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ENT271 MAIN TOPICS

ENT271 MAIN TOPICS

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ENT271 MAIN TOPICS

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  1. ENT271 MAIN TOPICS 1. Course Description - OK 2. Introduction to Drive Systems 3. Electricity and Electromagnetic Fundamentals 4. DC Generators 5. DC Motors 6. Transformers 7. Gears 8. Fluid Power: Hydraulic and Pneumatic 9. Valves

  2. DRIVE SYSTEM ENT271 2. Introduction to Drive Systems

  3. Contents • Fluid Power Drives • Historical background • Hydraulics and Pneumatics • Applications • Advantage and disadvantages • Learning Checklist • Methods for transmitting power • Electrical Drives • Definition • Historical background • Types • Applications • Basic components • Advantages and disadvantages • Mechanical Drives • Definition • Types of motion • Example of usage • Freedom and constraints • Cams, Gears, and Belt and Chain • Advantages and disadvantages

  4. METHODS FOR TRANSMITTING POWER There are three basic methods for transmitting power in all industrial process: • Electrical drive systems – (dc & ac motors, stepper motors and servo motors) • Mechanical drive systems – (pumps, gears, conveyors, chain, belt drives and cams) • Fluid power drive systems – (hydraulic pumps, pneumatic cylinders) In practice, most applications actually use combination of these three methods to achieve the most efficient overall systems

  5. 1. Electric Drives 1.1 A definition • Is a device to convert mechanical energy to electrical energy or vice versa • Motor: Device to convert electrical energy to mechanical energy • Generator: Device to convert mechanical energy to electrical energy • The difference between a generator and a motor is the direction of power flow • Motor: Electric power enters the windings and is converted to mechanical power that electromagnetic torque puts out through the shaft • Generator: Mechanical powers enter the shaft, overcomes electromagnetic torque, and is converted to electric power coming out of the windings

  6. Load pulleys Pulley Motor P M Belt Load 1 Load 2 Load n 1.2 Historical Background • Line shaft drive - oldest form of electric drive system Single-motor, multiple-load drive system

  7. 1.3 Most popular motors • Types: • DC motor • AC motor • Brushless motor • Step motor • Servo motor • Linear motor

  8. 1.4 Applications • Single-motor, single-load drives system – most common form of electric drive Electric vehicle (Venturi Fetish) Hard-disk drive Household appliance Golf cart

  9. Multiple-motor, single-load drives system – several motors are used to drive a single mechanical load. Industrial manipulator Airplane actuation system Mars rover Robot arm

  10. Power source Electronic converter Mechanical load Motor Controller • 1.5 Basic Components of An Electric Drive System Functional blocks of an electric drive

  11. A modern electric drive system has 5 main functional blocks • Power source – provides the energy the drive system needs. Two major type of power sources are used in industrial applications: alternating current (ac) and direct current (dc) • Converter – convert the electric waveform of the power source to a wave form that the motor can use. The converter adjusts the voltage (or current) to desired values. • Controller – supervises the operation of the entire system to enhance overall system performance, stability and efficiency • Often, mechanical load determined by the nature of the industrial operation, and the power source is determined by what is available at the site However, designer can decide other three components

  12. Motor - selection criteria: • Available power (DC or AC) • Operating condition • Starting characteristics (torque and current) • In the application for which a high starting torque is needed, dc series motor might be better than an induction motor • Operating speed • In constant speed applications, synchronous might be more suitable than induction or dc motors • Forward/reverse operation • Acceleration characteristics (depending on load) • Efficiency at rated load • Overload capability • Electrical and thermal safety • Life span and maintenance • Mechanical aspects (size, weight, noise level, environment) • Control complexity and cost

  13. 1.6 Advantages: • High voltage power lines provide the most convenient means of transmitting power over long distance • Machines and motors of various sizes can operate efficiently from the same power source • Easy to control – (e.g. by varying voltage or current) wide range of speed • High efficiency • Clean – does not pollute • Easy to store or transport energy

  14. 1.7 Disadvantages: • The potential for electrocution or electric fire is present whether the system is operating or not • The system is dependent on an outside source for a continuous supply of energy • Electrical energy is required by industrial process cannot be stored and must be used as it is delivered • To be efficient, electric motor must be run at high speeds and must be reduced by mechanical means these consume energy and wasteful

  15. 2. Mechanical Drives 2.1 Mechanical Drives – A definition • Mechanical Drives: Are motion converters. They transform power or motion from one form to some other required form. For example: • Transform linear motion into rotational motion • A linear reciprocating motion into rotary motion, as in internal combustion engine where reciprocating motion of the pistons is converted into rotation of the crank and hence the drive shaft 2.2 The example of usage / functions of Mechanical drive: • Force amplification given by levers • Change of speed given by gears • Transfer of rotation between axis using timing belts

  16. 2.3 Types of Motion • Translational • Rotational • Complex motion: combinational of translational and rotational including the components of the motion in three dimension Types of motion

  17. Y X 0 2.4 Freedom and Constraints • An important aspect in the design of mechanical elements is the orientation and arrangement of the elements and parts • The number of degrees of freedom (DOF)are the number of components of motion that are required in order to generate the motion • A rigid body in a plane • To determine the DOF of this rigid body we must consider how many distinct ways the bar can move. In two dimensional plane , there are 3 DOF. The bar can be translated along the x axis, translated along the y axis, and rotated about its centroid DOF of a rigid body in a plane

  18. Y X Z • A rigid body in a space • An unrestrained rigid body in space has six degrees of freedom: three translating motions along x, y and z axis and three rotary motions around the x,y,z axis respectively DOF of a rigid of a rigid body in space

  19. A joint is constrained to move a long a line; one degree of freedom shown in Fig. A • Fig. B shows a joint which has one translational degree of freedom and one rotational degree of freedom A) One degree of freedom B) Two degrees of freedom

  20. 2.5 Cams • A cam is a body which rotates or oscillates and in doing so imparts a reciprocating or oscillatory motion to a second body, called the follower with which it is in contact. As the cam rotates so the follower is made to rise, dwell and fall Fig.14 Cam and cam follower

  21. 2.6 Gears • Gears are widely use to transfer and transform rotational motion • Gears can be scaled to transmit power from small battery powered watch motor, up to the power from thousand horsepower gas turbine engines • Properly mounted and lubricated, gears transmit power efficiently, smoothly, and quietly • Gears classification • Gears may be classified to the relative position of the axis of revolution. The axis may be: • Parallel • Intersecting • Neither parallel nor intersecting (skull) • Gears function • Changing rotational speed • Changing rotational direction • Changing the angular direction of rotational motion • Multiplication or division of torque or magnitude of rotation • Converting rotational to linear motion and its reverse • Offsetting or changing the location of rotating motion

  22. 1. Spur gears • Gears for connecting parallel shaft External contact Internal contact 2. Parallel helical gears The leading edge of the teeth are not parallel to the axis of rotation 3. Herringbone gears (double helical gears)

  23. 4. Rack and pinion • Gears for connecting intersecting shaft 1. Straight bevel gears

  24. Neither parallel nor intersecting shaft 1. Crossed-helical gears 2. Worm gear

  25. 2.6 Belt and Chain Drives • Belt drives are just a pair of rolling cylinders with the motion of one cylinder being transferred to the other by a belt. Belt drives use the friction that develops between the pulleys attached to the shaft and the belt around the arc of contact in order to transmit a torque T1 rA rB B A T2 Driver Driven Torque on A (TA)=(T1-T2)rA Torque on B (TB)=(T1-T2)rB

  26. Types of belts • Flat • Produce little noise • Can transmit power over long distance • V • Used with grooved pulleys and less efficient than flat belts but a number of them can be used on a single wheel and so give a multiple drive

  27. 2.7 Advantages: • Parts have been standardized and are readily available for replacement • Pose little or no safety hazard when not operating (no pressured lines to burst or electric lines shock) • Generally simpler to troubleshoot and repair

  28. 2.8 Disadvantages: • The system’s response to change in motion is slow due to the inertia of material used • Moving pars need periodic lubrication and protection from oxidation and corrosion as well as repair or replacement • The distance where the power can be transmitted economically is limited due to the weight and inertia of the material

  29. 3. Fluid Power Drives 3.1 Fluid Power Drive– A definition • Is a technology that deals with the generation, control and transmission of pressurized fluids (either liquids or gasses) to provide force and motion to mechanisms • Fluid power drive is subdivided into the categories of hydraulics (medium used is a liquid, i.e. mineral oil or water) and pneumatics (is a gas, i.e. air or another inert gas) [Inert gas = any gas that is not reactive under normal circumstances]

  30. 3.2 Historical Background • Fluid power has been used since thousand of years ago • People used air and water to harness useful work • Sails were used to move ships, windmill were built to draw water to irrigate cropland. Water wheels were erected to process grain • Nowadays, we are using fluid power extensively

  31. 3.3 Hydraulics Vs Pneumatics • Cost: Pneumatics are considerably less to build and operate because no reservoir is needed to store fluid, no need to provide and recover fluid. When higher force or torque is needed, pneumatics needs large motors or cylinders than hydraulics. Usually, hydraulic is used when higher forces are needed • Precision: Fluid vary in controllability. Unlike gasses change volume significantly when pressurized. When precision is needed, usually, hydraulic is used • Safety: Gasses under high pressure are explosive. For lower force, usually pneumatics is used (in a range of 100psi (7bar)). Hydraulics is used for higher forces

  32. 3.4 Applications Hydraulic (1000-3000psi,messy/high maintenance) The Sarcos GRLA (General Large Robot Arm) 1.75 meter long from shoulder to wrist Hydraulic – up to 3000psi Shovel Dump Truck

  33. Pneumatic 60-100psi Pneumatic – McKibben Air Muscles 0-60psi

  34. 3.5 Components of a fluid power system: Hydraulic system • A tank (reservoir) to hold the hydraulic oil (A) • An electronic motor or other power source to drive the pump (B) • A pump to force the oil through the system (C) • Valves to control oil direction, pressure and flow rate (D-G) • An actuator to convert the pressure of the oil into mechanical force • or torque to do useful work (H) • Piping to carry the oil from one location to another

  35. 3.6 Components of a fluid power system: Pneumatic system • A pneumatic system also has 6 basic components: • An air tank to store a certain volume of compressed air • A compressor to compress the air coming from the atmosphere • An electric motor to drive the compressor • Valves to control air direction, pressure and flow rate • Actuators, to convert the air pressure into mechanical force or • torque to do useful work • Piping to carry the pressurized air from one location to another

  36. 3.7 Advantages: • Force levels can be made very high • This force can be transmitted over moderate distances at uniform intensity • The amount of force or motion can be controlled easily and accurately (ease and accuracy of control) • Motion can be reversed very quickly, started or stopped instantly with no slack or backlash • Systems can be assembled into self-contained, easily maintained, portable units (simplicity, safety, economy) • Constant force or torque

  37. 3.8 Disadvantages: • Hydraulic oils are messy and leakage is impossible to eliminate completely • Hydraulic lines can burst and might result in injuring people and damaging surrounding objects • Most hydraulic oils can cause fires when there is leakage • Compressive air for pneumatic systems can be dangerous if the pressure is too high • Air can be corrosive since it contains oxygen (about21%) and water • High pressure air (greater than 250psi) is typically not used due to the explosive dangers

  38. 4. Learning Checklist • Give the definitions and examples for the electrical drive, mechanical drive and fluid power drive • State the applications for electrical drive, mechanical drive and fluid power drive • What are the advantages and disadvantages in the usage of electrical drive, mechanical drive and fluid power drive? • What is the basic components of a fluid power drive and electrical drive system

  39. IF YOU CAN ANSWERS ALL FOUR, YOU MIGHT BE ABLE TO ANSWER EXAM QUESTIONS QUITE EASILY IF YOU CAN ONLY ANSWER ONE OR LESS THAN ONE, I SUGGEST YOU READ AGAIN TO AVOID PROBLEMS