For most elevator retrofits, the answer depends on the building, not on the drive catalogue. Choose closed-loop when levelling accuracy and ride comfort are hard requirements — typically in high-rise commercial buildings, hospitals and accessible buildings. Choose open-loop when the building is low-rise, the duty is simple and the budget must stay lean. The mistake is to treat the two as interchangeable: they differ in feedback, in speed accuracy and in what happens at zero speed.
This guide follows on from our comparison of open-loop vs closed-loop elevator drives and from the seven signs that a lift VFD is due for an upgrade. It turns that general comparison into a retrofit decision: what to check on the existing lift, what each control mode can and cannot promise, and how to budget honestly for the encoder feedback card.
Open-Loop vs Closed-Loop: What Actually Changes
In an open-loop drive, the drive commands a speed and estimates the motor’s actual speed from its electrical model. There is no encoder signal returning from the shaft, so the drive cannot correct a small speed error directly. In a closed-loop drive, an encoder on the motor shaft feeds the actual speed and position back to the drive, and the drive corrects continuously. That feedback loop is the whole difference — and it is why the two modes behave differently at low speed, at standstill and under changing load.
The practical consequences show up in three places. First, speed accuracy: without feedback, speed drifts more as load and temperature change; with feedback, the drive holds speed far more tightly. Second, zero-speed torque: a closed-loop drive can hold full torque at zero speed, which matters when the car is held at a floor with the brake released or during a precise levelling move. An open-loop drive cannot hold the same torque at zero speed, so levelling and holding behaviour are less secure. Third, ride comfort: tighter speed control means smoother acceleration, deceleration and floor approach, with less jerk felt by passengers.
None of this makes open-loop “bad”. For a low-rise residential lift with modest traffic, simple stops and tolerant passengers, open-loop control is often entirely adequate and simpler to commission. The problem only appears when a building needs the precision that open-loop cannot guarantee — and the retrofit is specified on price alone.
Why Levelling and Ride Comfort Are Hard Requirements in Tall and Hospital Buildings
In a low-rise building, a small levelling error is an annoyance. In a high-rise building, it compounds: longer travel, higher speeds, heavier cars and more frequent stops all expose speed error, and passengers notice every uneven stop. In a hospital, the stakes are higher still. A stretcher, a wheelchair or a bed must roll across the threshold without a step or a jolt; a lift that stops a few centimetres high or low is not merely uncomfortable, it is a handling hazard and an accessibility failure. The same applies in any accessible building, where step-free boarding is the point of the lift.
That is why levelling accuracy and ride comfort should be treated as specification requirements in these buildings, not as preferences to trade away. If the building brief says the car must level accurately under varying load and must ride smoothly at low speed, the retrofit decision is effectively made: only closed-loop feedback can be specified with confidence. An open-loop drive may happen to level well on a light day and drift on a heavy one — and “usually accurate” is not a specification.
Verified Example: 11LIFT Open-Loop and Closed-Loop Performance
The 11LIFT elevator inverter is a useful reference because the same platform is offered in open-loop and closed-loop versions, so the comparison is like-for-like. Its verified ratings are 2.2–37 kW, with 220V / 380V supply classes. The table below uses only the manufacturer’s published control figures.
| Control figure | Open-loop (SVC) | Closed-loop (FVC) |
|---|---|---|
| Starting torque | 180% at 0.5 Hz | 200% at 0 Hz |
| Speed control ratio | 1:100 | 1:1000 |
| Speed stability accuracy | ±0.5% | ±0.05% |
| Encoder feedback card | Not required | Required, priced separately |
Read the table as a retrofit guide, not as marketing. The closed-loop version’s 200% torque at 0 Hz is the zero-speed holding capability described above. Its 1:1000 speed ratio and ±0.05% accuracy are what make precise, repeatable levelling realistic. The open-loop figures — 180% at 0.5 Hz, 1:100 and ±0.5% — are respectable for simpler duty, but they are an order of magnitude looser on accuracy. If a building’s levelling tolerance is tighter than ±0.5% can guarantee, open-loop is the wrong choice however attractive its price looks.
The same open-loop / closed-loop platform logic appears in the Z3300 and D3300 family, where the open-loop and closed-loop models share a 2.2–45 kW platform, and the Z3300-P and D3300-P versions cover 2.2–37 kW in 220V / 380V classes with RS485 communication. The platform point matters for retrofits: changing control mode is not always a different machine, but it is always a different feedback and commissioning requirement.
The PG Card Budget Truth
Here is the budget item most retrofit quotations obscure. On the 11LIFT closed-loop version, the PG card — the encoder feedback card — is required and priced separately. It is not bundled into the closed-loop drive price, and it is not optional. A quotation that compares an open-loop drive price with a closed-loop drive price, without adding the PG card, understates the closed-loop cost and invites an awkward variation later.
The card must also match the encoder type. Different encoders need different PG cards — for example incremental ABZ encoders, SIN/COS encoders for synchronous motors, and absolute encoders each have their own card variant — so “a PG card” is not a single interchangeable part. Budget for the correct card for the motor’s encoder, confirm it before ordering, and confirm who supplies and commissions it. Because this is a quotation-based product range, no price is stated here; the point is structural, not numerical: closed-loop cost equals drive plus PG card plus encoder work plus commissioning, and any comparison that omits the card is incomplete.
How to Choose for a Retrofit: A Decision Checklist
- Write down the building type, the number of floors, the traffic pattern and who uses the lift. A hospital or accessible building starts with a different requirement from a warehouse goods lift.
- State the levelling and ride requirement in words before choosing a drive: must the car level accurately under full and empty load, and must boarding be step-free for wheelchairs and stretchers? If yes, closed-loop is the default.
- Inspect the existing motor. Does it have an encoder, what type is it, and is it in serviceable condition? Record the encoder type — it determines which PG card the closed-loop option needs.
- Check the existing drive’s symptoms against the modernization signals: repeated levelling faults, rough starts and stops, nuisance trips or obsolete parts. A drive that is failing for other reasons should not be kept alive by a control-mode debate.
- Compare like with like. Price the open-loop option against the closed-loop option plus its PG card, plus any encoder replacement or alignment work, plus commissioning. Do not compare a bare closed-loop drive price with a complete open-loop price.
- Confirm commissioning scope. Closed-loop commissioning includes encoder alignment, feedback verification and low-speed tuning; confirm who performs it and that it is in the quotation.
- Decide, and record the reason. “Closed-loop because levelling is a hard requirement in this hospital” is a defensible decision; “closed-loop because it is newer” is not, and neither is “open-loop because it is cheaper” in a building where levelling is a safety and accessibility requirement.
Retrofit Selection Matrix by Building Type
| Building type | Usual choice | Why |
|---|---|---|
| Low-rise residential | Open-loop | Short travel, simple stops and tolerant duty; open-loop accuracy is usually sufficient and commissioning is simpler. |
| Goods lift / warehouse | Open-loop, unless levelling is critical | Load handling matters more than passenger ride comfort, but check levelling if trolleys or forklifts cross the threshold. |
| High-rise commercial | Closed-loop | Long travel and high traffic expose speed error; tight levelling and smooth ride are expected by every passenger, every stop. |
| Hospital / accessible building | Closed-loop | Step-free, accurate levelling is a hard requirement for stretchers, wheelchairs and beds; zero-speed torque and ±0.05% class accuracy protect boarding safety. |
Use the matrix as a starting point, then apply the checklist above to the actual lift. A low-rise building with an accessibility-critical lift, or a warehouse lift that must level precisely for loading equipment, can reasonably move one row up the matrix. The building type guides the decision; the levelling requirement settles it.
Frequently Asked Questions
Does closed-loop always give a better ride?
In practice, yes for the same mechanical lift, because feedback corrects speed error continuously and supports smoother low-speed approach and levelling. But a closed-loop drive cannot fix worn guides, a faulty brake or a poorly balanced car — mechanical condition still sets the ride baseline.
Can I keep the existing motor and just change the drive?
Often, yes, provided the motor is in good condition and its ratings suit the new drive. For closed-loop, the motor must also have a suitable encoder, or one must be fitted. Check the motor nameplate, encoder type and mechanical condition before assuming reuse.
If the old motor already has an encoder, is closed-loop almost free?
No — this is the trap in many retrofit budgets. An existing encoder does not remove the PG card: the closed-loop drive still requires its own encoder feedback card, priced separately, and the card must match the encoder type. The existing encoder must also be checked for condition and alignment, and closed-loop commissioning and tuning are still required. The encoder saves the cost of fitting a new one; it does not make closed-loop free.
Is open-loop ever the right choice for a tall building?
Only in narrow cases — for example a low-traffic service lift where levelling tolerance is genuinely loose and passengers do not rely on step-free boarding. For a passenger lift in a high-rise commercial building, open-loop’s looser speed accuracy makes it a poor specification, even if it appears to work on a light-traffic day.
What should a retrofit quotation include for closed-loop?
The drive, the correct PG card for the motor’s encoder type, any encoder work or replacement, and commissioning including encoder alignment and low-speed tuning. If any of those lines is missing, ask for it before comparing quotations — an incomplete quotation is not a lower price, it is a later variation.
For a retrofit assessment, contact us with the building type, floor count, existing motor and encoder details, and the levelling problem you are trying to solve. That information is enough to confirm whether open-loop or closed-loop is the defensible choice — and which PG card the closed-loop option requires.