Dynamometer type Wattmeter

Dynamometer or electrodynamic type wattmeters are moving coil instruments in which the operating field is produced, not by a permanent magnet but by another fixed coil.

In dynamometer type wattmeter, the two coils i.e field producing stationary coil and moving coil are connected as shown.

The field producing coil is divided into two equal halves and positioned parallel to each other. The distance between the two halves is adjusted to obtain a uniform magnetic field.

The field producing coils have fewer number of turns of thick wire to keep its resistance as small as possible, ideally zero, so that whole load voltage appears across the moving coil.

The moving coil has large number of turns of thin wire and is connected in series with a high resistance R , so that it can have as high resistance  as possible, ideally infinite and the whole load current passes through the field producing coil stationary coil.

Dynamometer type Wattmeter Working Principle

When a current carrying coil is placed in a magnetic field, the coil experiences a torque.

Deflecting torque

Let the load current passing through the fixed coils be  I₁  and  the small current through the moving coil be I₂ .

The interaction of magnetic fields produced by  I₁  and  I₂  produces the necessary deflecting torque T_d .

As field producing coils are air-cored, therefore  

I₂  ∝ V   

or    I₂  = K₂ V                   ...(2)

As  deflecting torque, T_d = BINA

∴     by putting  (1) and (2),  T_d =( K₁I₁)( I₂ )NA=( K₁I₁)( K₂ V )NA 

or    T_d  ∝ I₁ V  

or    T_d  = K I₁V = K × Power

This  expression is for dc circuits where voltage V and current I₁ are constant.

For ac circuits, V and I₁  will  be varying continously  ∴ their instantaneous values  v and i respectively are taken .

∴  The instantaneous deflecting torque is given by   T_d  ∝ vi 

Let the values of v and i be given by

v = V_m sinθ

and  i = I_m sin(θ-Φ)    , Φ=angle of lag,assuming lagging power factor  load

Due to inertia of the moving system, the instrument will indicate  average power

∴  T_d  ∝ average of vi

So,   deflecting torque ∝ real power

Hence, we find that in case of ac circuits also, the deflection is proportional to the real power consumed by the load.

If the wattmeter is spring controlled, controlling torque T_c =K₄ ϴ

When pointer comes to rest ,  T_c = T_d

or     ϴ ∝ real power

∴ Dynamometer wattmeters have linear scales upto certain limit.

Its clear now that dynamometer wattmeters can be used for dc as well as ac.

Advantages :-

1. Highly accurate for medium and high power factors. 
2. Can be used for ac as well as dc.
3. The fixed coils are air cored , therefore, no hysteresis losses. 

Disadvantages and limitations  :-

1. Not useful for power measurements at low power factors as inductance of moving coil causes serious errors.  

Related concepts :-

1.  How to Connect a Wattmeter ?
2.  Two wattmeter method - Balanced load condition
3.  Hot wire Instruments