Topics Covered in Chapter 15 Magnetic Field around an Electric Current Magnetic Polarity of a Coil Motor Action between Two Magnetic Fields Induced Current
Lenz' Law Faraday's Law of Induced Voltage Relays Topics Covered in Chapter 15 (continued)
Straight Conductor A magnetic field is produced by the flow of current through a straight conductor. The magnetic field around a straight conductor is circular and perpendicular to the axis of the conductor. The polarity of the circular field is counterclockwise when viewed along the conductor in the direction of electron flow.
Field Directions Around Current-Carrying Conductors CCWCW
Left-Hand Rule When the fingers of the left hand curl around the turns of an electromagnet in the direction of electron flow, the thumb points to the north pole.
The fingers wrap the coil in the direction of electron current and the thumb points to the north pole. The Left-Hand Rule N
Motor Action Motion action is the result of two magnetic fields interacting with one another. The fields can attract or repel.
Motor Forces on Current-Carrying Conductors RepulsionAttraction
N S Motor action of a current carrying conductor in a field. Up
Generator Action When a moving conductor cuts across flux lines, a voltage is induced. The amount of induced voltage is proportional to: The conductor velocity The amount of flux The number of turns of wire The polarity of induced voltage is determined by Lenz’ law.
The induced voltage is to the rate of motion. N S Less voltage is generated when the conductor moves slowly.
The induced voltage is to the flux. N S More voltage is generated by the stronger magnet.
The induced voltage is to the number of turns. N S Increasing the turns generates more voltage.
Moving the field has the same effect. N S Most generators use a moving field and stationary coils.
Summary of Induced Voltage Faraday’s Law v ind = N d (Wb) dt(seconds) The induced voltage is directly proportional to the number of turns times d /dt. Increase the number of turns Increase the flux Decrease the time To generate more voltage:
Relay Contacts The switching contacts of a relay may be: Normally open (NO) Normally closed (NC) The movable arm on a relay is called the armature. When the coil is energized, the armature: Opens the contacts (NC relay) Closes the contacts (NO relay)
SPDT relay has both NO and NC contacts. When the coil is energized, the armature is attracted and opens the NC contacts and closes the NO contacts.
Relay Coil Currents The minimum amount of coil current required to energize a relay is called the pickup current. The minimum amount of coil current required to keep a relay energized is called the holding current.
The pickup current is greater than the hold current because of the air gap. The armature has moved and the air gap is gone. There is continuity between the main contact and the NO contact. Less current is now required to overcome the spring and hold the relay closed.