Transformers & PCB's Flashcards
Made by connecting one end of each winding.
Wye connection
Line and phase ____ are the same in a wye connection.
Currents
In a wye-connected system, the line voltage is higher/lower than the phase voltage by 1.732.
Higher
Three-phase voltages are ____ degrees out of phase with each other.
120
Delta connections are made by ____ separate inductive loads being connected.
3
T/F In a delta connection, line and phase voltages are different.
F, they are the same.
T/F In a delta connected system, the line current is lower than the phase current by 1.732.
F, higher
If line values of voltage (E) and current (I) are known, apparent power is calculated by:
VA = 1.732 x E (line) x I (line)
If phase values of voltage (E) and current (I) are known, apparent power is calculated by:
VA = 3 x E (phase) x I (phase)
Three phase watts (P) is calculated by:
P = 1.732 x E (line) x I (line) x PF (PF = 1 unless otherwise stated)
P = 3 x E (phase) x I (phase) x PF (PF = 1 unless otherwise stated)
T/F Wye connections are characterized by the fact that 1 terminal of each of the devices is connected together.
T
What is a magnetically operated machine that can change values of voltage, current and impedance without a change of frequency?
Transformer
What are the 3 transformer classifications?
isolation transformer
autotransformer
current transformer
Are all values of a transformer proportional to its turns ratio?
Yes
Turns ratio example:
4:1 means 4 turns of primary winds to 1 secondary wind
N(P)
of turns in primary winding
N(S)
of turns in secondary winding
E(P)
Primary voltage
E(S)
Secondary voltage
I(P)
Primary current
I(S)
Secondary current
Transformer formulas
EP/ES = NP/NS EP/ES = IS/IP
Where is the power input winding connected to incoming power supply (generator side)?
Primary winding
Where is the load or output winding connected to driven load?
Secondary winding
T/F In an isolation transformer, the secondary winding is physically and electrically isolated from primary winding.
T
What is it called when a coil induced a voltage into itself?
Self-induction
The ____ current is the amount of current required to magnetize the core of the transformer.
Excitation
T/F Mutual induction is the ability of two coils to induce a voltage into another coil.
F, one
What is a step-down transformer?
A transformer that has a lower secondary voltage than primary.
What is a step-up transformer?
A transformer that has a secondary voltage than primary.
A good rule of thumb for any type of transformer is that:
power in must equal power out.
Ratio formula
Ratio = E(S1)/E(P)
What transformer changes high voltage of power company distribution lines to the common 240/120 volts used to supply power to most homes and many business?
Distribution transformer
What transformer reduces line voltage to the value needed to operate control circuits?
Control transformer
These one-winding transformers use same winding for both the primary and secondary.
Autotransformers
What is the output of an open delta?
86.6%
What provides electric insulation between the windings and the case, helps provide cooling and to prevent moisture formation which can deteriorate the winding insulation?
Dielectric oil
Why are windings 120 degrees apart?
So power does not go below zero
Only ____ values of voltage and _____ can be used when calculating transformer values.
phase; current
What is the simplest circuit breaker?
Ordinary knife switch
What is fundamental to the success of a circuit breaker?
Plasma arc
How do air-blast breakers extinguish arc?
By a cooled dry air blast
T/F Sulfur hexafluoride must be kept cool if the circuit breaker will be used outdoors
F, heated
What is a circuit breakers primary function?
Protection
What do you use to extinguish an arc?
Speed
What are the 3 circuit breaker ratings?
Voltage class
Continuous current
Interrupting capacity
(or interrupting time)
What is the current that the breaker will carry indefinitely without exceeding a specified temperature rise?
Continuous current rating
How fast is interrupting time?
1/20th of a second
What 3 methods can a breaker be tripped free?
Pneumatically
Mechanically
Electrically
What are currents that operate at a frequency that is a multiple of a fundamental power of frequency?
Harmonics
