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A 3-phase balanced supply system is connected to a 3-phase unbalanced load. Power supplied to this load can be measured using: 1. two wattmeters 2. one wattmeter 3. three wattmeters Which of these statement/s is/are correct?

A 3-phase balanced supply system is connected to a 3-phase unbalanced load. Power supplied to this load can be measured using: 1. two wattmeters 2. one wattmeter 3. three wattmeters Which of these statement/s is/are correct? Correct Answer 1 and 3

Measurement of polyphase power: 3 - ϕ

1) For 3ϕ – 4 wire → neutral is present

2) As per Blondel’s theorem, if neutral is not present for ‘n’ wire system, then ‘n-1’ number of wattmeter’s are required for ‘n’ wire system.

Type of

Load

Star

3 - ϕ

3 - wire

Star

3 - ϕ

4 – wire

Delta

3 - ϕ

3 – wire

Balanced

2

1, 2, 3

2

Unbalnced

2

2, 3

2

 

  • The one-wattmeter method is used for the measurement of three-phase power in case balanced loads only
  • It is used for both star and delta connected loads
  • Two wattmeter method and three wattmeter methods are used for both balanced and unbalanced loads
  • Two wattmeter method is best suitable for three-phase three-wire system and three wattmeter method is suitable for the three-phase four-wire system

 

Note: If neutral is available we shouldn’t use a 2 – wattmeter method.

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