AC Control Equipment

Question 1

Describe a Nema 7 electrical enclosure.0

Answer 1

It is a hazardous location class/type of enclosure Used where flammable gases or vapors may be present.

Question 2

Describe the clapper mechanism in a magnetic motor starter.

Answer 2

A clapper type set of contacts has the armature hinged. As it pivots to the sealed in position the contacts on the armature close against the stationary contacts.

Question 3

What is a shading coil and what would happens to an armature without one?

Answer 3

A shading coil is a single turn of copper or aluminum mounted on the face of the magnetic core. The main magnetic field induces a current in the shading coil which is out of phase with he main.
Without the shading coil, the armature would vibrate, causing noise, generate heat, and the magnetic faces would wear rapidly.

Question 4

What voltage is the minimum potential difference that will cause the armature to seat against the pole faces of the core.

Answer 4

The seal in voltage.

Question 5

Which armature mechanism transforms the vertical action of the arm. To a horizontal motion of the contacts?

Answer 5

A bell crank mechanism

Question 6

What will happen if the potential difference across the coil is too great?

Answer 6

It will…..
Become very hot
Draw too much current
Produce too much magnetic force.

Question 7

What do the terms standard duty and heavy duty refer to in regards to pushbutton contacts?

Answer 7

It describes the amount of current the contacts can carry.

Question 8

NEMA type 1b flush mounted stations not having pilot lights should be mounted in a……

Answer 8

4 11/16” single outlet box.

Not a gem box or handy wall boxes.

Question 9

Which pushbutton station should be used where dangerous dusts and gases may be present?

Answer 9

NEMA type 7 and 9 stations for hazardous locations.

Question 10

The joy stick operator is designed only for …..

Answer 10

Use with an ac power source and one hole mounting.

Question 11

Which element of a precision snapswitch transmits the external operating force to the spring mechanism?

Answer 11

The actuator.

Question 12

Which factors contribute towards the temperature developed inside the switch?

Answer 12

It depends on the continuous current, the interrupting current, and the in rush current.

Question 13

What kind of actuator should be used when space limitations doesn’t allow for a standard limit switch?

Answer 13

Remote cable

Question 14

The current rating of a switch is based on….

Answer 14

The temperature generated in the switch through normal operation

Question 15

What are signs of a mercury switch failure?

Answer 15

Glass chips floating on the mercury
The glass becomes crazed
The mercury will string out/break into pools.

NOT change color

Question 16

Wiring for a plug in limit switch stays intact while the plug in portion is replaced.

Answer 16

True

Question 17

When it is necessary to change a control circuit from one sequence to another, a switch is used.

Answer 17

Transfer

Question 18

Plugging switches can operate electro-mechanically or electro-magnetically

Answer 18

True.

Question 19

Pressure switches are divided into groups according to…..

Answer 19

The kind of liquid or gas they measure.

Question 20

There are two kinds of mechanical temperature control switches. They are

Answer 20

Gas actuated ( capillary tube or direct immersion)
Thermostat

Question 21

Where a liquid level must be controlled, which type of switch should be used?

Answer 21

A float switch.

Question 22

What sort of timer is normally used in applications that do not require frequent changes in time settings?

Answer 22

Time delay relay.

Question 23

Each timers set of contacts open or closes according to its……..

Answer 23

Cam settings. M

Question 24

How many cams are located on the shaft of a percentage timer?

Answer 24

2 cams

Question 25

Which device perfor.s a control function in a machine that cannot be controlled by time?

Answer 25

A revolution counter.

Question 26

Which relay requires a temperature compensating network?

Answer 26

A voltage sensing relay.

Question 27

The purpose of a three phase sequence sensing relay is to…….

Answer 27

Detect phase reversal

Question 28

What is the maximum voltage for a NEMA Class B industrial ac relay?

Answer 28

A maximum of 600 volts

Question 29

In regards to hazardous locations what is a class II hazardous material?

Answer 29

Combustible dust

Question 30

An example of a non-electric hazard is

Answer 30

Sparking from a dropped tool

Question 31

What is needed where conduit leaves a Divosion 1 hazardous area and enters a nonhazarsous area?
Also where the conduit enters an enclosure that houses arcing or high temp equip.

Answer 31

Seals

Question 32

Equipment for Class 1, Division 1 areas has all arching parts immersed in……..

Answer 32

6” of oil.

Question 33

MI cable is hygroscopic (moisture attracting) and must installed in a way to avoid buildup

Answer 33

True

Question 34

What are class II hazardous materials?

Answer 34

Combustible dusts

Question 35

Regarding synchronous motors, a signal from the proper rotor speed circuit (rotor rpm) sets the proper speed flop circuit (correlation between rotor pole faces and RMF of stator .

Answer 35

This setting can cleared by either a stop signal from the operator or by a signal from the proper speed flop circuit

Question 36

Regarding automatic sequence relays ( multi speed controller for motor). How many relays are needed?

Answer 36

1 for every speed the operator can select.

Question 37

When an automatic autotransformer is used in the start circuit, the unit is accelerated in 2 stages.

Answer 37

The start contactor applies reduced voltage through a three phase autotransformer. The unit will eventually achieve full speed after full voltage is applied.

Question 38

When are part winding starters used in motors?

Answer 38

Used with motors having delta connected windings. The circuit will only work if neither winding is open.

Question 39

The open circuit transition y- delta starter includes
Three three pole starters
Adjustable Pneumatic timer
Three pole overload protection

*closed circuit starters have an additional Three pole contactor and a bank of resistors.

Answer 39

True

Question 40

What is the largest size starter that be placed in an MCC with drawout construction ( removable from front) ?

Answer 40

A size 4 starter is the largest

Question 41

Describe a NEMA 12 MCC enclosure.

Answer 41

It is very similar to a NEMA 1 gasketed except it is gasketed all the way around including doors, cover plates, around operating mechanism and on the bottom plate.

Question 42

Current transformer are shipped with shorting bars on the secondaries. They can be used to short circuit the secondaries is work is to be done after they have been energized. Secondaries must never be left without a load, potential difference could reach KVs…

Answer 42

True

Question 43

In a corrosive environment, what type of buss bars should be used?

Answer 43

Tin-plated copper.

Question 44

Insulated terminals should be used when……

Answer 44

Severe vibration or flexing of the wire will occur.

Question 45

Most wire nuts are made for copper to copper conductor connections. Do aluminum conductors require special wire nuts.

Answer 45

Yes they require a specially coated steel spring for any aluminum connection.

Question 46

Some wire terminals or lugs will indicate which conductor can be used, if no marking is obvious then it is for…….

Answer 46

Copper.

Question 47

The disadvantages of laced or tightly bound wire looms are overcome with…..

Answer 47

The use of wire ducting