1φ Xfmr Theory #1

Question 0

What is the turns ratio of a transformer with a 120V primary winding and a 24V secondary winding carrying a 500va load?

Answer 0

120 / 24 = 5:1

Question 1

40 turns of a 300 turn winding has a voltage of 32V. What is the applied voltage of of the entire winding?

Answer 1

32 / 43 = .8 per turn.

300 x .8 =240V

Question 2

A single phase transformer has a 480V primary and a 240/120V secondary with a center tapped neutral. The turns ratio of this transformer is ______?

Answer 2

2:1
120V isn’t the secondary phase voltage, it’s 240V.
120 is half the coil.

Question 3

If the primary voltage is double the secondary voltage, the current in the primary will be _______ _______ the current in the secondary.

Answer 3

One-half

Question 5

The iron core of a transformer is generally made of laminations rather than a solid iron core and as a result _____ _____ _____ _____.

Answer 5

Eddy currents are reduced

Question 6

An auto transformer is used in preference to an isolation transformer __

Answer 6

(c)

Question 7

When a step-up transformer bis used, it increases the ______.

Answer 7

Voltage

Question 8

Compared to the secondary winding of a step-down transformer, the primary has ___.

Answer 8

C

Question 9

A transformer is based on the theory that energy may be transferred by induction form one set of coils to another by varying magnetic flux, provided both sets of coils ______.

Answer 9

Are on a common magnetic circuit.

Question 10

In a transformer, the induced voltage per turn in the secondary winding______.

Answer 10

Is equal to the induce emf per turn in the primary winding ______

Question 11

A single phase transformer has a primary voltage of 480 V and a secondary voltage of 240/120V. If the current in the secondary is 100 amps the current in the primary will be _______ amps.

Answer 11

50.
480/240 = 2 ratio.
100/2 = 50A

Question 13

The input of a transformer is 1500va and its efficiency is 96%. What is the output va.

Answer 13

E = Out/In ➡️ .96 = O/1500 ➡️ 1500 x .96= 1440va

Question 14

What would be the expected efficiency of a transformer?

Answer 14

90%

Question 15

A 6900/230V transformer is drawing 12A of current in the primary winding. The secondary load is drawing 350A. The approximate efficiency of this single-phase transformer would be _____?

Answer 15

97%.
E = out/in
230 x 350 = 80500; 6900 x 12 = 82800
80500/82800 = .97222

Question 16

What is the formula to find the ratio between true power (W) and apparent power (va)?

Answer 16

PF = W/va

Question 18

What is the power factor of a 7460W load connected to a 240V 1φ source that draws 50A.

Answer 18

62%
240 x 50 = 7460W
PF = W/va
PF = 7460/1200 = .6216666

Question 19

A 1φ transformer has a primary voltage of 120, & a secondary voltage of 24. The load is 240va.
What is the turns ratio?

Answer 19

120/24 = 5

5:1 ratio.

Question 20

A 1φ transformer has a primary voltage of 120, & a secondary voltage of 24. The load is 240va.
What is the primary current?

Answer 20

P = I x E watts the same throughout
240/120 = 2A

Question 21

What is the PF of a 3hp motor that draws 34A @ 115V on the input?

Answer 21

3 x 746 =2238W
34 x 115 = 3910va
PF = W/va
2238/3910 =.57237
  57%

Question 22

A 1φ transformer has a primary voltage of 120, & a secondary voltage of 24. The load is 240va.
What is the va input?

Answer 22

240va.

Va in = va out @ 100% efficiency.

Question 23

A 1φ transformer has a primary voltage of 120, & a secondary voltage of 24. The load is 240va.
What is the secondary current?

Answer 23

P = I x E
240/24 = 10
10A

Ratio is 5:1 so 5 x 2A = 10A

Question 32

The efficiency of a transformer is the ______ of the _______ to the _______.

Answer 32

Ratio, output, input
output
Eff = ———-
Input

Question 33

A transformer has the secondary output of 4800va. The secondary draws 5000 va. What is the transformer efficiency?

Answer 33

o 4800va
E = —- = ——— = .96 or 96%
i 5000va

Question 34

A 1φ transformer has an Ip of .5A, a Vs of 12V, and a Is of 5A. What is the Vp?

Answer 34

Vp Is Vp 5
—- = —- ➡️ —- = —- ➡️ .5Vp = 60 ➡️ Vp = .5/60 = 120V
Vs Ip 12 .5

Question 35

A1φ transformer has Vp of 240V, a Vs of 120V, and a Is of 20A. What is the Ip?

Answer 35

Vp Is 240 20
—- = —- ➡️ —— = —– ➡️ 240 Ip = 2400/240 = 10 A
Vs Ip 120 Ip

Question 36

A 1φ transformer has an Ip of 1A, a Vp of 120V, and an Ip of 5A.
What is the secondary V?

Answer 36

Vp Is 120 5
—- = —- ➡️ —– = —- ➡️ 5Vs = 120 ➡️ Vs = 120/5 = 24V
Vs Ip Vs 1

Question 37

A 1φ transformer has an Ip of 2A, a Vp of 120 V, and a Vs of 12V.
What is the Is?

Answer 37

Vp Is 120 Is
—- = —- ➡️ —– = —— ➡️ 12Is = 240 ➡️ = Is = 240/12 = 20A
Vs Ip 12 2

Question 38

When the secondary winding of a transformer has less turns than the primary, the secondary winding current is _______?

Answer 38

More than the primary.

Less turns = less V. Less V = more I.

Question 39

A transformer has a 3:2 ratio. Its primary winding is designed for 240V. The secondary voltage of this transformer is ______ volts.

Answer 39

160V.

240/3 = 80 x 2 = 160

Question 40

A 7200/240V transformer has 1800 turns in its primary winding. The number of turns in the secondary is ______?

Answer 40

60 turns.

7200/240 = 30 ➡️ 1800t/30 = 60.

Question 41

A transformer is marked 120/24V and has two black wires on one side and two yellow wires on the other side. How can you determine which wires are the 120 primary and which are the 24V secondary?

Answer 41

Measure the resistance with an ohmmeter, the set of wires with a higher resistance is the 120V primary.
The higher the R the lower the I. The lower the I the higher the V = primary.