This instrument is similar to PMMC(Permanent Magnet Moving Coil) Instruments except in place of permannet magnets , there is a coil .
Construction: There is a coil which is divided into two sections ..this coil does not move during operation and remains stationary. So this coil is called as fixed coil.
These two fixed coils are placed adjacent to each other , side by side. These fixed coils are air cored means does not wound on iron or any magentic material to avoid hysteresis losses on A.C.
A coil is placed on the spindle and free to rotate known as moving coil. A pointer is also attached on the spindle to give the reading.
Working: When current flows through fixed coils than a magnetic field is set up in the instrument which on turn magentizes the moving coil .so moving coil moves in the space which causes pointer to rotate and give necessary reading.
Damping: Air friction (Pneumatic) damping is used as the field is very weak produced by air cored fixed coils. So eddy current damping can not be used.
Use: Widely used as wattmeter to measure power in AC circuits. Can be used for AC and DC measurements but these instruments are costly and inferior than PMMC instruments for DC measurements.
Advantages and disadvantages of Dynamometer instruments
- No hysteresis and eddy current losses
- Low senstivity due to low torque weight ratio
- Can be used both on AC and DC
- Expensive and inferior than PMMC instruments for DC measurements.
- These instruments are widely used as wattmeter to measure the power in AC Circuits.