Electrical Measuring Instruments
Overview
Electrical Measuring Instruments are devices used to measure electrical quantities like voltage, current, resistance, and power. For ITI Electrician Semester 2 (NCVT), students must understand instrument types, working principles, deflecting/controlling/damping torques, errors, and practical usage. This chapter is high-scoring and frequently tested in theory and practical papers.
Typed Notes
Types & Classification of Instruments
Electrical instruments are classified as: (1) Indicating instruments — show reading on scale (ammeter, voltmeter, wattmeter). (2) Recording instruments — draw a graph over time (chart recorder). (3) Integrating instruments — measure total quantity over time (energy meter/kWh meter). Indicating instruments are further classified by working principle: PMMC, Moving Iron (MI), Electrodynamometer, Induction type, Hot Wire, Rectifier type. PMMC is most accurate for DC. Induction type instruments are used only for AC. Exam tip: Energy meter is an integrating instrument — frequently asked.
Three Torques in Indicating Instruments
Every indicating instrument requires three torques for proper operation. (1) Deflecting Torque (Td): Produced by the electrical quantity being measured; causes pointer to move. (2) Controlling Torque (Tc): Opposes deflection and brings pointer to a definite position; produced by spring control or gravity control. Spring control gives uniform scale; gravity control gives non-uniform scale. (3) Damping Torque: Prevents oscillation of pointer and brings it to rest quickly; produced by air friction, eddy current, or fluid friction damping. At final steady position: Td = Tc. Exam tip: PMMC uses eddy current damping; MI uses air friction damping.
PMMC vs Moving Iron Instruments
PMMC (Permanent Magnet Moving Coil): Works on DC only; based on the principle that a current-carrying conductor in a magnetic field experiences a force (Lorentz force / D'Arsonval principle). Scale is uniform/linear. Damping: eddy current. Controlling: spring. Very accurate and sensitive. Cannot be used for AC directly. Moving Iron (MI): Works on both AC and DC; based on the principle of attraction or repulsion between iron pieces. Attraction type: single iron piece attracted into a coil. Repulsion type: two iron pieces (one fixed, one moving) repel each other. Scale is non-uniform (cramped at lower end). Damping: air friction. Robust and cheap. Exam tip: MI repulsion type is most commonly used.
Ammeters, Voltmeters, Wattmeters & Energy Meters
Ammeter: Measures current; connected in series; has very low resistance; range is extended using a SHUNT (low resistance connected in parallel with the meter). Voltmeter: Measures voltage; connected in parallel; has very high resistance; range is extended using a MULTIPLIER (high resistance in series). Wattmeter: Measures power (P = VI cosφ); Electrodynamometer type; has Current Coil (CC) in series and Pressure Coil (PC) in parallel. Reading = W = VI cosφ. Energy Meter (kWh meter): Induction type; integrating instrument; measures units of electricity consumed. 1 unit = 1 kWh. Disc rotates due to eddy currents. Exam tip: Shunt → Ammeter; Multiplier → Voltmeter — most repeated MCQ.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Ammeter and Voltmeter
- Ammeter: series, very low resistance
- Voltmeter: parallel, very high resistance
- Ranges + shunt/multiplier
Ammeter ko galti se parallel lagao — short circuit! Yeh classic viva question hai.
02Multimeter (AVO Meter)
- Voltage, current, resistance ek meter
- Range selection pehle
- Continuity test
Fuse soojha ya nahi — multimeter continuity mode se 2 second mein pata.
03Insulation Tester (Megger)
- 500 V / 1 kV test voltage
- Measures insulation resistance MΩ
- Winding aur wire health
Nayi wiring ki IR 1 MΩ se kam aayi to plaster se pehle hi fault dhundo.
04Energy Meter and Clamp Meter
- kWh meter — revolving/ digital
- Clamp meter: bina circuit todo current
- CT principle based
Clamp meter se live wire ka current bina disconnect kiye naap sakte ho.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Three essential torques in any indicating instrument: Deflecting Torque (Td), Controlling Torque (Tc), and Damping Torque (Td) — at balance, Td = Tc.
- 02PMMC (Permanent Magnet Moving Coil) instruments work on DC only; scale is uniform/linear; damping is eddy current type.
- 03Moving Iron instruments work on both AC and DC; scale is non-uniform (cramped at start); used for AC measurements in panels.
- 04Ammeter is always connected in SERIES (low resistance); Voltmeter is always connected in PARALLEL (high resistance).
- 05Wattmeter has two coils — Current Coil (CC) in series and Pressure/Voltage Coil (PC) in parallel with the load.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the purpose of a shunt in an ammeter, and how is it connected?
Why can a PMMC instrument not be used for AC measurements directly?
Explain the difference between spring control and gravity control in measuring instruments.
What is the working principle of an induction-type energy meter? Name its main parts.
Define 'Instrument Errors' and name the types of errors found in measuring instruments.
Electrical measuring instruments are classified as indicating, recording, and integrating types. Three torques (deflecting, controlling, damping) are essential for any indicating instrument. PMMC works on DC with uniform scale; Moving Iron works on AC/DC with non-uniform scale. Ammeters use shunts (series connection); voltmeters use multipliers (parallel connection). Wattmeters measure power; energy meters (induction type) measure kWh consumed.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which type of instrument is used to measure energy (kWh) consumed in a household?
Q2.A PMMC instrument has a full-scale deflection current of 1 mA and coil resistance of 100 Ω. What type of control is used in PMMC?
Q3.An ammeter must always be connected in ______ with the circuit, and it has ______ resistance.
Q4.In a Moving Iron repulsion type instrument, what causes the pointer to deflect?
Mark karo — dashboard par progress dikhega.