Line V-1 Flashcards
Name three types of mechanisms used for sensing pressure.
bourdon tube; bellows; diaphragm
With respect to pressure switches, what does the term differential mean?
In a pressure switch, differential is the difference between the cut-in pressure (switch closes)
and the cut-out pressure (switch opens)
For sump pumping, the float switch _________ the circuit at high liquid level, and ________ the circuit at low level
closes, opens
A paddle arm is sometimes associated with which type of two-wire pilot device?
flow switch
What type of switch is sometimes installed into the ducts of commercial air heating systems to prevent the duct heaters from energizing when there is no air movement in the duct?
air-flow switch
What device is used to activate the mercury switch contacts in a common thermostat?
bimetallic element
What type of switch is commonly used to respond to temperature changes in liquids?
remote bulb, temperature switch
What type of mechanical switch is used to prevent over-travel?
limit switch
With which type of switch are the terms transmitter and receiver commonly associated?
photoelectric switch
What other name is a plugging switch commonly known by?
zero-speed switch
What is the purpose of the lockout relay used with a plugging switch?
This relay prevents activation of the plugging switch contacts until power is present at the motor. It is sometimes also called a safety-latch relay
What is anti-plugging protection?
Anti-plugging uses a zero-speed switch to prevent a motor from reversing until it has come to a complete stop
State two advantages that across-the-line starting has over reduced-voltage starting for AC motors
Any two of the following: simple control; low component cost; maximum starting torque;
minimum acceleration time for motor
State two advantages that reduced-voltage starting has over across-the-line starting for AC
motors
Any two of the following:
reduced inrush current on starting; reduced torque on starting;
less disturbance (line drop) to the power system
List three methods of reduced-voltage starting.
Any three of the following:
primary impedance starters; autotransformer starters; wye-delta
starters; part-winding starters; solid-state starters
What is the difference between a reduced-voltage starter with an open transition and a reduced-voltage starter with a closed transition?
For open-transition starters, there is a momentary loss of voltage (power) to the motor
as the control transfers from the starting to the running stage. But for closed-transition
operation, there is no interruption of voltage
If an AC motor will not start under load with rated voltage, will you be able to start it with
reduced voltage?
No. If the voltage is reduced for starting, then the torque is also reduced (in fact, it is
proportional to the square of the voltage). If the motor will not start with full voltage, it
definitely will not start at reduced voltage, with the same load requirement
Why is it sometimes necessary to provide larger-than-normal contactors for some motors?
Because frequent (repetitive) operation under large starting inrush conditions will greatly shorten the life of the contacts
What type of motor duty is required for a service that must operate for definite, specified,
alternate intervals of load and rest?
intermittent duty
What is the most important consideration in determining whether or not a motor may be
started directly across the line?
permissible voltage fluctuations (disturbance) to the power distribution system
State two situations where it may be necessary to use starting reactors rather than resistors.
For high-voltage/high-current motors
where heating or physical construction of the resistors creates a problem
What is the main disadvantage of using reactors to start induction motors?
It worsens the already poor (lagging) power factor of induction motor circuits.
Describe how several steps can be added to the starting stage of the primary resistance
starter
by adding additional resistors in series, each with an accelerating contactor
What type of starter is the primary-resistance starter?
closed transition
What standard taps are available on motor-starting autotransformers?
50%, 65% and 80% of the full-line voltage rating
Autotransformer starters are sometimes referred to as _______ starters
compensator
Autotransformers used for starting motors are generally intended for intermittent duty. What does this mean?
Intermittent duty means a requirement of service that demands operation for specified alternate intervals of load and no-load, load and rest, or load, no-load and rest.
What protection is usually provided for the autotransformers?
Thermal (high-limit) switches equipped with heat sensors
What two common three-phase connections are used for starting motors with autotransformers?
three transformers connected in wye; or two transformers connected in open-delta
Why is the autotransformer method of motor starting considered to provide more starting torque per line ampere of current than any other type of reduced-voltage starter?
For all other starters, the line current and the motor current are the same value. In the autotransformer starter, the line current (primary) is a further reduction of the motor current
(secondary) through transformer action
What additional components are necessary to convert an open-transition, wye-delta starter
to a closed-transition type
an additional three-pole starting contactor and three power resistors
How many leads are run between the wye-delta starter and the motor?
six
Can all three-phase motors be used for wye-delta starting?
No. The motor connection box used must allow access to both ends of each phase winding
external to the enclosure, since the wye-delta starter starts the motor in a wye connection and switches the windings to a delta connection for running
Why is a wound-rotor motor normally started with resistance inserted in its secondary circuit?
With resistance in the secondary circuit, the starting torque is increased and the starting
current is reduced
