C1D1 Inverter Duty and Temperature switches

[JeffKiper]

I have a new plant that is trying to figure out how to satisfy the VFD needs, Temperature switches for surface ignition and field disconnects. Sorry that was a run in sentence.

We are going to use Allen Bradley Powerflex 525. AB says that the control circuit and power needs to be in separate conduits. The electrician said that motors only have 1 conduit inlet. We can't use Intrinsically safe inputs because we can't keep 2" separation in the pecker head.
Is there a code requirement for auxiliary contacts on field disconnects when using VFDs? I have seen a boatload of different installations where guys use 480vac and 24vdc in the same pipe for the field disconnect. Then the next contractor does 2 pipes. The next contractor uses VFD cabme with 2 control wires for the AUX. One old school contractor put relays in the panel so he could run 120 VAC with the 480 VAC in 1 pipe then have the relay control the 24VDC.
I haven't found any code requirement and it's probably one of the we've always done it this way answers. I'm the controls guy and we are just doing panels and start up. No pipe &wire or terminations.

 

[paulengr]

I have a new plant that is trying to figure out how to satisfy the VFD needs, Temperature switches for surface ignition and field disconnects. Sorry that was a run in sentence.

We are going to use Allen Bradley Powerflex 525. AB says that the control circuit and power needs to be in separate conduits. The electrician said that motors only have 1 conduit inlet. We can't use Intrinsically safe inputs because we can't keep 2" separation in the pecker head.
Is there a code requirement for auxiliary contacts on field disconnects when using VFDs? I have seen a boatload of different installations where guys use 480vac and 24vdc in the same pipe for the field disconnect. Then the next contractor does 2 pipes. The next contractor uses VFD cabme with 2 control wires for the AUX. One old school contractor put relays in the panel so he could run 120 VAC with the 480 VAC in 1 pipe then have the relay control the 24VDC.
I haven't found any code requirement and it's probably one of the we've always done it this way answers. I'm the controls guy and we are just doing panels and start up. No pipe &wire or terminations.

You are dealing with AB. They don’t know what they are talking about. If it needs physical separation why are they in the same peckerhead with just one hole? We are a UL motor shop. New motors come that way under a certain size. Not a C1D1 issue specifically.

There are three issues with running power and controls in the same conduit. The first is insulation and dealing with faults. All cabling in the same space must have the same insulation rating. So you can’t for instance run Ethernet and 4160 V together as an extreme example.

Second is interference. VFDs produce lots of high frequency noise and even standard motors can induce 60 Hz voltages on control cables. So running shielding and twisted pairs and DC only (no 120 VAC controls) pretty much eliminates the issue. I’m working with a bunch of RTDs running over 200 feet right now. I just used 3 conductor shielded cable grounded at ONE END ONLY (controls end), no issues at all. So if you have Klixxon temperature switches, use 24 VDC. If you have RTDs or PTCs use shielded cables. It’s pretty basic stuff here.

By way of example for DECADES AB had instructions in their manuals to ground both ends of communication cables. I have eliminated many control problems by going with the IEEE standards (one end only) instead on AB equipment.

If you HAVE to use 120 VAC controls, use a big machine tool relay. Something bigger than an ice cube relay. Maybe a contactor. The heavy current draw ensures that the inductive current is too small to operate the relay.

The third is philosophy. What happens if your power system gets damaged and you short it and a control cable together? This is why it’s considered bad practice for instance to put the line and load side wiring in the same conduit...it’s a vulnerability thing. No Code, just practice.

 

[JeffKiper]

Thank you for the knowledge.
I only ground in the control panel for shields. Way to many noise issues over the years.

I will have to verify what the motors have as a temperature switch or sensor. 99% of what I've seen are switches in a XP motor. I'm sure there is something else out there that others are familiar with.

If the motor has temp switches I'm looking at calling for 600v shielded for the enable in the PF525 in series with the field disconnect aux early break and temperature switches.
If the motor has PTC or RTD I'll spec 600V shielded. I still use the enable in the PF525 in the field disconnect aux.

Hope life is treating you well Paul. Haven't seen you on the PLC forums in a while.

 

[paulengr]

Thank you for the knowledge.
I only ground in the control panel for shields. Way to many noise issues over the years.

I will have to verify what the motors have as a temperature switch or sensor. 99% of what I've seen are switches in a XP motor. I'm sure there is something else out there that others are familiar with.

If the motor has temp switches I'm looking at calling for 600v shielded for the enable in the PF525 in series with the field disconnect aux early break and temperature switches.
If the motor has PTC or RTD I'll spec 600V shielded. I still use the enable in the PF525 in the field disconnect aux.

Hope life is treating you well Paul. Haven't seen you on the PLC forums in a while.

Be very careful with field disconnects on VFDs. That’s a great way to blow one up. I’m surprised AB didn’t balk at this.

With temperature switches you don’t need much if any voltage on them. Just use a barrier and perhaps an isolation relay to trigger the PLC input.

Truthfully though you don’t really need isolation relays except in special cases. The VFD must have built in overload protection. The special case is when standard motor modeling does not adequately protect a motor such as if it’s a submersible or ambient temperatures are unusual. Theoretically temperature sensors work better but in practice it’s rarely beneficial.

A C1D1 motor is designed with the idea that it is filled with the flammable gas that will eventually ignite under fault conditions. It is built to contain the pressure of burning gas within the frame without bursting and exhausts via a labyrinth that cools the hot escaping gas and prevents propagation of a flame.