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DC Motors and Generators

Instruction Plan

• A. How motors use magnetic force– 1. Constant opposing and attracting of magnetic forces are needed for a motor to run.– 2. Armature is on a rotating shaft

• a. Direction of current flow determines the magnetic poles of the armature

• b. Changes in the direction of current flow can occur while the armature rotates

– 3. Commutator reverses direction of current flow• B. Timing in a motor is controlled by the angle of commutation.

Motor Basics

The Permanent Magnet Motor

• A. Specific dc motors are used for different purposes.• B. Two permanent magnets which are apart create a

magnetic field between them.– 1. Magnetic poles of the magnets do not change– 2. Magnetic poles of the armature can change– 3. Motor can change direction if the voltage to the

armature changes• C. DC motors can operate in a clockwise or counter-

clockwise direction.• D. The strength of the magnetic field affects the speed

and operation of the motor.

The Series DC Motor• A. Motors which operate large loads require stronger

magnetic fields.1. Electromagnets are used instead of permanent

magnets.2. The field coil is wired in series with the armature

windings.• B. Counter emf is the voltage produced in the armature

when it cuts through the magnetic field1. Cemf opposes the power supply voltage.2. Cemf determines amount of current flow, torque and

speed.3. As motor speed increases, cemf increases

Compound Motors• A. Combination of series and shunt motors

– 1. Torque abilities of series motor– 2. Constant speed advantages of a Shunt motor

• B. Uses both a series and shunt field coil• C. Cumulative compound motor has series and shunt coil

connected to aid one another• D. When the speed is increased, the torque decreases• E. Current through the shunt field coil remains the same,

limiting the speed of the motor• F. Compound motors are not used as extensively as

series or shunt motors

Generator Action• A. Generators supply electrical energy to motors

– Convert mechanical energy to electrical energy– Voltage is induced by passing a wire through a

magnetic fielda. When the wire is moved up and down, the induced voltage

reverses direction

b. Amount of voltage determined by speed of the wire moving through the magnetic field

• B. Generators use coils of wire to produce electrical energy– Armatures are coil that rotate in a magnetic field– The faster the armature rotates, the greater the

amount of voltage produced