# Transformers theory NJATC 2008

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1. A material that is easily magnetized is what?
ferromagnetic
2. Permanent magnets last forever? true or false? Transformer Principles and Applications WB2
false.
3. What rule is used to determine the direction of magnetic flux around a conductor? The thumb identifies what? Transformer Principles and Applications WB2
Left hand rule and the thumb identifies north
4. List the magnetic units of measure that are anologous to voltage, current, and resistance. Transformer Principles and Applications WB3
• voltage: magneto motive force (mmf)
• Current: magnetic flux (theta)
• resistance: reluctance (R)
5. What term defines the measure of the ability of a material to conduct magnetic flux (current)? Transformer Principles and Applications WB3
Permeability
6. Is copper a ferromagnetic material, or a material that is easily magnetized? Transformer Principles and Applications WB 3

What materials are ferromagnetic?
No

Iron, nickle, and cobalt
7. Explain magnetic saturation. Transformer Principles and Applications Transformer Principles and Applications WB PP 3
All magnetic domains in the core allign with the magnetic field. PP8
8. When a magnetic material reaches saturation are further increases in magnetic flux density possible? Transformer Principles and Applications WB4
No
9. The graph showing a normal magnetization curve is called what? Transformer Principles and Applications WB4

What do the letters represented in this graph represent?

The knee illustrates what in the graph?
A B/H curve.

• B-flux density
• h-field density

That the field strength no longer has much effect on the magetic flux density
10. The turns ratio represents what? Transformer Principles and Applications WB 5
The ratio of voltage in the primary to the voltage in the secondary
11. Transformer loss is due to what? Transformer Principles and Applications WB5
Wire resistance, hysteresis, flux loss, and eddy currents
12. In a transformer the high and low voltage leads are marked with what? Transformer Principles and Applications PP13

The secondary nuetral is marked with what?
• high voltage- H
• Low voltage- X

Secondary nuetral X0 (zero)
13. A transformer is an electric device that uses electromagentism to change voltage from one level to another or to isolate one voltage from another through the process of ______________? Transformer Principles and Applications PP13
mutual induction
14. The property of a device or circuit that causes it to store energy in an electromagnetic field defines what? Transformer Principles and Applications PP15
inductance
15. reactance to oppose a changing current, and abiltity to generate a current are also known as what? Transformer Principles and Applications PP 15
• self induction
• mutual induction
16. The total power is the same, less, or more, in the primary and the secondary of a transformer? Transformer Principles and Applications PP15

This means what in regards to the voltage across primary and secondary?
same, except for some losses in the transformer

That when the voltage is higher in the secondary, the current is proportionally lower in the secondary.
17. Lenz's law states what? Transformer Principles and Applications PP15
That induced voltage opposes current changes
18. The counter voltage limits the circuit current, so the circuit current is determined by the....? Transformer Principles and Applications PP15

Represented by what formula?
the impedance and the difference between the source and the counter voltage.

• It=Es-Ec/Z
19. The first 900 of a cylce is spent doing what? Transformer Principles and Applications PP15
charging the inductor
20. When the magnetic field starts to collapse back into the inductor it aids the current provided by the source. Meaning what? Transformer Principles and Applications PP15

The formula for inductive reactance is what?
• That the current lages 900 behind the source. (inductive: Voltage leads Current / ELI) current lags voltage
• XL=2pieFL
• XL in ohms and L is inductance in henrys
21. The total opposition or impedance to a circuit is calculated using what formula? AC Theory PP 61 Transformer Principles and Applications PP 15
Z= square root of (R2+XL2)
22. Does a coil of wire have more or less impedance than a straight piece of wire? Transformer Principles and Applications PP17

The four factors that determine or control the inductance of a coil are what? using the acronym CoNTroL. Transformer Principles and Applications PP 17
More

• cross secton of the core
• number of turns
• type of the core
• length of the coil
23. What is the no-load current through a primary core? Transformer Principles and Applications PP 17 also known as what? Code Calcs PP295

This has how many components and what are they?

Inrush current can be as much as how many times the primary rated current? (code calcs pp295)
exciting current, or magnetizing current; also known as inrush current

2: true power no-load current (in KW) and reactive power that builds the field (in KVAR)

As much as 8-10 times the rated primary current
24. The single phase transformer primary and secondary current ratings can be taken form tables or can be calculated using transformer power equations. The equations for single and 3-phase are? Code calcs pp 296
• Single phase
• I = (KVA x 1000)/E

• 3 phase
• I = (KVA x 1000)/ E x 1.73
25. What rating is always the same on the primary and secondary side of a transformer? Transformer Principles and Applications PP18
The power rating
26. What rating of the coil must never be exceeded and why? Transformer Principles and Applications PP18
The voltage rating, because although the turns may be correct, the inductance of the coils is too low to provide required current limits
27. Define a coil tap. Transfomer Principles and Applications PP19
an extra electrical connection on a transformer that allows a varying number of turns of a coil to be part of the circuit. Usually 2.5 percent per tap
28. taps are used in the field for the turns ratio of a transformer to be modified to compensate for what? Transformer Principles and Applications PP19
low primary voltage. Usually 2.5 percent per tap
29. What causes the electrons to travel more toward the outside of the conductor? Transformer Principles and Applications PP 20
higher frequencies. This effect is known as "skin effect"
30. Eddy currents mean that at higher frequencies than 60HZ, heat produced by eddy currents increases as the ________. Transformer Principles and Applications PP20
square of the frequency. For example: the third harmonic (180Hz) has nine (32) times the heating effect of the fundamental 60Hz freq.
31. Hum as a fundamental frequency of what? Transformer Principles and Applications PP 34
Twice the applied frequency.
 Author: johnbowens ID: 56873 Card Set: Transformers theory NJATC 2008 Updated: 2011-12-18 23:55:49 Tags: transformers principles applications Folders: Description: transformer theory Show Answers: