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