a mechanical torque is developed between the armature and the stator
...
DERIVATION
Torque T = F × r (N-m) …where, F = force and r = radius of the armature
Work done by this force in once revolution = Force × distance = F × 2πr
circumference of the armature)
(where, 2πr =
Net power developed in the armature = work done / time
= (force × circumference × no
...
Putting these in the above equation 2
...
Ia
We know, Eb = PΦNZ / 60A
Therefore, Ta × 2πN/60 = (PΦNZ / 60A) × Ia
Rearranging the above equation,
Ta = (PZ / 2πA) × Φ
...
159 Φ
...
Thus, armature torque is
directly proportional to the product of the flux and the armature current i
...
Ta ∝ Φ
...
Some torque is lost, and therefore, shaft torque is always
less than the armature torque
...
(where, N is speed in RPM)
SPEED AND TORQUE EQUATION
Eb=V-IaRa=
V-IaRa= PΦNZ / 60A
For a DC motor the speed equation is TαφIa
φ constant(DC Shunt motor)
For a DC shunt motor torque is directly proportional to the armature current
...
TαφIa
TαIa2 (DC series motor)
CHARACTERISTICS OF DC MOTOR
Three characteristic curves are considered important for DC motors which are
(i) Torque vs
...
armature current
(iii) Speed vs
...
These are explained below for each type of DC motor
...
armature current (Ta-Ia)
In case of DC shunt motors, we can assume the field flux ɸ to be constant
...
Hence, the Ta-Ia characteristic for a dc shunt motor will be a straight line through the
origin
...
armature current
The shunt motor torque is proportional to flux and armature current
...
Ia
Armature current increases,torque also increases
...
torque
...
When the load torque increases the speed lightly decreases
SERIES MOTOR CHARACTERISTICS
(i) Torque vs
...
armature current
(iii) Speed vs
...
(i) Torque vs
...
torque is directly proportional to the product of armature current and field flux, Ta ∝ ɸ
...
In DC series motors, field winding is connected in series with the armature, i
...
Ia= If
...
Hence, before magnetic saturation Ta α Ia2
...
After magnetic saturation of the field poles, flux ɸ is independent of armature current Ia
...
Therefore, after magnetic
saturation,
ii)Speed vs
...
Hence, for small currents speed is inversely proportional to ɸ
...
Therefore, when
armature current is very small the speed becomes dangerously high
...
iii) Speed vs
...
This characteristic is also called as mechanical characteristic
...
COMPOUND MOTOR CHARACTERISTICS
DC compound motors have both series as well as shunt winding
...
And if the series flux is opposite to the direction of the shunt flux, then the motor is said
to be differentially compounded
...
APPLICATIONS
The various applications of DC shunt motor are in
Lathe Machines, Centrifugal Pumps, Fans, Blowers, Conveyors, Lifts, Weaving Machine,
Spinning machines, etc
...
THREE PHASE INDUCTION MOTOR
Stator - Stator Winding - Rotor - Rotor Winding Cooling Fan Bearings
Frame -Frame provides mechanical support to the stator and rotor
...
Stator: It is stationary part of induction motor
...
Rotor:- It is the rotating part of the induction motor
...
It has two ends, one is called Driving end and another is called non- Driving end
...
Stator Windings: - Star connected - Delta connected
Rotor Windings: It is wound as rotor bars and short circuited at both the ends through
end rings
...
Squirrel cage induction motor 2
...
The stationary rotor cut the revolving field and due to electromagnetic induction an
e
...
f
...
As the rotor conductor is short circuited current flows through them
...
The difference between the flux (Ns) and the rotor speed (N) is called slip
...
Its construction is simple and rugged
...
where
cost is most important factor
...
m
...
/phase
E2 = rotor e
...
f running condition
E2 αNs
E2 αNs-N
E2r αNs-N = S
E2
Ns
E2r=SE2
X2 = standstill rotor reactance/phase, f2 = rotor current frequency at standstill
When rotor is stationary i
...
s = 1, the frequency of rotor e
...
f
...
TORQUE SLIP CHARACTERISTICS
The Torque Slip Characteristic is represented by a rectangular hyperbola
...
Thus, it passes through the point of maximum torque when R2 = sX20
...
This torque is a measure of the short time overloading
capability of the motor
...
They are
given below
...
The torque equation of the induction motor is given below
...
The slip is directly proportional to the square of the torque
...
In unstable(BC) operating region the slip is further increased from Sm so the region is
unstable
...
Torque is inversely proportional to slip,slip increases and torque decreases
...
The region (AD) is called slip region The motor is operated continuously operated in this
region
...
(iii) Full load torque
(i)Starting torque:
The slip and speed is zero
At this condition the motor produces a torque is called Starting torque
...
(iii)Full load torque:
The point D corresponds to full load torque of the motor
...
Normally
full load torque is less than the maximum torque
...
The magnitude of the peak negative torque is larger than the positive torque, the
difference being a function of stator resistance
Operating Regions:
1
...
Generating region
3
...
MOTORING REGION(0
Speed decreases torque increases till breakdown is reached
...
GENERATING REGION(s<0)
The machine operates as a generator
The rotor rotates at a speed greater than synchronous speed in the same direction as that
of rotating magnetic field
...
3
...
This region occurs only when the stator field is reversed by changing the phase sequence
of the input supply of motor
...
So the motor quickly comes to stop
...
The rotor is the rotating part, the stator winding helps in rotating the rotor
...
Stator is also provided with a 'starting winding' which is used only for starting purpose
...
The alternating current flowing through stator winding causes induced current in the rotor
bars (of the squirrel cage rotor) according to Faraday's law of electromagnetic induction
This induced current in the rotor will also produce alternating flux
...
However, if the rotor is given a initial start by external force in either direction, then
motor accelerates to its final speed and keeps running with its rated speed
...
Double-Field Revolving Theory
The double-field revolving theory states that, any alternating quantity (here, alternating
flux) can be resolved into two components having magnitude half of the maximum
magnitude of the alternating quantity, and both these components rotating in opposite
direction
...
They are equal in magnitude but opposite in directions
...
The net torque in the motor is equal to the sum of the torque due to each of the two magnetic
fields
...
When
the stator is fed with a single phase supply, it produces alternating flux(which alternates
along one space axis only)
...
That is why a single phase induction motor is
not self starting
...
Single phase(Universal)
SPEED TORQUE CHARACTERISTICS
The torque of the induction motor is zero when the motor is driven slower than
synchronous speed, and it becomes braking torque at synchronous speed
...
The rotation region that provides maximum or high torque varies depending on the
electric resistance
...
STARTING OF AN INDUCTION MOTOR
The Single Phase Motor is not self starting and hence needs an auxiliary means or
equipment to start the single phase induction motor
The various Starting methods of a Single Phase Induction motor are as follows: Split Phase Motor
Capacitor Start Motor
Capacitor Start Capacitor Run Motor or Two value capacitor motor
Permanent Split Capacitor (PSC) or Single value capacitor motor
Shaded Pole Motor
...
It has a single cage rotor, and its stator has two windings known as main winding and
starting winding
...
The main winding has very low resistance and a high inductive reactance whereas the
starting winding has high resistance and low inductive reactance
...
It is mainly used in fans,blowers,centrifugal pumps,washing machines
...
This type of motor is not used for drives which require
more than 1 KW because of the low starting torque
...
The motors are used in mixer grinder, floor polishers
...
Capacitor start single Induction motor
The above fig shows the capacitor start single phase induction motor
...
I t is also used to get higher starting torque
...
The starting current Is leads the line voltage, because the capacitor present in the auxiliary
winding
...
Used in pumps and compressors
Used in the refrigerator and air conditioner compressors
...
CAPACITOR RUN MOTOR
In this motor, a capacitor is permanently connected is series with auxiliary winding
...
The capacitor value is between the range of 20-50µF
...
APPLICATIONS
Fans
Blowers
Centrifugal pumps
Capacitor start and capacitor run induction motor
The above fig shows the Capacitor start and capacitor run induction motor
...
In this motor, we get high starting torque,because two capacitor
...
The running capacitor is permanently connected in series with auxiliary winding
...
SHADED POLE INDUCTION MOTOR
The stator of the shaded pole single phase induction motor has salient or projected
poles
...
The poles are divided into two unequal halves
...
A single phase winding is wound on the salient poles
...
Due to this starting torque also developed
...
Construction is simple and robust because there is no centrifugal switch
...
The starting torque is very poor
...
The speed reversal is also difficult and expensive as it requires another set of copper rings
...
Breaking methods can be classsified into
Mechanical braking-The frictional force between the rotating part and brake drums
provide the brake
...
So it produce a negative slip
and negative torque whether machanical or electric braking,the braking of the drive
should be such that as to stop the motor at the specified point of time and location
TYPES OF BRAKING
Brakes are used to reduce or cease the speed of motors
...
Plugging type braking-The direction of rotation of an electric motor is dependent on
several variables
...
Dynamic braking-When an electric motor rotates a kinetic energy is stored in its rotating
mass
...
In this type of braking, the DC motor is
disconnected from the supply and a braking resistor Rb is immediately connected across
the armature
...
The machine
acts as generator
...
A limiting resistor R1 is used in the circuit for controlling dc excitation
The dc dynamic braking of induction motor is quite convenient in practice and commonly
used with active loads
...
When the three phase stator windings are connected in the RYB sequence the airgapnm
field rotates in one direction at synchronous speed
...
Uniform current loading
DISADV:
Energy loss is high
...
* (generator) braking – mech energy to electrical energy, oppose the motion
...
* Power developed by a motor is given by the product of speed & torque
...
So called
“forward motoring”
* Quadrant II – Power –ve, m/c works under braking opposing the motion
...
Direction of motor & load torques and direction of speed are marked by arrows
...
One end of a rope is tied
to a cage which is used for transporting material
...
* Weight of the counter weight chosen higher than the weight of an empty case but lower than a
fully loaded cage
...
This
torque is the diff
...
* Load torque TL2 in quadrants II & III is the speed-torque charal of an empty hoist
...
This is –ve because the counter
weight is always higher than the empty cage
...
It will be obtained if
motor produce +ve torque in CCW direction equal to TL
...
* Quadrant IV operation is obtained when a loaded cage is lowered
...
In order to limit the speed of the cage within a safe value,
motor must produce a +ve torque T = TL2 in anti clockwise direction
...
* Quadrant II is obtained when an empty cage is moved up since a counter weight is heavier than
a empty cage, it is able to pull it up
...
Since speed is +ve, developed power is, ve
...
Quadrant III – empty cage is lowered since empty cage weight is < counter weight motor
produce a torque in clockwise direction
...
Title: Drive motor characteristics
Description: This notes describes the characteristics of various motors on Dc and Ac for third sem mechanical engineering students.....
Description: This notes describes the characteristics of various motors on Dc and Ac for third sem mechanical engineering students.....