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[Galileo]

Hello,

I'm working on expanding my level of PLC programming knowledge. I know a lot of the basics so I think I would rate myself in-between a novice and an intermediate level student.

I'm looking for resources that help resolve the programming with the physical components required. I have some familiarity with a few of the 8 pin relays and motors starters used as control devices. I'd like to expand into different types of sensors used.

I'm particularly interested in learning how to control a vfd directly with a plc. Is it just a matter of getting a transformer to step the output of the plc down to match the inputs on the vfd?

 

[justthecowboy]

...

 

[_Dock_]

Typical industrial machinery has the following voltage hierarchy... This varies by industry obviously.

(In the USA)
High Voltage - Typically 480V, runs motors (Line Voltage of Building)
Control voltage - Typically 120V, runs the PLC, power supplies, enclosure fans, lighting, etc. (stepped down via tranformer from 480V to 120V)
Low Voltage - 24VDC runs sensor, small actuators, pneumatic solenoids. (24VDC comes from power supply with 120VAC input to 24VDC output)

To answer your VFD question specifically VFD's are given commands from the PLC via digital inputs or over a network using a protocol, EX Ethernet IP, modbus, ethercat, etc.

The VFD recieves its power from the high voltage side of the system (480) the PLC processes the programmed logic and turns on an output or gives a command via a the protocol. In the simplest form this is done with is 2 or 3 wire control to the VFD.

Inputs - (PLC Code) - Output 1 VFD direction, On for foward, off for reverse.
Inputs - (PLC Code) - Output 2 VFD Run, On for Run, Off for stop.

 

[drbitboy]

Inputs - (PLC Code) - Output 1 VFD direction, On for foward, off for reverse.
Inputs - (PLC Code) - Output 2 VFD Run, On for Run, Off for stop.

There are also

• enable Input (discrete), but that should be driven by a physical E-Stop or similar.
• speed reference Input (analog; PLC Code or other)

and sometimes the VFD has one input for [Run Forward] and a separate input for [Run Reverse]: no-run is 00; run fwd is 10; run rev is 01 (I dunno what happens with 11; maybe a fault?).

With separate direction and run inputs: no-run is 00 (fwd-off) or 10 (rev-off); run fwd is 11 (fwd-run); run rev is 01 (rev-run).

then there is configuring the VFD, a whole 'nother industry in itself almost.

 

[Ken Moore]

Some simpler machines that have all hard wired connections. They also do not have analog signals.
The VFD has preset speeds based upon which VFD DigIn is true. PLC output x0 = speed1
output x1=speed2, and so forth.

 

[GaryS]

I don't think there is one source that will give you what you want
Your best option is find a good mentor that will let you look over their shoulder and to work with on different projects. it takes years to learn and them they change everything
so you are always learning.
as for direct control of a VFD from the PLC you need to get the VFD manual on networking
read and understand it each manufacture is different there is no one size fits all
and sometimes you have to think out of the box
Welcome to the industry and good luck

 

[Galileo]

@GaryS
Thanks, Gary. Finding a good mentor is a real challenge. Around here, I am now sure who or how to approach people about the subject. Yes, I'm all about that out-of-the-box thinking. I need my own lab (dreading the cost of 480volt).

 

[Ken Moore]

FYI, there are small 2-3 HP drives available that have single phase input and three phase output.

In my area getting 3-phase power to a small shop is over 12,000 dollars just to the electric company.

 

[Galileo]

@Ken Moore
That's certainly a better option than 12 grand. Thanks for the heads up!

 

[ndzied1]

(In the USA)
High Voltage - Typically 480V, runs motors (Line Voltage of Building).

Using the term “high voltage” can bring some confusion. In some fields like power generation and transmission 480vac is considered low voltage.

This is also true of some motor manufacturers where the 480vac motors are classified as Low Voltage Motors.

See this reference
https://www.generatorsource.com/Articles/Generator-Info/High-Medium-and-Low-Voltage-Differences.aspx

 

[Corsair]

The PLC Class Outline at corsairhmi.com has some guidelines as to how to interface PLCs to VFDs.

I know the first choice is whether to do it through I/O or through communications. Communications gives a lot more options, especially when you want to know things like motor currents, fault types, and so on.

Control through I/O lets you have a lot more choices when the VFD goes bad and you have to change it out.

Currently I'm tending towards hybrid systems where the drive is started with a relay contact run signal and the speed is controlled with 4-20 milliamps. The network is hooked up for monitoring and diagnostic stuff. If you have to change the drive out for something different you'll lose the data but you will still be able to run.

 

[Jim3846]

plenty of ways to control vfds.
discrete IO inputs from plc to vfd
network interface from plc to vfd, devicenet, profibus, controlnet, ethernet, etc....
What if you have a brake contactor? additional parameters.
What if you have a brake resistor? additional parameters.
speed control from plc to vfd? additional parameters.

 

[Toine]

Yet another factor to keep in mind when deciding how to interface with VFD's: ease of troubleshooting and required tools for doing so. When you are an OEM and your products go all over the world and are typically used in rough places, then some kind of network or field bus controlled way of communicating with VFDs can prove hard to troubleshoot for a maintenance man who needs instruction before he can safely use a dmm. For those cases it might be preferred to have digital I/O to the VFDs. And an analog speed reference if such is needed (0..10V or 4..20mA). Troubleshooting any kind of field bus can be next to impossible for maintenance staff who know just enough to be dangerous. If it works: all fine and dandy. If it doesn't, well then the customer is up s##t creek. Without a paddle. Most of them don't appreciate.