Errors in Dynamometer type Instruments

Errors in Dynamometer type Instruments Points : Errors in Dynamometer type Instruments, Low torque / weight ratio, Frequency, Eddy currents, External magnetic fields, Temperature change The main sources of errors in a dynamometer type instruments are due to:
i) Low torque / weight ratio
ii) Frequency
iii) Eddy currents
iv) External magnetic fields
v) Temperature change 1. Low torque/weight ratio error The magnetic field produced by air-cored coils is essentially small and, therefore, flux linkages per ampere in the moving coil are few. Therefore, the deflecting torque is low. Thus to produce the reasonable deflecting torque, the mmf of the moving coil must be increased. The mmf of the moving coil can be increased either by increasing the current through the coil or by increasing the number of turns. The current cannot be increased beyond 200mA as otherwise the springs would be heated too much. On other hand an increase in the number of turns of the moving coil, will give rise to increased weight of the moving parts resulting in a low torque/weight ratio error. This error also increases friction losses in the electrodynamometer type instrument. Remedy This error can be minimized by careful design of the coils. 2. Frequency Error The frequency error of the electrodynamometer type instruments is due to largely as a result of variable of self-reactance of coils with frequency. In case of voltmeter, Remedy In order to reduce frequency errors in voltmeters the coil winding is made a very small part of the circuit. This makes the inductive reactance, a small fraction of total impedance and, therefore. 3. Eddy Currents Errors The effect of eddy currents is to produce a torque as a result of coupling between moving coil and the adjacent metal parts. Remedy The metal in the coil supports and structural part is kept to the minimum as possible. Any metal that is used is kept away and is selected to have high receptivity so as to reduce eddy currents induced in it. 4. External Magnetic Fields Error As the operating field in electrodynamometer instruments is weak, therefore, these must be protected from the external magnetic fields, otherwise these will produce error. Remedy Portable instruments use metal shields ‘which serve to isolate the coils from external influences. Precision type of instruments are unshielded as eddy currents are induced in the metal shields which introduce errors. In precision instruments the influence of external magnetic field can be minimized by using “Asiatic System”. These Astatic electrodynamometer instruments are constructed with two similar sets of fixed and moving coils mounted on the same shaft. The two pairs of fixed coils are so connected that their magnetic fields ate in opposition. The currents in the moving coils are also opposed and, as a result, the deflecting torques applied to the moving coils acts in the same direction. Since the two field are in opposite direction, therefore any uniform external field reduces the field of the one coil and increases by an equal amount, the field of the other coil. Therefore the deflecting torque produce by one coil is increased while that produced by the other coil is reduced by an equal amount, thus, the net torque on account of the external magnetic field. 5. Temperature Change Error High currents are carried by coils, which produce heat. This heating of coils produces errors. Remedy High accuracy instruments contain temperature compensating resistors, which tend to neutralize the effects of temperature changes.