0
\$\begingroup\$

enter image description here

A PC is connected to a data acquisition card which has single ended channels. One of the channels is outputting a constant DC voltage which can be controlled by a user via a program in the PC.

The output voltage is feeding a VFD. The VFD takes the voltage value and controls the frequency of the AC motor depending on the voltage applied.

There is an isolation amplifier between DAQ analog output and the VFD as in the figure. When I measure the analog output of the DAQ and the output of the isolation amplifier with a voltmeter I see the same value.

What might be the reason then for the isolation amplifier?

\$\endgroup\$

3 Answers 3

Reset to default
0
\$\begingroup\$

What might be the reason then for this amplifier?

The possible reason is that your motor controller (Variable frequency drive) is connected to AC mains power and may not have satisfactory protection to offer a signal that controls it. This might mean that the VFD can inject (unintentionally) noise and interference onto its input relative to ground (earth). This may upset the PC DAQ output or indeed cause it to fail or maybe just give the PC a few resets once per year.

There may also be a safety issue - the spec for the isolation device says it has a 250VAC operational isolation and is capable of being "tested" to 3.5kVAC. This sounds to me like it would protect against risk of electric shock and provide decent isolation for noise and ground loops from the VFD.

\$\endgroup\$
4
  • \$\begingroup\$ u mean for instance if there is a rapid current change or some fault in vfd pc's aq card may be harmed from the high current in case there is no isolation amplifier? do u mean that? \$\endgroup\$
    – user16307
    Commented Nov 22, 2013 at 13:00
  • \$\begingroup\$ Multiple grounds on AC circuits can lead to big earth fault currents going thru the daq card. \$\endgroup\$
    – Andy aka
    Commented Nov 22, 2013 at 17:19
  • \$\begingroup\$ i dont understand \$\endgroup\$
    – user16307
    Commented Nov 23, 2013 at 0:27
  • \$\begingroup\$ the pc is earthed yeah and the vfd is earthed if the motor smokes it might put ac to the pc via the vfd. Earth is enough to prevent electrocution but no enough to stop a few excess volts zapping the CPU hence isolation is cool. \$\endgroup\$
    – Andy aka
    Commented Nov 23, 2013 at 1:11
1
\$\begingroup\$

The VFD used a high voltage AC drive that does not share a common return or ground with the DAC, thus isoation is needed.

\$\endgroup\$
-1
\$\begingroup\$

Isolation amplifier has an amplifier and isolator in one package. The reason of using isolation amplifier here is to minimize return loss and amplify the power at the same time

\$\endgroup\$
7
  • \$\begingroup\$ so u mean it acts as a voltage follower? \$\endgroup\$
    – user16307
    Commented Nov 22, 2013 at 10:04
  • \$\begingroup\$ What is return loss? \$\endgroup\$
    – user16307
    Commented Nov 25, 2013 at 13:50
  • \$\begingroup\$ no it does not work as a voltage follower \$\endgroup\$
    – Dr. Arslan
    Commented Nov 30, 2013 at 13:10
  • \$\begingroup\$ see this link for return loss en.wikipedia.org/wiki/Return_loss \$\endgroup\$
    – Dr. Arslan
    Commented Nov 30, 2013 at 13:11
  • \$\begingroup\$ but i measure the voltage before and after it is the same value. it looks like it doesnt amplify the voltage. \$\endgroup\$
    – user16307
    Commented Nov 30, 2013 at 15:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.