DELMAR’S Standard Textbook of Electricity - 5 th Edition
1) Single-Phase Transformers a) Magnetically operated machine b) Change values of voltage, current , and impedance without changing the frequency i) All values are proportional to the turns ratio c) One of the most efficient machines known i) 90 to 99 percent efficient d) Three categories i) Isolation transformer ii) Autotransformer iii) Current transformer e) Transformer formulas i) Common variables
(1) N
P
= number of turns in the primary
(2) N
S
= number of turns in the secondary
(3) E
P
= voltage of the primary
(4) E
S
= voltage of the secondary
(5) I
P
= current in the primary
(6) I
S
= current in the secondary ii) E
P
/E
S
= N
P
/N
S iii) E
P
/E
S
= I
S
/I
P iv) N
P
/N
S
= I
S
/I
P v) E
P
X N
S
= E
S
X N
P vi) E
P
X I
P
= E
S
X I
S vii) N
P
X I
P
= N
S
X I
S f) Primary vs. secondary windings i) Primary is the winding connected to the incoming power supply (input side) a) Secondary is the winding connected to the load (output side)
Notes

2) Isolation Transformers a) Primary winding is physically and electrically isolated from the secondary winding i) “Magnetically” not “electrically” coupled b) Greatly reduces voltage spikes originating on the supply side of the transformer c) Improved user safety i) No connection between the load side and utility ground ii) “Line” isolation d) When NP/NS = 1, safety or line isolation is the only purpose for the transformer e) Laminated metal core is used to improve the magnetic coupling between primary and secondary windings f) Basic operating principles i) Self-induction
(1) Continually expanding and contracting magnetic fields produced by an AC current flowing in the primary induces an opposing voltage in the primary winding “self-induction”
(2) Limits the current flow of the primary ii) Excitation current
(1) Amount of current necessary to magnetize the core of the transformer
(2) Remains constant from no load to full load
(3) Usually ignored when making transformer calculations iii) Mutual induction
(1) Because the core is common to both the primary and secondary windings, a voltage and current is also induced in the secondary
(2) If N
P
= 240 and E
P
= 120 VAC then 120/240 gives a

volts-per-turn ratio of .5 or .5 volt per turn
(3) For that same transformer, if N
S
= 100 then the secondary voltage (E
S
) will be 100 X .5 or 50 V g) Transformer Calculations i) Step-down
(1) Given that N
P
= 240, N
S
= 60 and E
P
= 120 V, find E
S
(2) If the load = 5 ohms find I
S
and I
P
(3) Input VA must = output VA ii) Step-up
(1) Given that N
P
= 240, N
S
= 1200 and E
P
= 120 V, find
E
S
(2) If the load = 2400 ohms, find I
S
and I
P
(3) Input VA must = output VA h) Calculating isolation transformer values using the turns ratio i) Multiple-tapped windings and calculations i) Tapped primary – see example (page 757) ii) Tapped secondary – see example (page 757) j) Distribution transformers i) Used to step-down the high voltage utility company distribution lines to 240/120 VAC k) Control transformers i) Step-down 480/240 VAC to a lower voltage used in control circuitry (120 VAC or 24VAC typical) ii) Commonly contain 2 primary windings each rated at
240 VAC and 1 secondary winding rated at 120 VAC l) Transformer core types i) Core type
(1) Windings around each end of core material ii) Shell type
(1) Core has a central metal core piece around which the windings are placed – HV over LV iii) H-type core
(1) Surrounds the windings on four sides

(2) Improves efficiency
(3) Reduces flux leakage iv) Tape-wound/toroidal core
Flux leakage is minimized
(1) Most efficient core design available
3) Autotransformers a) One-winding transformer i) Step-up or step-down b) No line isolation
4) Transformer Polarities a) Describes the relationship between the different windings at the same point in time i) Usually when the maximum positive voltage is produced across the winding ii) Indicated on a schematic by a dot on one end of the winding(s) iii) Additive and subtractive polarities
5) Voltage & Current Relationships in a Transformer a) E
P
leads I
P
by 90 degrees b) E
S
lags I
P
by 90 degrees c) Therefore: E
S
is 180 degrees out of phase with E
P
6) Testing the Transformer a) Ohmmeter tests for opens and short circuits between primary and secondary and ground(case)

7) Transformer Nameplates a) NEC 450.11 requires the following information i) Name of manufacturer ii) Rated kilovolt-ampere kVA iii) Frequency iv) Primary and secondary voltage v) Impedance of transformers rated 25 kVA and larger vi) Required clearances of transformers with ventilated openings vii) Amount and kind of insulating liquid used viii) Temperature class for the insulating system of drytype transformers
8) Determining Maximum Current a) Secondary current i) I
S
= kVA/E
S b) Primary current i) I
P
= kVA/E
P
9) Transformer Impedance a) Factors controlling i) Size and type of core material ii) Winding wire size iii) Number of turns iv) Degree of magnetic coupling between windings
10) Constant current transformers a) Supply a constant current under varying load conditions i) Feature a stationary and movable coil ii) Coil spacing is inversely proportional to load current
(1) If load current increases, coils move farther apart

and output voltage goes down regulating current and vice versa
11) Series and parallel connection of transformer windings a) Primary b) Secondary