Dynamometer Type Moving Coil Instruments (Ammeter, Voltmeter) | Working | Construction | Advantages | Disadvantages

Dynamometer Type Moving Coil Instruments (Ammeter, Voltmeter):

It is a modification of PMMC instrument. Permanent magnet in PMMC is replaced by two fixed coils. Soft iron case in PMMC is also removed. Thus moving coil is also air-cored.

Princlple of working:

It works on the simple principle i.e. whenever a current i carrying conductor is placed in a magnetic field, force is exerted on the conductor. This is same as that of a PMMC instrument.
In this case fixed coils produce magnetic field whenever they are energized. Moving coil carrying the current to bei measured is suspended in this magnetic field.
Hence it experiences a torque. The moving coil then rotates i through an angle proportional to the current flowing through the moving coil.
As the pointer is attached to the moving coil. we get the pointer detlection proportional to the current to be measured.

Construction:

As shown in Fig. 4.6.1 F,. F are two identical circular, air cored coils. They are connected in series and these coils are placed parallel to each other.
They produce a uniform magnetic field. M isa light moving coil. It lies in the magnetic field produced by F. F, Moving coil is supported by a spindle and jewelled bearing. i Two control springs wound in opposite directions are used as leads to pass current in moving coil. These springs produce the controlling torque. Damping torque is obuained by using air-friction damping.
Dynamometer type instrument is current sensitive. The capacity of carrying current of moving coil is greater than that in PMMC instrument. Usually the moving coil can carry current upto 100 mA.
Dynamometer type instrument is used as a transfer instrument i.e. instrument is calibrated on d.c. source and then used without modification to measure a.c. It is equally accurate on both A.C. and D.C.)
Dynamometer is the most fundamental meter movement. It is versatile. It can be used as an ammeter, a volumeter, a wattmeter, power-factor meter or frequency meter. Double coil movement is used for polyphase wattmeter. Cross-coil movement is useful for power factor meter and frequency meters.

Advantages:

1. Use of iron is avoided. Hence it is free from hysteresis and eddy current losses.
2. It is uscful for both a.c. and dc. measurements. High degree of accuracy
3. It can be used as a transfer instrument.
4. It is often used as a standard instrument for calibrating ammeters and volt meters.

Limitations:

1.Operating field is weak as air-cored coils are used. To obtain sufficient torque, large turns on moving coil are used. Hence it becomes heavy. Therefore frictional error increases. Power loss of the instrument is more than that of a PMMC instrumment.
2. Scale is nonlinear.
3.Torque to weight ratio is small.
4.  It is an expensive instrument.
5. The instrument has low sensitivity
6. Shieding should be provided to avoid effect of stray magnetic field