DC Motors

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Shunt Field As ____ is produced by the spinning armature it begins to oppose the applied voltage and cause current flow to __

CEMF, Decrease

Commutator

Acts as an rotary switch, while maintaining correct direction of current flow through armature windings

DC Motor

operate on the principle of repulsion and attraction of magnetism, First motors invented

When magnetic lines of flux flow in the same direction they __ each other

repel

Three types of motors

Shunt, Series, compound

What is torque

Turning force of a motor

Power applied to ____ causes motor to turn

Armature

Shunt Field Consider no load:

Armature continues to increase in speed, CEMF is almost the same as the applied voltage

Shunt motor Counter- Electromotive Force (CEMF)

Armature winding spins through the magnetic field produced by the pole piece a voltage is induced into the armature that is opposite the polarity of the applied voltage and is know as _____

Shunt Field Consider load:

As load is applied the torque is not sufficient to support the load at the speed at which the armature is spinning ,The arm slows down and CEMF is reduced and more current flows through armature windings, This produces an increase in magnetic field strength and a increase in torque, This is the reason armature current increases when a load is added.

The ___ are used to supply power to the armature from external power source

Brushes

How is torque created

By magnetic field of pole pieces and magnetic field of the loop or armature

How do you determine torque

By magnetic strength of the pole pieces and magnetic strength of the armature

Shunt motor Limits the flow of current through the armature when the motor is in operation

CEMF

The ____ forces the direction of the current flow to remain constant through certain sections of armature as it rotates

Commutator

Device used to convert electrical energy into mechanical energy

DC motor

High starting torque

Driving heavy loads in starting conditions, Will have a starting torque as high as 500%, Used in blower fans, power seats

Magnetic repulsion and attraction

Electrons enter the loop through the negative brush flow around the loop and exit through the positive brush, As current flows through the loop a mag field is created around the loop

Dc motors Disadvantages

High initial cost, Increased operation and maintenance due to presence of commutator and brush gear, Cannot operate in explosive and hazard conditions due to sparking

Shunt motors Speed torque Characteristics

Inrush current can be high due to no CEMF being produced by the armature,

Shunt motor CEMF is proportional to three factors

Numbers of turns of wire in armature, Strength of the mag field of the pole pieces, Speed of armature

DC motors Advantages

Offers wide range of speed control both above and below the rated speed, used where variable speed is desirable

Series Motors

Only a series field is connected with the armature, Armature current flows through the series field

Increasing the number of loops

Provides a strong continuous turning force

Dc motor advantages

Quick starting, stopping, reversing and acceleration

The shunt motor

Shunt fields connected in parallel, Permits external power to be supplied to shunt field and maintain a constant mag field, Ver good speed characteristics, Full load speed generally remain within 10% of no load speed, Often called constant speed motors

Series Motors

Speed is controlled by load coupled to motor, A load increase causes a speed decrease and vice versa, Because both armature and series field current increase the torque increases by the square of the current, If current doubles torque increases four times

Shunt Field Speed regulation of a DC motor

The amount by which the speed decreases as the load is added is called the speed regulation and is proportional to the speed of the armature., The lower the armature resistance the better the speed regulation

Shunt field Only current limiting factor:

amount of wire resistance in the windings of the armature

When magnetic lines of flux flow in opposite direction they ____ each other

attract


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