Module C Flashcards
(31 cards)
How many contacts does a reversing magnetic motor starter require?
Two contactors – one for forward, one for reverse.
How is a 3-phase motor reversed using a starter?
By interchanging any two supply lines to the motor.
How many overload relays are required in a reversing starter?
One overload relay, as only one motor is used.
Why is mechanical interlocking required?
To prevent both contactors from engaging at the same time, avoiding a short circuit.
Why are electrical interlocks used?
To prevent coil burnout by blocking the opposite contactor from energizing.
How are electrical interlock contacts connected?
One NC contact from each contactor is wired in series with the other contactor’s coil.
What is the purpose of holding contacts?
They keep the coil energized after the start button is released.
How many holding contacts are used in a reversing starter?
Two – one for forward, one for reverse.
List the 7 control sections of a reversing starter:
a) General Stop
b) Forward Stop
c) Forward Start
d) Forward Holding
e) Reverse Stop
f) Reverse Start
g) Reverse Holding
What is jogging?
Repeated short bursts of full voltage to the motor with no holding circuit.
What is inching?
Starting the motor at reduced voltage to slowly creep to a position.
How is jogging achieved in a circuit?
By interrupting the holding circuit.
Three ways to achieve jogging:
a) NO/NC pushbutton with NC in the holding circuit
b) Maintained selector switch controlling jog/run modes
c) Control relay isolating jog and holding circuits
What is positive jogging?
A method that prevents the motor from running if the button is released too quickly; usually uses a relay for isolation.
What is a risk with NO/NC pushbutton jogging?
If the button is released too quickly, the coil may stay energized, causing the motor to run.
Can a single button be used for both jog and start?
Yes, if designed correctly.
Why must starter rating be higher when jogging?
Jogging causes high inrush current (2–8× full-load), which overheats smaller contacts.
What is a selector switch?
A maintained contact switch that changes circuit operation without rewiring.
Are selector switches momentary or maintained?
Maintained
Can selector switches have more than two positions?
Yes – e.g., Hand/Off/Auto.
What is AC-1 used for?
Heating and general distribution loads (low power factor).
Can a selector switch enable/disable multiple parts of a circuit?
Yes – they can control several devices simultaneously.
How are pilot lights connected?
In parallel with operating coils or in series with contacts.
