Dynamometer type measuring instruments are similar to PMMC instrument. Except that the permanent magnetic field coil is replaced by a coil which carries the current to be measured. Such instruments may also serve as transfer instruments. A transfer instrument is that which may be calibrated with dc source and then used on ac without any modifications. Because they have precision grade accuracy both for ac and dc measurements. It can be calibrated on dc and then used directly on AC circuit. By establishing a direct means of equating ac and dc measurements of voltage and current.
The coils of dynamometer are usually air-cored which avoids hysteresis, eddy currents and other errors when the instrument is used on AC. The fixed coil is divided into two halves connected in series with the moving coil. And placed together and parallel to each so as to provides a fairly uniform field within the range of the movement of the coil. However, the space between the two halves of the fixed coil must be sufficient enough to allow the movement of the moving coil shaft.
Two hair strings are used to create controlling torque in the instrument which also serves the purpose of lead to the moving coil a section damping is usually provided in such a instrument.
WORKING OF DYNAMOMETER
Dynamometer develops deflecting torque by the interaction of magnetic fields. One field is due to the current in a moving coil. And the other field is due to the current in the fixed coil. The field due to the coil is not constant but varies with the magnitude of the current flowing through the fixed coil. Hence the deflecting torque of this instrument is determined not only by moving coil current but also by fixed coil current.
This instrument with its fixed and moving coil connected in series is adapted for voltage measurement as shown in figure.
Dynamometer is rarely used as an ammeter by following reasons:
- The lead-in spiral to the moving coil can carry limited current.
- Frequency variation influence the inductance of the coil and introduce error.
- The resistance of the moving and fixed coil in series may cause undesirable high voltage drop across the stunt.
Since the deflecting torque is determined by variations in either the field or the moving coil currents. So the dynamometer instrument is a versatile measuring device for several other applications such as for measurement of power reactive volt-ampere. And with some modification for measuring power factor and frequency in AC circuits.
Though these instruments can be used both, for dc and ac measurements. But mostly it is used in Ac practically. Because of the
- These instruments have highest cost.
- Higher power consumption.
- Lower torque-weight ratio.
- Non uniform scale.
USAGE OF DYNAMOMETER FOR DIFFERENT MEASUREMENTS
For Small Current
To use the dynamometer as an ammeter for a measurement of small currents the moving coil and fixed coil are connected in series, as shown in the figure below. In this case current in fixed coil equal to the current in moving coil.
For High Current
When the dynamometer is to be used for measurement of higher currents, the fixed coils are wound with thick wire. The moving coil cannot be wound of thick wire. If it is wound it will be
Thus shunt resistance limits the current in the moving coil to only a small fraction of the current being measured. For measurement of higher currents by dynamometer type instruments, the instrument may be used by connecting the moving coil across a shunt and fixed coils as shown in the figure below.
To use the instrument as a voltmeter the coils are wound with high resistance fine wire. Fixed and moving coils are connected in series along with a high non-inductive resistance R as shown in the figure below. In this case, the
The torque of the instrument is dependent upon the strength of the magnetic field created by field coil and the moving coil which are proportional to current. Hence in ammeter, the torque is proportional to the square of the current under measurement. And in
Errors in dynamometer type instruments
- Frictional Error: Since the coils are air-cored, therefore the
magneticfield produced is of small strength. So they require a largenumber of ampere-turns to create necessary deflecting torque. This result in the heavymoving system. Therefore small torque-weight ratio. Thus the frictional losses in dynamo type instruments are somewhat larger as compared to other instruments.
- Temperature errors: Since the operation of dynamo type instrument required considerable power,
self heatingin these instrumentis appreciable. The error due to self heatingmay be much as 1% of full scaledeflection.
- Error Due to Stray Magnetic field: Since the operating magnetic field produced by the
fixedcoil. In these instruments is somewhat weaker in comparison to that in the instrumentof other type. The operation of these instruments is more sensitive to the stray magnetic field.
- Frequency error: The change in frequency causes error
- Due to change in reactance of operating coil.
- Due to change in magnitude of Eddy current setup in the metal part of the instrument near to operating portion.
Advantages of Dynamometer
- As the instrument has Square Law response so can be used on both the dc as well as on AC.
- These instruments are free from hysteresis and Eddy current errors. It is because of absence of iron in the operating part of the instrument.
- Ammeter up to 10A and voltmeter up to 600V can be constructed with precision grade accuracy.
- Dynamo type voltmeter are useful for accurate measurement of rms value of voltage irrespective of waveform.
- Because of Precision grade accuracy and same calibration for DC and AC measurement instruments are used as transfer and calibration instruments.
Disadvantage of Dynamometer
- The scale is not uniform as the instrument uses Square Law response. These instruments have small torque-weight ratio so the friction error is considerable.
- Owing to heavy moving system friction losses in these instruments are somewhat more than those in other instruments.
- As a result of measures taken to reduce the frictional errors, their cost is more in comparison to moving iron and PMMC instruments. They are more sensitive to overload and mechanical impact and are to be handled with care.
- Adequate screening of the movements against the stray magnetic field is essential.
- The sensitivity of the instrument is typically very low due to poor deflecting torque. The sensitivity of dynamo type wattmeter is 10 to 30 per volt in comparison to the sensitivity of 20-
kilo-ohmper volt in case of D’Arsnoval movement.
- The power consumption of
this instrument is comparatively high because of their construction.