Table 3-2. Effect of Distortion When Measuring Ac Voltages

TM 11-6625-537-14-1

Additional filtering will be required if ac com-

500 volt dc reference to change as much as 0.0125

ponents which affect the accuracy are ever en-

v o l t . This would cause the VOLTS meter to

countered.

deflect 1/10 full scale if the NULL switch was set

g. Effect of Distortion When Measuring Ac

to .1, or to deflect full scale if the NULL switch

was set to .01.

Volts. When measuring ac voltages, the elec-

tronic voltmeter indicates a value which is 1.11

Effect

Components

M e a s u r e m e n t s . The electronic voltmeter will

times the average of the input ac waveform. This

o p e r a t e well within specifications when ac

value is the same as the root-mean-square (rms)

components of a few hundred Hertz (Hz) are

value only if the input waveform is of a single

p r e s e n t on the unknown dc voltage being

frequency. Table 3-2 shows how the presence of

measured. Ac components which are a multiple or

harmonic distortion on the input voltage may

submultiple of 60 Hz may cause the VOLTS

cause the indication of the electronic voltmeter to

m e t e r needle to oscillate at the difference

vary from the true rms value. The error depends

frequency. For example, a 60 Hz ac voltage that

on the phase relationship between the harmonic

is 5 percent of the input voltage may cause an

a n d the fundamental and may be anywhere

error of approximately 0.01 percent (which is well

between maximum positive and maximum

within specifications). A larger ac component can

negative, including zero.

be tolerated at low frequencies other than 60 Hz.

Table 3-2. Effect of Distortion When Measuring Ac Voltages

Percent error

Harmonic

Percent distortion

Maximum positive

Maximum negatiue

Any even harmonic

0.1

0.000

0.0001

0.5

0.000

0.005

1.0

0.000

2.0

0.000

0.020

Third harmonic

0.033

0.1

0.033

0.168

0.167

0.5

0.338

0.328

1.0

0.687

0.667

2.0

Fifth harmonic

0.020

0.1

0.020

0.101

0.099

0.5

0.205

1.0

0.195

0.420

2.0

0.380

h. Ac Voltage Errors Due to Grounding. A

3-6. Initial Adjustments

0.47 microfarad capacitor (C1) is connected from

a. Advance OPERATE-CALIBRATE control

the lower input terminal to chassis ground within

a g a i n s t spring tension to CALIBRATE, and

the electronic voltmeter. The chassis is in turn

a d j u s t CALIBRATE control to obtain a zero

connected to power system ground through the

VOLTS meter needle deflection. Release

power cord. If it is desired to measure an ac

O P E R A T E - C A L I B R A T E control.

voltage where the lower input terminal is not at

NOTE

ground potential, a line cord adapter must be

M a x i m u m accuracy when using the

used to isolate the electronic voltmeter chassis

voltmeter as a differential voltmeter is

from power system ground. Otherwise, the 0.47

obtained only when the calibration of the

microfarad capacitor would place an ac load on

reference supply is carefully done. The

the circuit being measured.

reference supply may be adjusted

whenever necessary without disturbing

3-5. Preliminary Starting Procedure

a n y control settings or removing the

a. Check that power switch is in the down

input or output connections by repeating

position (OFF).

step a above.

b. Set front panel controls as follows:

b. Adjust the reference supply prior to each

(1) Set RANGE control to 500.

specific measurement for best accuracy until

(2) Set AC-de polarity control to +.

voltmeter has warmed to an equilibrium tem-

(3) Set voltage readout dials A, B, C, D and

perature (approximately one half hour).

E to O.

c. Allow one hour warmup time when making

(4) Set power switch to the up position (ON).

prolonged measurements so that the reference

( 5 ) Allow v o l t m e t e r to warm up for 10

supply does not shift.

minutes.

3-5