How to connect a Wattmeter ?

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 R_R . Line to line voltage be 100 V(rms). What will be the reading of the wattmeter ?

The wattmeter will read  

                             P_w=V_RB × I_R× cos Φ      
where, Φ is the phase difference between V_RB  and  I_R

      _Therefore,       P_w= 100 × (10/√ 3) × cos 30⁰

                                 = 500 Watt

But, is this the power resistor  R_R  actually  consume ?

Answer is No. Actual power consumption by R_R  =  (I_R)² × R_R

                                                                        ₌  (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.