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Ac/Dc Machinies motor midterm

Terms in this set (68)

A transformer that decreases the voltage on its secondary winding with respect to the primary is called a ______ transformer.
. Step down
A transformer that increases the voltage on its secondary winding with respect to the primary is called a ______ transformer.
Step up
A transformer that produces the same voltage on its secondary as is applied to its primary winding is called a ______ transformer.
Impedance
A transformer with one winding common to both primary and secondary circuits is a(n)
Autotransformer
A transformer works on ______ law of physics.
Faraday's law
A variac is a(n) ______ that produces a variable AC voltage from a fixed voltage AC supply.
Autotransformer
An instrument transformer that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary is a(n) __
Current transformer
Circulating currents, called ______ cause heating and energy losses within the transformer core decreasing the transformers efficiency.
Eddy currents
Eddy current losses within a transformer core can be greatly reduced and controlled by ______ the thickness of the steel core.
Reducing
If a dual winding transformer has two secondary windings, each rated at 120 volt, 50 amp, what would the secondary current rating be if these windings were connected in series?
100 amp
If a dual winding transformer has two secondary windings, each rated at 120 volt, 50 amp, what would the secondary current rating be if these windings were connected in parallel?
25 amp
If a dual winding transformer has two secondary windings, each rated at 120 volt, 50 amp, what would the secondary voltage rating be if these windings were connected in series?
240 volt
If a dual winding transformer has two secondary windings, each rated at 120 volt, 50 amp, what would the secondary voltage rating be if these windings were connected in parallel?
240 volt
If the primary of a transformer is supplied with 480 volts, and the output voltage of the secondary is 120 volts, and the load of the secondary is 4 amps, how much current will flow in the primary winding?
1 amp
In an ideal transformer (ignoring any losses), the relationship of power in the primary and secondary winding is
Equal
The ______ current transformer primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit.
Wound
The difference in voltage between the primary and the secondary windings is achieved by changing the ______ of coil turns in the primary winding compared to the ______ of coil turns on the secondary winding.
Number
The input winding of a transformer is the ______ winding.
Primary
The method of identifying the orientation or direction of a transformers windings is called the _____
Dot convention
The output winding of a transformer is the ______ winding.
Secondary
The process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it is __
Mutual induction
The windings of a transformer are wrapped together around a closed magnetic iron circuit called a
Core
Transformers are rated in ___
Volt-amperes
Voltage control of a transformer is achieved by changing ______ in the primary winding.
Direction
Voltage ratio of a transformer is equal to the
Turns ratio
When the transformer load is increased, the primary current will
Increase
______ has an advantage over ______ because it can be easily generated, transformed into a much higher voltage and then distributed around the country over long distances.
AC voltage, DC voltage
______ of a transformer is the change in secondary terminal voltage when the transformer loading is at its maximum, while the primary supply voltage is held constant.
Voltage regulation
A device that provides a path for pressure created by an electrical fault inside the transformer to prevent rupture of the tank is a (n) ______ .
Explosion vent
A disadvantage of the ______ connected transformer is that it requires the use of three windings, and if any one winding becomes faulted or disabled, the whole group is disabled.
Wye/Wye
A transformer where the complete core and coil assembly is enclosed in a metal tank and immersed in an insulating oil, and the oil conducts the heat from the core and coil by means of natural convection to the surface of the enclosure where it is dissipated by radiation into the surrounding air, is a ______ transformer.
Oil-Immersed Self-Cooling
A transformer where the cooling of the coil windings is accomplished by natural convection of the surrounding air and by the radiation of heat from the different parts of the transformer structure is a ______ transformer.
Self-Air-Cooled Dry-Type
How much increase rating does a functioning air blast give a dry type transformer?
33 %
If transformer pad pressure is low, use ______ to raise pressure
Pure dry nitrogen
In a three phase transformer the magnetic flux of the three phases are ______ apart.
120 degrees
The fact that each winding must be wound for the full line voltage is a disadvantage of the ______ connected transformer.
Delta/Delta
The fact that there is no neutral or common connection is a disadvantage of the ______ connected transformer.
Delta/Delta
What is the main concern for distribution type transformers?
Dissipation of heat
Which is a type of oil used in wet transformers?
Vegetable oil
_ is a system that provides the ability for a motor or process to keep running instead of shutting down after one phase has gone to ground.
High resistance grounding
A ___ is a machine component used to reduce friction and maintain clearance between stationary and moving parts.
motor bearing
A design ___ motor is an integral-horsepower 3-phase induction motor design made for full-voltage starting and high locked-rotor torque, with locked-rotor current not exceeding a specified value.
. C
A design ___ motor is an integral-horsepower single-phase motor design made for full-voltage starting and locked-rotor torque higher than for design M, N, and O motors.
L
A(n) ___ is a metal tag permanently attached to an electric motor frame that gives the required electrical ratings, operating ratings, and mechanical-design codes of the motor.
nameplate
AC motors will typically use voltage levels of 24 V, 48 V, 90 V, or 180 V.
False
Directly applying any frequency to a motor other than the nameplate rated frequency will change the motor's operating characteristics (speed and output torque) and can damage the motor.
True
Electric motors typically require either direct current (DC), single-phase alternating current (1f AC), or three-phase alternating current (3f AC).
True
Electrical ratings included on motor nameplates include a ___ rating and phase
all of the above
For every 1% voltage unbalance, there can be up to a 6% current unbalance.
True
For fractional-HP frame sizes, the frame number represents the distance from the bottom of the base to the center of the shaft in 8ths of an inch.
False
Insulation breakdown is the most common cause of motor failure.
True
Most motors include light-duty-rated or medium-duty-rated bearings.
True
Motor manufacturers produce only energy-efficient motors.
False
Since a nameplate has limited space to convey the ratings, most information is abbreviated or coded to save space.
True
The NEMA ___ letter is a code letter representing a National Electrical Manufacturers Association (NEMA) motor classification for the torque and current curves of a motor.
design
The ___ is a code letter signifying the maximum operating temperature of the insulation used in a motor.
insulation class
The ___ is the difference between the winding temperature of a running motor and the ambient temperature.
temperature rise
The ___ rating is a description of the expected or allowed application of a motor.
usage
The ___ rating is the amount of current a motor draws when delivering full rated power output
current
The ___, or duty cycle, is the amount of time a motor can be operated without being turned OFF to allow for cooling.
operating time rating
The actual voltage applied to a motor should be within 15% of the motor's nameplate rated voltage.
False
The list of motor nameplate locked-rotor indicating code letters begins with the letter "A" and ends with "Z."
False
The voltage applied to a motor must always exactly match the nameplate voltage rating.
False
Typical operating environments for totally enclosed motor enclosures include the following: wet locations, dirty locations, oily locations, corrosive locations, extremely low- and high-temperature locations, gas and other hazardous locations, and combustible-dust locations.
True
When the nameplate of a motor does not list a service factor rating, a service factor rating of 1.00 is assumed and there is no built-in safety margin for the motor.
True
___ current, or starting current, or inrush current, is the amount of current a motor draws on startup or when the rotor is locked.
Locked-rotor (LRC)
___ motors are used in applications such as compressors, waste-disposal systems, electric hoists, gate-opening motors, and other applications in which the motor need only be turned on for short time periods to meet the application requirements.
Intermittent-rated
___ type is the type of protection given to a motor to shield the motor from the outside environment as well as to protect individuals from the electrical and rotating parts of the motor.
Enclosure
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