Week 2

Question 1

Provide three advantages that three-phase systems have over single-phase systems.

Answer 1

Increase in power capability
Less pulsating on rotating generators and motors
Generators, transformers, and motors are smaller and less expensive

Question 2

How far apart are the three coils of a generator from one another?

Answer 2

120 degrees

Question 3

How many times does the voltage fall to zero in one cycle?

Answer 3

Three times

Question 4

If the system voltage is 12,470 volts, what would the coil voltage be in a WYE configuration?

Answer 4

7,200 volts

Question 5

If the phase current in a WYE system is 40A, what is the coil current?

Answer 5

40 amps

Question 6

If the coil voltage in a DELTA system is 12,000 volts, what is the system voltage?

Answer 6

12,000 volts

Question 7

Why could linemen work barehanded on the older DELTA systems back in the day?

Answer 7

Because there was no ground or neutral reference. If there was nothing touching the other phases on the circuit, like a tree limb or another lineman, then there was no short circuit.

Question 8

Why is it hard to tell the difference between a DELTA and an ungrounded WYE in the field?

Answer 8

Because neither system has a system neutral.

Question 9

What is meant by “phase sequence”?

Answer 9

Phase sequence is the direction a motor will spin, based on how it is connected to the system. If the sequence is unchanged, the motor will spin the same direction as the generator.

Question 10

You are reading the phase sequence on a 7,200/12,470-volt system with a high-voltage phasing meter. You read across an open cut-out (from top to bottom) and you read 12,470 volts. What does this mean?

Answer 10

The phase sequence is changed. DO NOT CLOSE THAT CUTOUT!