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Electromechanical drive systems Learning summary By the end of this chapter you should have learnt about: • Characteristics of loads • Linear and rotary inertia • Geared systems • Tangentially driven loads • Steady-state characteristics of loads • Modifying steady-state characteristics of a load using a transmission • Sources of mechanical power and their characteristics • Direct current motors and their characteristics Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two Electromechanical drive systems Learning summary • • • • Rectified supplies for dc motors Inverter-fed induction motors and their characteristics Other sources of power: pneumatics and hydraulics Steady-state operating points and matching of loads to power sources. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.3 Linear and rotary inertia – key points By the end of this section you should have learnt: • the similarities and differences between linear and rotational inertias and how they are analysed • the concept of moment of inertia • how to calculate moment of inertia for simple components made up of cylinders and tubes. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.4 Geared systems – key points By the end of this section you should have learnt: • the concept of a gear ratio • how to relate angular velocities, angular accelerations and torques between input and output shafts of a geared system (or other system involving belts, friction drives etc.) • the concept of ‘referred inertia’ • the effect of inefficiency on the transmission of torque and on the apparent value of referred inertia. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.5 Tangentially driven loads – key points By the end of this section you should have learnt: • how to calculate the referred inertia of a tangentially driven system • the inertia behaviour of screw-driven systems. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.6 Steady-state characteristics of loads – key points By the end of this section you should have learnt: • the different contributions to steady-state running characteristics of loads • how to express these different contributions to the load characteristics in the form of a mathematical expression, both for linear and rotational motion • that friction can have a beneficial role in some situations as well as having a detrimental effect on efficiency in other situations. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.7 Modifying steady-state characteristics of a load using a transmission – key points By the end of this section you should have learnt: • how to refer the torque–speed characteristics of a load to the input shaft of a transmission system in order to obtain the characteristics observed by the mechanical power source driving it • that a transmission will affect the combination of torque and speed required to drive a load but will not help to overcome a shortfall in the power available for providing the drive. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.8 Sources of mechanical power and their characteristics – key points By the end of this section you should have learnt: • the different types of mechanical power sources used within drive systems • the meaning of a torque–speed–SFC diagram for an internal combustion engine, and in particular understand the implications of its main features. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.9 Direct current motors and their characteristics – key points By the end of this section you should have learnt: • the operation of dc motors • how to derive the general torque equation • the different forms of dc motor • why the shunt motor produces virtually constant speed • how to vary the speed of a shunt motor • the characteristics of a series motor • how to control the speed of a series motor • why the speed of a separately excited motor is proportional to the armature voltage. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.10 Rectified supplies for dc motors – key points By the end of this section you should have learnt: • how to draw the current and voltage waveforms for both diode and thyristor bridge rectifiers with an inductive load • how to derive the dc output voltage for diode and thyristor bridge rectifiers with an inductive load • how to calculate the speed of a separately excited dc motor supplied via bridge rectifiers. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.11 Inverter-fed induction motors and their characteristics – key points By the end of this section you should have learnt: • the principles of operation of an induction motor • the operation of a simple inverter • the principles of pulse width modulation • induction motor torque is proportional to the applied voltage divided by frequency (Vp/f ) • at frequencies above ‘base speed’, the torque falls as the frequency increases • approximately linear torque–speed characteristics of induction motors can be obtained for relatively low values of slip. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.12 Other sources of power – key points By the end of this section you should have learnt: • the similarities and differences between pneumatic and hydraulic systems • why pneumatic or hydraulic actuators might be used in preference to electric motors • the ancillary equipment needed to power pneumatic and hydraulic systems • how hydraulics can be used to provide a variable-ratio drive • the cost and energy-efficiency issues associated with pneumatic power and compressed air. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two 4.13 Steady-state operating points and matching loads of power to sources – key points By the end of this section you should have learnt: • how mechanical power sources and loads interact • how a transmission system may be used to match the power source to the load • to calculate the combination of torque and speed at which a load and power source will operate under steady-state conditions • the function of a clutch with particular reference to starting a mechanical load from rest • why some sources of mechanical power are better suited to starting of loads than others. Unit 4: Electromechanical drive systems An Introduction to Mechanical Engineering: Part Two