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4th Year Electrical - Controls, Switching and PLC's > Controls and Switching Circuits > Flashcards

Flashcards in Controls and Switching Circuits Deck (58)
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1

A reversing magnetic motor starter for a three phase motor requires two contactors.

True or False?

True

2

How is reversing a three-phase motor accomplished with a forward/reverse starter?

The motor can be reversed by interchanging any two supply lines.

3

Why is mechanical interlocking required?

Mechanical interlocking is required to prevent both contactors from pulling in at the same time and causing a short.

4

Why are electrical interlocks used?

Electrical interlocks are used to prevent coil burnout.

5

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

6

What is the purpose of the holding contacts?

The holding contacts keep the coil energized after the start button is released.

7

How many holding contacts does a reversing magnetic starter employ?

Two holding contacts

8

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

9

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.

10

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

11

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.

12

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.

13

What part of the circuit must be affected to achieve jogging?

The holding circuit

14

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

15

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

16

It is possible for the same button to be used for a start function and a jog function?

True or False?

True

17

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.

18

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.

19

All selector switches have only two positions.

True or False?

False;

They may have several positions such as off, on and auto selector.

20

Selector switches are typically (momentary/maintained).

Maintained

21

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

22

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.

23

Automatic pilot device contacts are (momentary/maintained) contact devices.

Momentary

24

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

25

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.

26

Why is a control transformer used?

Electrically safety by stepping down the voltage

Isolating the control circuit from the power circuit

27

Name three (3) styles of control transformers

- outdoor enclosed
- indoor open
- indoor enclosed

28

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

29

What type of fuse is recommended to protect a control transformer?

a) fast blow
b) time delay

b) time delay

30

Name three (3) basic types of timing devices used in electrical circuits.

- spring would interval timers
- mechanical and electronic time switches
- timing relays