Dynamometer type instruments are widely used in electrical measurements, especially as wattmeters. However, with specific modifications, they can also function effectively as ammeters and voltmeters. This article explains how the range of these instruments can be extended for various measurement purposes in both AC and DC circuits.
Range Extension of Dynamometer Type Ammeter
The standard range of an unshunted dynamometer ammeter is typically 0–50 mA. This limitation is due to the current-carrying capacity of the moving coil, which cannot handle more than 50 mA. However, this range can be extended using the following methods:
1. Using Shunts
To measure higher currents, a shunt resistor is connected in parallel with the moving coil. This allows a portion of the current to bypass the coil, protecting it while still enabling accurate readings.
- In AC ammeters, shunts are generally used up to 0–5 A range.
- Beyond this range, due to the effect of inductance and resistance, current division becomes inaccurate.
2. Using Current Transformers (CTs)
For measuring large alternating currents, a current transformer (CT) is used along with a 0–5 A AC ammeter.
Example:
- Suppose a CT with a ratio of 50:1 is used.
- If the ammeter reads 4 A, the actual line current is:
- Line current = 4 A × 50 = 200 A
By using CTs of different ratios, multiple current ranges can be achieved efficiently and safely.
Range Extension of Dynamometer Type Voltmeter
Dynamometer voltmeters are primarily used to measure alternating voltages. Their measuring range can be extended using the following methods:
1. Using Multipliers
For voltage ranges up to 0–750 V, a high resistance multiplier (Rs) is connected in series with the voltmeter.
- This method is suitable for moderately high voltages.
- The multiplier limits the current through the instrument, allowing higher voltages to be measured without damage.
2. Using Potential Transformers (PTs)
When the voltage exceeds 750 V, the power lost in the multiplier becomes significant. In such cases, a potential transformer (PT) is used with a 0–110 V AC voltmeter.
Example:
- Suppose a PT with a ratio of 20:1 is used.
- If the voltmeter shows 100 V, the actual line voltage is:
- Line voltage = 100 V × 20 = 2000 V
By selecting PTs with different ratios, the voltmeter can measure various high voltage ranges accurately.
Conclusion
Extending the range of dynamometer type instruments increases their versatility for electrical measurements in both low and high power systems. By using shunts and current transformers for ammeters, and multipliers and potential transformers for voltmeters, these instruments can handle a wide spectrum of current and voltage levels.
FAQs
Q1. What is the main limitation of un-shunted dynamometer ammeters?
Ans: The moving coil can handle a maximum of 50 mA, limiting the range without additional components.
Q2. How does a current transformer help in measuring high AC current?
Ans: It steps down the high current to a lower, measurable value which can then be read by the ammeter and multiplied by the CT ratio.
Q3. Why are multipliers not used beyond 750 V in voltmeters?
Ans: Because the power loss in the resistor becomes excessive and inefficient at very high voltages.
Q4. Can dynamometer instruments be used in both AC and DC circuits?
Ans: Yes, one of their advantages is their ability to measure quantities in both AC and DC systems.
Q5. What is the typical voltage range of a dynamometer voltmeter without a transformer?
Ans: Typically up to 0–750 V using series multipliers.