Electrical Grid 2: Chapter 2 - Three-Phase Power - Possible Test Questions - WEEK 8 Flashcards

1
Q

Generating three-phase power is similar to generating:

A
  • single-phase power; the only difference is that instead of one coil of wire, there are three. To better illustrate the principle we will show a different type of magnet.However, the principle remains the same.
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2
Q

Each coil of wire on the generator will produce:

A
  • Its own voltage and current. The voltage and current on each coil will be 120 degrees out of phase with the others. This produces a three-phase sine wave.
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3
Q

Three-phase power results in:

A
  • much smoother and more efficient operation of equipment. Because three-phase power delivered to a load is constant, the three phase load can be much smaller than a single-phase load while providing the same output.
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4
Q

The three coils of a three-phase generator can be connected in:

A
  • either a DELTA configuration or a WYE configuration.
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5
Q

In a WYE system, coil current and phase current remain:

A
  • the same.
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6
Q

In a DELTA system, the system voltage is the same as:

A
  • the coil voltage.
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7
Q

Delta and three-wire Wye systems look:

A
  • the same in the field. Both have three phases and carry no neutral.
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8
Q

The four-wire WYE system is the most:

A
  • widely used system.
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9
Q

In the field, three-wire WYE systems and DELTA systems are:

A
  • identical in appearance.
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10
Q

All of the generators connected to a regional transmission network are all:

A
  • at the same angle of rotation at any given time.
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11
Q

When the phase sequence (shown as ABC in the graphic above) is maintained throughout the system:

A
  • three-phase motors will spin in the same direction as the generator.
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12
Q

If the phase sequence is unchanged:

A
  • rotation will remain unchanged.
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13
Q

If any two phases are reversed:

A
  • rotation will change.
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14
Q

A phasing meter is a high voltage voltmeter used for:

A
  • proving phase sequence. The meter consists of a phasing probe and a meter probe, which attach to hot sticks. An insulated jumper connects the two probes.
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15
Q

Phasing meters are used to ultimately prove that:

A
  • the sequence has not changed.
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16
Q

To prove phasing, the following steps are taken:

A

1) Verify that the meter is working properly. The manufacturer usually
provides a testing socket that is located in the phasing meter case.
2) Verify the correct voltage on both sides of the open point as depicted below.
3) Confirm proper phase sequence between the open points of the same phase as indicated by 0 (zero) volts on the phase meter. A zero reading can indicate that the phases are in sequence, or the meter is malfunctioning. To ensure that the meter is working properly, a cross- phase check of the meter should follow any zero reading.

17
Q

The direction that each individual motor needs to rotate is determined:

A

electricians and is dependant upon the equipment the motor is connected to.

18
Q

As each phase peaks in the sine wave:

A
  • the magnetic field rotates.
19
Q

Reversing two phases feeding a three-phase motor will:

A
  • reverse the rotation of that motor.