Electrical Test 1 - AC Power and Transformers

3 Basic Types of AC Loads

Resistive, Inductive and Capacitive

An isolation transformer has the same input and output voltages. True or False?

True

Transformer Core Types

Core-type Shell-type H-type Toroid

Cycle

One complete waveform

A tape-wound core is assembled by ___________

winding a long continuous tape into a spiral

What are the common efficiencies for transformers?

90% to 99%

Transformers

Magnetically operated machines that can change values of voltage, current and impedance without a change in frequency

Polarity

Means the direction of the induced voltages in the primary and secondary winding of the transformer

Peak-to-peak

Measured from max value in positive direction to max value in negative direction

A transformer supplying a house with 240/120 V has a secondary which is center tapped. The conductor connected to the center is called the _____ conductor.

Neutral

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

Primary winding is connected to the __________. The secondary winding is connected to the __________.

- Power Source - Load

Frequency

Number of complete cycles that occur in 1 second

Isolation Transformer

- Primary and secondary windings are physically and electrically isolated/separated form each other - Help filter voltage and current spikes - Operate on the principle of mutual inductance

Shell-Type Transformer

- Primary and secondary windings are wound around the center core piece with the low-voltage winding being closest to the metal core - This arrangement permits the transformer to be surrounded by the core and provides excellent magnetic coupling - When the transformer is in operation, all the magnetic flux must pass through the center core piece. It then divides through the two outer core pieces

Toroid Transformer

- Tape-wound core or toroid core is constructed by tightly winding one long continuous silicon steel tape into a spiral - Flux leakage is kept to a minimum - One of the most efficient designs

H-Type Transformer

- Surrounds the windings on four sides instead of two - This extra metal helps reduce stray leakage flux and improves the efficiency of the transformer - The H-Type core is often found on high-voltage distribution transformers

What affects transformer impedance?

- The amount and type of core material - Wire size used to construct the windings - The number of turns - Degree of magnetic coupling between the windings

Additive and Subtractive Polarities

- The polarity of transformer windings can be determined by connecting them as an autotransformer and testing for additive or subtractive polarity, often referred to as a boost or buck connection - done by connecting one lead of the secondary to one lead of the primary and measuring the voltage across both windings

Power Ratings

- The power handling capacity of a transformer is dependant 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 - Sometimes accomplished by immersing the transformer in oil, or by the sue of cooling fins or both

Control Transformers

- Used to reduce the line voltage to the value needed to operate control circuits - Most common type contains two primary windings (240 V) and one secondary (120 V) - When primary are connected in parallel, the same voltage is applied across both windings - Primary leads are generally cross-connected (so metal links can be used to connect primary for 240 V or 480 V operation

Sine Waves

- Produced by all rotating machines and contains a total of 360 electrical degrees - Voltage at any point along the waveform is equal to the maximum, or peak, value times the sine of the angle of rotation - Frequency is measured in hertz

What are the advantages of autotransformers?

- Generally less expensive than isolation type transformers because they contain only one winding - Generally exhibit very high efficiencies in the range of 98% to 99% - Can provide variable voltages

Multi Tap Transformer

- Is a step up or step down transformer that has multiple taps on either the primary winding or the secondary winding or both - Provides flexibility in your input and output voltage requirements

What are the disadvantages of autotransformers?

- Main disadvantage is that it does not provide line isolation from the power source. They are often used because of their ability to reduce voltage spikes and electrical noise that occurs in the power source. - They will transmit spikes and electrical noise to the load since there is no separation between the load and power source

Peak Values

- Measured from zero to the highest value obtained in either positive or negative direction (1/2 of peak-to-peak) - The maximum amount of voltage attained by the waveform

A transformer that has one winding, which is used for both the primary and secondary, is called a(n) _____.

Autotransformer

Determining Max Current

Because power in = power out, the current rating can be determined by dividing the kilovolt-ampere rating by the winding voltage

How can you increase flux density?

By using such materials such as soft iron or steel which will concentrate the magnetic lines of flux into a smaller area to increase flux density

Resistive Loads

Contain pure resistance Characterized by: - they produce heat - the current and voltage are in phase with each other

The industrial use transformer that normally has 240 V or 480 V primary and a 120 V secondary is called a(n) _____ transformer.

Control

How do you determine how much core material is needed?

Determined by the power rating of the transformer

Wavelength

Distance the radiating field or signal can travel during one cycle of alternating voltage

The transformer that converts a high voltage from the power company down to 240/120 V for homes and businesses is called a(n) _____ transformer.

Distribution

Isolation Transformers

Greatly reduce voltages spikes that originate on the supply side (before they are transferred to the load side)

What type of core is often found on high-voltage distribution transformers?

H-Type

If the resistance of a transformer winding is read with an ohmmeter, it will generally ______

Indicate a lower value (because of impedance)

*The continuous movement of magnetic filed cuts the conductors of the primary winding ____________

Inducing a voltage in the winding

What are the two main types of voltage transformers?

Isolation Transformers Autotransformers (Current Transformer - another type)

What is the single greatest advantage of AC? What are two other advantages?

It can be transformed and DC cannot - a transformer permits voltage to be stepped up or down - high-voltage transmission requires less current to produce the same amount of power

Saved Most transformer cores are assembled from many thin sheets of metal. This type of construction is called ____________

Laminated

Tapped and Multiple-winding transformers

- It is possible to use multiple taps (connection points) to achieve different voltage ratings - Different taps on the primary side determine the voltage delivered to the customer

Distribution Transformers Continued...

- Loads intended to operate on 240 V (water heater, electric heat), are connected directly across the lines of the secondary - Loads intended to operate on 120 V connect form the center tap, or neutral, to one of the secondary lines

A basic rule of transformers

"Power in equals power out" - This means the VA on the primary side must equal the VA on the secondary side

Squaer Waves

- AC reverses its direction of flow and has periodic intervals - AC waveforms can vary depending on how the current is produced

What are the basic laws of electricity?

- Anytime current flows through a conductor, a magnetic field is produced around the conductor. The strength of the magnetic field is proportional to the amount of current flow and the polarity is determined by the direction of current flow. - Anytime a conductor cuts the flux lines of a magnetic field, a voltage will be induced in the conductor

Average Values

- Are actually DC values - Found when a sine wave AC voltage is changed into DC with a rectifier - Found by multiplying peak value by 0.637 or the RMS value by 0.9

Transformer Impedance

- Determined by the physical construction of the transformer - Expressed as a percent and is measured by connecting a short circuit across the low-voltage winding of the transformer and the connecting a variable voltage source to the high-voltage winding - the variable voltage is then increased until rated current flows in the low-voltage windings - Determined by calculating the percentage of variable voltage as compared to the rated voltage of the high-voltage winding

Distribution Transformers (aka service transformer)

- Provides the final voltage transformation in the electric power distribution system, stepping down the voltage to the level used by the customer (commonly 240/120 volts) - Center tap is grounded and becomes the neutral conductor (common conductor)

Testing the Transformer

- Simple test for grounds, shorts or opens can be done with an ohmmeter - the ohmmeter should indicate infinity for shorts and grounds - When testing for continuity, the wire resistance will be indicated on the ohmmeter

General Rule

Secondary windings should not be connected in parallel

What do the type and shape of the core determine?

The amount of magnetic coupling between the windings and to some extent the efficiency of the transformer

Flux Leakage

The amount of magnetic flux lines that do not follow the metal core and are lost to the surrounding air

What must occur for current to flow through a winding?

The applied voltage must overcome the induced voltage

Excitation Current

The current supplied to the primary of a transformer when there is no load connected This is the current necessary to supply the power needed to overcome the core losses

Breaking Capacity/Interrupting Rating

The current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration

Transformer Efficiency

The ratio of power delivered to the load to the power delivered to the primary (Efficiency = Pout/Pin)

RMS Values

The square root of the mean of the square of instantaneous currents - "amount of AC power that produces the same heating effect as an equivalent DC power"

What happens when a load is connected to the secondary winding?

The voltage induced in the secondary winding causes current to flow through the load. This current flow causes a separate magnetic field to be created around the secondary winding. This magnetic field will however always be opposite in polarity to the magnetic field of the primary and because of this, causes the magnetic field in the primary to become weaker

Most cores are made from _____ to form a solid metal core

Thin steel punchings laminated together

What is the function of the neutral?

To carry the difference in current between the two secondary lines and maintain a balanced voltage

A tape-wound core is also called a(n) _____ core.

Toroid

All values of a transformer are proportional to its ________

Turns Ratio

Linear Waves

Voltages rises at a constant rate with respect to time

What is a common magnetic measurement?

Weber (1 Weber = 100,000,000 lines of magnetic flux)

* Just read a couple times *

When current rises from 0 towards a positive peak, a magnetic field expands around the coil winding of the primary and when it reaches its peak, the current is no longer increasing and the magnetic field stops expanding and goes back towards 0. The decrease in current causes the magnetic field around the primary to decrease also. When the current passes through zero and begins to go into the negative, the current is reversed. Magnetic field again begins to increase around the primary winding but the polarity is reversed. When it heads back towards 0, the magnetic field around the primary coil decreases back to 0 again.

Autotransformers

- One winding transformers (same winding for both primary and secondary) - no line isolation - Often used by power companies to provide a small increase or decrease to the line voltage - help provide voltage regulation to large power lines - Contained in a housing filled with transformer oil, which acts as a coolant and prevents moisture from forming in the windings

What is another basic law of electricity?

An induced voltage is always opposite in polarity to the voltage that produces it - This means that the voltage induced in the primary winding will always oppose the applied voltage. In an electric circuit, voltage is similar to pressure in a hydraulic system.

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

The strength of a magnetic field is determined by _________ and this is referred to as _________.

- the number of flux lines within an area - flux density

Transformer Polarities

- The two coil windings do have a distinct orientation of one with respect to the other - Either coil could be wound around the core clockwise or anticlockwise so the keep track of their relative orientations - "Dots" are used to identity a given end of each winding (dot convention) - Wound to have the relative polarity of the secondary voltage with respect to the primary voltage

Laminated Cores

Preferred because 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 core material

True Power (Watts)

Produced only when both current and voltage are either positive or negative

Constant Current Transformers (aka current regulator)

- Used to provide power to series connected loads - Designed 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 (done in series and not parallel because of the savings in wire) - require a single conductor to be connected from lamp to lamp instead of two conductors - Have a reactor coil connected in parallel with street lamps in case one fails

What does a weaker magnetic field in the primary winding cause?

Causes less counter voltage voltage to be induced in the primary winding. Since primary current is limited by this counter voltage, primary current will increase. If more load is added to the secondary, the secondary current increases producing a stronger magnetic field around the secondary and further weakening the primary magnetic field, causing an increase in primary current. This is the reason that primary current increases as load is added to the secondary.

Impedance

In an AC circuit the total current limiting effect (impedance) is symbolized by the letter Z. This is the self induced voltage in the primary winding

Voltage and Current Relationships in a Transformer (2)

- When the primary induces a voltage in the secondary, this induced voltage is proportional to the rate of change of current. The secondary voltage is max during te periods that the primary current is changing the most and it will be zero when the primary current is not changing (because the secondary lags the primary current by 90, and the applied voltage leads the primary current by 90, the secondary voltage in 180, out of phase with the applied voltage

Important Considerations for Isolation Transformers

1. Basic operating principles - self-induction: coil induces a voltage into itself 2. Excitation Current - amount of current required to magnetize the core of the transformer 3. Mutual Induction - ability of one coil to induce a voltage into another coil

What needs to be included on Transformer Nameplates?

1. Name of manufacturer 2. Rated kilovolt-ampere 3. Frequency 4. Primary and Secondary Voltage 5. Impedance of transformers rated 25 kilovolt-ampere and larger 6. Required clearances of transformers with ventilating openings 7. Amount and kind of insulating liquid where used 8. Temperature class for the insulating system of dry-type transformers

To an AC power source, the primary winding appears to be a resistance connected in series with an inductor and represents losses due to heat and these losses include:

1. Resistance of the wire that forms the primary winding. All sources of resistance produce heat, which is a loss 2. Eddy current losses - are circulating currents that are induced into the core material of the transformer. Produces heat. 3. Hysteresis Losses - caused by the continual reversing of polarity of the magnetic domains or magnetic molecules in the core material

What are the 3 factors that determine the amount of induced voltage?

1. Strength of the magnetic field. The more lines of flux cut by the conductor the greater the amount of induced voltage 2. Length of the conductor. This is often stated as number of turns of wire. The greater the surface area of the conductor, the more lines of magnetic flux that can be cut 3. Speed of the cutting action. The speed of the cutting action in an alternating current circuit is proportional to the frequency (one volt is induced in a conductor when it cuts magnetic lines of flex at a rate of 1 Weber per second

Non-ideal Transformers: Operational losses occur due to:

1. Winding resistance (causing power to be dissipated in the windings) 2. Hysteresis Loss (due to the continuous reversal of magnetic field) 3. Core losses due to circulating current in the core (eddy current) 4. Flux Leakage where boundary flux from the primary not link to the secondary

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

Another constant current method

- Uses a film cut-out device consisting of two pieces of metal separated by an insulator designed to puncture at a predetermined voltage. As long as the lamp is in operation, the voltage drop across the cut-out device is not sufficient to cause the film to puncture, if the lamp burns out, producing an open circuit, the entire circuit voltage will appear across the cut-out device and cause it to short circuit

Voltage and Current Relationships in a Transformer (3)

- When a load is connected to the secondary, current begins to flow - Because the transformer is an inductive device, the secondary current lags the secondary voltage by 90 - Because the secondary voltages lags the primary current by 90, the secondary current is 180 out of phase with the primary

Voltage and Current Relationships in a Transformer (1)

- When primary is connected to power but there is no load, current is limited by the inductive reactance of the primary - at this time, the transformer is essentially an inductor and the excitation current is lagging the applied voltage by 90 degrees

Core-Type Transformer

- Windings are placed around each end of the core material - The low-voltage winding is placed closest to the core and the high-voltage winding is placed over the low-voltage winding - High voltage conductors are smaller cross section than the low voltage conductors