AC Transformers II
Basic power rule of transformers
"Power in equals power out" - This means the VA on the primary side must equal the VA on the secondary side
Skin Effect
- Eddy current is produced into a conductor - cause electrons to be repelled toward the outer surface of the conductor - Is proportional to frequency and can be reduced by using conductors with a large surface area
Primary winding is connected to the __________. The secondary winding is connected to the __________.
- Power Source - Load
Isolation Transformer
- Primary and secondary windings are physically and electrically isolated/separated form each other - Operate on the principle of mutual inductance
Shell-Type Transformer
- Primary and secondary windings are wound around the center core piece
What are the common efficiencies for transformers?
88% to 92%
What is a transformer?
A transformer is a device used to change the voltage of an alternating power supply
A transformer that has one winding, which is used for both the primary and secondary, is called a(n) _____.
Autotransformer
Because the current in an AC circuit is continually changing in magnitude and direction, the magnetic field created in the primary winding is ______________
Continually expanding and contracting
Transformer Core Types
Core-type Shell-type Toroidal
What is a CT
Current Transformer
The power handling capacity of a transformer is dependent upon its ability to dissipate heat (measured in VA or Watts)
If the heat can safely be removed the power-handling capacity can be increased - use correct wire diameter - Sometimes accomplished by cooling
The equation for an ideal transformer is
Ip x Vp = Is x Vs Where: Ip - Current in primary coil Vp - Voltage across primary coil Is - Current in secondary coil Vs - voltage across secondary coils
What are the main types of voltage transformers?
Isolation Transformers Autotransformers Current Transformer - another
Most transformer cores are assembled from many thin sheets of metal. This type of construction is called ____________
Laminated
Transformers (definition)
Magnetically operated machines that can change values of voltage, current and impedance without a change in frequency
Mutual Inductance
The expanding and collapsing magnetic produced in the primary winding by the power supply will induce a voltage into the secondary winding Coil induces a voltage into another coil
The power factor in an ac circuit is equal to the _____ of the phase shift angle
cosine
Core-Type Transformer
- Windings are placed around each end of the core material
What are the advantages of autotransformers?
- smaller, lighter & less expensive than isolation type transformers because they contain only one winding - Can provide variable voltages
Toroid Transformer
- toroid core is constructed by tightly winding one long continuous wire into a spiral - One of the most efficient designs
Important Considerations for Isolation Transformers
1. Basic operating principles - self-induction: coil induces a voltage into itself 2. Mutual Induction - ability of one coil to induce a voltage into another coil
Non-ideal Transformers: Operational losses occur due to:
1. Copper Loss: Winding resistance (causing power to be dissipated in the windings) 2. Hysteresis Loss: due to the continuous reversal of magnetic field 3. Eddy current: Core losses due to circulating current in the core
Polarity
Means the direction of the induced voltages in the primary and secondary winding of the transformer
equation that relates the number of turns to the voltage
Np/Ns = Vp/Vs
What are the conditions for a STEP-DOWN transformer
Ns < Np
What are the conditions for a STEP-UP transformer
Ns > Np
How does a CT work
Produces an alternating current in its secondary which is proportional to the alternating current in its primary
3 Basic Types of AC Loads
Resistive, Inductive and Capacitive
what is the relationship between the number of turns in the primary and the number of turns in the secondary side of a step-up and a step-down transformer?
Step-up: more turns in 2ndary; V increases, I decreases Step-down: less turns on 2ndary; V decreases, I increases
What must occur for current to flow through a winding?
The applied voltage must overcome the induced voltage
Transformer Efficiency
The ratio of power delivered to the load to the power delivered to the primary (Efficiency = Pout/Pin)
Why must the supply to the primary coil be alternating current and not direct current
Transformers so not work with direct currents because they do not produce a varying magnetic field to induce an E.M.F on the secondary coil
All values of a transformer are proportional to its ________
Turns Ratio
How can eddy current losses be reduced?
by having a layered/laminated core
What is the purpose of the iron core in a transformer?
channels the magnetic flux
what are the 2 main losses in transformers?
copper losses and iron losses
Purpose of Laminated Cores
each lamination acts as an insulator to reduce the formation of eddy currents inside the core material
What are these two sub-types of Iron losses?
hysteresis losses and eddy current losses
What term describes the basis of transformer operation?
mutual inductance
There are 2 coils in a transformer called the primary and secondary. Which receives the input current and which produces the output current?
primary receives and secondary produces
Autotransformers
same winding for both primary and secondary coils
