TRANSFRMRPCB

Question 1

Why is speed a desirable quality to have in opening circuit breakers?

Answer 1

• facilitates extinguishing an arc
• minimizes damage caused by a fault
• reduces effects of a fault on lines and equipment not directly involved with the fault

Question 2

What is an isolation transformer?

Answer 2

A transformer that is magnetically coupled, but electrically and mechanically isolated.

Question 3

What are harmonics? When do they occur?

Answer 3

Voltages or currents that occur at frequencies that are multiples of 60 Hz. The are produced by non linear loads that draw current in pulses.

Question 4

What must be known about a transformer to determine the different values of voltage and current?

Answer 4

Turn ratio

Question 5

What part of the transformer is connected to the load and what part is connected to the power supply?

Answer 5

The secondary winding is connected to the load and the primary winding is connected to the power supply

Question 6

True or False:

In a step down transformer, the power in does not equal the power out.

Answer 6

False

Question 7

What is a transformer?

Answer 7

A magnetically operated machine that can change voltage, current, and impedance without changing frequency

Question 8

How are transformers rated?

Answer 8

kVA

Question 9

Name the three basic contacts of a PCB?

Answer 9

Butt, wedge, and bayonet

Question 10

What is the meaning of electrically trip free regarding a PCB?

Answer 10

The PCB will not allow the closing coil to close under a fault condition.

Question 11

What is the most common gas used in circuit breakers to quench an arc?

Answer 11

Sulfur hexafluoride (SF6)

Question 12

What is continuous current?

Answer 12

The amount of current that a circuit breaker can carry continuously

Question 13

What is interrupting time?

Answer 13

The time it takes a circuit breaker to open after the tripping coil is energized

Question 14

How many transformers make an open delta connection?

Answer 14

Two

Question 15

What is the frequency of the second harmonic?

Answer 15

120 Hz

Question 16

What is a triplen harmonic?

Answer 16

Odd multiple of the third harmonic

Question 17

What is the primary function of a circuit breaker?

Answer 17

Protection

Question 18

A transformer has a primary voltage of 240 V, a secondary voltage of 180 V, and the windings are connected subtractively. What voltage will appear across the entire connection?

Answer 18

240 V - 180 V = 60 V

Question 19

What is another name for an arc?

Answer 19

Plasma

Question 20

Two transformers rated at 100 kVA each are connected in an open delta connection. What is the total output power supplied?

Answer 20

.866(100kVA + 100kVA) = 173.2 kVA

Or

1.732(100 kVA) = 173.2 kVA

Question 21

What is a knife switch?

Answer 21

The simplest form of a circuit breaker

Question 22

How many seconds does it take for a circuit breaker to open?

Answer 22

1/20 of a second (about 3 cycles)

Question 23

A transformer has a primary voltage of 240 V and a secondary voltage of 80 V. What is the turn ratio?

Answer 23

Np/Ns = 240V/80V

Np/Ns = 3/1

3:1

Question 24

How are circuit breakers rated?

Answer 24

• interrupting current capacity
• continuous current capacity
• voltage class

Question 25

What are the 4 mediums that extinguish the arc in circuit breakers?

Answer 25

Dry Air blast, oil, SF6 and Vacuum

Question 26

A transformer has a turn ratio of 1:6 and a primary current of 18 A. What is the secondary current?

Answer 26

``` Np/Ns = Is/Ip 1/6 = Is/18 Is= 3 A ```

Question 27

True or False: Phase voltage and line voltage are 90 degrees out of phase on a three phase system.

Answer 27

False They are 120 degrees out of phase

Question 28

What two things are important about a wye connected three phase system?

Answer 28

Line current equals phase current and Line voltage is higher than phase voltage. E(Line) = E(Phase) x 1.732

Question 29

What two things are important about a delta connected three phase system?

Answer 29

Line voltage equals phase voltage and line current is higher than phase current by x 1.732 I(Line) = I(Phase) x 1.732

Question 30

True or False: Voltage, current, frequency, and impedance are things that can be changed by a transformer.

Answer 30

False Frequency can’t be changed

Question 31

In a transformer with a power output of 750 VA and a primary voltage of 120 V, what is the primary current?

Answer 31

P = I x E 750 VA = I x 120V I = 6.25 A

Question 32

What is the formula to find the phase voltage if given line voltage in a three phase wye connected system?

Answer 32

E(phase) = E(line) / 1.732

Question 33

An open delta three phase transformer system has one transformer center tapped to provide a neutral for single phase voltages. If the voltage from the line to center tap is 277 V, what is the high leg? Show the math.

Answer 33

277 V x 1.732 = 479.76 V

Question 34

What kind of transformer uses one winding for both the primary and secondary windings?

Answer 34

An autotransformer

Question 35

What is mutual inductance?

Answer 35

When a coil induces voltage into another coil

Question 36

A transformer has a turn ratio of 2.5:1 and a primary voltage of 220 V. What is the secondary voltage?

Answer 36

``` Np/Ns = Ep/Es 2.5/1 = 220/Es Es(2.5) = 220(1) Es = 220/2.5 Es = 88 V ```

Question 37

A transformer has a primary voltage of 240 V, apparent power of 360 VA, and a secondary voltage of 48 V. What is the secondary current?

Answer 37

P = Is x Es 360 VA = Is x 48 V Is = 7.5 A

Question 38

All values of a transformer are proportional to what?

Answer 38

Turn ratio of the primary windings to secondary.

Question 39

What is self-induction?

Answer 39

When a coil induces voltage onto itself

Question 40

What is the formula to find primary current in a transformer?

Answer 40

Np/Ns = Is/Ip = Ep/Es

Question 41

Name three things that dielectric oil does in a transformer.

Answer 41

Insulates, cools, and moisture prevention

Question 42

What is the disadvantage of SF6 in circuit breakers?

Answer 42

It breaks down into corrosive materials

Question 43

What is the disadvantage of oil circuit breakers?

Answer 43

They can explode and leak oil creating a fire hazard

Question 44

What are three ways a breaker can be tripped free?

Answer 44

Electrically, mechanically, and pneumatically

Question 45

Why are heaters used with SF6 breakers?

Answer 45

It liquifies readily at low temps

Question 46

What 8 things are on a transformer nameplate?

Answer 46

1. Name of manufacturer 2. Rated kilovolt-ampere 3. Frequency 4. Primary and secondary voltage 5. Impedance of transformers rated 25kVA and larger 6. Required clearances of transformers with ventilated openings 7. Amount and kind of insulating liquid where used 8. The temperature class for the insulating system of dry-type transformers

Question 47

A _____________ transformer has the same input and output voltage and is primarily used for the purpose of voltage spikes.

Answer 47

Isolation

Question 48

A good rule for any transformer in regards to power is __________________ .

Answer 48

Power in must equal power out.

Question 49

True or false: An autotransformer can be either step up or step down.

Answer 49

True

Question 50

What are the 3 purposes of the dielectric oil in a transformer?

Answer 50

Cooling, Insulation between windings, and prevent formation of moisture.

Question 51

If a breaker opens in 10 cycles, what is it's speed? (Assume 60hz)

Answer 51

1/6 of a second. T (time in seconds) = # of cycles / 60 (frequency)

Question 52

What is the phenomenon that makes breakers successful?

Answer 52

Plasma Arc

Question 53

The amount of time it takes for a breaker to open is known as _____________ .

Answer 53

Interrupting Time (rated in cycles)