Machines; Controls And Switching CCTs Flashcards
Be able to: 1) State sections involved in fwd/rev stop ctrl of 3-phase motors. 2)Design jogging and inching CCTs 3)Develop schematic diagrams for ctrl. CCTs
What are some examples of motors that have to be repeatedly reversed?
Planning machine and elevators
How many contractors does a reversing motor starter require and what are their functions?
Two: operate motor in forward, operate motor in reverse.
How many overload relays are required if one motor is used for both forward and reverse?
One
Describe the interconnections in a reversing starter power circuit. Draw it out if possible.
Forward: L1 to T1, L2 to T2, L3 to T3
Reverse: L1 to T3, L2 to T2, L3 to T1
What is it called when the action of one device affects the way another device acts?
The devices are INTERLOCKED.
How do we prevent both forward and reverse contactor coils from energizing at the same time in a reversing magnetic motor starter?
Mechanical interlocks.
Why is electrical interlocking required in addition to mechanical?
Electrical interlocking prevents coil burnout if both fwd and rev. buttons are pushed at the same time. (the mechanical interlock allows only one armature to pull in, causing the OTHER coil to sustain excessive current and burn out w/o this protection)
On a reversing starter, for electrical interlock, the forward NC contact is in ____________ (series/parallel) with the ___________ (forward/reverse) contactor coil CCT
Series, reverse
What is the purpose of the reverse NC contacts in a reversing motor starter?
To prevent the forward contactor coil from pulling in while the motor is already running in reverse. (Electrical interlock)
What style motor starters have electrical interlocks prewired to the contactor coils by the manufacturer?
NEMA-rated starters.
What style motor starter may have contractors, interlocks, and overload relays supplied as separate blocks?
IEC-style
What are the following contact numbers for an IRC reversing starter control?
1) fwd holding contact
2) rev. holding contact
3) overload
4) reverse electrical interlock
5) forward electrical interlock
6) motor
1) 13, 14
2) 13, 14
3) 95, 96
4) 21, 22
5) 21, 22
6) A1, A2
What are the following terminal numbers for a NEMA reversing starter control?
1) fwd holding contact
2) rev. holding contact
3) overload
4) reverse electrical interlock
5) forward electrical interlock
1) 2, 3
2) 4, 5
3) prewired
4) 6
5) 7
What is the purpose of the forward and reverse NO contacts?
To keep the coil energized after start button has been released (holding contacts)
How are the holding contacts wired with respect to their respective push button?
Parallel
What are the 7 sections of the control circuit if a reversing motor starter?
General stop Forward stop Reverse stop Forward start Reverse start Forward holding Reverse holding
Where is the reverse stop circuit in a reverse motor starter located?
Between the general stop circuit and the reverse start + reverse holding circuits.
What are push buttons with two sets of contacts used for in reverse motor control applications?
To change motor direction without having to depress the stop button (pushbutton interlocking)
What is an application where you would NOT want use interlocked pushbuttons to reverse the starter control circuit?
Flywheel or other high inertial loads. They do not like sudden application of reverse torque.
How is reversing a three phase motor accomplished (disregard control circuit)?
Changing any 2 leads
How many overload relays are required on a three phase reversing magnetic starter?
1
Why is mechanical interlocking required?
- prevent both contactors from pulling in at same time
- (thereby) prevent short-circuit
Why are electrical interlocks used?
Prevent coil burnout
where might the ability to jog a motor be applicable?
Cranes, hoists, printing processes, large lathes
Is it possible for the same button to be used for a start function and a jog function?
Yes
What is positive jogging?
When a motor can be jogged successfully regardless of time required.
What are the three most often used devices to open or close a circuit?
1) pushbutton contacts
2) selector switches
3) relay contacts
How is a selector switch wired to allow for jogging?
In series with the holding contacts
- With ‘run’ function allowing current to flow (closed switch)
- and ‘jog’ preventing current to flow (open switch) to the holding contacts.
How is a pushbutton wired to allow for jogging of a motor?
Interlocked NO/NC pushbutton.
- N/Oin parallel with both “start” and holding circuit
- N/C in series with holding contacts
(When N/O pushed to close, N/C pushbutton is opened thereby breaking current flow to holding contacts)
What is it called when a connection method prevents the motor from accidently going into run if the button is released very quickly
Positive jogging
When using a control relay to allow jogging in a motor circuit, how is the jog pushbutton wired?
Parallel with:
- start button
- holding circuit
CR contacts isolate jog pushbutton from:
- control relay itself
- holding contacts
What methods of jogging control provide positive jogging? Which do not?
Positive: relay controlled and selector switch controlled
Not positive: pushbutton controlled
What is the starting current of a 3 phase induction motor compared to it’s full load current?
2-8 times higher
When would you need to de-rate a magnetic motor starter?
When it is required for a jogging application
How might the required size of a NEMA rated starter change between normal duty and jogging duty?
Usually one size larger for jogging duty
Define the following use categories for IEC style starters:
1) AC-1
2) AC-4
3) AC-3
4) AC-4
1) high pf loads like heating & general distribution
2) starting, plugging, and inching SLIP RING MOTORS
3) starting/stopping SQUIRREL CAGE MOTORS that are stopped under normal current conditions.
4) Plugging/jogging SQUIRREL CAGE MOTORS.
Jogging vs. Inching?
Jogging = start/stop @ full rated voltage Inching = start @ reduced voltage to let machine creep to desired location.
Why must selected starter rating be greater when a motor is to be jogged?
High in rush current
Which part of the circuit must be affected to achieve jog?
Holding circuit
What do pilot lights indicate?
Power on to a motor controller
Contactor energized
Overload trip of the overload relay
How are pilot lights wired with respect to an auxiliary contact of a contactor or relay?
Series
How are pilot lights wired with respect to an oerating coil for contactor or relay?
Parallel
What kind of contact stays in its changed position after the actuating force has been removed?
Maintained contact
What is an example of a maintained contact?
- standard on / off switches
- selector switches
What are some examples of manually-operated pilot control devices?
Switches
Push buttons
True or false? The majority of automatic pilot devices are momentary contact devices
True. They return to their normal position once operating condition is removed.
What kind of devices detect a change in some element of the system and convert this change into opening or closing contacts?
Automatic pilot devices
Pilot lights are connected in parallel with _____________.
Operating coils
Pilot lights are connected in series with _____________.
Contacts
Are automatic pilot devices momentary or maintained contact devices?
Momentary
Name four types of automatic pilot devices
A pressure
B limit
C float
D temperature
A selector switch used in a control circuit can enable or disable more than two parts of the circuit at the same time. True or false?
True
