3.15 Transformers Flashcards
What is a transformer?
An electrical device that is used in AC circuits to change the voltage and current supplied to a circuit or device
What does a step-up transformer do?
Increases the voltage and decreases the current
What does a step-down transformer do?
Decrease the voltage and increases the current
How do transformers transfer energy?
Via a magnetic circuit with no electrical connection between input and output terminals
What is the general build up of a transformer?
Two coils and an iron core
In a transformer what is the input coil called?
Primary winding
In a transformer what is the output coil called?
The secondary winding
What makes up the iron core in a transformer?
Isolated thin iron layers (laminated core)
Why is it a laminated core in a transformer instead of a solid iron core?
To prevent eddy currents which waste power by heating the iron core
Does frequency change in a transformer?
No it doesn’t
The voltage phase relationship between the primary circuit and secondary circuit can be either?
0˚ or 180˚
In a transformer what determines whether its 0˚ or 180˚ ?
0˚ or 180˚how the secondary is ‘wound’ relative to the primary
What indicates whether a transformer is 0˚ or 180˚?
It is marked with phasing dots
On a drawing of a transformer what does it mean when the phase dots are in line?
It’s in phase (0˚)
On a drawing of a transformer what does it mean when the phase dots are not in line with each other?
It is out of phase (180˚)
What is the efficiency range of a small power transformer ?
80-90%
What is the efficiency range of a large commercial power line transformer?
98%
What are the two types of loss you can have in a transformer?
-copper
-iron
What is copper loss in a transformer?
Loss caused by direct current resistance in the primary and secondary windings
What is iron loss in a transformer?
Loss that’s caused by eddy currents and hysteresis loss (lag)
When a transformer losses some energy what is it converted into?
Heat energy
In copper loss how much power dissipated is directly proportional to?
-resistance of wire
-the square of the current (size of wire)
What is hysteresis loss?
When a conductor is passed through a magnetic field inside the conductor the poles in the core must align themselves however they can lag behind causing heat loss
When load is applied on the secondary winding of a transformer what happens?
A current flows in the winding which creates a magnetic field
Once the secondary current has a load and creates a magnetic field, what happens to the first and second magnetic field?
Because they’re opposite in direction it reduces the overall magnetic strength
In a transformer if the voltage increases what must happen to the current?
It must decrease
In a transformer if the current increases what must happen to the voltage?
It must decrease
What does a transformers ratio depend on?
-number of turns in the primary and secondary windings
-voltage across each winding
-current through the windings
When is a transformer under no load?
When there is no electrical load connected to its secondary winding and therefore no current flowing
Why is the input power always the same as the output power in a transformer ?
Because a transformer can only change voltage and current not power
Why does a transformer not suffer from friction or windage losses?
Because it has no moving parts
How can hysteresis losses be reduced?
By making the core from special steel alloys
If the primary voltage and load are connected in the same way, what happens to the phase?
No phase difference can be seen
In a transformer what does it mean if the primary winding has a red dot and black dot on the secondary winding?
It’s out of phase
In a transformer what does it mean if the primary and secondary winding both have a red dot?
It’s in phase
What is commonly used instead of a single phase transformer?
A three limbed transformer
What does a three phase transformer consist of?
-core
-core insulation
-primary winding
-insulation
-secondary winding
In a three phase transformer what is the first layer of insulation used for?
To protect the primary winding from damage by the metal core
What are the primary and secondary windings made from in a three phase transformer?
Insulated copper or aluminium
In a three phase transformer what is the insulation between primary and secondary made from?
Treated paper or cloth
Why is the insulation in a three phase transformer treated?
To reduce the possibility of fire
In a three phase transformer what is known as ‘tight coupling’?
When the secondary winding is wound on top of the primary
Why is tight coupling used?
It’s said to be 99.9% efficient
What are the three types of couplings used in a three phase transformer?
-tight
-medium
-loose
What is the benefit o using a star/Y connection?
You can have multiple voltages
What is the main benefit of a delta connection?
Higher level of reliability (if one fails open the other two can still maintain voltage)
How many windings does a three phase transformer have?
6 (3 primary, 3 secondary)
In a star connection you have Red, Yellow, Blue. What is the fourth connection?
Neutral
When labelling a three phase transformer what is indicated by capital A,B,C?
The primary windings
In a three phase transformer what is indicated by lower-case a,b,c?
The secondary windings
If you have a large power transformers and its unit are too high what do the unis turn into?
KVA (kilo volt-amperes) or MVA (Mega volt-amperes)
What is active power used for?
To generate real work
In a transformer, the power in must be the same as?
The power going out
Having different amount of coils on a side of a transformer causes what?
A step up or down transformer
How do you create a step up transformer?
You have more turns on the secondary winding
How do you create a step down transformer?
You have more turns on your first winding
How many coils does a auto transformer have?
One
How do the primary and secondary windings work in an auto transformer if it only has one coil?
They share a part of the winding
What is the major advantage of a auto transformer?
It’s small design
What is the main disadvantage of a auto transformer?
It doesn’t have the primary to secondary winding isolation of a normal transformer
What is a variable transformer known as?
Variac
What is a variable transformer?
The same as a auto transformer but with a slider arm to vary the output
What are current transformers?
Transformers with a built in AC current measurement so you don’t have to break the current carrying leads
What must current transformers always have?
A load
