Why a compressor is the hardest load for a VFD
A compressor is nothing like a fan or a pump. A fan starts almost without load — the air offers no resistance until the impeller spins up. A compressor is the opposite: at the moment of start the shaft has to move the piston or screw block against the back-pressure in the receiver and overcome high static friction in the oil film. That is why the starting current of a compressor motor easily reaches 5–7 times the rated value, and the starting torque required is close to 150–180 % of rated.
Hence the main practical advice we repeat to clients every day: a VFD for a compressor is sized by the motor's rated current, not by "the kilowatts on the nameplate". The catalogue power of a VFD is a guide for light loads. For a compressor, what matters is whether the inverter can withstand the current surge in the first seconds of the start. If it can't, you will get an overload fault before the machine even reaches operating speed.
Below is a practical sizing algorithm, the typical mistakes (above all OL2 at start) and the settings that actually work in compressor applications. All parameters are verified against the Veichi AC310 and AC10 technical manuals; where the series differ, it is stated separately.
Sizing a VFD by rated current, not by kW
The algorithm is simple and reliable:
- Take the motor's rated current from the nameplate (the "A" line for 380 V), not a guess based on kW.
- Choose a VFD whose rated continuous output current ≥ the motor current with a margin.
- For a compressor, allow a margin of 10–25 % above the motor's rating — this is exactly the margin that "absorbs" the starting surge.
- For hard-start applications (piston compressor, vacuum pump, crusher), take a VFD one frame up.
Indicative nameplate currents of 4-pole 380 V asynchronous motors (always check against the real nameplate — values depend on the manufacturer, cos φ and efficiency):
| Motor power | Typical rated current (380 V) | Min. VFD output current for a compressor (+10–25 %) | Comment |
|---|---|---|---|
| 4 kW | ≈ 8.8 A | ≈ 10–11 A | Screw — an even match is usually enough |
| 5.5 kW | ≈ 11.8 A | ≈ 13–15 A | Piston — take the upper end of the margin |
| 7.5 kW | ≈ 15.4 A | ≈ 17–19 A | A frequent "borderline" OL2 case |
| 11 kW | ≈ 22.5 A | ≈ 25–28 A | A safe upgrade for a 7.5-kW piston unit |
| 15 kW | ≈ 30 A | ≈ 33–37 A | — |
| 18.5 kW | ≈ 37 A | ≈ 41–46 A | Margin is critical for a hard start |
| 22 kW | ≈ 43 A | ≈ 47–54 A | — |
Why current and not kW: two "7.5 kW" motors from different manufacturers can have noticeably different rated current. A VFD protects itself and the motor by current — so it is the current that determines whether the protection trips during the start. Sizing "by kilowatts" is exactly what leads to the classic mistake below.
One more practical detail — reading the motor nameplate. A compressor motor plate usually has two current lines: for the "star" connection (Y, 380 V) and the "delta" connection (Δ, 220 V). Take the value for the voltage and connection the motor will actually run on from your VFD. If the motor is switched to "delta" 220 V (a common choice for powering a three-phase motor from a single-phase VFD input via a three-phase output), the rated current will be higher — and it is this value that must be the basis for sizing. The "I took the star current but wired it in delta" mistake is another typical reason for undersizing.
OL2 at start: what it means and how to fix it
In Veichi, overload faults are split: oL1 (code 14) is a motor overload, while oL2 (code 15) is an overload of the inverter itself (AC310, "Faults and diagnostics" section, fault code table). This is a fundamental difference.
When a compressor throws oL2 right at the moment of start, it is almost always a single diagnosis: the motor's starting current exceeds the VFD's capability. The inverter physically cannot deliver such a current surge, the hardware protection trips, and the start fails.
From practice: we had a case where a piston compressor on a 7.5 kW motor consistently dropped into oL2 at start even with maximum torque boost. The solution was moving to a higher-power VFD — from 7.5 to 11 kW. Only the current margin let the inverter "swallow" the starting surge. No torque-boost settings on a "borderline" VFD would have fixed it.
What to do when you see oL2 at start:
- Check the sizing by current. If the VFD output current only equals the motor's rating, that is the cause. You need a VFD one frame up.
- Extend the acceleration time (a slower start reduces the instantaneous current), but this is a palliative — on a compressor with back-pressure it helps only to a limited extent.
- Enable the SVC vector mode instead of plain V/F — for more accurate torque control at low speed (see below).
- Don't confuse oL2 with oL1. If it's oL1, the problem is in motor protection / overheating, not in the VFD size.
Single-phase compressor: why we move the client to 3 phases
A separate story is domestic and semi-professional single-phase 220 V compressors with a run capacitor. The request "fit a VFD to my single-phase compressor" comes up often, and the honest answer is usually no.
The reason is physical: the run (and start) capacitor creates a non-linear load that a VFD cannot adequately control. A VFD is designed to drive a three-phase motor by forming symmetrical three phases; a capacitor single-phase motor does not fit this model. Attempts to "shoehorn" a VFD onto such a motor give an unstable start, capacitor overheating and unpredictable behaviour.
The working solution we recommend: replace the single-phase motor with a three-phase 220/380 V one and drive it from the VFD. Then you get full torque-vector control, a soft start and real capacity regulation. This isn't "upselling" — it is the only engineering-correct path for a capacitor compressor.
Exception: a single-phase input to the VFD and a three-phase output is a normal and common scheme (powered from the 220 V domestic mains, with a three-phase motor on the output). There is no problem here; the "thin ice" is specifically the capacitor single-phase motor.
Screw and piston compressor: the difference at start
The compressor type directly affects the power margin:
- A screw compressor spins up relatively smoothly: the screw block has no sharp peak loads per revolution. An even VFD match by current with a minimal margin is often enough, and the soft start through the VFD almost eliminates the starting surge.
- A piston compressor works cyclically: every revolution the piston overcomes back-pressure, and the torque pulsates. The start is harder, especially "under pressure" (if the receiver is not vented). Here we allow the upper end of the margin (20–25 %) and, if needed, a VFD one frame up.
A practical detail for piston units: if the scheme allows starting from an unloaded receiver (pressure vented through an unloader valve), the starting torque drops sharply and the VFD requirements ease. This is worth considering at the sizing stage. More on the differences in the article VFD for screw and piston compressors.
Vacuum pump and generator power: torque and voltage sag
Two adjacent cases that regularly come up on compressor and vacuum units.
A vacuum pump is specifically a starting torque task. The pump has to move the shaft against vacuum and friction, so the key setting is start torque (torque boost on V/F or, better, the SVC vector mode). Insufficient starting torque means the pump "won't grab" the shaft, the current rises, and oL2 is possible.
Generator power (gen-set). The first question we ask the client is: "Are you powered from the mains or from a generator?" This is critical. A generator has a limited ability to withstand load surges: at the moment of compressor start the voltage sags, and the VFD may trip on an undervoltage fault (Lu) or behave unstably. If powered from a gen-set, allow an extra margin in generator power, extend the ramp and always check VFD compatibility with the source.
Setting up Veichi for a compressor: the key parameters
The parameters are verified against the manuals. Important note about codes: legacy Veichi series (the P-scheme) used parameters like P0.0x; the current AC310 and AC10 use F-codes. Below are the current F-codes.
| Task | Parameter (AC310 / AC10) | Value | Why for a compressor |
|---|---|---|---|
| Control mode | F01.00 | 1 = SVC (0 = V/F; 2 = FVC — AC310 only) | Accurate torque at low speed — critical for a start under pressure |
| RUN command channel | F01.01 | 0 — keypad / 1 — terminals | How to start: from the keypad or from external buttons/a controller |
| Torque boost (V/F) | F04.01 | tuned to fit | If you stay on V/F — it raises torque at start |
| Start mode | F07.00 | 0 — from start frequency; 2 — catch-on-the-fly | "2" (Flying Start) is needed if the shaft is still spinning after a stop |
SVC vector mode (F01.00=1). The motor control mode is set by parameter F01.00: 0 — scalar V/F, 1 — sensorless vector (SVC), 2 — closed-loop vector (FVC, available on the AC310 only; the AC10 range is 0~1, without FVC). For a compressor this is the most important setting after correct VFD sizing. The plain scalar V/F mode (F01.00=0) holds torque worse; sensorless vector control (SVC) gives more accurate torque at low speed — exactly where the hard start happens. For most compressors SVC is our default choice. For SVC to work correctly, after installation it is worth running a motor autotune (parameter self-learning): the VFD measures the real winding characteristics and builds the torque vector more accurately. Without autotuning the vector mode works "blind" on nameplate data, and on a hard start this is noticeable.
A separate note on acceleration and deceleration time. A compressor dislikes abrupt starts: too short a ramp increases the starting current and brings oL2 closer. A sensible compromise is a ramp where the pressure builds smoothly, without surges. When braking a large inertial compressor, the DC bus voltage may rise (risk of an overvoltage fault ou); if stops are frequent, a braking resistor is needed to dump the excess energy. For compressors running in a long, steady mode a braking resistor is usually not needed — coasting is enough.
Catching a spinning motor (F07.00 = 2). If the compressor stops by coasting and may be restarted before it fully stops, then without catch-on-the-fly the VFD will "hit" the still-spinning shaft and you'll get overvoltage or a current surge. The speed-tracking start mode (Flying Start) first determines the current speed and direction, then smoothly catches up. How to set this up step by step is in the article Flying Start on Veichi (F07.00=2).
Rotation direction. A compressor is critical to direction — the wrong direction of a screw block or oil pump is harmful. The direction is set by parameter F07.05; the reverse-run lockout is enabled there too. Details in the article reverse and start-reverse logic on Veichi (F07.05 / P0.09).
For the general principles of regulating compressor capacity with a VFD, see the articles using a VFD in a compressor system and controlling air compressors at variable speed.
When a VFD on a compressor is NOT needed (honestly)
So you don't waste money:
- A single-phase capacitor compressor without replacing the motor — a VFD won't solve the task correctly. First, a 3-phase motor.
- A compressor that runs in "start-stop" mode rarely and without a need to regulate capacity — then a soft starter, or even a direct start, is enough. A VFD is justified when you specifically need to regulate the air supply against variable demand.
- A budget "right at the limit" by current — better not to buy a VFD at all than to buy one too small: oL2 at start will cancel out the savings.
But if the task is to stabilise the network pressure, remove the "start-stop" cycles and save energy at partial load, a VFD pays off quickly. You can pick a model by current in the variable frequency drives catalogue; for compressor applications we usually recommend the Veichi range — it holds torque well in SVC mode and has clear F-codes for start and catch-on-the-fly.
FAQ
How do I correctly size a VFD for a compressor?
By the motor's rated current from the nameplate, not by kW. The VFD output current must be ≥ the motor current with a 10–25 % margin for a compressor. For piston and vacuum applications take the upper end of the margin or a VFD one frame up.
Why does the OL2 fault occur at compressor start?
oL2 in Veichi (code 15) is an overload of the inverter itself. At compressor start it means the motor's starting current exceeds the VFD's capability. The main solution is a higher-power VFD (for example, from 7.5 to 11 kW), not just a settings change.
Can I fit a VFD to a single-phase 220 V compressor?
On a capacitor single-phase motor — no, it creates a non-linear load that a VFD does not control correctly. The solution is to replace the motor with a three-phase 220/380 V one. A single-phase VFD input with a three-phase output, however, is a normal scheme.
Which control mode should I choose for a compressor on Veichi?
Sensorless vector — SVC (parameter F01.00=1 on AC310/AC10; 0 = V/F; 2 = FVC on the AC310 only). It holds torque at low speed better than plain V/F, which matters for a hard compressor start.
What to do if the compressor is powered from a generator?
Allow an extra margin in generator power, extend the acceleration time and check VFD compatibility with the gen-set. A generator sags in voltage on starting surges, so undervoltage faults (Lu) and unstable operation are possible.
How does sizing differ for a screw and a piston compressor?
A screw unit starts more smoothly — an even match by current is often enough. A piston unit has a pulsating torque and a harder start — allow a 20–25 % margin, and it's better to start from an unloaded receiver.
