Induction and Alternating Current

Induction and Alternating Current

Alternating Current, Generators, and Motors

Generators and Alternating Current

•  ______– a device that uses induction to convert mechanical energy to electrical energy

–  Commonly uses rotational energy by having steam or running water turn a ______

•  Steam may be generated by a coal or natural gas ______or from geothermal heat sources

–  The rotation of the turbine causes a wire loop to ______in a magnetic field

•  When a loop is parallel to a magnetic field, the charges are perpendicular to the magnetic field

–  Current is ______, induced emf is ______

•  When a loop is perpendicular to a magnetic field, the charges are parallel to the magnetic field

–  Current is ______, induced emf is ______

•  Induced emf versus time graphs as a ______curve

•  Maximum emf for a generator = number of loops * cross sectional area of the loops * magnetic field strength * angular frequency of rotation of loops

•  emfmax = NABw

•  Angular frequency = 2*pi*frequency

•  w = 2pf

•  Sample Problem: A generator consists of exactly eight turns of wire, each with an area A = 0.095m2 and a total resistance of 12W. The loop rotates in a magnetic field of 0.55T at a constant frequency of 60.0Hz. Find the maximum induced emf and maximum current in the loop.

•  f = 60.0Hz A = 0.095m2 R = 12W

•  B = 0.55T = 0.55V*s/m2 N = 8

•  emfmax = ? Imax = ?

•  w = 2pf

•  emfmax = NABw

•  I=emf / R

•  ______(ac) – an electric current that changes direction at regular intervals

–  Typically produced in ______

–  Is reflected in the ______nature of the graph

•  In the US, Canada, and Central America the current reverses itself at a frequency of ______or 60 reversals/second

•  In Europe and most of Asia and Africa, the frequency is ______

•  Since alternating current is constantly reversing, ______current and emf values are not as useful as they are in direct current

•  Of more importance are instantaneous and ______(rms) values

•  Rms current – the amount of ______current that dissipates as much energy in a resistor as an instantaneous alternating current does during a complete cycle

–  An ______value allowing for accurate comparisons between alternating and direct current

•  Power can be calculated by using the appropriate rms values in the equations given previously

Potential Difference / Current
Instantaneous values / Dv / i
Maximum values / DVmax / Imax
rms values / DVrms=DVmax/√2 = 0.707*DVmax / Irms = Imax/√2 = 0.707*Imax

•  Power = rms current squared * resistance

•  Power = one-half * maximum current squared * resistance

•  P = (Irms)2R = ½(Imax)2R

•  Ohm’s law still applies in ac circuits

•  Rms potential difference = rms current * resistance

•  DVrms = Irms*R

•  Sample problem: A generator with a maximum output emf of 205V is connected to a 115W resistor. Calculate the rms potential difference. Find the rms current through the resistor. Find the maximum ac current in the circuit.

•  DVmax = 205V R = 115W

•  DVrms = ? Irms = ? Imax = ?

•  DVrms = .707*DVmax

•  Irms = DVrms / R

•  Irms = .707*Imax

•  Alternating current can be ______in to direct current

•  The conducting loop in an ac generator must be free to ______while remaining part of the ______at all times

–  The ends of the conducting loop are connected to conducting rings called ______that rotate with the loop

–  Connections to the external circuit are made by stationary graphite strips called ______that stay in contact with the slip rings

–  Both the loop current and the output current are continuously changing ______

•  By replacing the two slip rings with a single split slip ring called a ______, the generator can produce direct current

–  The brushes change halves of the commutator at the same instant the current reverses so there is a ______which cancels out leaving the current flowing in a single direction

–  By using multiple loops and commutators, the ______from the individual loops are canceled out resulting in an almost constant output current

Motors

•  ______– convert electrical energy into mechanical energy

•  Reverse of a generator

–  Looks much like a dc generator

•  The coil of wire is mounted on a rotating shaft and is positioned between the poles of a ______.

•  Brushes make contact with a commutator, which ______the current in the coil.

•  The alternation of current causes the ______produced by the current to regularly ______and thus always be repelled by the fixed magnetic field.

•  The coil and shaft are therefore kept in continuous rotational motion

•  ______– the emf induced in a motor’s coil that tends to reduce the current powering the motor

–  The induced emf

–  If this did not occur, ______law would be violated

•  The faster the coil rotates, the ______the back emf

•  The ______available to supply current to the motor equals the difference between the applied potential difference and the back emf