Transformer Operation (single Phase)

Question 1

Describe transformer operation at No-Load

Answer 1

• Supply V on Pri coil causes Exciting current (Ipri), which causes Exciting Flux in core.
• V induced on BOTH coils by Exciting current
• Induced V on pri (Epri) OPPOSES supply V (Vpri)

(Open circuit test)

Question 2

List the 3 different flux that is induced on the Tx core during a short circuit test (Tx operation with load)

Answer 2

1st: Exciting Flux
2nd: Sec Current Flux
3rd: Pri Current flux

Question 3

What is exciting current?

Answer 3

A small amount of current on the primary winding

Question 4

When do we see secondary current flux?

Answer 4

When the Tx is connected to load, the exciting current flux induces V on Sec winding and produces secondary current flux

Question 5

Describe Primary Current Flux

Answer 5

Resultant flux (the difference between exciting current flux and secondary current flux) produces NEW primary induced V. 
Ipri increase and produces Primary Current Flux

Question 6

How do we calculate

%V regulation

Answer 6

%Vreg=

(Vsec NL - Vsec FL) / (Vsec FL) •100

Question 7

Eddy current losses and Hysteresis losses are

a) Copper losses
b) Iron Losses
c) Core losses

Answer 7

c) core losses

Question 8

What test is performed to determine core losses?

Answer 8

An open circuit test

Question 9

Copper losses are measured in

a) Watts
b) Ohms
c) VA

Answer 9

a) Watts

I squared R losses

Question 10

Copper losses are proportional to the square of the currents and vary from no-load to full load conditions

True or False

Answer 10

True

Question 11

How do we calculate Tx efficiency?

Answer 11

%Eff=

Pout / Pin •100

Question 12

How do we calculate Power Input?

Answer 12

Pin =

Pout + copper loss + core loss

Question 13

How do we calculate copper losses?

Answer 13

P = I squared R

Question 14

Which 3 factors affect Tx efficiency?

Answer 14

Load capacity
- less load = less efficiency

Power Factor
- increased pf = increased efficiency

Losses
- more losses = less efficiency

Question 15

The __________ voltage is the Vpri required to circulate the rated full load Isec in a Tx when the secondary winding is short circuited.

a) Supply
b) Induced
c) Impedance
d) AC

Answer 15

c) Impedance Voltage

Question 16

Which test determines Impedance Voltage?

Answer 16

Short circuit test

Question 17

The percent Impedance voltage (%Z or %IZ) is generally between _____% and _____%

Answer 17

The percent Impedance voltage is generally between 2% and 10%

Question 18

What is the formula to calculate %IZ

Answer 18

%IZ=

Impedance V /
Primary Rated Voltage

Answer•100

Question 19

How to calculate Available Short Circuit Current

Answer 19

Short-cct I =

FL Isec / %IZ

(Full Load Secondary Current)

Question 20

When Tx’s are connected in parallel, the Impedance (increases / decreases) and the available short-cct current can go (up / down) considerably.

Answer 20

When Tx’s are connected in parallel, the Impedance DECREASES and the available short-cct current can go UP considerably.

Question 21

When connecting Tx’s in parallel, which 4 requirements MUST be met?

Answer 21

1) NL Vsec must be equal
2) same tap settings must be used on both Tx’s
3) the %IZ must be within 5% of each other
4) windings must be connected so that the output voltages are in phase with each other

Question 22

Why should to transformers connected in parallel have the same rated percent impedance?

Answer 22

If they don’t have the same %IZ, they do not share the load proportionately. This creates excess heat.