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
Why is this resistance removed in steps as the motor accelerates?
to produce a smoother accelerating torque
What happens to the magnitude and frequency of the voltage induced in the rotor as the motor accelerates?
They decrease.
What type of device do you commonly use to manually control the resistance in the
secondary circuit?
a drum controller
What is meant by the term automatic, definite-time acceleration?
Definite-time starters automatically remove the resistance in steps at the same time
intervals each time the motor is started.
How is automatic, definite-time acceleration normally achieved?
Time-delay relays are normally used to provide the definite-time steps for removal of the
secondary resistance
What is the major disadvantage of automatic, definite-time acceleration?
It is not sensitive to the requirements of the load. Under heavy loads, the resistance may be
removed too quickly. Under light loads, the resistance may be removed too slowly
What is meant by the term automatic, speed-sensing acceleration?
Automatic, speed-sensing acceleration is a means of automatically sensing the speed of the
rotor in order to determine the proper time for removal of resistance from the secondary
winding
How is motor speed sensing normally achieved?
by using frequency-sensing relays
How do you reverse the direction of rotation of a wound-rotor motor?
by interchanging any two of the line leads to the stator
Can a wound-rotor motor be reversed by making changes to the rotor circuit?
No. To reverse the direction of rotation of the motor, you must reverse the direction of
rotation of the stator field
Explain the difference between a wound-rotor motor starter and a wound-rotor motor- speed regulator
With a wound-rotor motor-speed regulator, some of the secondary resistance may be left in the secondary circuit to control the speed of the motor. The resistors used with a speed regulator would have to have a higher wattage rating than those used in a starter
Why does speed regulation of a wound-rotor motor become very unstable below about 50% of synchronous speed?
The flat slope of the torque-versus-speed curve means that a small change in torque output
of the motor requires a large change in speed
What is the major advantage of a speed regulator that uses regeneration over a speed regulator that simply uses secondary resistors?
The efficiency of a regeneration type of controller is much higher than that of one that
utilizes secondary resistors
Which is generally more accurate: a regulator that uses regeneration or one that uses
secondary resistors? Why is this so?
A controller using regeneration is generally more accurate because it constantly compares the set speed to the actual speed and adjusts itself to bring the two together
What will happen to the amortisseur winding in a synchronous motor if the motor is permitted to operate at less than synchronous speed for an extended period of time?
The amortisseur winding will overheat.
What device in the controls is used to protect the amortisseur winding from the problem
stated in the above question?
the out-of-step relay
What is the purpose of the field discharge resistor?
The field discharge resistor protects the rotor field from having high voltages induced in it
when the motor is running at lower than synchronous speed
What is different about a field contactor compared to a normal contactor?
The field contactor has a positive overlap between the NO contacts and the NC contact, which a standard contactor does not have. That is, when the contactor pulls in, the NO contacts close before the NC contact opens. When the contactor drops out, the NC contact
closes before the NO contacts open
What does the abbreviation PFR stand for?
polarized field frequency relay
What are the three main functions of the PFR?
It monitors the rotor speed so that it can time the application of the DC rotor field current
to somewhere between 92% and 97% of synchronous speed.
It times the application of the DC excitation so that it will create a magnetic field in the rotor
circuit that will reach full strength when two opposite poles are aligned with each other.
It disconnects the DC field excitation if an overload or voltage dip causes the motor speed
to drop, and reapplies it if the speed returns to within synchronizing speed before the
out-of-step relay opens its contacts.
What is the purpose of the AC coil on the PFR?
It senses the speed of the rotor.
What is the purpose of the DC coil on the PFR?
It times the application of the rotor field excitation so the field reaches full strength when
opposite poles (rotor and stator) are adjacent
Explain the purpose of the field reactor coil.
It produces a voltage drop when alternating current is flowing in the rotor circuit. This voltage drop is necessary so the AC coil of the PFR can sense the AC in the circuit.
What is the purpose of the field rheostat?
It controls the DC through the rotor field. The power factor of the motor may be varied by adjusting the excitation current in the rotor field
What will happen if the resistance of the field rheostat is decreased while the motor is running?
The current through the rotor field will be increased, causing the power factor of the motor to become more leading (or less lagging).
Explain how the out-of-step relay protects the amortisseur winding against overheating.
The out-of-step relay is a timer relay that starts to time out whenever the rotor is turning slower than synchronous speed. If the relay times out, it opens a contact in the control circuit, which causes the line contactor to drop out and the motor to stop
Explain why the NC contact from the PFR is in series with the field contactor.
When the motor is first started, or anytime the rotor turns at lower than synchronous speed, the PFR pulls in, opening the NC contact. This prevents the DC field from being applied to the rotor.
As the motor accelerates, what happens to the magnitude and frequency of the AC induced
in the field coils?
They decrease.
At what percentage of synchronous speed is the DC excitation normally applied to the field windings?
It is normally applied somewhere between 92% and 97% of synchronous speed. The actual
percentage depends on the load applied to the motor.
What happens to the voltage induced in the amortisseur winding once the motor is operating at synchronous speed?
At synchronous speed, the relative speed between the rotating stator field and the amortisseur winding is zero. Therefore, no voltage is induced in the amortisseur winding
Why are synchronous motors often rated with a starting duty cycle?
The amortisseur winding is not designed to carry current for long periods of time. The amortisseur winding requires time to cool before restarting
How can the power factor of a synchronous motor be made more leading?
The resistance of the field rheostat must be reduced to allow more excitation current to flow
through the rotor field winding
Which action takes place first when the field contactor pulls in?
The NO contacts that apply the DC excitation to the field close.
Which takes place first when the field contactor drops out?
The NC contact in series with the field discharge resistor closes.
What is the reason for the positive overlap of the contacts in the field contactor?
The positive overlap ensures that the field will never be open-circuited. An open circuit in the field circuit can cause a high voltage to be induced in the DC windings of the rotor field
List three types of friction brakes.
drum, disk and band brakes
Are friction brakes normally electrically applied and spring-released?
No. Friction brakes are normally spring-applied and electrically released.
Where are disk brakes normally mounted?
A disk brake is normally mounted on the end bell of the motor.
Explain what is meant by the term plugging
Plugging is a control function that provides braking by reversing the motor line voltage
polarity or phase sequence so that the motor develops a counter-torque that exerts a
retarding force
Explain what is meant by the term dynamic braking
Dynamic braking is a method of retarding the motor speed or completely stopping the motor by making it act as a generator to supply a resistive load.
Explain what is meant by the term regenerative braking
Regenerative braking is a method by which the motor is made to act like a generator and feed energy back into the system
Which type of brake is automatically applied if there is a system power failure?
spring-applied type of friction brake
Which type of braking is generally considered to be the fastest deceleration method?
plugging
State a disadvantage of friction-type brakes.
Any of the following: the friction parts require periodic maintenance and replacement; the brakes tend to be bulky; the initial cost of the brake is high
Which braking method requires a reversing controller?
plugging
What is the main disadvantage of using the time-delay relay circuit for plugging compared to the zero-speed switch method?
The time-delay plugging circuit cannot sense the actual speed of the motor shaft. Therefore, any change in motor load would require a different setting on the time-delay relay
When using dynamic braking on a squirrel-cage induction motor, how can the braking torque be adjusted?When using dynamic braking on a squirrel-cage induction motor, how can the braking
torque be adjusted?
by varying the DC applied to the stator
When using dynamic braking, what can happen when there are too-frequent stops and starts?
The motor can overheat.
What is the essential difference between regenerative braking and dynamic braking when comparing energy conversion?
Regenerative braking delivers true power back to the electrical supply (recovers braking
energy), whereas dynamic braking expends most of the braking energy as heat dissipation
in resistors, etc.
When using eddy-current braking, how is the braking torque adjusted?
by varying the DC to the electromagnetic coils of the eddy-current brake
Which of the braking methods is considered to be the most expensive?
the eddy-current brake
