Transformers Flashcards
Primary winding
Winding connected to power source
Secondary winding
Winding connected to load
Core type transformer
Winding wrapped around two sides of the core
Shell type transformer
Winding wrapped around the center leg
Why are the primary and secondary windings wound on top of each other?
Much less leakage flux
Easier to insulate high voltage winding from core
The ____ voltage winding is on top of the ____ voltage winding
Low
High
What is the purpose of a unit transformer?
To step up the voltage at generation
What is the purpose of substation transformer?
To step down voltage from transmission to distribution
What is the purpose of distribution transformer?
Steps down distribution voltage to final voltage for usage
What is the purpose of potential/voltage transformer?
Change high voltage to low voltage, but can handle very small current
(used for instrumentation)
What is the purpose of current transformer?
Change the primary current to much smaller secondary current
(used for instrumentation)
Emf per turn for an ideal transformer is the same (T/F)
True
Mmf on each side for an ideal transformer is the same (T/F)
True
What is magnetization current iM of transformer?
Current required to produce flux in core of transformer
What is the core losses current (i h+e)of a transformer?
Current for hystersis and Eddy current losses
Magnetization current ____ (leads/lags) applied voltage
Lags by 90 degrees
Eddy currents are proportional to
Rate of change of flux
What is leakage flux of a real transformer?
Flux which escapes the core and passes only through one transformer winding
What is leakage flux modelled by
Primary and secondary inductances
What is magnetization flux of real transformer?
Flux which links with both windings
What assumptions do we make for idela transformer
No hystersis or eddy current losses in core
Ideal magnetization curve
Zero leakage flux (complete coupling)
Zero resistance of transformer winding
What are the losses in a transformer?
Copper, leakage, Eddy current, hystersis
Copper losses are proportional to
Square of current in the winding
Leakage flux is modelled by
Self inductance Ls and Lp
Eddy current losses are proportional to
Square of voltage applied to transformer
Hysteresis losses are a function of
Nonlinear function of applied voltage
What does Rc model
Eddy current and hysteresis losses (proportional to voltage and in phase)
What does XM model
Magnetization current proportional to voltage and lagging by 90 degrees
Why do we place XM and Rc inside after Lp and Rp
Voltage applied at core is input voltage minus internal voltage drop of winding
Why do we move XM and Rc forward in the approximate circuit?
The excitation branch current is very low and the the voltage drop is negligible
Why does output voltage vary with load even if input voltage is constant
Because of series impedance
Voltage regulation of lagging load is
Large positive
Voltage regulation of unity load is
Small positive
Voltage regulation of leading load is
Negative
Autotransformer is used to convert
Voltages of small ratio
Common winding of autotransformer is
Where both primary and secondary voltage appear
Series winding of transformer is
Where only one (primary or secondary) voltage appears and is in series with common
Why can autotransformer handle more power
Because not all power from primary to secondary goes through winding
Why can’t we reconnect an ordinary transformer as autotransformer
Because insulation of low voltage may not be able to withstand full output voltage of autotransformer
Series and common winding insulation of autotransformer is ____ and decided by
Same
Higher voltage (SE + C)
Autotransformers are used as
Low voltage tap variable transformer
What is the primary disadvantage of autotransformer
Electrical isolation of two windings is lost
How many taos are typically present
4 with spacing +- 2.5 % of full load
Transformer with voltage sensor that automatically adjusts is called
Tap changing under load (TCUL) transformer
