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.