EMC 225 motor control Flashcards
What is motor control
refers to the components and wiring of ccts used to start,stop and reverse, change speed of motors
What do typical motor ccts include
-power-rated devices to switch the motor or other power load
-overcurrent protection for power ccts and control ccts
-overload protection for the motor
What are 2 main types of starters
-manual
-electromagnetic
What is a combination starter
has a electromagnetic starter and a disconnect switch within an enclosure
What is a controller in a motor
-device or group of devices that regulate the operation of the apparatus to which is connected
What does a controller in a motor control do
-direction of rotation
-speed
-speed of acceleration or deceleration
What does murphys law state
if something can possiblt go wrong, it eventually will
In addition to actually controlling motor operation, motor control cct may include what other features?
-manual or automatic control devices
-indicators
-meters or displays
-saftey features to protect personelle
-preventative functions to avoid or limit equipment damage when abnormal condictions
motor control equipment is often applied to other types of power loads. What are 2 examples
-high capactity heating elements
-groups of light fixtures
What are these motor cct components used for?
a)manual starters and contactors
b)indicators
c)warning horns and lights,lockabke diconnects, ect.
a) switch the motor or other power load
b)visual information of status of equipment and allow remote monoriting
c)warn workers that equipment is about to start.lockable disconnects provide a isolation poing for workers that could be injured or killed if equipment was energized
What does an overload in a motor mean
-exxessive load current that will overheat conductors and equipment if it continues to run.
What does a short cct in a motor refer to
very high current associsated with a low impedance fault.
What is fault current and interrupting capacity?
-overcurrent device that provides short- cct protection and must be able to inturupt the max voltage and fault current the supply can provide
what code rule talks about ratings of protective and control equipment
14-012
Overcurrent protection refers to the ______________ that automatically opens the cct when the current exceeds a certain value
device
In ccts that do NOT supply motors how is the overcurrent device selected?
to provide both overload and short cct protection
-doesnt exceed the safe current rating of conductors or connectedf equipment
In motor ccts the overcurrent device is selected to carry the __________________- without ttripping and provide _________________ for the conductors and equipment
-motor starting current
-short-cct protection
What are diconects for a motor
-saftey devices where the load is a hazardous electrically and mechanically.
The number and loaction of diconnects in motors must satisfy which CEC rules
28-600,602,604
What are the requirements for a manually operated switch or breaker
-opens all ungrounded lines at the same time
-within sight
-9m of the motor and/or locked in the open position using an approved locking device
Air conditioning and refrigeration equipment must have a diconnect within ____________
3m
You must isolate the motor at the ___________-
diconnect
In a system with ________ the motor is automatically disconnected from the supply with the line voltage dips velow a “hold-in value” and is automatically restarted when the voltage returns to normal
low-voltage release
In a system with _____________ the motor is automatically disconnected from the supply when the line voltage dips velow the “hold-in” value and is NOT automatically restarted when the voltage returns to normal
low-voltage protection
if a manual motor starter with no magnetically controlled components what would happen in the situation of a lower than normal supply voltage that would usually allow a magnetic device to “drop out”
-motor stays energized but would stall
In the turn “drop out” what does it refer to
the contactor
when more precise protection from undervoltage condictions are needed what is added
an adjustable undervoltage relay
what are momentary contacts
-also called spring return contacts
-return to their at-rest state when the operating button or handle is released
what are maintained-contacts
-also called stay put contacts
-stay in their last positon after the operating handle is released
what does normally open (NO) or normally-closed (NC) refer to
-the status of the contcts when the operating device (handle,button,coil) is at rest in the off position or de-energized
how can normally open and mornally closed contacts operate if they are part of the same device with operate with no overlap?
NC contacts open BEFORE the NO contacts close
Do early make and late break contacts overlap?
-yes, early-male NO contact closes before a standard NC contact opens. A late break-NC contact opens after a standard NO contact closes
What are double-throw or change over contacts
-have one NO and one OC constact connected to a common terminal.
-the 2 contacts cant ve used in differne tccts bc they are not electrically isolated
-Only 3 terminals per contact pair
what are fleeting contacts
-chage state twice while the operatior is in motion
-may be usedf to supply a brief pulse to an electronic controller
what are piolet-duty contacts
-intended for typical control finctions only
-not rated for power load
-switches with low category numbers cant be used for electromagnetic fields
what are holding,holdin or seal-in contacts
-NO contacts controlled by an electromagnetic coil
-wired in // with the momentary contact start or reset button used to enrgize that coil.
-HOlding contact maintains the coil cct after the start button is realeased. BC the coil is energizxed through a contact it controls
-cant reset itself after a power loass theirfore provides low voltage protection
how are manual switches categorized
-contact type
-contact number
-style of operator used
What is a toggle switch
-used by tipping th ehandle one way or another.
what are pushbutton switches
-2 or more pushbuttons are combined to create a control station
when are selector switches used
-industrial controls have 2,3 or 4 positions which can be a mix of maintained (stays put) or momentary (spring return)
What do emergency stop switches look like
-pushbuttons with raised and or enlarged operators
in emergency switches what has to happen before the contacts return to the normal state
-have to be pulled or turned bc NC ccts fail in an open state and all Emergency switches use a NC contact so it stops the minute they are operated
how do automatic control switches work
-open and close their contact in response to changes like temperature or pressure
-ex)thermostat
-operates the controls when the temperature matches the set point
what is it and the purpose of a differential in an automatic control
-minimum change required before the contacts change state
-ensures there is always some delay between each contact operation
-if equipment is switches on an doff too often in a short period of time (short-cycling) it can stress the equipment, reduce efficency, cause operating failures
is a themostate a Automatic control? how does it work?
-yes
-senses temperature
-fluid versions called aquastats
are level controls and sensors an automatic control? how do they work?
-yes
-sense the level of liquids using floats, electrified probes,pressurized tubes
-or granular solids (using tilt switches,pressure plates,solar sensors
what are limit switches and are they an automatic control?
-yes
-also called position switches
-have levelers,rollers,flexible whiskers to activarte the contacts through direct contact with solid objects
what are proximity switches
-detect solid objects without physical contact
-used as limit or position switches
what do speed switches do
-monitor shaft speed.
-
-if contacts change state as the shaft comes to a stop they may be called ________________
zero speed switches
-if contacts change state when the shaft reaches high speed they may be called _______________
overspeed switches
zero speed switches are used for what
-control plug braking (braking by reversing the motor while its running)
-may be called plugging switches
-if they are used to prevent plugging they are called anti-plugging switches
automatic switches are used as _______– to start and stop motors
controls
_________ shut down equipment when obnormal condions occur
limits
What other names may be used for momentary and maintained contact operation?
-spring return
-stay put
When a manually operated momentary contact is listed as normally closed, does this mean that the contact is closed when the contact operator is activated or at rest
at rest
are the NO and NC contacts of a double-throw (changeover) pair ever closed at the same time?
no
What general resitriction applies to the poilet duty contacts
-non power loads
-coils in relays
-contactors
-solenoid valves
-indicator lights
are seal-in (or hold-in) contacts usec to provide low-voltage release or low-voltage protection?
low-voltage protection
What differences are there between typical modern emergency stops, and standard stop buttons?
modern- maintained orr latching action so they hold the cct open until they are manually reset
standard- stop buttons reset to their normal state immedatly when they are released
what is the set point of an automatic conteol like a thermostat?
-the value at which contacts change state, in a thermostate the contacts change at the termerature setting with some allowance for differiential
The symbols for mechanical limit switches can be difficult to interperatue because they can be NO,NC,NC held open, or NO held closed. What part of the symbol allows us to identify what type it is?
Viewing the “L” attached to the contact line as a spring that pulls on the contact
to interperate automatic control symbols should you assume the contact moves toward or away from the operating symbol when the sensed quantity increases?
assume the contact moves toward the operating symbol when the sensed quantity increases
In what way is a limit function different from a control function
limit function- stops the equoment when dangerous conditions occur
-control function- operates to keeps the equipment running normally
What are 2 common categories od 3 phase motors
-synchronous
-induction
Squirrel cage rotor performance depends on _____________________and reactance, and the balance between the two. Very small changes in these values have significant effects on starting current, starting torque, running torque, full load speed, etc.
-rotar resstance
motor starter ratings are based on what
common motor type, the Design B.
Design A or E—and motors marked “High Efficiency”—tend to have __________________ Higher rated overcurrent devices and contactors may be required.
unusually high starting currents.
what is jogged in a motor
energized briefly and repeatedly, such as for positioning
what is plugged in a motor
brought to a stop by reversing) an oversized contactor is required.
When a motor has to be jogged frequently reversed, or plugged what is needed
oversized contactor is required.
why do open frame motors have higher service factors than totally enclosed motors
cooling air is in direct contact with the windings and rotor.
what is a three-phase stator winding
the winding installed in the stationary part of the motor and connected to the three-phase supply
what does the direction of roatation depend on in a 3 phase motor
The direction of rotation depends on the phase sequence of the supply and the order of the motor connections.
how do you reverse the roation of a 3 phase motor
interchange the supply connections to any two phases. Reversing starters usually interchange L1 and L3.
how dpo three phase motors operate
principle of a rotating magnetic field which is as a result of the physical spacing of the three sets of windings on the motor stator and also by virtue of the out of phase relationship of the three supply conductor voltages.
when does single phasong happen in a 3 phase motor
when one of the three supply conductors for a three phase motors is disconnected.
what happens when one of the three supply conductors for a three phase motor is disconnected
-less smooth rotating magnetic field
- higher than normal current in the remaining two conductors
-a reduction in motor torque characteristics
what causes single phasing in a three phase motor
loose terminations, broken conductors, equipment failure,
what is “phase loss” protection devices in a three phase motor
act to disable the circuit if it senses either the loss of a phase or a serious imbalance of phase currents.
types of single phase motors
-shaded-pole
-series universal
-split phase
-capacitor run
-capactior-start
-two valve capacitor
-repulsion-start induction-run
-written-pole
when are series-univeral motors used?how do you control the speed?
how do you reverse?
-encorperated into an appliance or tool
-can be speed controlled by adjusting the voltage and
-reversed by switching the armature and field connections
when ate split-phase/resistance start motors used? How are they reversed? what are they used with?
-power tools like a bench saw,small pumps and compressors
-reversed by switchong the start winding connections but MUST BE STOPPED AND RESTARTED FOR A CHANGE IN DIRECTION
-used with field-installed manual or electromagnetic motor controls
where are capacitor run motors used? how are they controlled? how to reverse?
-used almost exclusively for fans.
-Larger models are likely to be controlled with manual or electromagnetic starters.
- reversed with a simple connection change, but they can be damaged if they are not stopped first.
What are capacitor start-motors? how are they controlled and reversed?
-the most common general-purpose single-phase motor above 1 horsepower
-. Larger models are controlled with manual or electromagnetic starters.
-Standard models must be stopped before they can be reversed. “Instant” reverse models are available for special applications, but only in the smaller sizes.
where are two-valve capacitor motors used? how are they controlled? what must happen before they are reversed?
-used for hard to start loads such as pumps and compressors
- can be controlled with manual or electromagnetic starters.
- Standard models must be stopped before they can be reversed.
where are repulsion-start induction run motors used?what are the advantages? how are they reversed and controlled?
- excellent starting torque and a good performance at reduced voltage. Because of their high cost, they are generally restricted to very demanding applications with long supply circuits, (e.g. barn cleaners and silo unloaders).
-Most are reversed by opening the motor and repositioning the brushes, but electrically reversible models also exist.
-They can be controlled with manual or electromagnetic starters.
What are the advantages of written pole motors
they provide superior performance at a premium price.
When electrically operated devices are used to control the motor or other power load, there is always a____________ in addition to the main power circuit
control cct
what does a power cct do
-supplies power to the motor or other power load, and operates at the load voltage
-The contacts of the primary switching devices which actually switch the motor current are part of the power circuit.
in a power cct that is manually operated what is used
-horsepower rated switches
-manual motor starters
in a power cct that is electrically operated what is used
-contactors
-hp rated relays
-electronic contactors,soft starters, or adjustable frequency drives
what is a control cct? what voltage does it operate at?
provides power and control for electrically operated switching devices and/or auxiliary circuits associated with the control system.
-operate at a lower voltage than the power cct
- some control circuits operate at the same voltage as the power circuit—up to 600 V!
A “power cct” supplies energy to a motor or other _______________
power loads
A “control circuit” supplies energy to electrically operated switching devices and/or auxiliary devices associated with the ___________
Control system
What is the difference between a “contactor” and a “starter”?
A contactor -is an electromagnetic switching device designed for power loads,
-a starter includes both a contactor and an overload device.
Is it safe to assume a control circuit is always operating at 120 V or less? Explain
NEVER assume a control circuit is operating at 120 V or less. Some systems operate at the same voltage as the power load, up to 600 V.
What motors can be controlled by an attachment plug
120v motors rated 1/3hp or less
if the motor has no built in overload protection it needs a ________
manual starter
Why are line-voltage controls added to a basic motor cct
-create simple but functional automatic control systems
What does the symbol that looks like a double question mark represent
overload element in the power cct
How does an overload element work
-built into the latching mechanistm that holds the contact closed
-When an overload occurs the overload device releases the latch and the contacts spring open
overloads are devices which respond to motor _________ and protect against overload
current
external motor overloads come in what 3 styles
-melting alloy
-bimetallic
-electronic
What type of overloads are bimetallic and melting alloy types known as
-thermal overloads
-both rely on heat production to trigger the trip operation
sustained overload current in a melting alloy heats the solder to the melting point, the spring loaded wheel is freeed, and the overload ______________
trips
what are interchangeable “heaters” used for
- adjust the overload setting and other tripping characteristics
what does a rest button on a manual starter or overload relay do
-reset the overload to a running state
-in thermals they wont latch until solder is solidified
can melting alloy overloads be set for automatic reset
-no
how do bimetallic overloads work
-use heater elements to monitor current.
-when current is applied to heating stgrip it bends and trips the overload mechanism
most bi-metallic overloads have an ambient temperature compensation to keep the tripping response ____________
constant
bimetallic overloads may have ______________sensitivity to unbalanced phase currents to brivide better protection for three ohase motors
increased
How do electronic overload protection work
-monitor current indirectly through electromagnetic induction
-electronic cct trips if it senses its too nhigh
class 20 of tripping is _______________
standard delay
class 10 of tripping is _____________
quick trip
class 30 of tripping is ______________
slow trip
a typical single-phase overload has ____________ element
one
three-phase overloads must have ______ elements
3 elements
-one for each ungrounded conductor
what are internal motor overloads
-heater driven bimetallic disks with directluy connected contacts
what is one advantage of external motor overload
-respond to a combination of motor temperature and current
what is the saftey concern with automatic restarting overload after reset
-starting motors unexpectadly can be a danger to people around it
how do you manually set an overload setting
-set it as low as possible and if it trips too much then raise it a little to prevent nusence tripping
in ccts thatr dont supply motors, the Overcurrent devices provides short cct and overload protection. in motors these functions are seperated. Why?
-o/c device sized high enough to start the motor so wont protect it or cct conductors
-seperate overload device is needed
What device protects the motor cct conductors from overload, the overcurrent device or the motor overload?
motor overload
what are 3 types of motor overload devices
-bimetallic
-melting alloy
-electronic
how do overload heaters de-energize the motor if the motor is controlled by:
a)manual starter
b)electronically operated starter?
a- mechanically releasing the contacts
b- opening a contact in a coil cct
when does a motor disconnect not have to be put in sight and within 9m of the motor?
-diconnect is an aproved lock off device
-not used for AC or refridgerant
disconnects for AC and refrigeration equipment must be within __________- meters of the equipment
3
what is electromagnetic switching
-one voltage is used to switch another
-energizing a magnetic device which activates movable components to make and break contact points
Do motor controls have to break all ungrounded conductors?
no unless used as a disconnect
when a starter contacter device is used in combination with an overload device they make up a ____________
starter
the contactor for a 3 phase motor has what 3 things
-magnetic coil rated to finction at a specific voltage
-3 sets of contacts rated for the motors power requirements
-at least one set of normally open contacts for “holding in” purposes
what are electromagnetic controls
-require seperate power and control ccts
-provide low voltage release or low voltage protection
what are undervoltage coils
-used for low-voltage protection
what can be added to the control cct of contactors or starters to provide precise adjustavle low voltage release or protection
undervoltage relays
when an undervoltage coil is installed in a manual starter or cct breaker the contacts open automatically when the coil voltage __________ the rated hold-in value. and cant be closed unlesss the undervoltage coil is energized
drops below
an undervoltage coil trips the device on ________
loss of voltage
a shunt trip coil trips when ___________
voltage is applied
what is an undervoltage relay
-auxilery relay usedf to monitor the cct vo0ltage
when are LVR and LVP used
-saftey for personnel
-saftey for motors
-safteyu for the product
a lower than normal voltage applied to a magnetic coil will result in the coil __________
dropping out
if the motor doesnt restart automatically it is a low voltage __________
protection
if the motor restarts automatically then it is a low voltage _________
release
What are some examples of line voltage controlls?
-thermostats
-lighting controlls
is there contuimity through an overload after it trips
no
-if there is it would be a low resistance element not a fuse
if a breaker or manual starter is equipped with an undervoltage coil, what condition must be met before the breaker or starter can be closed
-coil must be energized
the control devices used with two-wire controls have ___________- contacts
maintained
Descrive the meaning of “hand”, “off” and “auto” as they apply to typical H-0-A control system
hand-turned on by manual control
off-control cct is denergized
auto-equipment controlled by serperate automatic control
the control devices used with three-wire controls have a ____________ contacts
momentarty
why is it dangerous to connect control devices on the ground size of a control coil?
-accidental ground will prevent motor from tripping from an OL
what is the meaning of “I” AND “O” markings on the IEC equipment?
i- closed
o-open
if a maching has a seperate system for some essential function like lubrication or cooling, interlocking contacts from this system are connected into the control cct for the main drive motor. what is the purpose of these interlocks
-ensure main motor doesnt run without the essential functions
2 basic variations of magnetic control
- 2wire control
-3 wire control
3 standard tests for all electromagnetic ccts
- basic function test
- LVR/LVP test
- overload response test
what are indication devices
visual or audiotry
-piolet lights
-horns
what do pilot lights indicate
-control power indication
-motor energization
-machine motion
-overload trip condition
what do alternating relays do
-alternate the duty of energized equipment
when 2 peices of equipment are interlocked the operation of one is ___________ on the other
dependant
power ccts sometimes mist endure breaking of locked rotar currents which are typically _____________ times the full load amp rating of the motor
6 times
what is a power cct
part of the cct that energizes the motor windings
what is a control cct
facilitates the activation of the power cct
what term is used to describe excessive load current that will overheat the conductors of a motor?
overload
A manually operated switch or breaker that opens all ungrounded lines simultaneously must be within signt and within ________- meters of a motor
9
moter-operated machinery like saws and routers or other moving machinery may create a hazard on return voltage after stopping due to failure of voltage. what type of protection from a hazard must be provided?
a) low voltage release
b) low voltage protection
b
in a motor control cct, what device is normally used as a START button
normally open spring return
what type of device is used to detect solid objects without physical contact?
proximity switch
a contactor device is used in combination with an overload device is commonly refered to as a ________
starter
the contact that is connected in // with a start button that maintains a control cct is called the __________
holding
true or false. A motor controller need not open the cct in all ungrounded conductors to a motor unless it also serves as a disconnecting means
true
true or false. when it is necessary or desirable that a motor stop on failure or reduction of voltage and automatically restart on return of voltage, the motor controll device will provide low voltage protection
false
true or false. the disconnecting measn of a motor may be electrically operated either automatically or by remote
false
