From IGBT to PLC Relay Output: What I Learned Buying an ABB VFD and a Siemens PLC Module

Back in March 2024, our maintenance manager dropped a requisition on my desk. We needed a replacement drive for a conveyor line, a PLC module, and an IGBT spare. I've been the office administrator who handles purchasing since 2020, so this was a normal week for me—until it wasn't.

I manage about 60 to 80 orders a year across 12 vendors, mostly for a 400-person manufacturing site. I report to operations and finance, which means I'm supposed to keep costs down and also make sure the plant doesn't stop. Those two goals don't always point to the same buying decision.

Here's the story of how a simple replacement order turned into a crash course in IGBT vs. thyristor, PLC relay output specifications, and what a variable frequency drive can and can't do as a phase converter.

The Request That Started It

The requisition had three lines:

• An ABB ACS880 variable frequency drive for a 22 kW conveyor motor
• A Siemens S7-1200 PLC module with order number 6ES7214-1AG40-0XB0
• One IGBT 100A module as a spare

I knew the ABB drive was not going to be the cheapest option. But our process engineer has a good reason for specifying ABB: the ACS880 series has been reliable for us, and the authorized distributor we use carries a wide range of ABB VFD models. The PLC part number looked familiar. The IGBT 100A line, though, made me pause.

"Why are we buying an IGBT?" I asked the electrician. "Is that a spare for the old drive?"

"It's a power semiconductor," he said. "If you're buying a VFD, grab one rated for the same current class as the output stage. It's cheap insurance."

So I started searching, and that's where the first mistake almost happened.

The Twist: IGBT and Thyristor Are Not the Same Thing

If you search for "igbt thyristor" the way I did, you'll find both terms used loosely on surplus parts sites. I almost ordered a thyristor because the listing said "VFD spare part." I didn't know the difference. An IGBT is a transistor that switches on and off many times per second; a thyristor is a latching semiconductor often used in power rectifier or braking circuits. In a typical VFD, the main switching devices are IGBTs, not thyristors. They are not drop-in substitutes.

What most people don't realize is that a part can be called a "VFD component" and still be completely wrong for your drive. The order number, the voltage rating, and the current rating all have to match. For the IGBT 100A module we needed, I had to confirm the module style and the drive model before anyone on the maintenance side would touch it.

PLC Relay Output: The Quiet Spec That Changes Everything

Then came the PLC module. The purchase order said 6ES7214-1AG40-0XB0. I assumed that was the complete description. A vendor called me and said, "We have a 6ES7214 in stock, but it's the relay output version. Is that okay?"

I almost said yes. The phrase "PLC relay output" sounded like an upgrade to me—a relay can switch more current, right?

Good thing I checked. The 6ES7214-1AG40-0XB0 is the CPU 1214C DC/DC/DC version. It has 14 digital inputs, 10 transistor outputs, and 2 analog inputs. Transistor outputs switch faster and have no mechanical contacts to wear out. For the application the engineers designed, that was the correct spec. The relay-output version, which does switch AC directly, would have changed the wiring and the response time. Same CPU family, different beast.

Here's something vendors won't tell you: the last part of the order number is not a detail. It's the difference between a project that wires up in one shift and a project that runs a second emergency trip to the electrical supply house.

The Variable Frequency Drive Phase Converter Question

While we were getting quotes, I also looked into something I'd been asked about before: a "variable frequency drive phase converter." We're in an old building with a few single-phase circuits. For some smaller equipment, the maintenance supervisor thought we could use a VFD as a phase converter instead of buying a separate rotary converter.

The short version, based on a phone call to our ABB distributor's technical support: a VFD can be used as a phase converter in limited cases, but the drive has to be specifically rated for single-phase input and the motor size has to account for derating. A generic VFD is not always designed for that. For our 22 kW motor on an existing three-phase line, the right move was a normal three-phase ABB ACS880, not a single-phase input drive pretending to be a phase converter.

That call took eleven minutes. It probably saved us thousands of dollars and a long argument with the electrician.

What I Actually Paid For: Certainty

The finance director almost had a heart attack when I approved $380 in expedited freight on top of the drive and PLC order. I don't normally do that. But the conveyor was down, and our maintenance supervisor estimated the downtime cost at roughly $4,000 an hour in delayed shipments. Paying extra for a vendor that could commit to a date was a no-brainer.

In 2022, I had a bad experience with a supplier who promised a "guaranteed Friday" delivery and showed up the following Wednesday. We missed a customer deadline, and I had to explain to the VP why I'd picked a cheaper vendor. The lesson: in an emergency, the uncertain cheap option is more expensive than the certain one.

Lessons I Carry Into Every Order

1. Don't rely on the part family name

The 6ES7214-1AG40-0XB0 has a look-alike with relay outputs. Same "6ES7214" prefix, different variant. Now I ask the supplier to repeat the full order number and output type before I create a purchase order.

2. IGBT and thyristor parts are not interchangeable

Searching for "igbt thyristor" as if it's one category will get you lots of results, but only one of them is right for your drive. Confirm the component type, current rating (e.g., IGBT 100A), and manufacturer. If I can find a matching factory part instead of a generic, I let the engineer decide. Not my call.

3. Know your power source before asking about a VFD phase converter

A variable frequency drive phase converter setup makes sense for some small single-phase applications. It's not a universal solution. Talk to a licensed electrical engineer or a manufacturer technical support line before you commit.

4. Budget for delivery certainty when downtime is severe

I don't pay rush fees for everything. But if the alternative is missing a production date, "maybe" is the most expensive word in the supply chain.

Before ordering, I verified the PLC specs against the Siemens S7-1200 product information for 6ES7214-1AG40-0XB0 and the ABB ACS880 hardware manual. Both are public sources, and it's worth checking again before you buy—part revisions change more often than you'd think.

Final Thoughts and One Limitation

My experience is based on one plant's purchasing patterns. If you're running a small workshop or a very different application, your experience might differ. I'm not an engineer, and I can't tell you whether a particular IGBT 100A module will work in your drive. What I can tell you is that industrial procurement is full of small details that turn an average order into a costly one.

If you're sourcing an ABB VFD, a Siemens PLC module, or a spare IGBT, take the time to verify the exact model and the delivery date. The number of people who will blame you for the delay is a lot larger than the number of people who remember you saved $120 on a part. Take it from someone who has explained a late Friday delivery to a vice president.