UNIT-1 Drive Characteristics

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 1

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 2

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 3

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 4

1.3 Equations Governing Motor Load dynamics 1.3.1 Fundamental Torque Equations : J= Moment of Inertia of Motor Load kg/m 2 ω m = Angular Velocity of Motor shaft Rad /sec T = Motor Torque N-m T L = 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 dω m / dt) is dynamic Torque Unit -1 Drive Characteristics SAB Page 5

1.3.2 Load with Rotational Motion J 0 = Moment of Inertia of Motor Load and load directly coupled to shaft kg/m 2 ω m = Angular Velocity of Motor (Speed) T L0 = Load Torque (Load connected to motor shaft) J 1 = Moment of Inertia load connected to gear kg/m 2 ω m1 = Angular Velocity of Load coupled to the gear (Speed) T L1 = 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 ωm 2 Unit -1 Drive Characteristics SAB Page 6

Sub/: (1) in (2) Where η = Transmission efficiency of the gear Power at the loads and motors must be equal By dividing above eqn by ω m Sub/: (1) in (4) Unit -1 Drive Characteristics SAB Page 7

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,.. η n Then (5) becomes 1.3.3 Load with Translational Motion J 0 = Moment of Inertia of Motor Load and load directly coupled to shaft kg/m 2 ω m = Angular Velocity of Motor (Speed) T L0 = Load Torque (Load connected to motor shaft) M 1 = Mass of the load with translational motion F 1 = Force of the load with translational motion V 1 = Speed of the load with translational motion Unit -1 Drive Characteristics SAB Page 8

The Kinetic energy due to equivalent inertia is equal to kinetic energy of various moving part Dividing the above eqn by ω m 2 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 9

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,.. η n 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 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 The point of operation is restored,even if the speed is increased or decreased.hence the point A is stable point of equillibrium. Unit -1 Drive Characteristics SAB Page 10

Examine the stability of point B aq 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 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 (1) Due to a small disturbance in speed Δω m, results ΔT and ΔT L disturbance in T and T L (2) Unit -1 Drive Characteristics SAB Page 11

Subtract (2)-(1) (3) Here dt/dω m & dtl/dω m can be found by using slope in the below diagram 4 5 Unit -1 Drive Characteristics SAB Page 12

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 T F The Friction will be present at the motor shaft and in load. The friction torque T F is equal value of various torques referred to the motor shaft b) Windage Torque T W When motor runs, the wind generates a torque opposing the motion.this torque is known as windage torque c) Mechanical torque T M 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 13

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 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 ii) Torque proportional to speed (generator type load) 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 14

iii) Torque proportional to square of the speed (fan type load) The speed torque characteristics for this type of load is given by T α ω 2 T= K ω 2 Examples for this load are fans, Rotary pumps, compressors and ship propellers. 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 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 15

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) T L : negative (clockwise) Second quadrant: Forward Braking Power : Negative Speed : positive T: negative (clockwise) T L : positive (Anticlockwise) Third quadrant: Reverse Motoring Power : Positive Speed : Negative T: negative (clockwise) T L : positive (Anticlockwise) Fourth quadrant: Forward motoring Power : Negative Speed : Negative T: positive (Anticlockwise) Unit -1 Drive Characteristics SAB T L : negative (clockwise Page 16

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 (T L = T M ) 2. Acceleration including starting (T M > T L ) 3. Deceleration including stopping (T L > T M ) Unit -1 Drive Characteristics SAB Page 17

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 ω m1 Now motor speed is changed to ω m2, the speed torque characteristics adjusted to curve 2, so that the motor and load torque equals 2. Acceleration including starting (T M > T L ) 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 ω m1 is to be moved to operating point B at speed at ω m2. 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 18

3. Deceleration including stopping (T L > T M ) 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 ω m1 is to be moved to operating point C at speed at ω m3. 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 19

Permanent Magnet Synchronous Motor: Construction: 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 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 Unit -1 Drive Characteristics SAB Page 20

Surface Mounted Type: a. Projected 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. b.inset Type 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 2. Interior 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 21

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 90 0 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. Unit -1 Drive Characteristics SAB Page 22