Motors Lvl 2 Flashcards
Why are Wound-Rotor motors used for more industrial applications such as hoists and elevators?
Because they have have very high starting torque.
How are wound-rotor motors and squirrel-cage motors different?
The construction, Squirrel Cages have bars, while wound has wraps of conductors, which terminate to brushes via slip rings.
T/F: The construction of stators for wound-rotor and squirrel-cage motors is different.
False.
T/F: Rotors for Wound-Rotor motors can be wired for high or low voltage.
False, They can only be wired Wye or Delta to match their stator.
What are brushes made out of?
Carbon-Graphite mix
What table in the code gives Full-Load Current for 3-phase AC motors?
Table 430.250
Adding resistance to a Wound-Rotor motor during startup (through slip rings and brushes) keeps what two things in phase?
Current and Voltage
T/F: Resistance is not always connected to the Wound-Rotor Motors through the brushes.
True, but resistance is sometimes used to keep current and voltage in phase during startup.
Would resistors added to the circuit for the purpose of speed regulation be connected to the stator or rotor? (Wound Rotor)
The Stator
What are the three types of regulators?
- ) Drum Switches
- ) Rheostats
- ) SCR’s
What table in the code is used for determining the maximum rating or setting of protection for branch circuit short-circuits and ground faults?
Table 430.52
How are single-phase motors different from wound-rotor motors?
What power they operate on, and the way that they start.
What are the three types of single-phase motors?
- ) Shaded Pole
- ) Split Phase
- ) Capacitor Motor
The three different types of single-phase motors represent the three ways to do what?
To start rotation one way or another, so that it doesn’t just sit and “single phase’ or shake without rotation.
Shaded Pole motors are usually fractional, 1/__ hp or less.
1/20 hp
Why won’t single phase motors rotate without a starter or operator to push it in a definitive direction?
Because single phase power is switching polarity 60 times a second, and poles are placed 180 degrees apart, so there is not rotating magnetic field to start rotation on its own.
How do you reverse direction of a single phase motor?
Switch the two start leads.
Shaded poles consist of __ wrap(s) of short circuited wire on either side of the stator.
1, a single wrap
What is the most common type of single-phase motor, being used for fans, tools, and machines where starting is easy.
Split-Phase
Split Phase motors are available in sizes from 1/__ hp to 1/__ hp.
1/30 to 1/2 hp
A split phase motor has two windings, a(n) ____ winding and a(n) ____ winding.
Run winding and start winding.
Does the run winding or start winding have more resistance? (Split-phase motor)
The start winding, the resistance is what causes the phase shift to start rotation.
What are the three types of capacitor motors?
- ) Capacitor-Start
- ) Capacitor-Run
- ) Capacitor-Start-and-Run
Whenever a capacitor is used to start a motor, it is disconnected from the circuit at 60-80% by a ____.
Centrifugal Switch
The capacitor in a start winding causes the current to lag the applied voltage by about ____ degrees.
40 Degrees
Capacitor Motors have higher starting ____ than Split-Phase Motors.
Torque
____ Capacitors are larger and are designed to be disconnected from the circuit after startup.
Electrolytic
____ Capacitors are smaller and are designed to remain connected to the circuit during the entire duty cycle.
Oil-Filled.
How do you reverse direction of a Shaded-Pole Motor?
You have to take the rotor out and flip it over.
How do you reverse direction of a Capacitor-Start Motor?
Reversing either the start winding or the run winding.
How do you reverse the direction of a Capacitor-Run Motor?
Switch the Capacitor from one winding to another.
What is an Overload?
An excessive current that is confined to the normal conductor path.
What is an Overcurrent?
Any current over the normal level.
What is a ground fault?
Contact between an ungrounded conductor and any grounded raceway, box, enclosure, or fitting.
What is a short circuit?
When current takes a shortcut back to the source, Current rises to hundreds of times what it should be.
OCPD’s are for protection of what types of faults?
Large faults such as Short Circuits and Ground Faults
____ is designed to protect the motor from long durations of excess current, to avoid damage from overheating.
Motor Overload Protection
OCPD’s are divided into what two categories?
- ) Fuses
2. ) Circuit Breakers
Where in the NEC is the Table “Standard Ampere Ratings for Fuses and Inverse Time Circuit Breakers”?
Table 240.6(A)
Time-delay fuses are ____-element fuses.
Dual.
What are the 3 types of Overload Protection?
- ) Thermal
- ) Bi-Metallic Strip
- ) Electronic
The maximum amount of current a device can interrupt safely is known as its ____.
Interrupting Rating
T/F: In a DC motor, both the Armature and the Field windings are connected to the source.
True
What is the rotating part of a DC Motor called?
The Armature
What is the stationary part of a DC Motor?
The Field Windings. (Field Frame consists of the Field Windings and Housing)
A ____ is a ring made of insulated segments that keep the armature in the correct polarity.
Commutator
How do you change the direction of rotation in a DC Motor?
Change the polarity of either the Field Windings or the Armature Windings. If the both are reversed there is no change.
Is it easier to reverse polarity of the armature leads or the field winding leads?
Armature leads, because they’re usually accessible at the brush rigging.
What are the four types of DC Motors?
- ) Series
- ) Shunt
- ) Compound
- ) Permanent Magnet
Why are series DC motors called “Suicide” Motors?
Because if there is no load, they will keep speeding up until they fail.
In a ____ DC motor, the field winding and armature are wired in Parallel.
Shunt
A compound motor has attributes of both ____ and ____ motors.
Series and Shunt
Series Motors have ____ starting Torque.
High
Shunt Motors are used in applications where ____ and ____ speed is needed.
Constant and Adjustable
What type of DC Motor doesn’t have coils of wire in its field windings?
A permanent-magnet motor. It’s field windings are permanent magnets and it does not need a field created there by a coil.
What are the three types of DC Generators?
- ) Series-Wound
- ) Shunt-Wound
- ) Compound-Wound
What is the difference between AC Alternators and DC Generators?
DC Generators have a Commutator to keep all of the current positive so it creates pulsating DC instead of AC.
What type of DC motor can self-brake?
Permanent-Magnet
Why is reduced voltage starting a common way to start motors?
To avoid stressing your power system with large starting currents
What is the difference between Open-Circuit and Closed-Circuit transitions?
Open-Circuit completely disconnects for a brief moment before connecting it it line voltage.
Closed-Circuit never fully disconnects from the motor.
What are the two types of Autotransformer starting?
- ) Three-Phase (Open Transition)
2. ) Two-Phase (Closed Transition)
____ starting involves putting a resistor in series with the motor power circuit until motor is up to speed.
Primary-Resistor
____ starting is a temporary connection to only part of the windings during startup.
Part-Winding
T/F: Only Dual Voltage motors can be used for Part-Winding starting.
True
T/F: Any motor can be started with the Wye-Delta starting method.
False, it must be a motor that is able to be configured in Wye or Delta
T/F: It is possible to eliminate Inrush current when starting motors.
False.
