connect 6008 gnd to ac neutral ?

[Jan_Nielsen]

Hi,

I want to control an actuator that has a 3-wire interface: 24 V AC for power, 0-5 V DC for control, and Common.

For this I have a 230 V to 24 V AC transformer, and an NI USB-6008.

Now, first I am unsure if I can simply connect the 6008 gnd (and other DC gnds in the system) to the AC neutral ?

Second, this must be delivered to a customer so I can not be 100% sure if they will connect the AC neutral and line correct. What happens if they are swapped - and is there a way to protect the circuit, possibly so the system is not running if not connected correctly ?

Thanks, Jan

[johnsold]

Jan,

First, a clarification: The 3 wires on the actuator are 24 VAC in, Control in (0-5 VDC), and Common? That means that within the actuator the input power and the control signal share connections to common, right?

Second, you refer to 230 V and AC neutral.  I am guessing that this means you are in Europe.  Electrical systems in the U.S. are typically wired somewhat differently.  Does your power system have a grounded conductor?  Is the AC neutral grounded?

What kind of computer will be connected to the USB-6008? Desktop, laptop, industrial rack-mount?  Note that the USB-6008 grounds are connected to the computer DC ground.  Whether that is connected to the mains depends on the type of computer and power supply.

Does the 230 V to 24 V transformer have isolated primary and secondary windings or is it an autotransformer?

What you need to avoid is a connection which allows or causes large ground currents to flow through the USB cable.  That will destroy something. The DAQ device. The computer. Maybe the actuator.  And it could result in injury to the operator.

What kind of device is the actuator? How much power does it draw when "actuating?" What happens if it gets mechanically jammed?

Lynn

[Jan_Nielsen]

Hi Lynn, thanks for all your good questions.

The actuator is a Danfoss AME 130, to use for controlling a valve. There is a spec here including a wiring diagram on page 2. Nominal power consumption is 1.3 VA. It is protected against overload by force switch-off.

You are right that this is meant for Europe. I am based in Denmark, but will install the system in the UK. So we have 230 V, neutral, and earth.

The computer will be a HP laptop. I will also have some analog inputs to the 6008 (both 0-5 V RSE and 0-10 V floating), and expect to wire all gnds together.

Note that this is all for a prototype system, to run for a limited time. This means that in case of any errors, it's OK if the system stops working - eg. if a fuse is broken. The transformer is currently fused with 250 mA, I could exchange that for a smaller. The important thing of course is to prevent damage to people and electronics.

I am not too sure about the isolation of the transformer. It is a Comatec TBD2/035.24/F5 (spec here), and as mentioned with a fuse.

[johnsold · Accepted solution]

Jan,

The information you provided is quite useful.

The reason for my questions about the actuator is that I was concerned that the power to do the actuating might result in some kind of interference on the control side.  This can sometimes occur with large relays and contactors for example. It should not be a problem with your device.

On the third page of the transformer spec is a diagram which shows no connection between the primary and secondary windings.  That is what I was calling isolation.  An "isolation transformer" has defined charactersitics regarding the insulation strength between primary and secondary and often additional shielding to minimize capacitive coupling between the windings.  That degree of isolation is not necessary here.  With the transformer you have the 24 V circuit is not connected to the mains circuit.

I would suggest exactly one connection between the USB-6008 ground and the earth connection. That will prevent the voltages on the measurement system from becoming dangerous.  By using only one connection to earth, no current carrying loops are created. If the laptop power supply has an earth connection, check to see if it is connected to the USB ground. If so, that is the connection.  Otherwise add a wire to earth.

Lynn

[Jan_Nielsen]

Hi Lynn,

it seems I had somehow misunderstood the idea of an isolated transformer. I was afraid that I would be connecting the 6008 directly to the AC grid.

Anyway, things are much clearer now, I have just wired it up and it runs perfectly.

Thanks a lot for your answer, and for taking your time to read the specs !

Best regards, Jan

[johnsold]

Jan,

Glad to be of assistance. I do not want to see anyone hurt or equipment damaged because of a lack of understanding, so I try to comment when I see something like that.

Lynn