Electrodynamometer Type Instruments

It is also called an electrodynamic instrument. It is a type of moving coil instrument that can be used on both AC and DC supplies on the same scale. Such an instrument can be used as an ammeter or a voltmeter but is more commonly used as a wattmeter.

Parts of Dynamometer

• Fixed Coils
• Moving Coil
• Spindle
• Control Spring
• Damping Chamber & Vane
• Pointer & Scale

Working Principle

The working principle of this instrument is that when two conductors are placed close to each other and a current is passed through them, a chemical reaction is induced in them due to the interaction of the magnetic field of the conductors. If the two parallel conductors have currents flowing in opposite directions (as shown in Fig "a"). Assume that this "X" sign means that the current is flowing in or downwards in the conductor while the dot (meaning the current is flowing outwards or upwards) the direction of the magnetic flux or magnetic lines around the two 

conductors will be the same, thus the magnetic fields of the two conductors will repel each other. As a result, a force of repulsion will arise between the two conductors. If the direction of current in both the parallel conductors is the same as shown in Fig "b" then the direction of magnetic flux around the two conductors will be opposite to each other. Due to this force of attraction will be created between the two conductors. In such instruments, the torque is generated as a result of the interaction of the magnetic field generated in the two coils according to the above principle.

Working Method

When the current to be measured passes through the fixed coils of the dynamometer-type instruments, it creates a magnetic field, and when the current passes through the moving coil, a magnetic field is created around it. A deflecting torque is generated in the coil which rotates the entire moving system (spindle, moving coil, pointer and damping vane) in the opposite direction to the spring. Thus the pointer moves on the scale and shows the reading. It is noteworthy in such instruments that when the direction of the current passing through the fixed coils changes, the direction of the current passing through the coil also changes. Thus the direction of the deflecting force does not change even though the polarity of the supply is changed. Hence these instruments can be used on both AC and DC supplies.

Uses

1. Similarly, ammeters are used. (directly for very low current and shunt in parallel with moving coil for high current measurement) 
2. These are used as voltmeters. (with a multiplier for a greater range) 
3. These are mostly used as watt meters. 
4. With a little modification, they can also be used as frequency meters and power factor meters.

Advantages and Disadvantages

The Electrodynamometer type instruments have the following advantages and disadvantages.

Advantages

1. Dynamometer works on both ac and dc current.
2. Since iron is absent in the working parts of the instruments, these instruments are free from hysteresis and eddy current errors.
3. Ammeters up to (10 A) and voltmeters up to (600 V) can be constructed.

Disadvantages

1. Scale is not uniform.
2. Torque-weight ratio is small. Friction errors tend to be serious.
3. These instruments are most expensive than the other types of instruments.