Unit 20 Flashcards
Dual-voltage ac motors are made with two field windings, each rated for the ____ of two possible operating voltages
Lower
In a motor, the stationary field winding of a motor is mounted on the _____, which is the part of the motor that does not turn
Stator
What are the three main characteristics of a series-wound dc motor?
Poor speed control
Slow down considerably when a load is applied
Run at very high rpm’s without a load
The actual current drawn by the motor is dependent upon what two things?
Load on the motor and actual operating voltage at the motor terminals
Universal motors are ______ horsepower motors that operate equally well on ac and dc
Fractional
The most prevalent type of ac motor is the ______ motor
Induction
This type of motor has no physical connection between its rotating member (the rotor) and the stationary member (the stator)
Squirrel-Cage Induction Motor
How would you reverse the rotation of a three-phase ac motor?
Swap any two of the three line conductors
In a dual-voltage ac motor the field windings are connected in _______ for low-voltage operation and in ____ for high-voltage operation.
Parallel ; Series
If a motor is ______, or it operates at a _____ below its rating, the operating current can increase to a value above the motor FLA rating.
Overloaded ; voltage
The rotating part of the motor is referred to as either the ____ or the _____
Rotor ; Armature
If the rotating part of the motor winding is jammed so that it cannot rotate, no CEMF will be produced in the motor winding, which will cause the motor to what? And what is the main reason for it?
Operate at locked-rotor current (LRC) ; Low impedance on the conductor
The FLA rating is the current rating in amperes that the motor _____ while carrying its rated horsepower load at its rated voltage
Draws
One of the great advantages of the shunt-wound dc motor is its ability to to what?
Maintain a constant speed under load
How many watts are there in one horsepower?
746
When voltage is first applied to the field winding of an induction motor, what is the only thing opposing the flow of current through the motor winding?
Conductor resistance
When voltage is first applied to the field winding of an induction motor it will have a very large inrush of current, how much current will there be?
A minimum of six times the FLA rating.
DC motors use brushes with springs to hold them in contact with a _______
Commutator
A motor must have ______ opposing magnetic fields in order to rotate
Two
When a motor is operating at LRC, it is at least ____ times the running current value
Six
In a synchronous motor, the _____ is locked in step with the rotating _____ field and is dragged along at the synchronous speed of the rotating magnetic field
Rotor ; Stator
What is the formula for calculating the Full-Load Ampere rating of a three-phase motor?
FLA = (Motor HP x 746) / (E x 1.732 x Eff x PF)
In a Shunt-Wound dc motor, the armature and field are in ______
Parallel
The NEC requires standard overload protection devices to be sized at ____ to _____ percent of the motor FLA rating.
115 ; 125
What is the formula for calculating the Full-Load Ampere rating of a single-phase motor?
FLA = (Motor HP x 746W) / (E x Eff x PF)
According to the NEC, electrical components must be sized in accordance with what as listed in the NEC Tables?
Full-Load Current Rating
(T/F) Induction-type ac motors depend on commutation for operation
False
The output mechanical work of a motor is measured or rated in what?
Horsepower
Wound-rotor induction motors only operate on what type of power?
Three-phase ac power
What does FLA stand for?
Motor Full-Load Ampere
To reverse the rotation of a dc motor you must reverse either the _____ or the _____
Stator field ; Armature magnetic field
Once the motor begins turning, the rotor-bars (winding)) will be increasingly cut by the stationary magnetic field, resulting in an increasing what?
Counter-electromotive force
Series-wound dc motors have very good torque characteristics and are used as _____ motors for engines
Starter
What is the problem with commutation?
As the motor operates, the motor parts rub against each other and the motor wears itself out
In an ac induction motor, the stator produces what?
A rotating magnetic field
Wound-rotor motors can be used in applications that require some _____ control
Speed
