Chapter 9 - Three Phase Systems

Question 1

Give 2 types of 3 phase connections

Answer 1

1. Star Connected 3 Phase System.
2. Delta Connected 3 Phase System.

Question 2

Describe the Star Connected 3 Phase System?

Answer 2

In a Star Connection, the finish end of each phase is connected to one central point which is called a Star Point.

Question 3

Describe the Delta Connected 3 Phase System?

Answer 3

• The Delta Connection Configuration is another common Three Phase System.
• The three phases are connected in series to form a triangaler loop.
• There are no neutral points so delta connected systems must be balanced.

Question 4

List the Advantages of a Three-Phase Systems?

Answer 4

Advantages

• A 3 Phase Generator will produce more power than a Single Phase Generator of the same size.
• Due to the connections of the 3 phase Systems Power can be transmitted with greater economy since 3 or 4 of the conductors are required, rather than 6 if it was three seperate single phase Generators.
• 3 Phase systems Make self starting induction motors possible because 3 Phase voltages can create a rotating magnetic field.

Question 5

Explain the One Wattmeter method of measuring 3 Phase Power?

Answer 5

The 1 Wattmeter Method.

• One Wattmeter can be used to measure the total power dissipated in a Balanced 3 Phase Load.
• Total Power = 3 x Pa
• Pa = power indicated by the WattMeter

Question 6

Explain the Two Wattmeter method of Measuring 3 Phase Power?

Answer 6

The 2 Wattmeter Measuring Method.

• Again for measureing the total power dissipated in a 3 Phase Load, but the Load can be _Unbalanced or Balanced. _
• Works on both Star And Delta 3 Phase Connected Loads.

Question 7

Explain the purpose of using an unearthed electrical System onboard ship.

Answer 7

• The Reason the Royal Navy uses unearthed Systems is for Equipment Reliability not Personnel Safety.
• The electrical system is not directly earthed to the Ships Hull but current flow still occurs.
• Current flow happen due to a small amount of capacitance between the Supply Cables and Hull
• the Impedance is still fairly high so not much current flows
• By using unearthed systems, Fires and Damaged equipment are prevented should a piece of equipment become earthed to the hull by flooding, Rubbed Wiring, etc.

Question 8

Explain Why it is Lethal to touch a bare Conductor in an unearthed system?

Answer 8

• Because of the cabling to Hull Capacitance and the relatively low resistance of human enough current will flow to kill a person touching a live conductor.