DC Motors
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