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.
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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 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.
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 following These instruments have highest cost. Higher power consumption.
Electrodynamometer type Instruments - Construction and Operation
This type of instrument is known as Elctrodynamometer type instrument. We will discuss each of them one by one. Fixed Coil Fixed coil is provided for the sake of production of magnetic field. This fixed coil is divided into two sections so that a uniform magnetic field may be achieved at the centre.
This deflecting torque deflects the moving coil to such a position where the controlling torque of the spring is equal to the deflecting torque. Operation with AC Let, i1 and i2 be the instantaneous values of the current carried by the coils. As the quantity i2 is always positive, the current varies and the instantaneous torque also varies. But the moving system due to its inertia cannot follow such rapid variations in the instantaneous torque and responds only to the average torque. The average deflecting torque over a complete cycle is given by: where T is the time period for one complete cycle. If the pointer scale is calibrated in terms of the square root of this value, i.
Twitter Electrodynamometer type instruments: The necessity for the AC calibration of moving iron instruments as well as another type of instruments which cannot be correctly calibrated requires the use of a transfer type of instrument. A transfer instrument is one that may be calibrated with a d. This requires the transfer type instrument to have the same accuracy for both a. Electrodynamometer instruments are capable of serving as transfer instruments.
Dynamometer Type Instrument Construction and Working
Circuit Globe Electrical Measurement Electrodynamometer Wattmeter Electrodynamometer Wattmeter Definition: The instrument whose working depends on the reaction between the magnetic field of moving and fixed coils is known as the Electrodynamo-meter Wattmeter. It uses for measuring the power of both the AC and DC circuits. The working principle of the Electrodynamometer Wattmeter is very simple and easy. Their working depends on the theory that the current carrying conductor placed in a magnetic field experiences a mechanical force. This mechanical force deflects the pointer which is mounted on the calibrated scale. Construction of Electrodynamometer Wattmeter The following are the important parts of the Electrodynamometer Wattmeter.