isolation
I'm dealing with a module where the block diagram is given as follows:
The datasheet can be found both at here and an alternative one here.
When I perform a continuity test with a multi-meter between I/O Common and the Power Common pins, there is continuity and the multi-meter shows 0 Ohm.
But it seems according to what the datasheet tells and so the block diagram there must be isolation between these pins. Can I be wrong making conclusions about isolation by using a multi-meter?
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.
Completely isolating the controller from both primary and secondary sounds strange.
Here are the common approaches.
Controller on the primary side. The secondary side is isolated. The output power flows through the transformer. The feedback signal is brought back to the controller on the primary side. Typically, an opto-isolator used to bring the feedback signal back to the controller across the isolation barrier.
Controller on the secondary side. The half-bridge remains, of course, on the primary side. Since both output voltage and controller are on the secondary side, the feedback signal doesn't need to cross the isolation barrier. However, the gate control signals need to cross the isolation barrier from the controller to the Half-bridge. Gate-drive transformers or opto-couplers are used for that. The controller also needs an auxiliary power supply be able to start up.
Block diagrams taken from: http://powerelectronics.com . The block diagrams show flyback converters, rather than half-bridge. But, they still represent the isolation schemes.
Best Answer
Looks like there are jumpers, that are not shown as installed, but are installed by default on your back panel:
Given the comment Peter Smith noted about keeping voltage between I/O common and power common low (< 3V), we might assume there is something like a TVS in there, and the intended purpose of the isolation is just to prevent ground loops, so it's intended to be connected at some defined external point.