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

Question 1

Generating three-phase power is similar to generating:

Answer 1

• 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.

Question 2

Each coil of wire on the generator will produce:

Answer 2

• 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.

Question 3

Three-phase power results in:

Answer 3

• 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.

Question 4

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

Answer 4

• either a DELTA configuration or a WYE configuration.

Question 5

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

Answer 5

• the same.

Question 6

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

Answer 6

• the coil voltage.

Question 7

Delta and three-wire Wye systems look:

Answer 7

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

Question 8

The four-wire WYE system is the most:

Answer 8

• widely used system.

Question 9

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

Answer 9

• identical in appearance.

Question 10

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

Answer 10

• at the same angle of rotation at any given time.

Question 11

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

Answer 11

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

Question 12

If the phase sequence is unchanged:

Answer 12

• rotation will remain unchanged.

Question 13

If any two phases are reversed:

Answer 13

• rotation will change.

Question 14

A phasing meter is a high voltage voltmeter used for:

Answer 14

• 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.

Question 15

Phasing meters are used to ultimately prove that:

Answer 15

• the sequence has not changed.

Question 16

To prove phasing, the following steps are taken:

Answer 16

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.

Question 17

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

Answer 17

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

Question 18

As each phase peaks in the sine wave:

Answer 18

• the magnetic field rotates.

Question 19

Reversing two phases feeding a three-phase motor will:

Answer 19

• reverse the rotation of that motor.