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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
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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