Download Notes 6

Chapter 20
Motor Control Devices and
Circuits
Objectives
After studying this chapter, you will be able
to:
• Discuss several types of control devices
• Identify the proper usage of different types
of control devices
• Discuss the different types of controlled
devices
Objectives (cont’d.)
• Identify the proper usage of different types
of controlled devices
• Describe different methods of starting,
controlling speed, reversing, braking, and
stopping motors
• Develop control circuits to perform the
function of starting, controlling speed,
reversing, braking and stopping motors
Motor Control Devices
• Push buttons
– Manual control devices
– Momentary contact push button
– Mechanically held push button
– Rated as standard duty or heavy duty
• Rotary switches
– Switches operated manually with a turning
motion, also called selector switches
Motor Control Devices (cont’d.)
– Used to open or close one or more circuits
simultaneously
• Limit switches
– Automatic devices
– Consist of actuator, body and one or more
contact blocks
– Used to sense presence or absence of an
item or to verify position
Motor Control Devices (cont’d.)
• Proximity switches
– Noncontact-type switches used to sense
presence or absence of an object without
contact
– Two types: inductive and capacitive
• Inductive senses ferrous and nonferrous metals
• Inductive maximum sensing distance is 0.4 inches
• Capacitive senses conductive and nonconductive
material
Motor Control Devices (cont’d.)
• Photoelectric switches
– Noncontact sensors
– Consist of transmitter, receiver and switching
device
Operating Methods
• Thru-beam
– Transmitter and receiver are housed
separately
– Item to be sensed breaks the light beam
• Retro-reflective
– Transmitter and receiver housed in single unit
– Two types: standard and polarized retroreflector
Operating Methods (cont’d.)
• Diffuse
– Uses the reflectivity of the object being
sensed to return light from transmitter to
receiver
– Several different types: standard, long-range,
fixed-focus, wide-angle, and background
suppression sensor
Controlled Devices
• Relays
– Electromechanical switch
– Coil of wire wound around an iron core
– Two contacts: one fixed and one moveable
– When relay is energized, magnetic field
around the coil causes contact to move
– Time delay relays provide time delay before
contact changes state
Motor Starters
• Electromechanical switch similar to a relay
• Also contains overload protection for the
motor
• Available for AC or DC voltage
• Two different types: standard and
reversing motor starter
– Reversing allows the motor’s direction of
rotation to be changed
Annunciators
• Signaling apparatus that may be audible
or visual
• Examples: bell, chime, horn, loudspeaker
siren, indicator light, or strobe light
• Used as a warning device or to indicate
current condition of a machine or process
Motor Control Circuits
• Two-wire controls
– Method of motor control in which manual
devices are used to stop and start motor
• Three-wire controls
– Use some type of electromechanical switch to
provide a latching, seal-in, memory or holding
function
– Power failure will stop the motor
Multiple Start/Stop Controls
• Control two motors from one location, or
one motor from several different locations
• Forward/reverse controls
– Circuit can allow a motor to run in either the
forward or reverse direction
– Example: Figure 20-32A
Multiple Start/Stop Controls
(cont’d.)
• Speed control
– Controls speed of the motor
– Pilot lights can be used to indicate speed
• Jog control
– Motor may be jogged or bumped slightly
– Helpful when using motor to position object
– Momentary switch applies power to motor for
as long as switch is closed
Multiple Start/Stop Controls
(cont’d.)
• Hand-off-automatic control
– Allows circuit to function under the control of
either automatic device or manual control
Multiple Motor Starter Control
• Allows independent
control of multiple
motors
– If first motor does not
start, the second and
third will not either
– If first motor starts and
second motor fails to run,
third motor will not run
Sequential Starting Control
• Start a second motor a certain time after
starting the first motor
• Example: cutting tool with coolant pump
Various Starting Methods
• Primary resistor starter
– Resistors limit the inrush of current to the
motor windings
• Autotransformer starter
– Separate start relay and run relays
– Start relay energized first, then after time
delay, start relay is de-energized and run
relay is energized
Braking
• Plugging reverses the direction of a
running motor
– Braking action is very quick
– Should be reserved for emergency situations
since it will cause motor damage
• Dynamic braking
– Coasting motor acts as a generator
– Braking effect decreases as motor slows
Summary
• Types of motor control devices include
push buttons, rotary switches, limit
switches, proximity switches and
photoelectric switches
• Some operating methods include thrubeam, retro-reflective, and diffuse
• Relays are types of electromechanical
switches
Summary (cont’d.)
• Motor control circuit types include two-wire
and three-wire, speed control and handoff-automatic control, and those with
multiple start/stop capabilities
• Various starting methods exist, including
the primary resistor starter, and the
autotransformer starter
• Plugging is a method of braking