Induction Wattmeter is used to measure power in AC Circuits. We all know Power = VI Cos(Theta) where V is the voltage , I is the Current and Theta is the phase difference between Voltage V and Current I.
Construction: Two electromagnets (electromagnets means coil wound on steel core so when current passes through the coil it magentizes the steel core ultimately becoming the magnets)
One Electromagnets is E type and another is U type. These electromagnets are made up of thin laminations of steel just like transformer to reduce eddy current losses.
The coil of the U type electromagnet are connected in series with the load and carries the load current hence called series magnet or coil, current coil.
The coil of the E type electromagnet are connected in parallel or across the load and carries the current proptional to the load voltage hence called shunt magnet or coil , voltage coil.
An aluminium disc is placed between both these electromagnets on the spindle to which pointer is also attached .this disc is free to rotate.
Few copper shading bands are also placed on the central limb of the E type electromagnet so as shunt magnet flux lag behind the voltage by 90 degree.
Working: when series magnet or coil carries the load current and shunt magnet or coil carries the current proptional to the load voltage. Both the magnets set up magnetic field . This magnetic field is cut by the alumnimum disc and emf is proudced in the disc that causes disc to rotate. The pointer attached to the disc also rotates which gives necessary reading.
Spring control mechanism is used to provide necessary control torque . The scale is uniform.
Use: Can be used upto 100 amp load current to measure the power above that can be used in conjunction with current transformers
Advantages and disadvantages of induction Wattmeter
- Uniform scale
- No effect of outside/stray field
- No frequency errors
- Have temperature errors