alternating current 26201 11 page 1 30 -...

Alternating Current 26201‐11 ‐ 30

• Calculate the peak and effective voltage of current values for AC• Calculate the phase relationship between two AC waveforms• Describe the voltage and current phase relationship in resistive AC circuit• Describe the voltage and current transients that occur in an inductive circuit.• Define inductive reactance and state how it is affected by frequency• Describe the voltage and current transients that occur in a capacitive circuit• Define capacitive reactance and state how it is affected by frequency.• Explain the relationship between voltage and current in the following types of

AC circuits:• RL circuit• LC circuit• RLC circuit• Explain true power, apparent power, power factor and basic transformer

action.

• Alternating current (AC) and its associated voltage reverses between positive and negative polarities and varies in amplitude with time.

• One complete wave form or cycle includes a complete set of variations with two alternations in polarity.

• Many sources of voltage change direction with time and produce a resultant waveform. The AC waveform is knows as a sine wave.

• 1. In electric current in a conductor creates a magnetic field that surrounds a conductor.

• 2. Relative motion between a conductor and magnetic field. When perpendicular or opposed it will create voltage.

A sine wave represents the conductor through a magnetic field in AC

Direction

Force

Induction

The left hand rule for generators will help you determine which direction the current will flow in the conductor.

Important factors to an electrician are a conductor, a magnetic field, and relative motion.

• The conductor generates induced voltage across the magnetic flux…

• Three Factors Affecting Magnitude Of Voltage Generation:• 1. The Strength of The Magnetic Field• 2. The Length of The Conductor• 3. The Rate at Which The Conductor Cuts Across The Flux Field

• If the magnetic field and the length of the conductor are constant, the voltage produced will vary…

• The rate at which the conductor cuts through the magnetic flux depends on the revolutions per minute = Rate! More rate, more flux…

• Once the generator is at a constant rate, the voltage produced at that moment will depend on the angle of the conductor at that instant.

• When the conductor is parallel to the lines of magnetic flux (zero angle or Theta), the voltage will be zero.

• The magnitude of the voltage produced is directly related to the sine of the angle. Sine is trigonometric function, each angle has a sine value in voltage.

• The sine of zero is 0. It increase from 0 degrees to 90 degrees and decreases from 90 degrees back to 180 degrees, then it increases.

Peak

Zero Volts

Peak

Zero Volts

• Voltage is proportional to the sine angle, as it reaches 180 degrees the polarity reverses itself causing alternating current.

• The conductor is now cutting lines of flux in the opposite direction.

Peak

See Fig. 3 Pg. 2

The angles of a right triangle are directly proportional to it’s length.

The length of the side changes the angles of the right triangle.

You can find the Hypotenuse by using Pythagorean theorem.

The Hypotenuse is the longest side of the right triangle, the other sides are known as adjacent and opposite angles.

NOTE: All three sides of a right triangle should equal 180 degrees.

1. In order to find the degree of “A” for sine you would divide the opposite distance by the hypotenuse distance.

2. In order to find the degree of “A” for Cosine you would divide the Adjacent distance by the Hypotenuse distance.

3. In order to find the degree of “A” for Tangent you would divide the Opposite distance by the Adjacent distance.

http://youtu.be/HsAN0DhfSjIRun Short Video To Show Steps….

• The value voltage at any point along the sine wave can be calculated if the angle and the maximum angle are known. We use a formula to determine this…

• Formula Used:

• E = E(max) Sine 0

• E = E (300 volts) x Sine (22 degrees)

• E = 300 x .3746 (sin of 22 degrees) note: hit 22 on calculator then SIN

• E = 112.328 volts or the voltage value at 22 degrees. Go To Page 3 to see calculated examples…

300 Volts

• In the next series let’s look at AC Voltage Terms:

• Frequency: Number of times per second pattern repeats itself.• Period: The inverse of Frequency, time required to complete one cycle.• Wave Length: Distance traveled by waveform during one period. • Peak Value: The maximum value of voltage or current• Average Value: Calculated from values in sine wave on a half cycle.• Effective Value: AC voltage wave that indicates the same value in DC volts.

There is also square, Triangle, & Saw‐tooth wave form… See figure 8 Pg. 9

• Frequency: The frequency of a waveform is the number of times per second an identical pattern repeats itself. It’s unit is Hertz (Hz)

• Two Alternations form one complete cycle.

• One cycle equals = 1 hertz.

• AC power is 60 hertz / 60 cycles per minute alternating current.

• Period: The period of a waveform is the time required to complete one cycle. The period is the inverse of frequency or 1 over frequency.

• Wavelength: The wavelength is the distance traveled during one period. Since electricity travels at the speed of light (186,000 miles per second or 300,000 Kilometers) the wavelength equals the product of the period x speed of light.

• Peak Value: The peak value is the maximum value of voltage or current.

• Example Above:

• A sine wave has a peak voltage of 170v (alternating) or a peak to peak voltage of 340 volts. It’s maximum peak value is 170v.

170v

• Average Value: The average value is calculated from all the values in a sine wave for one alternation or half cycle. In a 180 degree sine wave, the sine values add up to .637 x peak = Average Value

• Example Above:

• Average Value = .637 x 170 which equals = 108v

170v

• Effective Value: Meters used in AC circuits use a value called effective value. The AC current (wave) that indicates the same energy transfer as a DC current or voltage.

• By using a root mean square value (45 degree) of .707 x the peak value we can find the effective value.

•• Example: RMS = .707 x170v or 120v

170v

See Pg. 8 Examples

© 2005 Refrigeration Training Services ‐ E1#2 AC and DC Current v1.0

21

0º 90º 180º

Effective Voltage

Peak Voltage 170 v

120 v

0 v

Effective voltage = .707 x Peak voltage

.707 x 170 = 120 v

Note: Meters measure effective voltage

• There is a phase lead or lag with of one with respect to the other unless they are alternating in unison, which means they are in same phase.

170v


23

Magnet

SOUTH

NORTH

Magnet

Current is generated in each conductor as it passes through the magnetic field.


24

0º 90º 180º 270º 360ºMagnet

SOUTH

NORTH

Magnet

120º 240º

Each sine wave starts 120° after the other.


25

0º 90º 180º 270º 360ºMagnet

SOUTH

NORTH

Magnet

120º 240º

Windings 120° out of phase give 3Ø motors their high starting torque.


26


27

0º 90º 180º 270º 360º

CURRENTVOLTAGE

Induction: When voltage leads current.

Reactance: The resistance encountered during the change of flow.

• According to Ohms law, current equals voltage divided by resistance.

• When E is an (RMS) value, I is also a (RMS) value in relation to the resistance.

• In an AC circuit with only resistance, the current variations are in phase with the applied voltage.

• Unless otherwise noted, AC circuits are in RMS values.

• RMS power = True Power or P = I2R

• What Is Inductance: It is the characteristic of an electric circuit that opposes the change of current flow.

• It is a result of the expanding and collapsing magnetic field caused when the current alternates.

• When this happens, flux cuts across the conductor (wire) producing induced voltage in the wire itself. (Like a transformer)

• An inductor is a coil of wire that may be wound on a core of metal paper or be self supporting.

• The inductance of a coil depends on its physical construction.• Number of Turns• Coil Diameter• Length of the Core• Core Material• Winding the coil in layers

See Pg. 11 for further…

A transformer with coils of copper wire is an example of an inductor in a circuit…

See Pg. 13 for further…

• Induced voltage is an actual voltage that can be measured and produced while the current is changing.

• The current that flows in an inductor is induced by the changing magnetic field that surrounds the inductor.

• The overall effect is that the current is out of phase and lags the applied voltage by 90 degrees.

• This can easily be remember as Voltage (E), Inductance (L), and current (I) or ELI and in capacitance its ICE.

• A capacitor is a device that stores an electrical charge in a dielectric material.

• Capacitance is the ability to store a charge. In storing a charge, a capacitor opposes a change in voltage.

• Capacitors are determined by:

• 1. Area of Plates• 2. Distance Between Plates• 3. Dielectric Permittivity

Pg 16 NCCER

• Connecting capacitors in series will increase the distance between plates, thus reducing total capacitance.

• Connecting capacitors in parallel will increase the plate area thus increase the total capacitance.

L1 NPg 564 Delmar

• In applications where a lower voltage rating is permissible, more capacitance can be obtained using a smaller Electrolytic Capacitor.

The actual capacitor should be as close to rated voltage to produce the oxide film necessary for capacitance.

DC capacitors will neutralize due to leakage over time.

• The charge on a capacitor is continuously charging, just after the 270 degree when voltage decreases the capacitor must loose its electrons from the negative plate

• In a capacitor circuit current leads voltage, unlike in an inductance circuit where voltage leads current. ELI / ICE MAN

L1 Load

Capacitors offer a real opposition to current flow. This opposition arises because of storage capacitance of the actual capacitor.

Calculating capacitive reactance is Xc:

Pg 19 NCCER

Pg 19 NCCER

A 35uf capacitor is connected to a 120v / 60hz line. How much current will flow in this circuit?

Delmar Pg. 593 Explanation:

1. Start with 10^(‐6) 2. A negative power simply means to convert the whole number into a fraction [Therefore you have (1/10)^6]. 3. (1/10)^6 = 1/(10x10x10x10x10x10) = 1/1,000,000 = .000001

AC circuits often contain inductors, capacitors, and or resistors connected in series or parallel.

The impedance (Z) of a circuit is defined as the total opposition to current flow.

Pg 19 NCCER

RL circuits combine resistors and inductors in a series or parallel or series‐parallel configuration.

In a pure inductive circuit, the current lags the voltage by an angle of 90 degrees.

Pg 21 NCCER

In a parallel RL circuit, the resistance and inductance are connected in parallel across the voltage source.

Pg 23 NCCER

http://vimeo.com/6421100

The End

alternating current 26201 11 page 1 30 -...

Documents