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Unit -1 Drive Characteristics SAB Page 1
UNIT-1 Drive Characteristics
DEFINITION:
• Systems employed for motion control are called as DRIVES • Drives may employ any of the prime movers such as diesel or petrol engine , gas or
steam turbines, steam engines , hydraulic motors and electric motors for motion control.
• Drives employing electric motors are known as ELECTRIC DRIVES
Parts of Electric drives:
PARTS OF ELECTRICAL DRIVES:
1. Sources 2. Power Modulators 3. Electrical Motors 4. Control Unit 5. Sensing Unit
1. Sources :
There are two types of sources a)AC Source b)DC Source
a)AC Source
• In India 1 – Phase and 3-Phase 50 Hz ac supply is available • For Low power drives 1-Phase Source is used • For High Power Drives 3-Phase source is Used. • Most Drives are Powered from ac source either directly or through converter link • For Traction drives even at high power levels 1-Phase Source is used. • For 50 Hz ac supply maximum speed of induction and synchronous motor will be 3000
rpm
Unit -1 Drive Characteristics SAB Page 2
• For higher speeds, supply should be converted to higher frequency • Low and Medium power motors are generally fed from 400 V supply. • For High power motors,3.3kV ,6.6kV,11kV and higher is used. • In India 25 kV 50 Hz supply is used for traction.
b) DC Source
• In underground traction 500 to 750V Dc Supply is used.
• Some drives are powered from battery.
• Depending on the size battery voltage may change as 6V,12V,24V,48V and 110V dc.
2. Power Modulators It is classified into three types (a) converters (b) Variable Impedances
(c) Switching Circuits a) Converters
i)ac to dc converters ii) Inverters (dc to ac)
iii)AC Voltage controllers (ac to ac) iv) DC choppers (dc to dc) v) Cyclo converters
b) Variable Impedances • Variable resistors are commonly used for low cost ac and dc drives for dynamic braking • In high power applications liquid rheostats (slip regulators) are used • Inductors are used for limiting the starting current
3)Motors:
Commonly used motors are dc-shunt,series, compound and permanent magnet Induction Motors: Squirrel cage, wound rotor and linear Synchronous motors: wound field and permanent magnet Brushless DC motors , Stepper motors and Switched reluctance motors can be used.
1.2 Types of Electrical Drives
Depending upon the mode of connection between Electric drive motor and
mechanical load, the electrical drive motor is classified as;
1. Group drive/Line shaft drive.
2. Individual drive.
3. Multi-motor drive.
Unit -1 Drive Characteristics SAB Page 3
1. Group drive / Line shaft drive:
The group drive consists of a single motor which drives several loads It is also called as ‘Line shaft Drive’. The various loads are connected to the same shaft (line shaft) with different speed. This is possible with multi-stepped pulley and gear. The size of the line shaft pulley and load shaft pulley determines the speed of the loads.
Advantages: • Installation cost and cost of one large motor will be much less than a number of small motor • The efficiency and power factor of a large group motor will be higher • In group motor drive operations can be stopped simultaneously Disadvantages:
• The breakdown of large motor causes all operations to be stopped • If the most of the machines are idle then the main motor will operate on load with less
efficiency • Noise level at the work site is quite high • Speed control of individual machines are not Possible
2. Individual Drive:
Unit -1 Drive Characteristics SAB Page 4
The individual drive system consist of only one electric motor for one machine.
This single electric motor is connected to all individual mechanical load through
different energy transmission devices.
This single motor activate only a single mechanical load at a time.
Advantages:
The machines can be installed at any desired position
If there is fault in one motor other machines will not affected
Speed of each machines can be controlled
The absence of belts and line shafts reduces the risk of accidents to operator
Disadvantages:
Initial high cost
For a single machine such as lathe , one single motor is used to control various
mechanism by means of mechanical parts like gears and mechanism so power loss will
take place
3. Multi-motor Drive
In multi motor drives, separate motors are used for operating different parts of same
mechanism
Each motor is used to drive only one of the many working mechanisms in a machine
Examples are metal cutting machine tools,paper making machines and rolling mills , etc.,
Unit -1 Drive Characteristics SAB Page 5
1.3 Equations Governing Motor Load dynamics
1.3.1 Fundamental Torque Equations :
J= Moment of Inertia of Motor Load kg/m2
ωm = Angular Velocity of Motor shaft Rad /sec
T = Motor Torque N-m
TL = Load Torque N-m
Fundamental Torque is given by
by Differentiating (1) We get
For Constant Inertia Drives dJ/dt=0, So above eqn becomes
The above equation is the torque developed by motor.
Here( J dωm / dt) is dynamic Torque
Unit -1 Drive Characteristics SAB Page 6
1.3.2 Load with Rotational Motion
J0= Moment of Inertia of Motor Load and load directly coupled to shaft kg/m2 ωm = Angular Velocity of Motor (Speed) TL0 = Load Torque (Load connected to motor shaft) J1= Moment of Inertia load connected to gear kg/m2 ωm1 = Angular Velocity of Load coupled to the gear (Speed) TL1 = Load Torque (Load connected to gear)
Gear teeth ratio is given by
Where n and n1 is gear speed
The Kinetic energy due to equivalent inertia is equal to kinetic energy of various moving part
Dividing the above eqn by ωm2
Unit -1 Drive Characteristics SAB Page 7
Where η = Transmission efficiency of the gear
Power at the loads and motors must be equal
By dividing above eqn by ωm
Sub/: (1) in (4)
Sub/: (1) in (2)
Unit -1 Drive Characteristics SAB Page 8
If n loads are connected directly to motor with moment of inertias J1 , J2 ,…….Jn and having
gear teeth ratio a1,a2,……an ,
Then (3) becomes
If n loads are connected to gear with torques TL1 ,TL2….TLn and having gear teeth
ratio a1,a2,……an ,and transmission efficiency η1, η1,….. ηn
Then (5) becomes
1.3.3 Load with Translational Motion
J0= Moment of Inertia of Motor Load and load directly coupled to shaft kg/m2 ωm = Angular Velocity of Motor (Speed) TL0 = Load Torque (Load connected to motor shaft) M1 = Mass of the load with translational motion F1 = Force of the load with translational motion V1 = Speed of the load with translational motion
Unit -1 Drive Characteristics SAB Page 9
The Kinetic energy due to equivalent inertia is equal to kinetic energy of various moving part
Dividing the above eqn by ωm2
Where η = Transmission efficiency of the gear
Power at the loads and motors must be equal
Dividing the above eqn by ωm
Unit -1 Drive Characteristics SAB Page 10
If n loads are connected directly to motor with Mass M1 , M2 ,…….Mn and having Velocities
v1,v2,……Vn ,Then (1) becomes
If n loads are with translational motion with Force F1 ,F2….Fn and having Velocities
v1,v2,……Vn ,and transmission efficiency η1, η1,….. ηn Then (2) becomes
1.4 Steady State Stability:
When motor torque equals load torque then drive is said to be steady state stability of
equilibrium. Steady state stability is examined using following Examples
The point of operation is restored,even if the speed is increased or decreased.Hence the
point A is stable point of equillibrium.
Examine the stability of point A
Decrease in speed:
Let point A be the equilibrium point
Consider that speed is decreased due to a small disturbance
Now motor torque will be greater than load torque
Due to this motor accelerate and operation will be restored to point A
Increase in speed:
Let point A be the equilibrium point
Consider that speed is increased due to a small disturbance
Now load torque will be greater than motor torque
Due to this motor decelerate and operation will be restored to point A
Unit -1 Drive Characteristics SAB Page 11
aq
The point of operation is not restored, even if the speed is increased or decreased. Hence the point
B is unstable point of equilibrium.
The condition for stability is given by
Mathematical condition for stability
Fundamental torque equation is given by
Due to a small disturbance in speed Δωm , results ΔT and ΔTL disturbance in T and TL
Examine the stability of point B
Decrease in speed:
Let point B be the equilibrium point
Consider that speed is decreased due to a small disturbance
Now load torque will be greater than motor torque
Due to this motor decelerate and operation will be move away from point B
Increase in speed:
Let point B be the equilibrium point
Consider that speed is increased due to a small disturbance
Now motor torque will be greater than load torque
Due to this motor accelerate and operation will be move away from point B
(1)
(2)
Unit -1 Drive Characteristics SAB Page 12
Subtract (2)-(1)
Here dT/dωm & dTL/dωm can be found by using slope in the below diagram
(3)
4
5
Unit -1 Drive Characteristics SAB Page 13
Sub/: (4) & (5) in (3)
The above equation is first order linear differential equation and the solution is given by
1.5 Classification of Load Torque :
a) Active Load Torque
Load torque which supports the motion under equilibrium conditions are
called Active load torque
Torque due to force of gravity and torque due to tension ,compression and
torsion undergone by an elastic body come under this category.
b) Passive Load Torque
Load torque which opposes the motion are called passive load torque
Torque due to friction ,cutting come under this category.
1.6 Components of Load Torque
a) Friction Torque TF
The Friction will be present at the motor shaft and in load.
The friction torque TF is equal value of various torques referred to the motor shaft b) Windage Torque TW
When motor runs , the wind generates a torque opposing the motion.This torque is known as windage torque
c) Mechanical torque TM
The nature of this torque depends on type of load. It may be constant and independent of speed , it may be some function of speed, it may be time
invariant or time variant and its nature may also vary with the change in the load
Unit -1 Drive Characteristics SAB Page 14
1.7 Load torque characteristics of Various Drives/Load
i) Constant torque type load ii) Torque proportional to speed (generator type load) iii) Torque proportional to square of the speed (fan type load) iv) Torque inversely proportional to the speed (constant power type load)
i) Constant torque type load
ii) Torque proportional to speed (generator type load)
The speed torque characteristics for this
type of load is given by
T=K
Machines used for shaping , cutting ,
grinding or shearing requires constant
torque irrespective of speed.
Similarly cranes during the hoisting and
conveyors handling constant weight also
exhibits same characteristics
The speed torque characteristics for this
type of load is given by
T α ω
T= K ω
Separately excited dc generators
connected to a constant resistive
load,eddy current brakes have this type
of characteristics
Unit -1 Drive Characteristics SAB Page 15
iii) Torque proportional to square of the speed (fan type load)
iv) Torque inversely proportional to the speed (constant power type load)
The speed torque characteristics for this
type of load is given by
T α ω2
T= K ω2
Examples for this load are fans ,
Rotary pumps , compressors and ship
propellers.
The speed torque characteristics for this
type of load is given by
T α 1/ω
T= K / ω
Examples for this load are lathes ,
boring machines , milling machines ,
steel mill coiler and electric traction
load
Unit -1 Drive Characteristics SAB Page 16
1.8 Multi quadrant Dynamics :
A motor operate in two modes – motoring and braking
a)Motoring: During motoring it converts electrical energy into mechanical energy, which supports motion b)Braking: During braking it converts mechanical energy into electrical energy, which opposes motion and it work as generator. A motor can provide motoring and braking both in forward and reverse direction. In I quadrant forward motoring takes place. In II quadrant forward braking takes place . In III quadrant reverse motoring takes place . In IV quadrant reverse braking takes place
First quadrant:
Forward motoring
Power : Positive
Speed : positive
T: positive (Anticlockwise)
TL : negative (clockwise)
Second quadrant:
Forward Braking
Power : Negative Speed : positive T: negative (clockwise) TL : positive (Anticlockwise) Third quadrant:
Reverse Motoring
Power : Positive Speed : Negative T: negative (clockwise) TL : positive (Anticlockwise) Fourth quadrant:
Forward motoring
Power : Negative
Speed : Negative
T: positive (Anticlockwise)
TL : negative (clockwise
Unit -1 Drive Characteristics SAB Page 17
First Quadrant :
In this mode Forward motoring operation takes place.
Here the loaded cage should be moved up , So the speed will be positive.
As motoring operation takes place Power will be positive.
Here motor torque is positive (Anticlockwise ) and Load torque is Negative (Clockwise).
Second Quadrant :
In this mode Forward braking operation takes place.
Here the Empty cage should be moved up , So the speed will be positive.
As braking operation takes place Power will be negative.
Here motor torque is negative (clockwise ) and Load torque is positive (Anticlockwise).
Third Quadrant :
In this mode Reverse braking operation takes place.
Here the Empty cage should be moved down , So the speed will be negative.
As braking operation takes place Power will be negative.
Here motor torque is negative (clockwise ) and Load torque is positive (Anticlockwise).
Fourth Quadrant:
In this mode Reverse Motoring operation takes place.
Here the Loaded cage should be moved down , So the speed will be negative.
As motoring operation takes place Power will be positive.
Here motor torque is negative (clockwise ) and Load torque is positive (Anticlockwise).
1.9 Modes of Operation :
• An electrical drives operates in three modes of operation
1. Steady state (TL = TM )
2. Acceleration including starting (TM > TL )
3. Deceleration including stopping (TL > TM )
Unit -1 Drive Characteristics SAB Page 18
1. Steady State Operation :
• The steady state operation can be realized from the speed torque characteristics such
that the motor and load torque equals
• Curve 1 shows the speed torque of the drive for given speed ωm1
• Now motor speed is changed to ωm2 , the speed torque characteristics adjusted to
curve 2, so that the motor and load torque equals
2. Acceleration including starting (TM > TL )
Electric drive operates in acceleration mode whenever an increase in speed is required.
For Acceleration the motor torque should be greater than load torque
Increase in motor torque also increases the motor current.
Care must be taken to restrict the motor current within a value which is safe for both motor and power converter.
In applications where acceleration is required for long duration, current must not be allowed to exceed the rated current value of the motor
In applications where acceleration is required for short duration, current can be allowed to exceed the rated current value of the motor
Here the operating point is A at speed ωm1 is to be moved to operating point B at speed at ωm2.
The operating point is shifted from point A to B through the path A D1 E1 B
Starting is a special case of acceleration where motor speed change from zero to desired speed
IN some applications the electric motor should accelerate smoothly , without any jerk.
This can be obtained when the starting torque can be increased steplessly from zero value. Such type of start is known as soft start
Unit -1 Drive Characteristics SAB Page 19
3. Deceleration including stopping (TL > TM )
• Electric drive operates in deceleration mode whenever an decrease in speed is required. • For Deceleration the load torque should be greater than motor torque • Deceleration can be attained by applying mechanical brake or Electrical brake. • Care must be taken to restrict the motor current within a value which is safe for both
motor and power converter during electrical braking • In applications where electrical braking is required for long duration, current must not
be allowed to exceed the rated current value of the motor • In applications where electrical braking is required for short duration, current can be
allowed to exceed the rated current value of the motor • Here the operating point is A at speed ωm1 is to be moved to operating point C at speed
at ωm3. • When the electrical braking is used, operating point is shifted from point A to C through
the path A D2 E2 C • When the Mechanical braking is used, operating point is shifted from point A to C
through the path A D3 E3 C • For smooth and quick stops the electrical braking is applied
• In electric train , for smooth stopping , Electrical braking is applied.
Unit -1 Drive Characteristics SAB Page 20
Permanent Magnet Synchronous Motor:
Construction:
Stator: • It is a stationary member . • It has the armature winding. • It is made up of continuous strips of steel , which is laminated • The thickness of lamination depends upon the frequency of source voltage. • The thickness of lamination also depends on the cost • The yoke of the machine completes the magnetic path • Armature winding are generally double layer and lap wound. • Individual coils are connected to form the phasor groups. • The phasor groups are connected together in series/parallel to form star or
delta connection. • The coils , phasor groups and phase should be separated from each other by
insulating them Rotor:
• Rotor is made up of Permanent magnet • Usually ferrite magnets are used. • Sometimes Rare earth magnets (CobaltSamarium) is used ,which is very
expensive and it is used to reduce the weight of motor Types of Rotor:
1. Surface mounted a. Projected type b. Inset Type
2. Interior type
• It has Stator and Rotor • The armature winding in the stator
and Permanent magnet of the rotor is so designed that flux density distribution is sinusoidal
• Due to the presence of permanent magnet, Slip rings and field rings are absent
Unit -1 Drive Characteristics SAB Page 21
Surface Mounted Type: a. Projected Type:
b.Inset Type
2. Interior Type :
In this type the magnets are placed on the surface of the outer periphery of rotor laminations
This arrangement gives highest air gap flux density
This type rotors are not preferred for high speed applications.
In this type the magnets are placed on the grooves of the outer periphery of rotor laminations.
This arrangement provides a uniform cylindrical surface
This arrangement has much more robustness than projected type
In this type of arrangement the permanent magnet are placed inside the periphery.
This arrangement gives more robustness.
It is suited for high speed applications
The manufacturing of this arrangement is more complex than surface mounted type
Unit -1 Drive Characteristics SAB Page 22
Principle of operation :
The PMSM has stator which is a classic three phase stator and has permanent magnets
in rotor.
The motor is driven using 3 ϕ inverter which is fed from a rectifier.
When stator is supplied with 3 ϕ supply , the armature draws a current .
The armature current depends on the rotor position and the turning on process of the
devices in the control circuit .
The output voltage is to be applied to 3 phase winding in such a way that angle between
the stator flux and rotor flux is kept close to 900
For proper operation the motor requires electronic control.
The rotor position sensor is required for accurate tracking of the speed in order to
prevent the motor from pulling out of step and to avoid instability
The armature supply frequency is changed in proportion to rotor speed,
It has features of like excellent dynamic performance and low torque ripple .
The PMSM drive is widely used in high performance servo drives in spite of its high cost.