Damping torque or damping forces is the speed deviation of an electromechanical torque deviations of a machine while the angle deviation is called synchronizing torque [1]. If the damping force is too large, then the pointer will come to rest slowly and this is called as over damping.

In order for an ammeter to measure a device’s current, it must be connected in series to that device. This is necessary because objects in series experience the same current. Ammeter in Series: An ammeter (A) is placed in series to measure current. All of the current in this circuit flows through the meter.

DC measurement instruments measure the average value. In AC current, the average value is zero as the current oscillates. Therefore DC instruments can not measure the Ac current.

There are two types of moving coil instruments namely, permanent magnet moving coil type which can only be used for direct current, voltage measurements and the dynamometer type which can be used on either direct or alternating current, voltage measurements.

There are two general types of moving-iron instruments namely:

• Repulsion (or double iron) type (figure 1)
• Attraction (or single-iron) type (figure 2)

Errors in Moving Iron Instruments

• (a) Hysteresis error.
• (b) Stray magnetic field error.
• (c) Temperature error.
• (d) Friction error.
• (a) Frequency error.
• (b) Error due to reactance of the instrument coil.
• (c) Error due to eddy current.
• (d) Error due to waveform.

eddy current damping

When a current flow through the coil, it generates a magnetic field which is proportional to the current in case of an ammeter. Damping is caused by the eddy current set up in the aluminium coil which prevents the oscillation of the coil. …

Damping – The damping torque is used for keeping the movement of the coil in rest. This damping torque is induced because of the movement of the aluminium core which is moving between the poles of the permanent magnet.