Current Readings with the vb Instrument

The information provided is for help with Commtest Instruments products:

vbSeries® instruments

QUESTION

Can I take current readings with my vb instrument?

ANSWER

Yes. Both old and newer vbSeries instruments are capable of taking current measurements.

Later Model vbSeries® Instruments

(firmware version 4.30 and higher)

vb3000 instruments using firmware version 4.30 and higher can accept current sensors as well as other sensor types. Sensor sensitivities from 0.1 to 2300 mV/A are supported and the recordings are displayed in Amps. Both Waveform and Spectrum type recordings can be taken, but note that the inputs are AC coupled so any DC level will be ignored.

Commtest's vbOnline device also supports current sensors in addition to selectable AC or DC coupled inputs. The setup parameters for taking current measurements are fully described in your software and instrument reference guides. (The latest versions of these can be downloaded for free from our website.)

Earlier Model vbSeries® Instruments

(prior to firmware version 4.30)

The following information is provided for owners of vbSeries instruments such as the vb1000, vb2000 and any vb3000 instruments running firmware versions prior to version 4.30 (note that vb modules with serial numbers below 11500 can only run firmware versions 3.xx).

These instruments only have an accelerometer input; however, they can be fooled into taking current measurements.

You will need a clamp style ammeter with a voltage output suitable for plugging into a multi-meter, and a cable to allow connection from the ammeter to the BNC connector on the vb instrument.

SETUP

(for earlier model vbSeries instruments)

From the Set Parameters Menu set the Mode to acceleration.
From the Set Parameters Menu press [9] to display the Sensor Setup Menu.
Turn drive current OFF and adjust the sensor's sensitivity to match the ammeter output (as mV/g instead of mV/Amp).

When using an accelerometer to measure velocity or displacement the instrument must perform a transformation on the input signal. When measuring acceleration no signal integration is required so the displayed chart is an accurate representation of the input signal.

The drive current is used to drive the accelerometers but will not be needed by the ammeter. Although unlikely, the drive current could cause false readings or even damage to the ammeter if left on. Typically these meters give an output of 10 mV/A. By setting the accelerometer sensitivity to 10 mV/g there will be a direct ratio of mV to g. The display on the vb instrument will read 'g' instead of 'A', but the values will be correct. At 50 or 60 Hz line frequencies the instrument will be able to read up to approximately 35 A rms with these settings.

Some ammeters give an output of 1 mV/A. In this case you should still set the accelerometer sensitivity to 10 mV/g. Now each 'g' displayed on the vb screen will represent 10 A. For example 0.10 g = 1 A. The instrument is able to measure up to approximately 350 A rms with these settings.

Most current readings are measured in rms so you should also set this to be able to compare readings with other ammeters.

The above description can be used with other voltage-output devices, subject to the following restrictions:

The input to the vb instrument is AC coupled - it will only read down to about 1 Hz (its 3 dB point).

The maximum voltage swing permissible on the instrument input is 1 V peak-peak at 50 to 60 Hz (this restriction only applies to instruments with serial numbers lower than 11500). This increases to 16 V peak-peak at 1 kHz.

MORE INFORMATION

For more detailed information you can purchase the technical paper 'Motor Current Analysis' from Technical Associates of Charlotte, P.C.

FAQ ID: 11644 Last Reviewed: 3 April 2006