Fareed Mohmed · Engineering
AC Induction Motor Selector
Power · Start · Real motors · Starter
Work out the shaft power a conveyor, pump, fan or hoist needs, describe how it starts and how it is used, and shortlist real Bonfiglioli induction motors that pass. Each motor is checked for power, speed, starting torque, start time, starts per hour, brake and efficiency class, and then matched to a starter or VFD rating.
This is a first selection for a single motor on a 400 V, 50 Hz supply. It does not replace the maker’s selection, which also checks the speed-torque curve, the voltage and winding, the cooling and the mounting. Motor data are Bonfiglioli catalogue figures; other brands are not loaded yet, and the starter and VFD ratings are general rules, not product data.
Start Here: Choosing an Induction Motor
A three-phase induction motor is chosen on four things, not one: the power the load needs at its speed, whether the motor can start the load, how long it takes to run up to speed, and how often it can start without overheating. These six steps give the order. Read them once, then use sections 01 to 05.
Find the power the load needs
Work out the shaft power from the machine: a conveyor, a pump, a fan, a hoist or a known torque and speed. Use the calculators and worked examples.
Start in section 01 →Describe the start and the duty
How hard the load is to start, how much inertia it has, how often it starts, the ambient temperature and how you want to start it: direct, star-delta, soft starter or VFD.
Fill section 02 →Let the tool shortlist real motors
Each motor is checked for power, speed, starting torque, start time, starts per hour, brake and efficiency class. Only motors that pass are listed, smallest first.
Find a motor in section 03 →Size the starter or drive
Pick one of the passing motors to see the contactor, overload and soft-starter or VFD current it needs.
See section 04 →Know what the start method does
A star-delta start cuts the current to a third, but it also cuts the torque to a third. Read the comparison before you choose.
Compare in section 05 →Confirm before you order
Check the speed-torque curve, the winding and the exact order code with the maker. This page is a first selection, not a maker’s quotation.
Common mistakes
Brands that make this product
Outlined gold: motor data loaded from the maker’s catalogue (Bonfiglioli). Grey dashed: not loaded yet. Confirm every model against the maker’s catalogue before you order.
Bonfigliolimotor data
ABBnot yet
Siemensnot yetCROMPTON GREAVESCrompton Greavesnot yet
SEW-Eurodrivenot yet
Nordnot yetBrand names and logos are trademarks of their owners and are shown only to identify catalogues.
Required Power
Everything starts from the power the load takes at the motor shaft. Pick the load type to see the formula, a worked example and a button that fills the example values. The result goes into the field at the bottom, which you can also type into directly.
Worked example for this load type
A belt conveyor carries a total of 5,000 kg at 1 m/s. Rollers and belt friction give a coefficient of about 0.04, the belt is flat, and the gearbox and belt are 85 % efficient. The force is 5,000 × 9.81 × 0.04 = 1,962 N, so the shaft power is 1,962 × 1 ÷ 0.85 = 2.3 kW. A conveyor is a constant-torque load.
A centrifugal pump delivers 60 m³/h of water against a 30 m head, and the pump is 65 % efficient. The shaft power is 1,000 × 9.81 × (60 ÷ 3,600) × 30 ÷ 0.65 = 7.5 kW. A pump is a quadratic-torque load: its torque rises with the square of the speed, so it starts easily.
A fan moves 12,000 m³/h against 800 Pa, and the fan is 65 % efficient. The shaft power is (12,000 ÷ 3,600) × 800 ÷ 0.65 = 4.1 kW. A fan is also a quadratic-torque load, but a large fan has a large inertia, so check the start time.
A hoist lifts 500 kg at 0.2 m/s with an 80 % efficient drive, so the shaft power is 500 × 9.81 × 0.2 ÷ 0.8 = 1.2 kW. This is the running power only. A hoist must also be held by a brake and needs the maker’s advice.
A mixer needs 40 N·m at the motor shaft at 1,440 rpm. The power is 40 × 1,440 ÷ 9,550 = 6.0 kW.
You already know the shaft power (for example from a machine drawing): 5.5 kW. Skip the calculators and type it in the field below.
Starting and Duty
Starting is where induction motors fail. Describe how hard the load is to start, how heavy it is to accelerate, how you will start it and how the motor will be used. Each field has a default you can change.
Don’t know the load inertia? Calculate it
Inertia is how hard a rotating part is to speed up. Enter the part that turns, and the gear ratio between the motor and that part. The result is reflected to the motor shaft, because the motor sees the inertia divided by the ratio squared. Use Add to build up several parts one at a time.
Find a Motor
The tool tests each Bonfiglioli motor against sections 01 and 02. It takes the catalogue’s rated power, rated speed, current, efficiency, starting torque and inertia, then works out the power used, the start time, the inrush current and the starts per hour. Passing motors are listed smallest first.
Starter or VFD for the Chosen Motor
Pick one of the passing motors. The table shows the contactor, overload and drive current each start method needs. These come from standard IEC practice and the motor’s rated current. They are not catalogue figures for a particular starter brand.
What Each Start Method Does
Torque and current are shown as a share of the direct-on-line values. The tool uses these factors in section 03. The VFD figure is an assumption you can change in section 02.
| Method | Torque factor | Current factor | Inrush | What it does | Use it for |
|---|---|---|---|---|---|
| Direct on line | 1 | 1 | 6 to 9 times the rated current | Simple and cheap. Full torque. Hard on the supply and the machine. | Small motors, light starts, a strong supply. |
| Star-delta | 1/3 | 1/3 | About 2 to 3 times the rated current | Starts in star, then switches to delta. Torque and current are one third. A jolt at the changeover. | Light loads (unloaded or fans and pumps) where the supply is weak. Needs a 400 V delta winding. |
| Soft starter | k² (k is the start voltage) | k | Set by the start voltage | Ramps the voltage. Smooth, adjustable, no changeover. Torque falls with the square of the voltage. | Conveyors, pumps and fans where smooth starting matters. |
| VFD | About 1.5 (limited by the drive) | About 1.5 (limited by the drive) | Close to the rated current | Gives full control of acceleration and speed. Needs motor cooling at low speed and costs more. | Speed control, many starts, long runs at partial speed. |
The torque factor is applied to the catalogue’s starting torque (Ms) and mean accelerating torque (Ma). The voltage dip you set in section 02 is applied as well, squared. The catalogue gives the locked-rotor current as a multiple of the rated current (Is/In), 6 to 9 times for most motors.
Reference Tables
Ambient temperature and power
| Ambient temperature | Allowed power, % of rated |
|---|---|
| 40 °C or lower | 100 |
| 45 °C | 95 |
| 50 °C | 90 |
| 55 °C | 85 |
| 60 °C | 80 |
Bonfiglioli’s table, for up to 1,000 m. For a derating above 15 %, the maker asks you to consult them.
Power factor for short-time and intermittent duty
| Duty | 10 min or 25 % | 30 min or 40 % | 60 min or 70 % |
|---|---|---|---|
| S2, limited time | 1.35 | 1.15 | 1.05 |
| S3, intermittent | 1.25 | 1.15 | 1.1 |
A motor may carry the rated power times this factor for the duty shown. S3 cycles must not exceed 10 minutes. For S4 to S9, contact the maker.
Starts-per-hour load factor, Kd
| Load, Pr / Pn | Kd at 15 % duty | Kd at 50 % | Kd at 100 % |
|---|---|---|---|
| 0.3 | 0.61 | 0.74 | 0.92 |
| 0.4 | 0.59 | 0.70 | 0.84 |
| 0.5 | 0.58 | 0.65 | 0.76 |
| 0.6 | 0.56 | 0.60 | 0.65 |
| 0.7 | 0.55 | 0.54 | 0.52 |
| 0.8 | 0.53 | 0.46 | 0.36 |
| 0.9 | 0.50 | 0.37 | 0.19 |
| 1.0 | 0.47 | 0.28 | 0.00 |
The catalogue prints Kd only as a chart. These values were read from the straight lines of the chart and are good to about ± 0.03. The tool interpolates between them. The permitted starts per hour are Z = Z0 × Kc × Kd ÷ KJ, with KJ = (Jm + Jc) ÷ Jm and Kc = (Ma − ML) ÷ Ma.
Efficiency classes
| Class | Name | What the regulation requires |
|---|---|---|
| IE1 | Standard efficiency | Allowed outside the EU rules. Bonfiglioli BN motors are the IE1 range; not every BN size carries an IE1 mark. |
| IE2 | High efficiency | EU minimum from 16 June 2011. Also the minimum when the motor is on a VFD. |
| IE3 | Premium efficiency | EU minimum for 7.5 to 375 kW from 1 January 2015, and for 0.75 to 375 kW from 1 January 2017 (IE2 if fed by a VFD). |
From the catalogue’s summary of EC Regulation 640/2009, which covers 2, 4 and 6-pole single-speed 50 or 60 Hz cage motors of 0.75 to 375 kW for continuous duty. It does not cover brake motors. Check the current regulation for your country.
Formulas used
| Quantity | Formula |
|---|---|
| Rated torque | Mn (N·m) = 9,550 × P (kW) ÷ n (rpm) |
| Synchronous speed | ns = 120 × f ÷ poles. 3,000, 1,500, 1,000, 750 rpm at 50 Hz |
| Start time | t = (Jmotor + Jload) × 2πn ÷ 60 ÷ (Ma − ML,mean) |
| Mean load torque while accelerating | Constant-torque load: ML. Fan or pump: about ML ÷ 3 |
| Load torque of a fan or pump | Torque rises with speed squared, power with speed cubed |
Explanatory Notes
How the shortlist works
For each motor the tool first applies the ambient and duty factors to the rated power and compares it with your load power (at the motor’s own speed, plus your margin). It then checks the speed, the starting torque against the load’s breakaway torque (plus 10 %), and the mean accelerating torque against the mean load torque. The start time follows from the motor and load inertia. Starts per hour use the catalogue formula. The brake, efficiency class and pole filters apply last.
What the catalogue prints
Bonfiglioli prints, for each motor at 400 V and 50 Hz: rated speed, torque and current, efficiency at 100, 75 and 50 % load (BN 8-pole at 100 % only), power factor, locked-rotor current (Is/In), starting torque (Ms/Mn), a torque ratio Ma/Mn, rotor inertia and mass. The catalogue calls Ma the mean breakaway torque. It is always lower than Ms, so the tool treats it as the mean torque available while accelerating. That is the safe reading. It prints no pull-out (maximum) torque, so the tool cannot check a load peak.
Which motors are loaded
157 single-speed motors at 400 V and 50 Hz. BX (IE3, 4 poles, 0.75 to 355 kW, including the cast-iron K frames from 30 kW), BE (IE2, 2, 4 and 6 poles, up to 22 kW) and BN (IE1 range, 2, 4, 6 and 8 poles, up to 30 kW). BX is only listed with 4 poles in this catalogue, so for an IE3 2 or 6-pole motor there is no match here. Dual-speed motors, 60 Hz and NEMA data, and motors for other voltages are not loaded.
Brake motors
The brake tables give the brake torque, the rotor inertia with the brake, and the permitted starts per hour. In some BE and BN rows the printed inertia with brake is lower than the plain motor, and the tool uses it as printed. DC brakes have normal (NB) and fast (SB) rectifiers with different starts per hour. The tool uses the normal figure, or the fast one where it is the only one printed. BX motors have no starts-per-hour figure, and for them the check shows n/a.
Starts per hour
Z0 in the catalogue is for 50 % duty and no load. The tool takes it from the brake version of the same motor, even if you chose no brake, and says so by showing it in the starts column. The Kd factor is read from a chart, so the result is approximate. If you need more than a few starts an hour, ask the maker.
Printed value left out
The BE 112M 4-pole row printed at 3.7 kW shows 27 N·m torque, which is the figure for the 4 kW motor on the same frame (3.7 kW at 1,445 rpm is 24.5 N·m). The tool leaves that row out and keeps the 4 kW row.
Star-delta and the winding
The catalogue lists 230/400 V motors as standard up to frame 132, and 400/690 V from frame 160. On a 400 V supply, star-delta starting needs the 400/690 V winding. So the tool removes the smaller frames when you choose star-delta. A 230/400 V motor can still be soft-started or run on a VFD.
VFD assumptions
The 150 % starting torque from a VFD, and the starter and drive current rules in section 04, are common practice and not catalogue data. Bonfiglioli says its standard motors may be used with PWM inverters up to 500 V at the transformer input, and that standard fan-cooled motors need torque derating, or a separate fan, at about 30 Hz and below. The derating curve is not loaded. The catalogue also lists maximum speeds above base frequency, for example 4,000 rpm for 4-pole BX 80 to 180.
Hoists, altitude and other limits
The catalogue asks you to contact the maker for lifting, for high hourly braking energy and for S4 to S9 duty. It gives no derating above 1,000 m, and no figure for high inertia loads beyond the starts formula. The tool leaves these to the maker.
Gearmotor catalogues
Nine other Bonfiglioli files were supplied. They cover gearboxes and gearmotors (C, VF-W, HDP, 3/H, TIR planetary, the ATEX and AS ranges). They rate the gearbox output, and are not used by this tool. They will suit a gearmotor selector.
Not loaded
ABB, Siemens, Crompton Greaves, SEW-Eurodrive and Nord. Their motors will be added as their catalogues are provided. Starter and VFD makers’ own products are not loaded either, so section 04 gives ratings, not part numbers.
Sources
All motor figures were read from the Bonfiglioli BN-BE-BX catalogue in October 2026. They are for 400 V, 50 Hz, S1 duty, and the maker’s tolerances apply. Confirm the order code and the current revision with Bonfiglioli before you order.
Motors
Bonfiglioli, Three-phase asynchronous motors BN-BE-BX Series, catalogue BR_CAT_BNEX_STD_ENG R06: motor rating charts BX (printed pages 58 to 62), BE (75 to 81) and BN (88 to 97).
Rules
The same catalogue: section 6.3 ambient temperature (table F20), 6.12 type of duty (table F26), 6.13 inverter-controlled motors, 6.14 permissible starts per hour with figure F29, section 2 efficiency classes, and the voltage table in section 6.1.
Standards
IEC 60034-1 (ratings and duty), IEC 60034-30-1 (efficiency classes) and EC Regulation 640/2009, as summarised in the catalogue. IEC 60947-4-1 for contactors and overload relay classes, as general practice.