Motor controls review Flashcards
Two dissimilar metals bonded together have?
different expansion rates
If electrical contacts are permitted to open and close slowely
poor contacts are permitted to open
A metal that remans in liquid state at room temperature.
Mercury
An RTD is made out of
Platinum
A bellows type thermostat works on change in
pressure principle
A device that emplys the heating of two metals to produce a voltage is known as
thermocouple
Two or more thermocouples connected in series
thermopile
A device that works on principle of change in resistance with a change in heat is known as
Resistance temperature detector
A hall generator is constructed as a
Integrated Circuit
A hall sensor is used to sense the presense of a
Magnetic Field
If a disk with magnetic poles around its circumferance is attatched to a rotating shaft , and a hall sensor is mounted near the disk
An AC voltage is produced
Most proximity detectors will easily detect what type of metal?
IRON
A proximity detector that can detect the presence of non-metallic devices usually operates on a change in
capacitance
A photo conductive device changes
its resistance with light
Magnetic control systerm usually operates on
120 volts
A control transformer is often used to step
down 600v or 208v to 120v
ANOTC contact must be connected
on-delay relay if its to be time delayed when closing
Many electronic timers can be used as
DOE or DODE
Pulse timers
continually turn off and on at regular intervals
The most accurate way of plugging to stop a motor is with a
zero speed switch
3 Main methods of braking
Mechanical - Dynamic - Plugging
DOE stands for
delay on energization
Typically , SCIM and their locked rotor currents cary from
2 1/2 to 10 times the FLA
What kind of coupling is required for a Plugging switch
Direct Flexible
SCIM, 2 Pole @ 50 hz, ac scource will run at RPM
RPM = F x (120/ poles)
SCIM, 6 poles @ 60hz , ac source will run at
RPM = F x (120/ poles)
DOE Stands for
Delay on energization
the shaft of a plugging switch is most often
mechanically, connected to the shaft on the machine it controls
pluggin cotrol provides
rapid stop of motion
plugging switches are deigned to be added to control circuits
piolet device
Most machines in which plugging is done are driven by
SCIM
The rotating motion of a motor transmitted to plugging switch contacts either by a magnetic induction or by a
centrifugal mechanism
A plugging switch acts like a link between the motor and
reversing started
a starter used for a plugging switch will
interchange 2 of the 3 moter leads for a 3 phase motor
reverse direction of current through armature for a dc motor
when changing the direcion of motor rotation, the electrician should change connections at the scource of power or motor
scource of power or motor
squirrel cage induction motors are charcterized by
simplicity
reliabilty
ruggedness
With regard to SCM, the current
The current drops as motor speed increases
When a squirrel cage motor is connected directly to the plant distributionsystem at full voltage
either magnetic or manual
With a SCM, when voltage is reduced to start the motor the
current is also reduced and so is the machines torque
Which is the simplest of all starting methods
Across the line
Most popular motor
3 phase SCIM
initial inrush current is not comparable to
locked rotor current
With a reactr or primary resistor type starters, reactors, or resistors are
connected in series with a motor
It is desirable to limit the starting current of a motor if full voltage starting cause serious line disturbances on
lighting ccts
computer and electronics
fire alarm system
Large inrush current during starting of large motors may cause
Utility line disturbances
closed transition starting means the motos is
never disconnected from the line until the motor is operating at full voltage
Conditions where it is not acceptable to start a motor across the line due to excessive
starting current or torque
primary resistor starters consume
high energy and start with high power factor
Split phase motors will disconnect the start winding from the power line using a
centrifugal switch
If the motor is disconnected between the start and run stage, the starting method is refferd to as
open transition
Most large HP motors are started at
65%
some large motor circuits may use several starting because of
limitiations by the utility
torque limits of machine
length of acceleration time required
Autotransformers starters are considered
inductive loads
Some resistor and reacors starters use multiple steps of starting.
This is accomplished by tapping the resistor or reactor to provide different values of
resistanve and inductance reactance
autotransformers are generally employed to start what type of motor
SCIM
Motor started witha autotransformer reduced voltage starter
less current less torque
When an autotransformer is used as a voltage starter
an adjustable timer controls when the motor recieves full voltage
Another name for Star delta
wye-delta
with a star delta, if the starter used has the required number of properly wired controls the motor can be
started in wye delta
a star delta moto must be wound in such a manner that it iell run it
stator connected ran in delta
autotransformer used as a reduced vltage starter must have a minimum of
3 taps
with star delta motors
the full stator is used with no limiting devices
star delta motors are used on loads having
high inertia, short acceleration periods
the speed of a star delta depends on
frequency and number of poles
care shouldbe taken when connecting the stator windings of a star delta motor the the load, contacts will otherwisw
reverse directions when the windings are changed from wye to delta
the main reason to use a closed transtion starting method with a star-delta motor is to
Prevent line disturbances
If a set of normally closed and normally open contacts share a common terminal the contact configuration is refered to ass
single pole double throw
If a stator winding are connected in delta during the starting period , the starting current will be how many times greator than the current values if started in wye
3 times
when selecting a relay for a particular application, one must determine
coil voltage
#of contacts
contact rating
Most capacitive proximity detectors have a maximum sensing distance of
25 mm - 1inch
capacitive proximity detectors can sense the presence of
wood copper or brass
ultrasonic detectors emit a
high frequency sounds
Photodetectors with both transmitter and reciever require protection when unts are mounted in same housing
use reflector for operation
In canada, most 3 phase industrial motors operate on voltages ranging
208v or 600v
common control systems operate on
120 volts
conrol transformers are used to step down
600v or 208v down to 120v
when using a 240v/480v to 120v control transformer on a 240v line (lower voltage)
Both primary windings are connected in parrallel
when using a 240v/480v to 480v control transformer on a 240v line
(higher voltage)
Both primary windings are connected in series
which is not a methof of interlocking
thermal interlocking
A method by which starters are connected so that one cannot be started until the other is started is known as
sequence control
To protect people, machines and production processes a
lockable disconnect
which type of single phase induction motor cannot be used with a variable speed controller?
Motors with a centrifugal switch
Variable frequency motor controllers must have some method of
reducing the applied voltage to the stator as frequency is reduced
Most VFDs provide
low speeding torque
current limiting
automatic speed regulation
A device could be installed to input accurate current draw data to a VFD?
Current transformers
A device used to accuratley moniter the speed of a motor
Hall effect sensor
Ramping refers to
accelereation and deeceleration to control motor
In a VFD, the maximum speed of the motor is controlled by the
maximum hertz
In a addition to their primary function of providing speed control for c motors , most variable frequency drivees can also provide
low speed torque control
current limiting
automatic speed regulation
What device could be installed to input accurate current draw data to a vfd?
current transformers
a device that should be used to acccurately moniter the speed is
hall effect sensor
ramping refers to
acceleration and deacceleration control of a motor
Many transistor- controlled VFD now employ a special type of a transistor called
insulated bipolar transistor
Ina VFD, the ——– setiting controls the maximum speed of the motor
maximum hertz
