➢ Constructional details of dynamometer type wattmeter:-
• Fixed Coils [F1 & F2]. : fixed coil is divided into two sections to give a uniform field near the center [F1 & F2]. The operating field is produced by the fixed coil
• Moving coil [M]:-The moving system consists of a coil mounted on the spindle which is free to rotate in the space between the two fixed coils. The coil is made up of thin copper wire and is air cored to avoid hysteresis.
• Control torque provided by two spiral springs. They also act as connecting leads for the moving coil. Pointer is mounted on the spindle.
• Mirror is provided to avoid parallax error.
• Damping is provided by air friction damping
Construction: It essentially consists of the fixed and moving coil. The fixed coil is split into two equal parts which are placed close together and parallel to each other. Moving coil is pivoted in between two fixed coils.The fixed and moving coils are exited / connected as shown in figure. The moving coil is attached to the moving system so that under the action of deflecting torque the pointer moves over the scale.
Working: When the instrument is connected in the circuit operating current flow through the coils due to this mechanical force exists between the coils. The result is that moving coil moves the pointer over the scale to give reading. When direction of current reverses, then it reverses the direction of current of fixed as well as moving coil so that the direction of deflecting torque remains unchanged hence these instruments can be used for measurement of A.C. & D.C. power.
Working Principle of Wattmeter: It consists of two stationary coils, called current coils and one moving coil, called voltage or potential coil. The moving coil is mounted on the spindle, in the gap between two stationary coils, as shown. The current coils are connected such that they carry the current proportional to (or equal to) the load current and the voltage coil is connected in such a way that it carries the current proportional to the load voltage. The interaction between two magnetic fields causes the production of force on moving system, which is proportional to the product of voltage and current i.e. power. The meter can be calibrated directly to indicate the power in watts.
OR
• The dynamometer wattmeter works on the motor principle
• When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction as per Fleming’s left hand rule.