Delta Electronics has published a technical note that explains the motor control mode codes on its variable frequency drives (VFDs), such as IM VF + PG, IPM FOC and PM TQC. It describes how each mode works and which applications it suits. The note is written for qualified engineers who select and commission drives.
Reading the control mode code
Each mode name has three parts: motor type, control algorithm and feedback mode. “IM FOC PG”, for example, means an induction motor with field-oriented control and encoder feedback. Not every combination exists. Availability depends on the drive model and may change, so Delta advises checking the user manual for each drive.
Motor types
- IM (induction): the most common type. It is robust and affordable, but it has slip, and its efficiency is reaching the limits that newer regulations require.
- PM (permanent magnet): a synchronous motor with higher efficiency. The rare-earth magnets make it more expensive, and commissioning takes more effort. EC (brushless DC) motors use the same control modes.
- IPM: magnets inside the rotor, which makes the motor more expensive to build.
- SPM: magnets on the rotor surface. It is cheaper to build but needs a larger air gap, so its efficiency is slightly lower than an IPM motor’s.
- SynRM (synchronous reluctance): the most efficient type and cheap to build, but with very limited overload capacity. It suits applications such as centrifugal pumps.
Control algorithms
- V/f: the default mode. The drive sets voltage and frequency from a table and ignores the actual rotor speed. It is enough for centrifugal pumps and fans.
- SVC (sensorless vector control): estimates rotor speed from the motor current and compensates slip automatically. It gives better speed and torque accuracy at low speed, for example on conveyors and positive displacement pumps.
- FOC (field-oriented control): servo-like behaviour based on Clarke-Park transforms. It handles high starting torque at very low speed (stone crushers, hoists) and precise speed, torque and position control.
- TQC (torque control): built on FOC. The drive regulates torque instead of speed, for example in tension control.
With PG (encoder feedback), the controller uses the measured encoder speed instead of the drive output frequency. In PM FOC modes, the encoder also provides the rotor angle.
Performance by mode
| Mode | Speed range | Starting torque |
|---|---|---|
| IM VF / IM VF + PG / IM SVC | 1:50 | 150% @ 3 Hz |
| PM SVC | 1:20 | 100% @ frated/20 |
| IM FOC sensorless | 1:100 | 200% @ 0.5 Hz |
| IPM FOC sensorless | 1:100 | 150% @ 0 Hz |
| PM FOC sensorless | 1:50 | 100% @ frated/50 |
| IM FOC + PG | 1:100 | 200% @ 0 Hz |
| PM FOC + PG | 1:1000 | 200% @ 0 Hz |
| SynRM sensorless | 1:20 | 100% @ frated/20 |
frated is the rated motor frequency (parameter 01-01). The speed range is based on the maximum frequency (parameter 01-00). In V/f mode with a 100 Hz maximum, the usable 1:50 range runs from 2 Hz to 100 Hz. Below that range the motor still runs, but control quality drops. Starting torque is given relative to rated torque (100%). All values depend on correct tuning and on site conditions.
Why it matters for engineers
Choosing the wrong mode is a common cause of a drive that cannot start a loaded machine or hold speed at low frequency. For pumps, fans and lightly loaded conveyors, V/f or SVC is usually enough. When full torque is needed at near-zero speed, FOC is required. For very wide speed ranges or position control, FOC with an encoder is needed. In Delta’s table, PM FOC + PG gives the best results: a 1:1000 speed range and 200% torque at 0 Hz. Check the torque needed at each speed before you select the drive.
The full technical note can be downloaded from Delta EMEA’s AC motor drives section. One example of a FOC drive for both induction and synchronous motors is the C2000 series. For more news from the manufacturer, see the Delta Electronics product news on Automation Fair.com