Motor Control Flashcards
Define a contactor
An electro mechanical device that opens and closes when the coil is energized
What two devices are found in a contactor?
A coil, and contacts
What three devices are found in a starter?
A coil, contacts, and OLR
Coil and contacts are ‘the contractor’
What is LVP and UVP and it’s purpose?
Low voltage protection, under voltage protection
Prevents machinery from restarting without pushing start button. Safety purpose.
What does N.C and N.O. Mean?
Normally closed - energized closed, denergized open
Normally open - denergized open, energized closed
What is overcurrent protection?
Fuses, breakers, devices that trip when current above its normal path is detected
What is overload protection?
Heater relays
What is 3 wire control?
LVP / UVP
What is a 2 wire control circuit?
LVR. Restarts equipment without pressing start
What is a power circuit?
Current path to motor
What are other names for holding contacts?
Seal in contact, maintaining contact.
Device that prevents chattering in coils
Shading coils
What are three common types of manual motor starters?
Toggle switch, push button start, drum switch
Three ratings found on manual motor starters
Voltage, amperage, horsepower
Drum switch ratings
1.5hp @ 120V, 3hp @ 240V
Which AC motor starter does not have overload protection?
Drum switch
What type of rating do contacts of manual motor starter have?
Horsepower
Why does a manual motor starter for a 240V motor have two power contacts?
For each ungrounded conductor
What problem can occur if a starter is constantly being reset?
Overloads will wear out and will cause nuisance trips
Heater selection for motors is based on what?
Full load current
Typical inrush current is approximately how large?
3-6 times normal running current
What is a drum switch used for?
Manually control speed, change motor direction.
What happens when a motor overload occurs?
Like current increases, heater elements warm, heater trips the O.L contacts and stops motor.
In the control circuit, what function does the NC contact have?
OLR, ectronical interlock
What does “full voltage control” mean?
Line voltage for control circuit
How do you change the direction of a motor?
Interchange any two leads of the motor
Difference between jogging and inching
Jogging is a momentary connection to full line voltage
Inching is a momentary connection to reduced line voltage
Over current protection protects the _________ and overload protection protects the _________
Conductors, equipment/motor
Overloads should trip at ___% to _____% of full load current
115 - 125%
What is a magnetic motor starter?
Electromagnet is used to operate a set of contacts for making and breaking the electric circuit.
What does across the line mean?
The motor is started with full line voltage.
What is a fuse called that provide both short circuit protection and overload protection?
Dual element time delay fuse
Two types of starters
Clapper type, vertical lift type
If the motor starter has tripped because of an overload, what must you do?
- Find out cause of overload
- Wait for OLR to cool down
- Reset OLR
Low voltage starters are rated by the EEMAC, what is their lowest and highest rating?
Size 00 (2hp max), size 8 (900hp max)
What elements does a power circuit usually contain?
Disconnect switch, NO contacts (go rated contacts) overload heaters, fuses (overcurrent protection)
Isolation switches are horsepower rated, T/F?
False. They are not.
What is the fundamental difference between a two wire and three wire control circuit?
The additional NO contact in parallel with the start button in the three wire switch.
When are TX’s used in a control circuit?
CT’s are used to step down the line voltage from the power circuit to operate at a safer level
What do wiring diagrams show?
Shows the physical relation of all devices and each conductor termination between each device.
What do ladder diagrams show?
Symbols and lines identifying components in the electrically connected way
What is the dangerous version of a jogging circuit called?
Double block, or blocking
Name two versions of jogging circuits that work and are safe.
Control relay, jog run selector switch
Are shading coils needed for DC coils?
No
What is the purpose of a motor starter?
Stop, start a circuit, and provide OLP
Four enclosure types for motor starters.
Open (not enclosed), explosion proof, waterproof, dust proof
What is a downside to using bimetallic strips for overload relays?
High ambient temperatures can cause nuisance trips
Disadvantage if manual motor starters compared to magnetic motor starter?
Manual motor starters do not provide low voltage protection
Why must starters be connected to the ungrounded conductor?
Ground condition could bypass the motor running protection
What are the two different types of contacts in a magnetic starter?
Stationary, moving
Fuses act quicker than overload relays. T/F?
False. OLR act quicker.
What type of contact is the overload relay in the control circuit?
Normally closed auxiliary contact
Magnetic type overload relays are _______ type
Solenoid
Thermal overload relays are _______ type
Heater
Why are components rated in horsepower?
So that they can safely handle in rush current and full load current
What is the purpose of a motor starter?
Stop, start, provide overload protection
Define in-rush current
Locked rotor current. Current drawn by the motor when first energized.
What are push button starters rated at?
5HP @ 230V
Why is a motor rated in HP and not just current?
Must be able to interrupt locked rotor current
Example of a two wire pilot device
Float switch, thermostat, flow switch
What is electrical interlocking?
Connection of contacts in control circuit to prevent forward and reverse from being energized at the same time
Why are both electrical and mechanical interlocks used?
Electrical interlocks act quicker than mechanical interlock, but electrical interlocks can fail. Arching can cause an uneven surface on the contacts which can prevent them from acting normal.
What are two different types of time delay relays? What are their differences?
Hydraulic and Pneumatic. Hydraulic is oil filled, pneumatic is air filled and uses bellows
Advantages of timer relays
Automatic repeat of functions, multiple trip points available, long timing functions
The overall pressure over which a pressure switch can function is called the ______
Differential
Starter size required for a motor load depends on _____
Motor duty cycle
There’s types of pressure switches
Bellows, diaphragm, bourdon tube
What type of oil is used in a dashpot?
Silicon
What are the two types of proximity switches?
Inductive: detects presence of metals
Capacitive: detects presence of almost any material