A wattmeter is a four terminal device. We usually
connect it in power circuits by shorting one current coil and one potential
coil terminal thereby making it a three terminal device.
wattmeter connection diagram
The point to focus is - wattmeter connection is very important to get correct reading.
What is the correct connection for a wattmeter ?
By correct connection we mean, the current coil must
be connected in series and the pressure
coil must be connected in parallel to
the load whose power is to be measured.
Ideally, current coil and pressure
coil have zero and infinite resistance
respectively. Therefore, the current through the current coil of the wattmeter will
be equal to load current and the voltage across the pressure coil of the wattmeter will be equal
to load voltage and the wattemeter measures the average real power absorbed by
the load.
Lets take an example to make it clear.
Let a wattmeter be connected in 3 phase balanced system
as shown to measure power absorbed by the resistance RR . Line to
line voltage be 100 V(rms). What will be the reading of the wattmeter ?
The wattmeter will read
Pw=VRB
× IR× cos Φ
where, Φ is the phase difference between VRB and IR
where, Φ is the phase difference between VRB and IR
Therefore, Pw= 100 × (10/√ 3) × cos 300
= 500 Watt
But, is this the power resistor RR actually
consume ?
Answer is No. Actual power consumption by RR =
(IR)2 × RR
= (10/√ 3) 2 × 10
=
333.33 Watt
So, what is wrong ?
The problem here is wrong wattmeter connection . Remember
? the current coil must be connected in series and the pressure coil must be connected in
parallel to the load. Therefore, correct
connection would be to just connect terminal P to N, keeping all other connections
as it is.
