1

CHAPTER CHAPTER 55

EMT 113: V-2008EMT 113: V-2008

School of Computer and School of Computer and Communication Engineering, UniMAPCommunication Engineering, UniMAP

Prepared By: Prepared By: Amir Razif b. Jamil AbdullahAmir Razif b. Jamil Abdullah

AC Meter.AC Meter.

2

5.1 Introduction to AC Meters.5.1 Introduction to AC Meters.5.2 D’Arsonval Meter Movement with 5.2 D’Arsonval Meter Movement with

Half-Wave Rectification. Half-Wave Rectification. 5.3 D’Arsonval Meter Movement with 5.3 D’Arsonval Meter Movement with

Full-Wave Rectification. Full-Wave Rectification.

5.0 AC Meters.5.0 AC Meters.

3

5.1 Introduction to AC 5.1 Introduction to AC Meters.Meters. Five principal meter movement that are commonly

used in ac instruments;(i) Electrodynamometer.(i) Electrodynamometer.(ii) Iron-Vane.(ii) Iron-Vane.(iii) Electrostatic.(iii) Electrostatic.(iv) Thermocouple.(iv) Thermocouple.(v) D’Arsonval (PMMC) with rectifier.(v) D’Arsonval (PMMC) with rectifier.

The d’Arsonval meter is the most frequently used meter movement, event though it cannot directly measure alternating current or voltage.

In this chapter it will discuss the instruments for measuring alternating signal that use the d’Arsonval meter movement.

4

(a) (a) AC Voltmeters and AmmetersAC Voltmeters and Ammeters AC electromechanical meter movements come in two basic

arrangements: (1) Based on DC movement designs.(2) Engineered specifically for AC use.

Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC.

Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage, Figure 5.1.

Figure 5.1: D’Arsonal Figure 5.1: D’Arsonal Electromechanical Meter Electromechanical Meter Movement.Movement.

Cont’d…Cont’d…

5

(b) (b) DC-style Meter Movement for AC application.DC-style Meter Movement for AC application. If we want to use a DC-style meter movement

such as the D'Arsonval design, the alternating current must be rectified into DC, Figure 5.2.

This can be accomplished through the use of devices called diodes.

The diodes are arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant direction throughout all portions of the AC cycle:

Figure 5.2: Rectified D’Arsonal Electromechanical Meter Movement.

Cont’d…Cont’d…

6

(c) (c) Iron-Vane Electromechanical.Iron-Vane Electromechanical. The AC meter movement without the inherent polarity

sensitivity of the DC types. This design avoid using the permanent magnets.

The simplest design is to use a non-magnetized iron vane to move the needle against spring tension, the vane being attracted toward a stationary coil of wire energized by the AC quantity to be measured, Figure 5.3.

The electrostatic meter movements are capable of measuring very high voltages without need for range resistors or other, external apparatus.

Figure 5.3: Iron-Vane Figure 5.3: Iron-Vane Electromachanical Meter Electromachanical Meter Movement.Movement.

Cont’d…Cont’d…

7

(d) (d) AC Voltmeter with Resistive Divider.AC Voltmeter with Resistive Divider. When a sensitive meter movement needs to be re-

ranged to function as an AC voltmeter, series-connected "multiplier" resistors and/or resistive voltage dividers may be employed just as in DC meter design, Figure 5.4.

Figure 5.4: AC Voltmeter with Resistive Figure 5.4: AC Voltmeter with Resistive Divider.Divider.

Cont’d…Cont’d…

8

(e) (e) AC Voltmeter with Capacitive Divider.AC Voltmeter with Capacitive Divider. Capacitors may be used instead of resistors,

though, to make voltmeter divider circuits. This strategy has the advantage of being non-

dissipative; no true power consumed and no heat produced. Refer to Figure 5.5.

Figure 5.5: AC Voltmeter with Figure 5.5: AC Voltmeter with Capacitive Divider.Capacitive Divider.

Cont’d…Cont’d…

9

In order to measure the alternating current with the d’Arsonval meter movement, we must rectify the alternating current by use of diode rectifier .

Figure 5.6 is the DC voltmeter circuit modified to measure AC voltage.

The diode, assume to be ideal diode, has no effect on the operation of the circuit .

For example if the 10 V sine-wave input is fed as the source of the circuit, the voltage across the meter movement is just the positive half-cycle of the sine wave due to the rectifying effect of the diode.

5.2 D’Arsonval Meter 5.2 D’Arsonval Meter Movement with Movement with Half-Wave Half-Wave Rectification.Rectification.

Figure 5.6: DC Voltmeter Circuit Modified to Measure AC Voltage.

10

The peak value of 10 Vrms sine wave is,

or

If the output voltage from the half-wave rectifier is 10V only, a dc voltmeter will provide an indication of approximately 4.5 V.

From the above equation,

rmsp

ave

pdcave

EE

E

EEE

*45.0

*318.0

mdc

rmsm

dc

dcs R

I

ER

I

ER

45.0

peakrmsp VVE 14.14414.1*10

dcac SS 45.0

Cont’d…Cont’d…

11

Example 5.1:Example 5.1: D’Arsonval Meter Half-Wave Rectifier. D’Arsonval Meter Half-Wave Rectifier.Compute the value of the multiplier resistor for a 10 Vrms ac range on the Compute the value of the multiplier resistor for a 10 Vrms ac range on the voltmeter shown in voltmeter shown in Figure 5.7Figure 5.7..

Solution:Solution:Find the sensitivity for a half wave rectifier.

.

KV

V

RRangeSR

VISS

macacs

fsdcac

2.43001

10*

450

*

4501*45.045.0

Figure 5.7: AC Voltmeter Using Half-Wave Rectification.

12

Commercially produced ac voltmeters that use half-wave rectification have an additional diode and shunt as shown in Figure 5.8, which is called instrument rectifier.

.Figure 5.8: Half-Wave Rectification Using an Instrument Figure 5.8: Half-Wave Rectification Using an Instrument

Rectifier and a Shunt Resistor.Rectifier and a Shunt Resistor.

Cont’d…Cont’d…

13

The full-wave rectifier provide higher sensitivity rating compare to the half-wave rectifier.

Bridge type rectifier is the most commonly used, Figure 5.9.

5.3 D’Arsonval Meter 5.3 D’Arsonval Meter Movement with Movement with Full-Wave Full-Wave Rectification.Rectification.

Figure 5.9: Full Wave Bridge Rectifier Used in AC Voltmeter Circuit.

14

Operation; Operation; (a) During the positive half cycle (red arrow), currents flows

through diode D2, through the meter movement from positive to negative, and through diode D3. - The polarities in circles on the transformer secondary are for the positive half cycle.- Since current flows through the meter movement on both half cycles, we can expect the deflection of the pointer to be greater than with the half wave cycle. - If the deflection remains the same, the instrument using full wave rectification will have a greater sensitivity.(b) Vise-versa for the negative half cycle (blue arrow).

Cont’d…Cont’d…

15

From the circuit in Figure 5.9, the peak value of the 10 Vrms signal with the half-wave rectifier is,

The average dc value of the pulsating sine wave is,

Or can be compute as,

The AC voltmeter using full-wave rectification has a sensitivity equal to 90% of the dc sensitivity or twice the sensitivity using half-wave rectification.

peakrmsp VEE 14.14*414.1

VEE pave 9636.0

VVEE rmsave 910*9.0*9.0

dcac SS *9.0

Cont’d…Cont’d…

16

Example 5.2Example 5.2:: D’Arsonval Meter Full-Wave Rectifier. D’Arsonval Meter Full-Wave Rectifier.Each diode in the full-wave rectifier circuit in Each diode in the full-wave rectifier circuit in Figure 5.10Figure 5.10 has an average has an average forward bias resistance of 50 Ohm and is assumed to have an infinite resistance forward bias resistance of 50 Ohm and is assumed to have an infinite resistance in the reverse direction. Calculate,in the reverse direction. Calculate,(a) The multiplier Rs.(a) The multiplier Rs.(b) The AC sensitivity.(b) The AC sensitivity.© The equivalent DC sensitivity.© The equivalent DC sensitivity.

Solution:Solution:(a) Calculate the current shunt and total current,

.

mAmAmAIII

and

mAmA

R

EI

mshT

sh

msh

211

1500

500*1

Figure 5.10: AC Voltmeter Using Full-Wave Rectification and Shunt.

17

The equivalent DC voltage is,

(b) The ac sensitivity,

(c.) The dc sensitivity,

.

KmA

V

I

ER

VVVE

T

dcT

rmsdc

5.42

0.9

0.910*9.010*9.0

K

RR

RRRRR

shm

shmdTs

15.4500500

500*50050*24500

2

VVRange

RS Tac /450

10

4500

VVS

S

or

VmAI

S

acdc

Tdc

/5009.0

/450

9.0

/5002

11

Cont’d…Cont’d…