PMMC Instrument Quiz

Identify the torques that operate in the Permanent Magnet Moving coil (PMMC) instrument

• Deflecting, controlling and hydraulic torques

• Deflecting, damping and hydraulic torques

• Deflecting, controlling and damping torques

• Deflecting, hydraulic and control torques

Answer explanation

In a PMMC instrument, the three main torques are deflecting torque, which causes the pointer to move, controlling torque, which stabilizes the movement, and damping torque, which reduces oscillations for accurate readings.

The diagram below shows the internal structure of a moving coil instrument. Which part is responsible for generating the deflecting torque in this instrument?

• The Scale

• The Coil

• The Spring

• The Pointer

Answer explanation

The coil in a moving coil instrument generates the deflecting torque when current passes through it, creating a magnetic field that interacts with the external magnetic field, causing the coil to rotate.

In an under damped system, the pointer oscillates before coming to rest. Which of the following best explains this behaviour?

• The system has no friction, allowing the pointer to oscillate indefinitely

• The damping force is so strong that it stops the pointer immediately, preventing oscillations.

• The system has moderate damping, causing the pointer to oscillate several times before eventually returning to its rest position.

• The system is critically damped, allowing the pointer to return to its rest position without oscillation.

Answer explanation

The correct choice is that the system has moderate damping, which allows the pointer to oscillate several times before eventually returning to its rest position, characteristic of underdamped systems.

In an electromechanical measuring instrument, the deflecting torque Td​ is produced by the interaction between the current in the coil and the magnetic field. Which of the following best explains how this deflecting torque is generated?

• The current flowing through the coil creates a magnetic field that interacts with the external magnetic field, producing a force that causes the coil to rotate.

• The resistance of the coil creates a heat effect, which generates torque and drives the pointer.

• The external magnetic field directly causes the coil to move, irrespective of the current flowing in it.

• The voltage across the coil produces a torque that aligns the coil with the magnetic field, causing rotation.

Answer explanation

The correct choice explains that the current in the coil generates a magnetic field, which interacts with the external magnetic field, resulting in a force that causes the coil to rotate, producing the deflecting torque.

Identify the statement which is NOT TRUE about DC voltmeter

• The parallel resistor in DC voltmeter Is known as multiplier resistance.

• The series resistor in DC voltmeter is known as multiplier resistance

• The multiplier resistance in DC voltmeter is used to limit the current flow through basic construction PMMC instrument.

• The multiplier resistance in DC voltmeter is used to extend the voltage range of the meter.

Answer explanation

The statement 'The parallel resistor in DC voltmeter Is known as multiplier resistance' is NOT TRUE. The multiplier resistance is actually a series resistor used to extend the voltage range and limit current in a PMMC instrument.

Identify the multiplier resistance for DC voltmeter with sensitivity of 1KΩ/V, internal resistance 1kΩ and range 0-15V.

• 1.4KΩ

• 14KΩ

• 140Ω

• 14Ω

Answer explanation

To find the multiplier resistance, use the formula: Multiplier Resistance = (Range x Sensitivity) - Internal Resistance. Here, (15V x 1kΩ/V) - 1kΩ = 15kΩ - 1kΩ = 14kΩ. Thus, the correct answer is 14KΩ.

A basic D'Arsonval movement with full scale deflection of 100mA and internal resistance of 300Ω is used as a voltmeter. Calculate the value of the multiplier resistance needed to measure a voltage range of 0 - 100V.

• 750Ω

• 700 Ω

• 1K Ω

• 7K Ω

Answer explanation

To measure 100V with a 100mA full scale, use Ohm's law: V = IR. The total resistance needed is R = V/I = 100V/0.1A = 1000Ω. Subtract the internal resistance (300Ω) to find the multiplier: 1000Ω - 300Ω = 700Ω.

Calculate the multiplier resistance Rs, for 80V range voltmeter that uses an inner resistance of a 10K Ω for its motion with full scale range of 200µ A.

• 260K Ω

• 370K Ω

• 380K Ω

• 390K Ω

Answer explanation

To find the multiplier resistance Rs, use the formula Rs = (Vmax / Imax) - Rm. Here, Vmax = 80V, Imax = 200µA, and Rm = 10KΩ. Calculating gives Rs = (80V / 0.0002A) - 10KΩ = 390KΩ, which is the correct answer.