When it comes to electrical measurements, selecting the right type of measuring instrument is crucial. The three commonly used types are:
- Permanent Magnet Moving Coil (PMMC) Instruments
- Dynamometer Type Instruments
- Moving Iron (MI) Instruments
Each of these has its own construction, working principle, applications, and suitability for AC or DC measurements. Below is a detailed comparison of moving coil, dynamometer, and moving iron voltmeters and ammeters based on various parameters.
Comparison Table
| S.No. | Feature/Parameter | Moving Coil (PMMC) | Dynamometer Type | Moving Iron (MI) |
|---|---|---|---|---|
| 1. | Construction | Delicate construction | Heavy moving system | Simple |
| 2. | Cost | Very high | High | Low |
| 3. | Power Consumption | Very low | High | Less than dynamometer type |
| 4. | Scale | Uniform | Non-uniform | Non-uniform |
| 5. | Torque-to-Weight Ratio | High | Small | More than dynamometer type |
| 6. | Suitable for | DC only | AC and DC | AC and DC |
| 7. | Effect of Stray Magnetic Fields | Not affected | Affected | Not affected |
| 8. | Application | Voltmeters and Ammeters | Mostly for Wattmeters | Voltmeters and Ammeters |
| 9. | Accuracy | High | Low | Reasonable |
| 10. | Sensitivity | High | Poor | Reasonable |
Key Takeaways
- PMMC Instruments are highly accurate and sensitive but limited to DC measurements only. They are expensive and delicate but provide a uniform scale.
- Dynamometer Type Instruments can work on both AC and DC and are commonly used in wattmeters. However, they have poor accuracy, high power consumption, and are sensitive to stray magnetic fields.
- Moving Iron Instruments are the most cost-effective and simple in design. They can be used for both AC and DC but offer moderate accuracy and sensitivity with a non-uniform scale.
Frequently Asked Questions (FAQs)
Q1. Which instrument is most suitable for accurate DC voltage measurements?
Ans: The moving coil (PMMC) instrument is best for accurate and sensitive DC voltage measurements.
Q2. Can dynamometer type instruments measure AC current?
Ans: Yes, dynamometer instruments are suitable for both AC and DC current and voltage measurements.
Q3. Why are moving iron instruments preferred in general-purpose AC measurements?
Ans: Because they are cost-effective, simple, and can handle AC and DC, making them ideal for general applications.
Q4. Which instrument has a uniform scale?
Ans: Only the moving coil (PMMC) instrument provides a uniform scale, making readings easier and more precise.
Q5. Are moving coil instruments affected by magnetic fields?
Ans: No, they are not affected by stray magnetic fields, which enhances their measurement reliability.