Three Phase Flashcards
To reverse the rotation direction of the magnetic field
Just reverse any two leads of a three phase motor
A four pole three phase motor has how many coils
12
The torque on the rotor depends on how fast the rotor drifts…
Backwards through the rotating magnetic field
The speed at which the magnetic field rotates is called the
Synchronous speed of the motor
On starting, the slip of the motor is at …
100%
In general the power factor of an induction motor increases with the load until the maximum power factor is reached at
Full load
Maximum torque is always developed when the phase angle is at
45 degrees
The rotor of a squirrel cage induction motor has four basic parts…
Core, conductors, end rings, shaft
Low starting torque is a result of the low resistance and high…
Reactance of the rotor
The wound rotor motor is always started with blank blank in the rotor circuit
Maximum resistance
The life of a motor is determined largely by the
Life of its insulation
A synchronous motor requires two sources of electric power…
One ac and one D.C.
When the D.C. Power is applied to the rotor, a set of what is produced
North and south poles
With a low D.C. Field current the ac current in the armature blanks the applied ac potential difference
Lags
Slipping a pole causes a blank of current in the D.C. Line during the blank period
Surge, slipping
Increasing the D.C. Current in the rotor winding increases the phase angle in the
Lagging direction thus reducing the power factor
A motor with a single winding can be connected for either…
Four pole or eight pole operation
Variable torque motors do what
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 so the power delivered by the motor remains unchanged
A two parallel Y connection requires a blank potential difference than a series Y connection
Lower
Avoid using too much solvent because it can
Flow into places too difficult to remove, and if left too long could damage insulation
The standard temp for measuring insulation resistance is
40 degrees C, 104 degrees F
The first symptom of an open rotor circuit is a lack of
Torque, the speed decreases and the motor makes a growling noise
Angular loads are a combination of
Radial and thrust loads
For the average service conditions bearings may be safely operated at temperatures up to…
95 degrees C, 200 degrees F
The capacity of the power supply limits the size of a blank that may be started across the line
Motor
At full load a motor will develop its rated power, draw its rated current, and run with its rated power factor, but with that there will be a
Rise in the motors internal temperature
The max torque a motor can deliver is usually
150 to 225% of the rated full load torque
Ex. Rated at 2HP full load can make 3 to 4.5 HP
Connecting an inductor in series with the motor windings is another method of
Reducing the starting current
The inductor adds inductive reactance to the circuit which is the basis for the name
Reactor starter
To ensure that a three phase motor control equipment is in good operating condition you should
Maintain a regular and systematic plan of inspections
When the starter contacts drop out when the potential difference across the line falls too low, is called…
Low voltage release
During plugging the current in an induction motor is higher than the locked rotor current. Blank blank is about three times the loss during normal starting
Power loss
What provides both backspin protection and surge protection when the motor stops
Off delay relay
What provides surge protection when the motor starts
On delay relay
The blank provide a reduced potential difference across the motor windings for starting
Autotransformers
Which method of starting produces the smoothest starting conditions of any of the methods of starting
Reactance starting
When a large number of steps is required to start what must be used
Resistance starting
What kind of winding can be a part of either the rotor or the stator in the alternator
Supply winding
The armature of an alternator always has the blank number of poles as the blank winding
Same, field
The current an alternator can conduct without overheating is determined by the size of the
Conductors in the winding
If the load is inductive the current blanks the voltage
Lags
If the armature of an alternator has four poles, how many poles does the field winding have
Four
Interlocks can be used in motor starting circuits so the motors can be started in a sertain sequence, the largest motor is started…
First
Power failure relays act if the potential difference across any phase drops below blank or less of the rated value
70%
Whenever an auxiliary generator starts it should run until when
All parts reach their normal operating temperature
When only one cranking period is provided it duration should be how long
At least 15 seconds
If more than one cranking period is provided each period should last how long
At least 7 seconds
When making a schedule for inspecting generators match the schedule to the maintenance schedule for the
Prime mover
The potential difference at which the electric systems of the utility company and the plant are connected
Service voltage
The major benefits of system grounding are
Longer insulation life, improvement in protective relaying, extra protection
A relay that operates faster as the current increases is said to have…
Inverse time characteristics
In a cascade system of large air gap circuit breakers the number of cascades steps should not exceed…
Two
In general three phase motors will give satisfactory service when operated at blank or more of their rated potential difference
90%
