3 Phase Systems Flashcards
A four-pole, three-phase motor has _____ coils.
12
The speed at which the magnetic field of an induction motor rotates is called the:
Synchronous speed
On starting, the slip of the induction motor is ____% of the synchronous speed.
100%
In general, the power factor of an induction motor increases with the load until the maximum power factor is reached at:
Full load
In an induction motor, maximum torque is always developed when the
Reactance and the resistance of the rotor are equal
Specifically, the core, the conductors, the end rings, and the shaft are the main parts of the squirrel-cage:
Rotor
When taking air-gap measurements on large motors, you should:
- keep a record fo the measurements and compare regularly
- take measurements about 90% apart
- take one measurement in the direction in which the load bears on the bearing
- all of the above
ALL OF THE ABOVE
In a squirrel cage motor, low starting torque is a result of
Low resistance and high reactance
A wound-rotor motor is always started with _________ in the rotor circuit.
Maximum resistance.
To improve a motors service life, you should do all of the following EXCEPT:
Remove the windings insulation
A synchronous motor requires:
Both AC and DC power
When DC power is applied to the rotor of a synchronous motor….
A set of north and south poles is produced.
With a low DC field current, the AC current:
Lags the potential difference
Slipping a pole causes a(n):
Surge of current in the DC line
The power factor of a synchronous motor will be less than 100% if the DC current in the rotor winding is:
Either too high or too low
A motor with a single winding can be connected for:
Either four-pole or eight-pole operation
Variable torque motors are designed to give:
An increase in torque with an increase in speed
In a constant horsepower motor, the torque:
Doubles when the speed is cut in half
When you reconnect the coils of an induction motor designed to run on a potential difference of 240V so that it will run on 480V
You reconnect the coils in series
A two-parallel-Y connection:
Requires a lower potential difference than a series-Y connection
When removing grease and oil from a motor, you should avoid using too much solvent because it
Can damage the insulation
40degrees C is the
Standard temp for measuring insulation resistance
A lack of torque, a decrease in speed, and a growling noises are all indications of a(n)
Open rotor circuit
Angular loads are caused by forces applied to the bearing
Both parallel and perpendicular to the shaft
200degrees F (95C) is the
Maximum temp at which bearings can be operated safely
The capacity of the power supply does NOT limit the
Normal running speed of the motor
At full load a motor connected to the line will do all of the following except:
Decrease its internal temperature
A motor rated at 2HP at full load can develop ___to_____ hp if it delivers at max torque
3 to 4.5
An inductor connected in series with the motor windings is found in a(n)
Reactor starter
To ensure that three phase motor control equipment is in good operating condition, you should:
Maintain a regular plan of inspections
Which of the following stops a motor when the potential difference falls too low and automatically restarts the motor when the potential difference returns to normal?
Low-voltage release
During plugging, the current in an induction motor is higher than the locked-rotor current causing a(n)
Power loss
A controller that provides both surge and backspin protection for a motor that drives a pump has a(n)
On-delay relay plus an off-delay relay
Reduced voltage starters for three phase motors change the potential difference across the motor by means of:
Auto transformers or resistors
When smooth starting is necessary but more than two steps are required, ________ is used.
Resistance starting
The supply winding in an alternator
Can be part of either the rotor or the stator
If the field winding has 4 poles, how many poles does the armature of the alternator have?
4
There is no current in the armature of the alternator when the
Regulator is disconnected
The size of the conductors in the winding determines the
Current an alternator can conduct without overheating
If the load driven by an alternator is purely inductive, the current:
Lags the potential difference.
When interlocks are used in motor-starting circuits to overcome voltage drop problems, which motor in the circuit is started first?
The largest
In most automatic installations, power failure relays act if the potential difference drops to _______ of the rated value
70% or less
Whenever an engine starts, it should run until all parts reach:
Their normal operating temperatures
In order to avoid damaging an engine when only one cranking period is provided, the cranking attempt should last at least _______ seconds.
15
In establishing for inspecting generators you should match the schedule to the maintenance schedule of the:
Prime mover
The potential difference where the electric system of the utility company and the plant are connected is called the ________ voltage.
Service
All of the following are benefits of system grounding, EXCEPT:
Allows service to be maintained in entire circuit until the fault is located
A relay with inverse-time characteristics operates:
Faster as the current increases
In a cascade system having large air-gap circuit breakers, there should not be more than ___ cascade step(s).
2
In general, three phase motors will give satisfactory service when operated at what percent of their rated potential difference?
90%
