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How to Size a VFD Correctly: Avoiding Over- and Under-Spec Issues

by Bryan Hellman

 



A practical, step-by-step guide for maintenance leads, controls engineers, and buyers.

What you’ll learn:
  • How to size to current (FLA) first, not horsepower
  • When to choose VT vs CT duty—and when to upsize
  • How temperature, altitude, enclosure airflow, and carrier frequency force derating
  • When you need a brake resistor or a regenerative front end
  • Wiring/protection notes you can hand to an electrician

Who this is for

If you’re replacing a failed drive, speccing a new machine, or comparing “same HP, different price” options, this walkthrough gives you a confident, repeatable path to the right VFD.

Before you start: collect these 5 facts

  1. Motor nameplate FLA & voltage (e.g., 8.4 A @ 460 V)
  2. Load type (Variable-Torque fan/pump vs Constant-Torque conveyor/mixer/PD/extruder)
  3. Starts/stops (gentle ramps vs heavy, frequent starts; vertical/overhauling?)
  4. Environment (panel temp, altitude, airflow/ventilation, cabinet space)
  5. Stop method (coast, controlled decel, dynamic brake resistor, regenerative)
Pro tip: Note the carrier/switching frequency (kHz) and motor cable length. Both affect drive heating and motor stress.

The 5-step sizing process

Step 1 — Size to current, not horsepower

Choose a drive whose continuous output current ≥ motor FLA at your voltage. HP is a label; current tells the truth.

If this…
  • You only know HP
  • FLA varies between motors of “same HP”
Then do this:
  • Find the nameplate FLA (or vendor FLA table) before buying
  • Match the drive’s continuous amps to FLA, not HP

Step 2 — Choose the duty class (VT vs CT)

Pick the duty class that matches the torque profile. VT saves heat and cost when you don’t need high overload; CT provides the headroom for tough starts.

Feature VT (Variable-Torque) CT (Constant-Torque)
Typical loads Centrifugal fans/pumps Conveyors, mixers, PD pumps, extruders
Overload capability ~110–120% for ~60 s 150% for ~60 s
Pros Lower cost & heat when high torque isn’t needed Handles heavy starts & impact loads
When to upsize Rarely; only if derating cuts amps below FLA If you need >150% or >60 s, move up a frame

Step 3 — Apply derating (real-world conditions)

Hot panels, high altitude, tight cabinets, and higher carrier frequency reduce available current. Use the vendor’s derating tables to confirm the current you still have after derating meets (or exceeds) the motor FLA.

Rule of thumb: If derating knocks your available amps below FLA, upsize one frame or lower the carrier frequency (if acceptable acoustically).

Step 4 — Decide braking (so you don’t trip)

Fast decel or overhauling/vertical loads push energy back into the DC bus. Without a path to dissipate/return it, you’ll see overvoltage faults.

  • Coast: simplest; longest stop
  • Controlled decel: may work on high-inertia fans with modest ramps
  • Dynamic brake resistor: dumps energy as heat for faster, reliable stops
  • Regenerative front end: returns energy to the line; best for frequent, heavy decels

Step 5 — Wiring & protection

Motor-side conductor sizing usually keys off motor FLA; line-side conductors and OCPD are based on the drive’s input rating and the manufacturer’s instructions. Long motor leads? Consider dv/dt or sine-wave filters.

Mini-worksheets

Worksheet A — Variable-Torque Fan

  • Motor: ____ HP @ ____ V, FLA = ____ A
  • Duty: VT (~110–120%/60 s)
  • Environment: ____ °C, altitude ____ ft, ventilation: ____
  • Carrier frequency: ____ kHz
  • Stop method: ☐ coast ☐ controlled decel
  • Drive continuous current ≥ FLA? ☐ Yes ☐ No → If No, upsize
  • After derating, still ≥ FLA? ☐ Yes ☐ No → If No, upsize

Worksheet B — Constant-Torque Conveyor

  • Motor: ____ HP @ ____ V, FLA = ____ A
  • Duty: CT (150%/60 s)
  • Starts: ☐ heavy ☐ frequent ☐ sticky product
  • Environment: ____ °C, altitude ____ ft, ventilation: ____
  • Carrier frequency: ____ kHz
  • Braking: ☐ brake resistor ☐ regen ☐ none
  • Drive continuous current ≥ FLA? ☐ Yes ☐ No
  • Overload spec meets need? ☐ Yes ☐ No → If No, upsize
  • After derating, still ≥ FLA? ☐ Yes ☐ No → If No, upsize

Guided Examples (so you can see the logic)

Example 1 — 30 HP Cooling Tower Fan (VT)
  • Nameplate: 30 HP, 460 V, FLA 40 A
  • Duty: Variable-torque; gentle starts/stops
  • Environment: 45 °C panel in summer, sea level
  • Carrier frequency: 8 kHz (quiet operation)
  • Braking: Controlled decel, not aggressive

Decision path:

  1. Base current ≥ 40 A.
  2. VT duty acceptable (no heavy starts).
  3. Derating: At 45 °C and 8 kHz, many drives lose ~5–10% capacity. Choose a frame that still delivers ≥40 A after derating (e.g., a model with 44–46 A nominal that derates to ~40–42 A).
  4. Braking: No resistor needed with modest decel.

Result: VT-rated drive one frame above the bare minimum, sized on current after derating, not HP on the nameplate.

Example 2 — 5 HP Conveyor (CT) with Sticky Product
  • Nameplate: 5 HP, 460 V, FLA 7.6 A
  • Duty: Constant-torque, heavy starts; occasional stop/start
  • Environment: 40 °C, sea level
  • Carrier frequency: 4 kHz (no acoustic requirement)
  • Braking: Needs quicker stops to prevent product pile-up → brake resistor

Decision path:

  1. Base current ≥ 7.6 A.
  2. CT duty with 150%/60 s overload required.
  3. Derating: Minimal at 4 kHz and 40 °C—still confirm the table.
  4. Braking: Add a resistor kit sized per the drive manual.

Result: CT-rated drive whose overload spec explicitly states 150%/60 s, plus a brake resistor. If the nearest frame only offers 120%/60 s, go up one size.

Common pitfalls (and how to dodge them)

  • “Same HP, cheaper drive.” The cheaper one often has lower continuous current or lower overload. Always compare amps and duty class.
  • Ignoring carrier-frequency heat. Bumping switching frequency to quiet a motor can derate output current below what you need. If you must run high kHz, upsize the drive.
  • Aggressive decel with no energy path. This invites DC-bus overvoltage. Add a brake resistor or use regen.
  • Long motor leads with standard motors. Reflected waves raise peak voltage at the motor. Use dv/dt or sine filters, or stick to shorter cables/inverter-duty motors.
  • Altitude and hot cabinets. If you’re at elevation or in a cramped panel, derating can turn a “correct on paper” drive into a chronic tripper. Plan airflow—or upsize.

Quick myth-busting

Myth: “If HP matches, I’m good.”
Reality: Two 20 HP motors can have different FLAs; two 20 HP drives can have different current ratings. Match current and overload, not just HP.

Myth: “CT drives are always better.”
Reality: CT gives headroom you may not need. For fans/pumps, VT duty saves money and heat.

Myth: “Higher carrier frequency is always better.”
Reality: It’s quieter, but hotter. If you don’t need near-silent operation, keep it moderate and preserve current capacity.

FAQ (short and practical)

Do I ever size above FLA if my duty is light?
Yes—when derating (temp/altitude/carrier) cuts available amps below FLA, or when you want extra thermal margin for summers or clogged filters.

Can I run multiple motors from one VFD?
Yes, if the sum of FLAs is covered and you meet overload needs. Provide individual overload protection for each motor and confirm the drive supports multi-motor.

Do I need an “inverter-duty” motor?
Preferred, especially with long leads or high switching frequencies. Standard motors can work with filters, shorter leads, and modest carrier frequency.

A simple decision tree (in words)

  1. Is the drive’s continuous output ≥ motor FLA at your voltage?
    No → pick a larger frame
    Yes → go on
  2. Is your load VT (fan/pump) or CT (conveyor/mixer/PD/extruder)?
    VT → verify ~110–120%/60 s overload
    CT → verify 150%/60 s overload
  3. Do environment and settings force derating (temp, altitude, kHz, enclosure)?
    Yes → confirm available current after derating still ≥ FLA; if not, upsize
    No → go on
  4. Will you decel quickly, stop often, or lower/overhaul a load?
    Yes → add brake resistor or regen
    No → coast/gentle decel is fine
  5. Any long motor leads or standard motors?
    Yes → consider dv/dt or sine filter
    No → proceed
  6. Line vs motor wiring sized per drive manual/code?
    Yes → done
    No → adjust conductor/OCPD as required

Need help confirming size or accessories?

Send your motor nameplate photo + application notes, and we’ll validate VT vs CT, derating, and braking in minutes.

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