Electricity 7 Of 7 Flashcards
Faraday used Oersted’s discovery, that electricity could produce motion, to build the worlds first ______________ in 1821.
Electric motor
What is the left hand rule?
It shows the direction of the flux lines around a wire that is carrying current.
What is the right hand rule for motors?
It shows the direction that a current carrying wire will be moved in a magnetic field.
What fingers point to what when using the right hand rule for motors.
Middle finger points in direction of current.
Forefinger=magnetic field
Thumb= motion of the conductor.
The four principle parts of an elementary dc motor are?
1-magnetic field
2-a movable conductor
3-a commutator
4- brushes
You can make the basic dc motor start by itself by giving it an armature that contains?
Two or more loops.
The ___________ is the rotating element of the dc motor.
Armature
The leads from the armature coils are connected to the raised portions of the commutator segments known as __________.
Risers
Brushes are made of soft carbon material containing a large amount of?
Graphite
Why are brushes designed to wear out faster than the armature?
Because they are cheaper to replace than to service the armature.
The field assembly in a dc motor consists of ______________ and ___________.
Pole pieces
Field coils
Most motor housings contain these three parts?
A field frame and two end frames
What are lap windings used for?
Low-voltage, high-current motors
What are wave windings used for?
High-voltage, low-current motors
What is cemf.
Counter electromagnetic force
Since the _________ opposes the line voltage, it acts to cut down the short circuit current through the _______ resistance armature.
Cemf, low
What gives electric motors a built in system for regulating their own speed?
CEMF
What happens to the motor when a loads is connected?
Motor slows down
Cemf goes down
EMF goes up
Speed goes up
Motors are rated in terms of the load they can drive. What is the rating called?
Their power output.
Power output is a measure of _________ Power delivered by the motor at full load and is given in ____________.
Mechanical power,
Horsepower
When a series motor has no load in needs little _________.
Torque
How does a series motor with no load reduce armature current?
By speeding up to build up cemf to reduce the armature current
Series motors are used when?
Sudden changing loads are put on the motor.
How do you reverse the direction of a dc motor?
Reverse either the field or the armature current, but not both
The _________ makes it easy to operate a motor-driven device that needs to be constantly changed in direction and speed.
Drum controller
The natural rate at which the magnetic field rotates about the stator is called?
The synchronous speed
What does a stator have that insures a rotating magnetic field is created.
Shaded poles
As the stator field rotates, the _________ is dragged along at this synchronous speed.
Rotor
Single phase motors can be started by hand, or they can be made with damper windings on their rotor to be?
Self starting
A two-pole motor with 60hz freq. has a synchronous speed of ____________.
3600 rpm
Rpm= (Hz x 120)/#of poles
For the induction motor to work, the rotor must __________ and ______ and a rate slightly less than that of the rotating field.
Slip
What does the double squirrel cage provide?
Higher starting torque
The stator and rotor of an induction motor act like?
Windings of a transformer
Since the induction motor relies on electromagnetic induction, it’s rotor cannot?
Run at synchronous speed
How are hysteresis losses created?
When molecules of an iron bar remain aligned after the position of the bar in the field has been changed
How does a ac repulsion motor resemble a dc motor?
It has a:
Drum-wound armature, a commutator, and brushes.
How does a ac repulsion motor resemble an induction motor?
I has a stator with distributed windings.
The hysteresis synchronous motor uses a _______ rotor to develop a greater torque.
cobalt-steel
