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Regenerative Drives Explained: Turning Braking Energy into Savings

by Bryan Hellman

 



When your machine slows down, that energy doesn’t have to turn into heat. With regenerative drives, you can push it back to the line—or share it with other axes—and lower your operating costs.

What you’ll learn
  • What regenerative drives are and how they work
  • When regen beats dynamic braking resistors
  • How to estimate energy savings and payback
  • Architectures you can actually buy today from IAC
  • Key pitfalls (harmonics, decel profiles, utilities) and how to avoid them

Shop Regenerative Drives (All Brands)

What is a regenerative drive?

A standard VFD converts AC line power to DC, then back to variable-frequency AC for the motor. During fast stops or with overhauling loads, the motor behaves like a generator and raises the DC-bus voltage. Most drives dump that energy into a braking resistor (heat).

A regenerative drive (or a drive paired with a regen-capable line/AFE module) provides a path to push that energy back—either onto your plant’s AC line or into other axes on a shared DC bus. Result: real kWh savings and less panel heat.

When does regeneration make sense?

Frequent/fast decel

Cycle machines, presses, winders/unwinders, centrifuges, and test stands with short decel times or repeated start/stop.

Overhauling loads

Elevators, hoists, and vertical axes where gravity assists motion—energy flows back every lower cycle.

Multi-axis systems

One axis brakes while another accelerates; a shared DC bus can “recycle” energy internally before returning any surplus to the line.

How regeneration works (plain English)

  1. Motor generates: During decel, the motor becomes a generator and pushes energy back into the DC bus.
  2. DC bus rises: Without a path, DC voltage spikes and the drive trips on overvoltage.
  3. Energy path: A braking resistor dissipates energy as heat; a regen line/AFE converts it back to AC and returns it to the plant power system.
  4. Sharing across axes: With a common DC bus, braking energy from Axis A can accelerate Axis B before anything is sent to the grid.

Architectures you can buy from IAC today

Approach How it works Best for Shop
Common DC-Bus (share regen internally) Multiple drives tie DC buses; braking on one axis feeds another. Multi-axis machines, lines with alternating accel/decel. Siemens SINAMICS S120
Active/Smart Line Modules (return energy) Line module converts DC-bus energy back to AC line power. High-duty regen, vertical axes, short decel times. 6SL3130-7TE23-6AA3 · 6SL3130-6TE23-6AA3 · 6SL3130-7TE25-5AA3 · 6SL3130-7TE28-0AA3 · 6SL3130-7TE31-2AA3 · 6SL3130-6TE21-6AA4
Regen-ready family (Allen-Bradley) PowerFlex 755T/755TS platform supports AFE & common DC-bus. Rockwell plants, users standardizing on PF 755T. PowerFlex 755T/755TS

Browse all Regenerative Drives & Modules →

Will it really save money? A quick back-of-the-envelope

Two levers determine savings: how much kinetic/gravitational energy you shed and how often you shed it.

Simple estimation flow
  1. Energy per stop (Wh) ≈ Average regen power (kW) × decel time (s) ÷ 3600.
  2. Daily energy (kWh) = Energy per stop × stops per day.
  3. Annual kWh = Daily kWh × operating days per year.
  4. Annual $ saved = Annual kWh × your $/kWh.

Example: A 30 kW motor decelerates hard (≈10 kW average back onto the DC bus) for 3 s, 200 times per hour, 16 hours/day, 260 days/year. Energy per stop ≈ (10×3)/3600 ≈ 0.0083 kWh. Daily ≈ 0.0083×(200×16) ≈ 26.6 kWh. Annual ≈ 26.6×260 ≈ 6,916 kWh. At $0.12/kWh, that’s ≈ $830/year for one axis—before HVAC savings and resistor maintenance.

Regen Savings Calculator

Estimate annual energy savings from switching from braking resistors to regeneration.

Estimated Annual Savings: $0
(Excludes HVAC and maintenance reductions)

Common pitfalls (and how to dodge them)

  • “We’ll just add a big resistor.” Works, but wastes energy and heats the area. In high-duty cycles, regen is usually the smarter lifecycle choice.
  • No place for the energy to go. Tight decel without resistor or regen invites DC-bus overvoltage faults.
  • Ignoring harmonics and interconnect rules. Coordinate with power quality/utility early, especially with high-duty regen.
  • Untuned decel ramps. Over-aggressive ramps cause nuisance trips; tune ramps and braking thresholds.
  • Forgetting panel thermal limits. Resistors demand spacing, ventilation, and high-temp wiring clearances.

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.

FAQ (short and practical)

Does regen always return power to the utility?
Not necessarily. In a shared DC-bus system, much of the energy can be reused by other axes first; only surplus is exported.

Will regen affect my machine controls?
It changes your energy path, not your motion profile. You’ll still tune decel ramps and braking thresholds to avoid DC-bus trips.

What should I buy first?
For Siemens-based systems, start with SINAMICS S120 and the 6SL3130 Active/Smart Line Modules above. For Rockwell, see PowerFlex 755T/755TS.

A simple decision tree (in words)

  1. Do you brake fast/frequently or have overhauling loads?
    No → Resistor may be sufficient
    Yes → Consider common DC-bus and/or regen line modules
  2. Multi-axis machine?
    Yes → Tie a common DC bus; add regen only if surplus remains
  3. Power quality or utility constraints?
    Strict → Prefer active/“smart” line modules (improved input current)
  4. Panel heat limits?
    Tight → Avoid big resistors; regen preferred
  5. Payback math penciling out (kWh × $/kWh)?
    Yes → Proceed with regen; tune decel to avoid trips

Want help picking the right regen approach?

Share your motor nameplate, decel profile, and how often you stop. We’ll estimate savings and recommend the best modules.

Shop Regenerative Drives Shop SINAMICS S120 Shop PowerFlex 755T

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