Controls and Switching Circuits Flashcards
A reversing magnetic motor starter for a three phase motor requires two contactors.
True or False?
True
How is reversing a three-phase motor accomplished with a forward/reverse starter?
The motor can be reversed by interchanging any two supply lines.
Why is mechanical interlocking required?
Mechanical interlocking is required to prevent both contactors from pulling in at the same time and causing a short.
Why are electrical interlocks used?
Electrical interlocks are used to prevent coil burnout.
How are electrical interlocking contacts connected in the circuit?
On NC contact of each of the contactors is connected in series with the coil of the other contactor
What is the purpose of the holding contacts?
The holding contacts keep the coil energized after the start button is released.
How many holding contacts does a reversing magnetic starter employ?
Two holding contacts
Identify seven (7) sections in the control circuit of a reversing magnetic starter.
a) general stop
b) forward stop
c) forward start
d) forward holding
e) reverse stop
f) reverse start
g) reverse holding
How many overload relays are required on a three-phase reversing magnetic starter?
One relay that is responsive to all three motor conductor currents is required.
Jogging allows a motor to be stopped and started repeatedly with one pushbutton without the motor starter holding in the run position.
True or False?
True
Jogging and inching are identical terms.
True or False?
False;
If the motor is to be repeatedly started and stopped at its full rated voltage, it is called jogging.
Jogging uses reduced voltage to allow a machine to creep to a desired position.
True or False?
False;
If the motor is started at a reduced voltage to let the machine creep to the desired location, the function is called inching.
What part of the circuit must be affected to achieve jogging?
The holding circuit
List three (3) ways to achieve jogging.
a) NO/NC pushbutton with the NC contact connected in the holding circuit.
b) maintained manual pilot device (selector switch) in the holding circuit
c) a control relay with contacts in the holding circuit
Which method of jogging could allow the magnetic coil to hold in if the pushbutton is operated very quickly?
Jogging with a NO/NC pushbutton
It is possible for the same button to be used for a start function and a jog function?
True or False?
True
Why must the selected starter rating be greater when a motor is to be jogged?
Repeated starting and stopping subjects the starters power circuit to a high inrush current that is many times the full-load current of the motor and overheats the contacts.
All selector switches have only three terminals.
True or False?
False;
They may have several terminals, depending on the number of points that can be selected.
All selector switches have only two positions.
True or False?
False;
They may have several positions such as off, on and auto selector.
Selector switches are typically (momentary/maintained).
Maintained
A selector switch used in a control can enable or disable more than two parts of a circuit at the same time.
True or False?
True
Pilot lights are connected in parallel with ___________ or in series with ____________.
Pilot lights are connected in parallel with operating coils or in series with contacts.
Automatic pilot device contacts are (momentary/maintained) contact devices.
Momentary
Name four (4) types of automatic pilot devices.
a) pressure switch
b) limit switch
c) float switch
d) temperature switch
e) proximity switch
f) motion sensor
g) other devices responding to specific conditios
Automatic pilot devices are shown on a drawing in a condition with the systems they are monitoring full and energized.
True or False?
False;
Automatic pilot devices are shown on a drawing in a condition with the systems they are monitoring empty and de-energized.
Why is a control transformer used?
Electrically safety by stepping down the voltage
Isolating the control circuit from the power circuit
Name three (3) styles of control transformers
- outdoor enclosed
- indoor open
- indoor enclosed
The primary of a control transformer has dual windings with a rating of 240/480V. The supply to the transformer is 120V. What is the configuration of the voltage links?
a) series
b) parallel
b) parallel
What type of fuse is recommended to protect a control transformer?
a) fast blow
b) time delay
b) time delay
Name three (3) basic types of timing devices used in electrical circuits.
- spring would interval timers
- mechanical and electronic time switches
- timing relays
Describe the three methods of controlling the timing of a timing relay.
- pneumatic
- fluid dash pot
- electronic
What does TDOE mean?
Time Delay On Energization
What does TDOD mean?
Time Delay On DeEnergization
When a TDOE timing relay is first energized, what would be the condition if:
a NO timed contact
(open/closed)
Open
When a TDOE timing relay is first energized, what would be the condition if:
a NC timed contact
(open/closed)
Closed
When a TDOE timing relay is first energized, what would be the condition if:
a NC, IC contact
(open/closed)
Open
When a TDOD timing relay is first energized, what would be the condition if:
a NO timed contact
(open/closed)
Closed
When a TDOD timing relay is first energized, what would be the condition if:
a NC timed contact
(open/closed)
Open
When a TDOD timing relay is first energized, what would be the condition if:
a NC, IC contact
(open/closed)
Open
When a TDOE timing relay has timed out, what would be the condition if:
a NO timed contact
(open/closed)
Closed
When a TDOE timing relay has timed out, what would be the condition if:
a NC timed contact
(open/closed)
Open
When a TDOE timing relay has timed out, what would be the condition if:
a NO, IC contact
(open/closed)
Closed
When a TDOD timing relay has just been deenergized, what would be the condition if:
a NC, IC contact
(open/closed)
Open
When a TDOD timing relay has just been deenergized, what would be the condition if:
a NC timed contact
(open/closed)
Open
When a TDOD timing relay has just been deenergized, what would be the condition if:
a NO timed contact
(open/closed)
Closed
Name three (3) additional functions that may be provided by a multifunction timing relay:
- interval timing
- one-shot timing
- repeat cycle timing
When a multifunction timing relay that is set for interval timing has timed out, what would be the condition of:
a NO timed contact
(open/closed)
Open
When a multifunction timing relay that is set for interval timing has timed out, what would be the condition of:
a NC timed contact
(open/closed)
Closed
When a multifunction timing relay is set for one-shot timing, how is the timing period initiated?
a signal at the start terminal
Briefly describe a repeat cycle timing function
Repeat cycle timing has a timed repeating changed and normal period for the timed contacts. The cycles repeat until the relay is deenergized.
Why is a timing relay used in a wye/delta starter?
To allow the motor to come up to speed before it automatically changes over to delta.
The mechanical friction braking solenoid is connected:
a) in parallel with the motor starter contactor coil
b) in series with the motor starter contactor coil
c) a) in parallel with the motor leads
d) in series with the motor leads
c) a) in parallel with the motor leads
What controls the amount of braking force applied to an ac motor that is using dynamic braking to stop the motor?
The amount of current supplies to the stator field
A dynamic electric brake for ac motors stops the motors by
Applying a dc voltage to the stator windings.
Why are motor breaks used on large ventilation fans?
To prevent the fan from rotating backward when the motor is deenergized.
What is an anti plugging circuit used for?
An anti plugging circuit is used to protect a motor against being reversed before the motor has slowed to a specific rotational frequency (usually near zero r/min).
Is an anti plugging switch a normally open or normally closed switch?
Normally closed
A three-phase motor is using a plugging switch for breaking. While the motor is operating, what position are the normally open contacts of the plugging switch in?
Closed
