THREE PHASE MOTORS and ALTERNATORS
Phase rotation:
The direction of rotation of a manetic field produced by the voltage of a poly-phase power system.
Frequency:
The number of complete cycles of AC voltage that occur in 1 s.
A dual voltage motor connected to operate on high voltage (460v) will have its windings connected in series or parrallel?
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A dual voltage motor connected to operate on low voltage (208-230) will have its windings connected in series or parrallel?
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Dual Voltage Motor
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How often can a motor be operated that has a nameplate that states the time rating (duty) is 30 minutes?
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How often can a motor be operated that has a nameplate that states: Duty continuous?
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How to find the Percent Slip?
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If a three phase motor is running at full load and it starts single phasing, what will happed?
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If single phase power is applied to the stator of a three phase motor at start up, what will happen?
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The rotor of a three-phase squirrel-cage induction motor will turn:
Faster than the rotating magnetic field of the stator.
Synchronous:
The speed at which the magnetic field rotates.
MEGGER
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Motors are designed to operate within _ % of its rated name plate voltage(s)?
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Power factor
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Power factor correction
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Service factor
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Single Phasing
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Squirrel Cage Induction Motor:
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What does the arcronm NEMA stand for and what is its purpose?
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What is the purpose of 3 capacitors connected to a three phase motor?`
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Why can an induction motor never reach synchronous speed?
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The nameplate on a 3HP motor states SF=1.25. 1. What percentage above the nameplate rating can this motor be loaded for a short period of time without causing serious degradation? 2. What is the maximum horsepower this motor can deliver for a short period of time without causing serious degradation? 3. Can this motor produce its service factor continuously?
1. 25% 2. 3.75HP 3. No
What are two ways to determine the direction of rotation of a three-phase motor?
1.Make an electric connection to the motor before it is mechanically connected to the load. 2.Use a Phase rotation meter.
What are the three basic types of three-phase motors?
1.Squirrel cage Inductor motor 2.Wound rotor Inductor motor 3.Synchronous motor
How is the rotor constructed in a three-phase squirrel-cage induction motor?
A cylindrical device constructed of heavy wire. A shaft placed through the center of the cage permits the cage to spin around the shaft.
Prime Mover:
A device supplying the turning force necesary to turn the shaft of a generator or alternator.
Alternator:
A machine the generate AC by a rotating conduchtor through a magnectic field.
Revolving-field-type Alternator:
Alternator that uses a stationary armature and a rotating magnectic field to AC generator.
Revolving-armature-type Alternator:
An alternator that uses a rotating armature with sliprings to generate AC.
How can the direction of rotation of a three-phase motor be changed?
By reversing two of its stator leads.
Phase rotation meter:
Compares the phase rotation of two different three phase connections. (determine the rotation of a motor).
The nameplate speed in RPM's listed on an induction motor is the speed of the rotor at _ load?
Full
What does the nameplate code letter indicate?
KVA/HP
Stator:
Stationary armature located in the revolving-field type alternator.
What main part of all three phase motors is basically the same?
Stator
Excitation Current:
The DC current used to produce electormagnetism in the rotor of an alternator.
What three factors cause the magnetic field to rotate in a three phase motor stator?
1.The fact that the voltage in a three phase system are 120 degrees out of phase with each other. 2.The fact that the three voltage change polarity at regular intervals. 3.The arrangement of the stator windings around the inside of the motor.
What three factors determine the output voltage of a three-phase Alternator?
1.The length of the armture of stator conductors (number of turns) 2.The strength of the magnetic field of the rotor. 3.The speed of rotation of the rotor
What two factors determine the output frequency of a three-phase Alternator?
1.The number of stator poles. 2.Speed of the rotation of the rotor.
What three factors determines the amount of induced voltage in the rotor of a squirrel cage induction motor?
1.The strength of the magnetic field of the stator. 2.The number of turns of wire cut by the magnectic field (in the case of a squirrel-cage rotor, this will be the number of bars in the rotor).\ 3.The speed of the cutting action.
A dual voltage three phase motor nameplate indicates a rated voltage of 208-230/460. Can this motor be connected to a three phase 208 volt system? Can this motor be connected to a three phase 480 volt system?
1.Yes 2.Yes
Phase shift:
A change in the phase relationship between two quantities of voltage or current.
What is the principle of operation of all three-phase motors?
Rotating magnetic field
Why is locked rotor amps (LRA) high when a three phase squirrel-cage induction motor is first started?
The fact that the rotor is not turning when power is first applied to the stator. Because the rotor is not turning, the squirrel-cage bars are cut by the rotating magnetic field at a fast rate.
Percent Slip:
The percent of speed difference between the speed of the rotor and the speed of the rotation magnetic field of a motor.
Squirrel Cage Rotor
The rotor of an AC induction motor constructed by connecting metal bars together at each end.
What main parts of all three phase motors are totally different?
The type of rotor used.
When is maximum torque developed in a three phase squirrel cage induction motor?
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Why does the current in the rotor and stator as the speed of the rotor decreases?
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Why does the current in the rotor and stator decrease as the speed of the rotor increases?
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A dual voltage three phase motor nameplate indicates a rated voltage of 230/460. Can this motor be connected to a three phase 208 volt system? Can this motor be connected to a three phase 480 volt system?
1.NO 2.Yes
What three factors determine the amount of torque in a three-phase squirrel-cage induction motor?
1.Strength of magnetic field of stator. 2.Strength of magnetic field of rotor. 3.Phase angle difference between stator flux and rotor flux.
What two factors determine synchronous speed?
1.The number of stator poles (per phase). 2.The frequency of the applied voltage.
Which type of alternator is the most common?
Revolving-field-type Alternator.
Rotor:
Rotating member of the machine.
