Construction and working of Repulsion Type Moving Iron Voltmeter and dynamometer type wattmeter

Repulsion type Moving Iron instruments and Dynamometer type wattmeter

1. Construction and working of  Repulsion Type Moving Iron Voltmeter/Instruments.

Construction :

One variation of moving iron instrument is repulsion type instrument. Here we keep the shaft of the moving system along the axis of the coil. We attach a soft iron bar or rod to the shaft so that it can move with the movement of the moving system. We fix another soft iron bar or rod with the inner coil body as shown. When there is no current in the coil, both iron bars are nearest to each other.

Working :

The current in the coil produces the magnetic field. The magnetic field induces same magnetic poles to similar ends in both of the iron bars or rods. Naturally, these two iron bars repel each other. The repulsive force causes the deflection of the pointer attached to the moving system. This is because due to the repulsive force, the bar attached to the moving system tries to go away from the bar fixed to the inner coil structure. This repulsion force is approximately proportional to the square of the current through the coil. Since this force is proportional to the square of the current, deflection of the pointer is irrespective of the direction of current in the coil that means the deflecting torque is unidirectional. This is the reason we can use a repulsion type moving iron instrument in DC and AC both.

Sometimes, to increase the uniformity of the scale, we use a tongue-shaped iron strip in place of the fixed rod.

Deflecting Torque of Repulsion Type Moving Iron Voltmeter

the force between two similarly magnetized iron rods or sheets produces the deflecting torque in this instrument.  So, the deflecting torque is proportional to the pole strength of both bars.

 Repulsive force between similar Poles

m1.m2

In other words, we can say that the deflecting torque is proportional to the product of pole strength of both bars. Again, pole strength is proportional to the magnetic field strength H. Hence, we can write the instantaneous deflecting torque is proportional to the square of magnetic field strength.

Again, the magnetic field strength is directly proportional to the current in the I2

As the deflecting torque is proportional to the square of the measuring current, the scale of the instrument is not uniform or even. Although if we use a properly shaped iron sheet instead of the iron rod in the instrument, the scale becomes quite even. We normally provide gravity control to repulsion type instruments but in modern days some manufacturers provide spring control also. We cannot provide eddy current damping in this type of instrument. Because the presence of the permanent magnet in eddy current damping system may disturb the magnetization of the fixed and moving iron pieces. This may cause serious error in the reading of the instrument.

Advantages of Repulsion type moving iron instruments: 

Suitable for a.c. as well as d.c. measurements. Simple and robust in construction as the moving system (soft iron piece) is free from current leads. Good accuracy.

Cheaper in cost as compared to permanent magnet moving coil instrument.

A large scale of a length of about 2409. The instrument has high torque to weight ratio.

The instrument can be used for low frequency measurements also. 

Disadvantages : 

Power consumed by the instrument is high as compared to that of the permanent magnet moving coil instrument. The scale is non-uniform.

Due to the presence of iron part in the operating system, error due to the hysteresis effect is introduced. To reduce this effect nickel iron alloys are used.

Similar to other instruments, this instrument also suffers from the errors introduced by friction, temperature variation and lack of balance in the moving system. 

The operating magnetic field being weak, the stray fields affect the working of the instrument. To reduce this effect it

is sometimes necessary to provide a magnetic shield to the instrument.

2. Construction and working of Dynamometer Type Wattmeter

Construction :-

The dynamometer type Wattmeter is one kind of dynamometer type instrument. Here we use two coils.

Current Coil

One coil is the current coil. We connect the current coil in series with the circuit. So, the main current can flow through this coil. We divide the current coil into two equal halves. Then we place these two halves side by side.

Voltage Coil

Then, we place another coil in the space available between two halves of the current coil. We connect this second coil across the source. This coil is a voltage or potential coil. Sometimes we also call it as pressure coil. Also, we connect one high resistance in series with the voltage coil. This resies resistance limits the current through the coil. Also it minimize the inductive effect of the voltage coil. We generally provide spring control with pneumatic damping in a dynamometer type wattmeter.

Working Principle :

The basic working principle of a dynamometer type instrument is somewhat similar to that of a permanent magnet moving coil instrument. The only difference is that here instead of using permanent magnet we use a current carrying coil for producing the operating magnetic field. The coil used for producing the magnetic field has two halves. We place both the halves side by side. The coil is an air-cored coil. Then, we place a moving coil in between two halves of the fixed field coil. The Dynamometer type instrument is normally spring controlled. We attach a pointer with the moving system.

Advantages of Dynamometer Type Wattmeter

1. A carefully designed such an instrument can give a very accurate measurement. So, we usually use this type of instrument as a standard instrument for calibrating other instruments.
2. Electrodynamometer type wattmeter can measure AC and DC power with equal accuracy.

Disadvantages and Error in Dynamometer Type Wattmeter

1. Inductive effect of voltage coil causes a serious error in this type of wattmeter. Although a non-inductive resistance in series with the voltage coil improves the accuracy of reading. Because, this arrangement reduces the phasing effect of the voltage coil inductance.
2. Besides that, there is always some voltage drop across the current coil. Also, the voltage coil takes a current from the system. There may be an error in the dynamometer type wattmeter due to these two reasons. Although, we can overcome this problem by connecting an inductor in series with the voltage coil. Also we place this additional inductor in such a way that it opposes the field flux created by the current coil.