VFD Parameters, V/f vs Vector Control and Braking Resistor Sizing
Commissioning parameters of an AC variable frequency drive, V/f vs vector control, drive current selection and braking resistor ohm and watt sizing.
An AC variable frequency drive (VFD, frequency inverter) sets induction motor speed by changing supply frequency: `n = 120 × f / number of poles`. With drives built to the IEC 61800 series, good results depend on entering the motor nameplate data, picking the right control mode and, where needed, sizing the braking resistor correctly.
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1. Drive selection: current, not just kW
- The drive's rated output current must be ≥ the motor nameplate current; matching kW alone misleads with 6–8 pole and older motors.
- Heavy duty (constant torque): conveyors, mixers, hoists; 150% current for 60 s. Normal duty (variable torque): fans and centrifugal pumps; 110–120% for 60 s.
- A single-phase 230 V input drive outputs 3 × 230 V. A 230/400 V motor must have its terminal links set to delta (Δ); left in star it gives only about one third of its power.
- On a three-phase 400 V drive the same motor is connected in star (Y).
2. Parameters entered at first start-up
Parameter numbers differ by brand; the content is the same:
| Parameter | What to enter | If wrong |
|---|---|---|
| Motor rated voltage, current, frequency, speed, cos φ | From the nameplate | Overheating, low torque, false overload alarms |
| Motor thermal (overload) current | Nameplate current | Unprotected motor or nuisance trips |
| Acceleration / deceleration ramp | 3–30 s depending on inertia | Short ramp: overcurrent or DC bus overvoltage fault |
| Minimum / maximum frequency | e.g. 20–50 Hz | Pump runs dry, fan overspeeds |
| Command and reference source | Keypad, terminals, fieldbus | Drive will not start |
| Carrier (switching) frequency | 2–16 kHz | High value: quiet motor, more heat and leakage current |
| Motor identification (autotune) | Mandatory in vector mode | Weak torque at low speed |
Above 50 Hz the output voltage cannot rise further, so the motor enters field weakening and torque falls inversely with frequency.
3. V/f control vs vector control
| Control mode | Principle | Low-speed torque | Suitable load |
|---|---|---|---|
| V/f (scalar) | Constant voltage/frequency ratio (400 V / 50 Hz = 8 V/Hz) | Weak below 3–5 Hz | Fans, pumps, several motors on one drive |
| Sensorless vector (SVC) | Motor model separates flux and torque current | Up to 150% at 0.5–1 Hz | Conveyors, mixers, extruders, compressors |
| Closed-loop vector | Encoder feedback | Full torque at zero speed | Hoists, lifts, winders, precise speed |
A quadratic V/f curve saves energy on fans and centrifugal pumps. When one drive feeds several motors, use V/f only and give each motor its own thermal overload protection.
4. Braking resistor sizing
A motor decelerating a load quickly acts as a generator and pumps energy into the DC bus. When bus voltage passes a threshold the brake chopper switches the resistor in. The chopper threshold is typically 750–800 V DC on 400 V class drives and about 380–400 V DC on 230 V class.
- Ohmic value: never go below the minimum resistance in the drive manual; too low a value destroys the chopper transistor. Peak braking power is `P = U² / R`: `780² / 100 Ω ≈ 6.1 kW`.
- Continuous power (W): `P continuous = P braking × duty cycle (ED)`. Example: 3 kW of braking for 6 s in a 60 s cycle → ED = 10% → `3,000 × 0.10 = 300 W`. Choose 400–500 W for thermal margin.
- Load type: hoist lowering and long braking push ED to 40–100%, so resistor power approaches motor power.
Connect the resistor to the `B+ / BR` (or `P / PB`) terminals with a short screened cable. Its surface can exceed 200 °C, so mount it in free air away from flammable material. Simply lengthening the deceleration ramp removes the need for a resistor in most fan and pump applications.
5. Wiring and protection
- Use screened motor cable with the screen bonded 360° at both ends; EMC limits are defined in IEC 61800-3.
- Do not open a contactor between drive output and motor while running.
- If an RCD is required, use Type B on three-phase drives and at least Type F or A on single-phase drives.
- For fault messages see ATV320 Fault Codes and SINAMICS V20 Fault Codes.
Frequently asked questions
Can a three-phase motor run from a single-phase 230 V supply?
Yes. A 230 V single-phase input drive outputs 3 × 230 V; a 230/400 V motor connected in delta delivers full power. In practice this is used up to about 2.2–3 kW.
Should I use V/f or vector control?
V/f is enough for fans and pumps. Conveyors, mixers and lifting applications that need high torque at low speed call for vector control with motor identification completed.
When is a braking resistor needed?
When a high-inertia load must stop quickly or the load drives the motor (hoist lowering, inclined conveyors). A DC bus overvoltage fault during deceleration is the typical sign.
What happens if the braking resistor value is too low?
Chopper current exceeds its limit and the brake transistor fails. The resistor must be equal to or higher than the minimum ohmic value in the manual.
Can a motor run above 50 Hz on a drive?
Yes, up to its mechanical speed limit, but torque falls above 50 Hz. Check the motor maker's maximum speed for bearing and fan limits.