Instruments & Electrical Measurements Lecture 11

Working Principle of Voltmeter and Types of Voltmeter What is Voltmeter? Voltmeter is a voltage meter. Which measures the voltage between the two nodes. We know the unit of potential difference is volts. So it is a measuring instrument which measures the potential difference between the two points. Working Principle of Voltmeter The main principle of voltmeter is that it must be connected in parallel in which we want to measure the voltage. Parallel connection is used because a voltmeter is constructed in such a way that it has a very high value of resistance. So if that high resistance is connected in series than the current flow will be almost zero which means the circuit has become open.

If it is connected in parallel, than the load impedance comes parallel with the high resistance of the voltmeter and hence the combination will give almost the same the impedance that the load had. Also in parallel circuit we know that the voltage is same so the voltage between the voltmeter and the load is almost same and hence voltmeter measures the voltage. For an ideal voltmeter, we have the resistance is to be infinity and hence the current drawn to be zero so there will be no power loss in the instrument. But this is not achievable practically as we cannot have a material which has infinite resistance.

Classification or Types of Voltmeter According to the construction principle, we have different types of voltmeters, they are mainly - 1. Parmanent Magnet Moving coil (PMMC) Voltmeter. 2. Moving Iron (MI) Voltmeter. 3. Electro Dynamometer Type Voltmeter. 4. Rectifier Type Voltmeter 5. Induction Type Voltmeter. 6. Electrostatic Type Voltmeter. 7. Digital Voltmeter (DVM). Depending on this types of measurement we do,we have- 1. DC Voltmeter. 2. AC Voltmeter.

For DC voltmeters PMMC instruments are used, MI instrument can measure both AC and DC voltages, electrodynamometer type, thermal instrument can measure DC and AC voltages as well. Induction meters are not used because of their high cost, inaccuracy in measurement. Rectifier type voltmeter, electrostatic type and also digital voltmeter (DVM) can measure both AC and DC voltages.

Extension of Range in a PMMC Voltmeter In the PMMC voltmeters we have the facility of extending the range of measurement of voltage also. Just connecting a resistance in series with the meter we can extend the range of measurement.

Let, V is the supply voltage in volts Let, V is the supply voltage in volts. Rv is the voltmeter resistance in Ohm. R is the external resistance connected in series in ohm. V1 is the voltage across the voltmeter. Then the external resistance to be connected in series is given by

Attraction type MI Instrument Principale If an unmagnetized soft iron is placed in the magnetic field, it is attracted towards the coil, if a pointer is attached to the systems and current is passed through a coil as a result of the applied voltage, it creates a magnetic field which attracts iron piece and creates deflecting torque as a result of which pointer moves over the scale.

Répulsion type MI Instrument Principale When two iron pieces are magnetized with the same polarity by passing a current which done by applying a voltage across the voltmeter than repulsion between them occurs and that repulsion produces a deflecting torque due to which the pointer moves. The advantages are it measure both AC and DC, it is cheap, low friction errors, Robust etc. It is mainly used in AC measurement because in DC measurement error will be more due to hysteresis.

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