3. OVERVIEW
• Introduction of 3 – Phase
• Advantage of 3-phase power over 1-phase
• Power Measurement in 3 – Phase Circuit
• 3 – Wattmeter method
• 2 – Wattmeter method
• 1 – Wattmeter method
• Error due to wattmeter connection
4. Introduction of 3 – Phase:
Phase Angle = 360 / number of poles.
= 360 / 3
= 120
5. ADVANTAGE OF 3-PHASE POWER OVER 1-PHASE
3 – Phase Power 1 – Phase Power
Output is greater than 1-phase. Output is less than 3-phase.
3-phase motor are self start. 1-phase motor are not self start.
Torque of 3-phase motor is uniform. Torque of 1-phase motor is pulsating.
Power factor of 3-phase machine is higher. Power factor is lower than 3-phase machine.
Efficiency is higher than 1-phase
machine.
Efficiency is lower than 3-phase machine.
More reliable and capable than 1-
phase machine.
Less reliable and capable than 3-phase
machine.
Required less amount of copper, For the same output. Required greater amount of copper, For the same output
6. Power Measurement in 3 – Phase Circuit:
THREE-WATTMETER METHOD:
3 wattmeter's are connected in such a
way that each has its current coil in one
line and its potential coil between that
line and some common point.
Total power : w1 + w2 + w3
7. TWO-WATTMETER METHOD:
Current coils of two wattmeter's are
connected in any two lines and the
potential coil of each joined to the
third line
Total power : w1 + w2 + w3
8. ONE-WATTMETER METHOD
If the load (3-phse) Y or Delta is
balanced,
the power in any phase can be
measured by a single wattmeter.
Total power = 3 X Power per phase
= 3 X wattmeter reading
9. ERROR DUE TO WATTMETER CONNECTION:
There are two methods to connect a
wattmeter in the circuit whose power
consumption is to be measured. They
are shown in the below figures.
