Unit 27: Single-Phase Transformers

In a transformer, how does the input power correspond to the output power?

# VA in = # VA out

Constant-current transformer

- "Current regulator" - design to deliver a constant output current, generally 6.6 A, under varying load conditions. - most often used to provide power to series-connected street lamps.

Magnetic domains

- Can be affected by outside sources of magnetism.

What is transformer impedance a major factor in determining?

- The amount of voltage drop a transformer will exhibit between no load and full load. - Determining the amount of current flow in a short-circuit condition.

Why do transformers have such high inrush currents when first connected to power?

- The core of a transformer does not have an air gap so that a very long reluctance path is produced for the magnetic lines of flux. - The domains remain in their set position once the magnetizing force has been removed but it "remembers" where it was last set on the waveform at which current is disconnected from the primary winding.

Autotransformer

1-winding transformer. - often used by power companies to provide a small increase/decrease to line voltage to help provide voltage regulation to large power lines.

Greatly reduces any _______1________ that originally on the ______2______ side before they are transferred to the load side due to ___________3___________?

1. Voltage spikes 2. Supply 3. Rise time of current through an inductor.

Edison connection

Grounding a center tap so it becomes the neutral conductor. - becomes the center point of the voltage produced by the secondary winding.

All values of a transformer are proportional to its...?

Turns ratio

In isolation transformer, the continuous expanding and collapsing magnetic field cuts the primary's...?

Windings and induces a voltage into it.

Multiple-tapped windings

Windings with more than one set of load wires connected to the primary or secondary to make the transformer more universal so that it can be connected to a different primary voltages without changing secondary voltage values or vice versa.

Transformer

A magnetically operated machine that can change values of voltage, current, and impedance without a change of frequency.

Mutual induction

Ability of one coil to induce a voltage into another coil. - The magnetic field produced by the primary's winding also cuts into the secondary's winding due to both windings being wound around the same core.

Flux leakage

Amount of magnetic flux lines that do not follow the material core and are lost to the surrounding air.

Distribution transformer

And changes the high voltage of power company distribution lines to the common 120/240V used to supply homes.

When does true power increase?

As load is added to the secondary winding.

Function of neutral conductor:

Carries the difference of current between the two secondary lines and maintains a balanced voltage.

Tape-wound core/ torrid core

Constructed by tightly winding one long continuous silicon steel tape into a spiral. - Flux leakage is kept to a minimum due to it being 1 continuous length of metal.

Parallel transformer connections

Increases current capacity. - Connecting secondary windings in parallel can cause high circulating currents and/or very unbalanced currents that can lead to transformer failure. - If all parts of transformer information are not the same, transformer failure may occur.

The primary winding is the power ____________ winding.

Input

How is an isolation transformer coupled?

Magnetically

Transformer core types

Most are made from thin steel punchings laminated together to form a solid metal core. - A thin layer of oxide forms on the surface of each lamination and acts as an insulator to reduce the formation of eddy currents inside the material's core.

Disadvantage of autotransformers

No line isolation between incoming power and load do to load being connected to one side of powerline.

The secondary winding is the power ____________ winding.

Output

Control transformer

Reduces line voltage to value needed to operate control circuits.

Which side should be disconnected first when servicing parallel transformers?

Secondary

Isolation transformer

Secondary winding is physically an electrically isolated from the primary winding.

Series connection of transformers secondaries

Secondary windings can be connected in series to produce a higher output voltage. - If the polarity is not correct, the output voltage will be 0.

Spike voltages and currents are generally a very short duration, meaning...?

That they increase in value very rapidly.

Excitation current

The amount of current required to magnetize a transformer's core. - Remains constant from no load to full load.

In isolation transformer, the faster the current attempts to increase, ...?

The greater the opposition to the increase there is.

What do the dots on transformers signify?

The polarities are the same at that point and each winding.

The amount of voltage induced into the secondary is determined by...?

The ratio of the number of turns of wire in the secondary to those in the primary.