unit 4: electromechanical drive systems an introduction to mechanical engineering: part two...

Unit 4: Electromechanical drive systems

An Introduction to Mechanical Engineering: Part Two

Electromechanical drive systemsLearning 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



Electromechanical drive systemsLearning 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.



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.

4.4 Geared systems – key points


• 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.



4.5 Tangentially driven loads – key points


• how to calculate the referred inertia of a tangentially driven system

• the inertia behaviour of screw-driven systems.



4.6 Steady-state characteristics of loads – key points


• 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.



4.7 Modifying steady-state characteristics of a load using a transmission – key points


• 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.



4.8 Sources of mechanical power and their characteristics – key points


• 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.



4.9 Direct current motors and their characteristics – key points


• 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.



4.10 Rectified supplies for dc motors – key points


• 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.



4.11 Inverter-fed induction motors and their characteristics – key points


• 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.



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.



4.13 Steady-state operating points and matching loads of power to sources – key points


• 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...

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