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ELE 351 > Transformers > Flashcards

Transformers Flashcards

1
Q

Primary winding

A

Winding connected to power source

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2
Q

Secondary winding

A

Winding connected to load

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3
Q

Core type transformer

A

Winding wrapped around two sides of the core

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4
Q

Shell type transformer

A

Winding wrapped around the center leg

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5
Q

Why are the primary and secondary windings wound on top of each other?

A

Much less leakage flux
Easier to insulate high voltage winding from core

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6
Q

The ____ voltage winding is on top of the ____ voltage winding

A

Low
High

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7
Q

What is the purpose of a unit transformer?

A

To step up the voltage at generation

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8
Q

What is the purpose of substation transformer?

A

To step down voltage from transmission to distribution

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9
Q

What is the purpose of distribution transformer?

A

Steps down distribution voltage to final voltage for usage

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10
Q

What is the purpose of potential/voltage transformer?

A

Change high voltage to low voltage, but can handle very small current
(used for instrumentation)

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11
Q

What is the purpose of current transformer?

A

Change the primary current to much smaller secondary current
(used for instrumentation)

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12
Q

Emf per turn for an ideal transformer is the same (T/F)

A

True

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13
Q

Mmf on each side for an ideal transformer is the same (T/F)

A

True

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14
Q

What is magnetization current iM of transformer?

A

Current required to produce flux in core of transformer

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15
Q

What is the core losses current (i h+e)of a transformer?

A

Current for hystersis and Eddy current losses

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16
Q

Magnetization current ____ (leads/lags) applied voltage

A

Lags by 90 degrees

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17
Q

Eddy currents are proportional to

A

Rate of change of flux

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18
Q

What is leakage flux of a real transformer?

A

Flux which escapes the core and passes only through one transformer winding

19
Q

What is leakage flux modelled by

A

Primary and secondary inductances

20
Q

What is magnetization flux of real transformer?

A

Flux which links with both windings

21
Q

What assumptions do we make for idela transformer

A

No hystersis or eddy current losses in core
Ideal magnetization curve
Zero leakage flux (complete coupling)
Zero resistance of transformer winding

22
Q

What are the losses in a transformer?

A

Copper, leakage, Eddy current, hystersis

23
Q

Copper losses are proportional to

A

Square of current in the winding

24
Q

Leakage flux is modelled by

A

Self inductance Ls and Lp

25
Q

Eddy current losses are proportional to

A

Square of voltage applied to transformer

26
Q

Hysteresis losses are a function of

A

Nonlinear function of applied voltage

27
Q

What does Rc model

A

Eddy current and hysteresis losses (proportional to voltage and in phase)

28
Q

What does XM model

A

Magnetization current proportional to voltage and lagging by 90 degrees

29
Q

Why do we place XM and Rc inside after Lp and Rp

A

Voltage applied at core is input voltage minus internal voltage drop of winding

30
Q

Why do we move XM and Rc forward in the approximate circuit?

A

The excitation branch current is very low and the the voltage drop is negligible

31
Q

Why does output voltage vary with load even if input voltage is constant

A

Because of series impedance

32
Q

Voltage regulation of lagging load is

A

Large positive

33
Q

Voltage regulation of unity load is

A

Small positive

34
Q

Voltage regulation of leading load is

A

Negative

35
Q

Autotransformer is used to convert

A

Voltages of small ratio

36
Q

Common winding of autotransformer is

A

Where both primary and secondary voltage appear

37
Q

Series winding of transformer is

A

Where only one (primary or secondary) voltage appears and is in series with common

38
Q

Why can autotransformer handle more power

A

Because not all power from primary to secondary goes through winding

39
Q

Why can’t we reconnect an ordinary transformer as autotransformer

A

Because insulation of low voltage may not be able to withstand full output voltage of autotransformer

40
Q

Series and common winding insulation of autotransformer is ____ and decided by

A

Same
Higher voltage (SE + C)

41
Q

Autotransformers are used as

A

Low voltage tap variable transformer

42
Q

What is the primary disadvantage of autotransformer

A

Electrical isolation of two windings is lost

43
Q

How many taos are typically present

A

4 with spacing +- 2.5 % of full load

44
Q

Transformer with voltage sensor that automatically adjusts is called

A

Tap changing under load (TCUL) transformer

ELE 351 (3 decks)

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