Question

A Dynamometer-type instrument is primarily used as a "Transfer Instrument" because:

• A. It has high sensitivity
• B. It has a linear scale
• C. It gives the same calibration for both AC and DC
• D. It is very inexpensive

Hint:

Transfer instruments bridge the gap between DC standard laboratories and practical AC field measurements.
Electrodynamometer wattmeters are the most common transfer instruments used to calibrate standard AC wattmeters.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks why a electrodynamometer-type instrument is designated and used as a "Transfer Instrument" in calibration systems.
A transfer instrument is an instrument that can be calibrated using a highly accurate DC source and then used to measure AC quantities with identical precision.

Step 2: Key Formula or Approach:
The deflecting torque (\( T_d \)) of an electrodynamometer instrument is given by the product of the currents in its fixed and moving coils:
\[ T_d = I_1 I_2 \frac{dM}{d\theta} \] Where:
\( I_1, I_2 \) are the currents in the fixed and moving coils respectively.
\( M \) is the mutual inductance between the coils.
For ammeters and voltmeters, both coils carry the same current (or a proportional current), making \( T_d \propto I^2 \).

Step 3: Detailed Explanation:
Let us explore the properties of the electrodynamometer that qualify it as a transfer instrument:

• Identical AC and DC Torque Equations:
- Under DC conditions, the deflecting torque is proportional to the square of the DC current: \( T_{d,\text{DC}} \propto I_{\text{DC}}^2 \).
- Under AC conditions, the instantaneous torque is proportional to the square of the instantaneous AC current. Due to the inertia of the moving system, the meter reflects the average torque: \( T_{d,\text{AC}} \propto I_{\text{RMS}}^2 \).
- Because the physical mechanism of torque production is identical for both DC and RMS AC, the deflection is exactly the same for a given DC value and the equivalent RMS AC value.

• Absence of Iron Core Errors:
- Electrodynamometer instruments are air-cored.
- This design eliminates errors due to hysteresis, eddy currents, and magnetic saturation, which typically vary between AC and DC.
- Consequently, the calibration curve obtained with a highly accurate DC standard remains perfectly valid for AC measurements.

Step 4: Final Answer:
A dynamometer-type instrument is used as a transfer instrument because it gives the same calibration for both AC and DC.