Motor theory
WHAT IS THE BASIC OPERATING PRINCIPLE OF AN AC INDUCTION MOTOR
Field induces voltage into the rotor
LIST THE OPERATING CHARACTERISTICS OF A COMPOUND MOTOR
Good speed regulation and good torque
LIST THE OPERATING CHARACTERISTICS OFA SHUNT MOTOR
Good speed regulation, low starting torque
Shunt
High resistance small wires
5 LIST THE OPERATING CHARACTERISTICS OF A SERIES MOTOR
High torque, less resistance, thick wires and poor speed regulation
LIST DANGERS OF A COMPOUND MOTOR
If the shunt field burns out motor can run away
IN AN AC INDUCTION MOTOR HOW IS THE ROTOR CONNECTED TO THE STATOR
Its not connected
WHAT IS THE PURPOSE OF THE INTERPOLES
Limit sparking
Series
Low resistance thick wires
A 1 HP motor with a Service Factor of 1.0 can produce
1 HP without overheating.
If a motor were 100% efficient, it would produce 1 horsepower for every ___________watts.
746
WHAT DO THE MARKINGS ON THE MOTOR LEADS A1-A2, S1-S2, F1-F2 MEAN
A= armature s = series f = shunt
In a DC motor, the stator is referred to as the
field.
The nameplate speed listed on an induction motor is the
full-load speed.
The purpose of post-installation tests on a motor is to
gather baseline data for comparison when doing preventive maintenance or troubleshooting.
Motor losses all show up as ____________.
heat.
Reducing the field current in a shunt motor will
increase motor speed.
An oversized motor will operate
inefficiently.
Fully enclosed motors are normally
likely to be larger and more expensive than open motors.
The startup torque of most motors is ____________.
more than full load torque.
Two different NEMA 142T motors will have the same
mounting and shaft dimensions.
The Service Factor listed on a motor's nameplate indicates how
much overloading the motor can tolerate without damage.
Thermal protectors built into a motor usually
need to be manually reset after tripping.
Brushes will spark if they are
not seated into the commutator curvature.
If the supply voltage to an induction motor is more than 10% above or below the nameplate voltage, the motor will probably
overheat.
An induction motor that is running slower than its rated speed on full rated voltage is most probably
overloaded
Motors burn out when ____________.
overloaded for long periods
Compound motors have both
parallel and series field windings.
The brushes and commutator in a DC motor
reverse the current in one armature coil at a time
Which kind of DC motor has the poorest speed regulation?
series
When the load on a series DC motor is removed
shaft speed may become dangerously high.
Which motor is best protected for outside operation?
splash-proof.
A motor will overheat quickest when it is ___________.
stalled.
One NEMA design induction motor should not be replaced with any other NEMA design because
starting torque and slip characteristics are different.
If the shunt winding of a cumulatively compounded motor burned out, the motor would_______________.
tend to run away or over speed
A voltmeter connected across the armature terminals of a running DC motor will measure _________________.
the armature supply voltage
The current to the armature of a DC motor decreases when
the motor speeds up.
If the listed FLA of a replacement motor is lower than that of the replaced motor,
the overload breakers must be recalibrated.
Compared to class A or B insulation, Class F insulation will
tolerate higher temperatures.
Most DC motors have
two, four, or six field poles.
NAME THE MAJOR PARTS OF A MOTOR
End bells, frame, bearings, stator and rotor
WHAT DETERMINES THE SPEED OF AN AC INDUCTION MOTOR.
Frequency and stator poles
IFI WEAKEN THE SHUNT FIELD WHAT WILL THE MOTOR DO
Speed up
When installing a new motor, always be sure
adjustable mounting brackets are tight, and allow no motor movement under changing loads.
The environment in which a motor operates must not be hotter than the motor's
ambient temperature rating.
The field coils in a shunt motor
are wound with many turns of fine wire.
Most motors operate at maximum efficiency __________.
at or near full load
All motors include ____________.
at some way to reverse the current in the stator or the rotor.
As the load on most motors increases _____________.
both current and torque will increase.
A motor with a 20 minute duty rating
can produce its rated horsepower for 20 minutes in each hour.
Reversal of current in an AC motor occurs when_____________.
current in the power lines reverses.
LIST DANGERS OF A SERIES MOTOR
Can run away if unloaded
A series motor will not automatically provide regenerative braking because
Counter EMF cannot rise above source voltage
DC motors draw high armature current at startup because
Counter EMF is 0
WHAT DETERMINES THE SPEED OF A DC MOTOR.
Current
IN AN AC INDUCTIÖN MOTOR MORE POLES WILL INCREASE OR DECREASE THE SPEED OF THE MOTOR.
Decrease
ALL MOTORS MUST HAVE SOMETHING TO DO WHAT
Reverse the polarity in the feilds or armature
LIST DANGERS OF A SHUNT MOTOR
Run away because of residual magnetic
WHAT ARE THE FOUR TYPES OF DC MOTORS
Series, shunt, compound and permanent magnet
WHAT IS MEANT BY DIFFERENTAL COMPOUND
Shunt and series working against each other
WHAT IS MEANT BY CUMMULATIVE COMPOUND
Shunt and series working together
INCREASING THE FIELD WILL DO WHAT TO A DC MOTOR
Speed will decrease
HOW DOI CHANGE THE DIRECTION OF ROTATION OF A PHASE AC INDUCTION MOTOR.
Switch 2 supply leads
ALL MOTORS WORK ON WHAT PRINCIPLE
The ability to switch poles/magnatism
A motor with a high LRA will
draw high current when starting or stalled.
Compared to a motor with a low power factor, a high power factor motor will
draw less current.