What to compare when sizing a replacement drive

Start with the supply, because it disqualifies candidates faster than anything else. Record the supply voltage, phase count, and frequency at the drive input, then hold each candidate against those three values. Getting this field wrong is the classic error, because power figures look almost identical across supply classes on a listing page, and by the time the mismatch shows up the crate is open.

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The reference filter speeds the shortlist, and the nameplate confirms it. The arrows are a comparison order, not an electrical connection.

Part of the answer sits in the reference itself. The D block in 22C-D038A103 and the -4 suffix in ACS580-01-09A5-4 both select a supply class, though the nameplate stays the confirming source. Treat the reference as a fast filter and the plate as the evidence, because sellers transcribe references wrong more often than they reprint nameplates.

A candidate with the right power figure but the wrong input class is out before any other comparison begins. Write the supply line first in the comparison record and let it remove candidates cheaply. Every later line costs more time to check, so the cheapest disqualifier runs first.

Where the supply is unusual, say so plainly in the record instead of rounding it to a common value. Sites with generators, long cable runs, or mixed 50 and 60 Hz equipment exist, and a supply line that reads as expected when it is not will poison every later comparison. The supply line is the foundation of the checklist, and foundations get written down as measured, not as assumed.

Compare output current against the motor nameplate current, never against kilowatts. Two motors with the same power rating can differ in current by a wide margin depending on speed, pole count, and efficiency, and the drive is the component that carries that current. This is the line where brand loyalty and price tags matter least and arithmetic matters most.

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The comparison uses printed nameplate values on both sides. It does not measure the installation, and it does not replace the manufacturer's sizing data.

Write the motor's rated current and the candidate's continuous output current side by side, and take both from nameplates rather than machine files. The nameplate on the motor is the source here, because the machine file may have drifted over several replacements. If the two figures meet with little margin, note that too, because the margin question comes back at overload.

The same check applies in reverse when the motor has been changed after the drive was installed. A bigger motor on an unchanged drive is a downgrade nobody wrote down, and the drive has been carrying it on its rating ever since. The intake photos from the nameplate guide make the mismatch visible before it becomes a delivery decision.

Keep the comparison on the continuous rating first and the overload rating second, in that order, because the two figures answer different questions. Continuous current decides whether the drive can run the motor at all; the duty class decides whether it survives how the machine uses that motor. Mixing the two is how a candidate looks fine in a catalog table and fails on its first start.

Check the overload duty class as its own decision, separate from continuous current. Drives are commonly offered in a normal duty class for loads that stay near rated current and a heavier class for loads that exceed it during starts or process shocks. A pump that accelerates gently and a crusher that stalls into hard rock can share a motor yet need different duty ratings.

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The observation describes one recorded cycle, not a characterization test. Duty class definitions come from the candidate manufacturer's data.

Record what your load actually does during starting, and this stops being a guess. If you are unsure, watch one full start cycle and note where the current peaks and for how long, then write those observations down with a date. The duty finding changes the shortlist immediately: a genuinely light duty load makes a smaller frame viable, while a heavy duty load forces a higher continuous rating or a stated overload allowance.

That second path usually changes frame size, price, and lead time, which is why the duty decision belongs in the record rather than in someone's memory. Discovering the mismatch during commissioning is the expensive version of the same discovery. Write the duty decision down now, while the load is observable and the shortlist is still cheap to change.

List every control interface the machine actually uses, then check each candidate against that list. Analog inputs, digital inputs, relay outputs, and encoder feedback each need a matching terminal, and network communication often depends on an option card you order separately rather than one built in. A candidate can match every electrical rating and still fail here if it has no slot for the network your machine speaks.

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The list documents which interfaces the machine uses today. It does not confirm that a candidate's interface behaves identically, which needs the candidate's manual.

Photograph the existing control wiring before removal, because a terminal photo is the fastest way to rebuild that list. A photo taken before removal costs nothing and saves an hour of tracing wires in a drawing that may not match reality. The same photos later serve the commissioning crew, who otherwise start from the cabinet alone.

Option cards deserve their own line, with the exact module reference where one is fitted. A card that is no longer available for the new family is a sourcing question, and it is much easier to answer while the old card is still bolted in and photographable. The interface list is also where compatibility claims from sellers need the most checking, because interface names look similar across families while the behavior is not.

When the machine speaks a network the candidate cannot serve, record that as unmatched rather than searching for a workaround in silence. An adapter, a protocol converter, or a controller-side change are all legitimate answers, but each one adds engineering and downtime, and each one belongs in the decision with a cost attached. The interface list is where hidden project scope shows up first.

Record the braking arrangement as part of the sizing evidence. If the machine decelerates a high inertia load, the installed drive may pair a braking chopper with an external resistor, and the replacement must either include that capability or accept the same external hardware. This is a matching line like any other, and it is skipped most often because the resistor hides at the back of the cabinet.

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The check compares documented capability, not measured braking performance. Resistor values must suit the candidate's ratings, which the manufacturer's data states.

A candidate without braking support turns every stop into an overvoltage risk on the DC bus, the drive's internal voltage link. The failure mode is a trip, usually during deceleration, and it shows up after the new drive has already been commissioned. That is the most expensive moment to learn that the machine brakes harder than the brochure assumed.

The braking article in this series covers what to photograph and record on the resistor side, including printed ohm and power values. One combination is itself a finding worth recording: a resistor bolted in the cabinet and no braking option on the drive. Bring both sides of that pair into the comparison, because the candidate has to satisfy the machine's stopping behavior, and running behavior alone is not enough.

Note the deceleration ramps in the drive settings next to the hardware evidence, because a long ramp can mask a marginal braking setup for years. A change in process speed or a new operator can shorten that ramp, and the margin that used to absorb it is gone. The comparison record should carry the settings and the hardware together, since the pair is what actually stops the load.

Measure the space before you shortlist anything. Note the installed drive's dimensions, its mounting clearances, the position of its cable entries, and its enclosure protection class. Compact and book-frame designs spread the same rating differently across width, height, and depth, so a candidate that wins on paper can still fail to fit under a cabinet shelf or beside a contactor.

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The fit line compares documented dimensions and required clearances. It does not verify build quality or thermal behavior, which need the installation instructions of the exact frame.

Airflow clearance requirements come from the manufacturer's installation instructions, so record the installed clearances and check the candidate against them. Manufacturers differ on how much clearance their frames need, so never copy the old gaps onto the new unit without checking. A candidate that runs cooler but needs more clearance can still be the wrong answer for the same cabinet.

Turn the whole comparison into a line-per-item record: supply, motor current, duty, interfaces, braking, dimensions, protection class. Each line reads matched, unmatched, or unknown, and the unmatched lines are exactly where the sourcing conversation should focus. A claim that something is a drop-in replacement stays a claim until every line reads matched, and the record is what lets someone else re-check that judgment later.

Protection class gets its own line, matched against the environment article's findings rather than against habit. A sealed cabinet may make a lower protection class acceptable, while a washdown corner may demand more than the installed unit ever offered. The environment record and the sizing record answer each other here, and the comparison is complete when that exchange is visible on paper.

• Match supply voltage, phase count, and frequency before anything else, from the nameplate rather than memory.
• Size against motor nameplate current, and record the overload duty your load really needs.
• List control interfaces and braking hardware, including any separately ordered option cards.
• Record dimensions, clearances, and protection class, one matched or unmatched line at a time.

• the VFD reference hub
• environment checks before a swap
• the parameter backup guide