WorksheetsĐề Mr Sơn
Total questions: 24
Worksheet time: 12mins
How the Current transformers (CT) allow the measurement of AC currents
By without breaking the current −carrying leads
By breaking the current −carrying leads and connect serially with CT
By without breaking the current −carrying leads and connect parallel with CT
Transformer Principle show: If two coils are wound around a common soft magnetic iron coil core but have no electrical connection. In case of Coil 1 generates a magnetic flux and Fix, rotate Coil 2 to 90 degree (compare with Coil 1)
Coil 2 will have a no voltage
Coil 2 will have a lower voltage
Coil 2 will have the maximum possible voltage
When switching off coils, special caution is needed due to
At the moment of switching off the coil, the change of the magnetic field is very rapid and very high
At the moment of switching off the coil, the change of the magnetic field is very slowly and very low
10 second After switching off the coil, the change of the magnetic field is very rapid and very high
Characteristics of AC is
The current I varies in direction (polarity) and in quantity in the circuit over a period of time t
The current I leads the flow in the same direction at the same quantity (size) over a period of time t
The current I varies in direction (polarity) and in the same quantity (size) over a period of time t
The symbol for frequency is
f
λ
β
For the voltage, the calculation is as follows:
u = Upeak * sin (2π *f * t) (M3.11 P212)
i = Ipeak * sin (2π *f * t)
λ = C*T
Peak−to−peak value means twice the peak value, in other words the value measured between:
Positive maximum value and negative minimum value of a sine curve
Negative maximum value and Positive maximum value of a sine curve
Positive maximum value and negative maximum value of a sine curve
If a wire loop spins in a magnetic field, a sinusoidal AC voltage is induced in the wire loop. The size of the AC voltage depends on
The spare between the wire loop and the magnetic field source
The of the wire loop area in the magnetic field
The angle of the wire loop towards the magnetic field
The frequency of a sinusoidal AC depends on the rotational speed of the generator. The frequency of the output voltage increases as
The generator speed increases
The generator speed decreases
In not depend on the generator speed
The frequency of a sinusoidal AC depends on the rotational speed of the generator. If the magnetic wheel of a bipolar device (one pole pair) rotates 50 times per second
an AC with a frequency of f = 100 Hz is generated
an AC with a frequency of f = 25 Hz is generated
an AC with a frequency of f = 50 Hz is generated
When connecting a load to two of the phases e.g. A and B, the Line to Line Voltage is available.
It is √3 times the voltage of a single Line
It is 3 times the voltage of a single Line
It is 1/3 times the voltage of a single Line
If neutral conductors are connected to the three systems, a standard three−phase system is obtained with a so called
Star connection
Delta connection
Line to Line connection
The true power of an effective resistance is the product of the RMS (Root Mean Square ) values of voltage and current. The true power is measured in
watt (W)
Voltage ((V)
H ( Hertz )
How many coil are there in the An autotransformer with voltages is stepped down
There is one coil for both input and output side
There is one coil for input side and one coil for output side
There is two coil for input side and one coil for output side
Current transformers (CTs) are special transformers which allow the measurement of AC currents without breaking the current−carrying leads
Current transformers must always have a load (ammeter)
Current transformers can have no load (circuit is Open)
The connector for the measuring circuit must be removed if there is current in the current-carrying lead
A three-phase transformer supplies a phase-to neutral voltage of 115 volts on all the three phases. On phase A the current is I = 30 amps, on phase B: I = 20 amps, on phase C:I = 50 amps. The total power output is
11500 VA
5750 VA
Zero VA
To control the permanent mag net generators (PMGs),the generator control unit (GCU) do:
Rectifies a AC armature current and sends a DC voltage to the exciter field winding
Convert a DC current and sends a AC voltage to the exciter field winding
Rectifies a AC armature current to DC voltage of PMG output
For a parallel operation of generators, some conditions must be met
The generators must be synchronized
Reactive power must be kept as large as possible
Real power output must be distributed differently
The synchronous motor is used to achieve a constant rotational speed. In order to produce a constant magnetic field, It requires
A AC current for the Rotor
A DC current for the Rotor
A DC current for the Stator
A synchronous motors are the most important three−phase motors .The motor consists of two major parts
The stator (stationary part) and the rotor (rotating part)
The DC power part and AC Power part
The magnetic flux generator and exciter generator
A synchronous motors are the most important three−phase motors, Whether the rotor is required power supply or Not
Yes, require AC power supply to magnetic flux generator
Yes, require DC power supply to exciter generator
No, do not require power supply
How to reverse the rotation direction of Asynchronous three−phase motors
Connect once stator phase to lower voltages power supply
Interchanging two of the three stator phases
Cannot reverse the rotation direction
In Asynchronous three−phase motors, the rotational speed of the rotor is
Always higher than the rotational speed of the rotating field
Always equal the rotational speed of the rotating field
Always lower than the rotational speed of the rotating field
Which kind of power supply to operate the capacitor motor
Single−phase AC voltage
Two−phase AC voltage
Three−phase AC voltage
