Three Phase Transformers Flashcards
(48 cards)
What is an isolation transformer?
Zero electrical or physical connection between the primary and secondary windings, works off of mutual induction
What are the four different application ratings?
Power Supply (over 500kVA)
Distribution (500kVA and under)
Control (Steps down to 120v)
Instrument (CT’s and PT’s)
Why is the isolation transformer most common?
It is the best for controlling and limiting current spikes
What are the properties of the high voltage winding?
More turns, thinner wire, thicker insulation
What are the properties of the low voltage winding?
Fewer turns, larger wire, thinner insulation
What are the properties of the transformer core?
Soft iron, high permeability, low retentivity, high resistance
When connection leads are on the same on the left hand side (H1, X1) the windings are additive or subtractive?
Subtractive.
When connection leads are different on the left hand side (H1, X2) the windings are additive or subtractive?
Additive
What is ‘excitation current’ or ‘magnetizing current’
When there is no load on the secondary, the primary winding will ‘self induce’ a current into the secondary winding. Usually 2-5% of the primary current.
What are the three rules for connected transformers in parallel?
Must have the same impedance, connected polarities must match, and same voltage ratings
T or F: transformers must be sized for the largest rated coil?
True
How do you find efficiency of a transformer?
Power out / power out + losses
How do you find fault current?
Rated amps of secondary / %Z
Which test will find copper losses?
Short circuit test
Which test will find core or iron losses?
Open circuit test
What is a short circuit test?
Full load current is measured to calculate copper losses. Current losses increase as load increases because copper losses are directly proportional to magnitude.
What is voltage regulation?
The change in voltage from no load to full load.
How do you find fault current?
I rated / impedence
I rated is full load amps of the secondary
Impedance voltage is usually ___% of rated voltage?
3-5%
Impedance voltage is also called ____
Short circuit voltage.
%Z = E short circuit / E rated
What are the three advantages of three phase TX’s?
- Cheaper equipment and installation
- Smaller equipment, weighs less and easier to install
- More efficient
When the polarity of a wye connected secondary is reversed, what happens to the line voltage?
It will read phase voltage (which is root 3 less)
On a dual winding TX, how do you find the transformation ratio?
Use only one set of windings.
Ex) primary winding is connected in series and each winding is seeing 240V, 480V in total for the TX. The secondary winding is connected in parallel, each winding is seeing 208V, 208V in total for the TX.
Use 240V / 208V to find the ratio, do not use 480V / 208V
Which connection will all X1’s be neutral connected?
Delta to Wye step down, or Wye to Delta step up
