KEVOS® MAKEOS · Power Transmission
Motor and Gearbox Selection
What gear ratio, motor rating and starting torque does this load need, and is the inertia match reasonable?
Selection of an induction motor and gearbox for a load: ratio from the motor speed (pole count and slip) and load speed, running torque and power through the gearbox efficiency, reflected inertia and the acceleration torque to reach speed in the time required (the same method as the reflected inertia tool), the next standard IEC motor rating with a service factor, and its starting torque against the starting requirement.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load torque | N·m | Torque the load needs at its own shaft. |
| Load speed | rpm | Required load speed. |
| Load inertia | kg·m² | Load inertia at the load shaft. |
| Acceleration time | s | Time allowed to reach speed. |
| Motor poles (50 Hz synchronous speed) | choice | Motor pole count (synchronous speed at 50 Hz). |
| Full-load slip | % | Slip at full load (from the motor data). |
| Gearbox efficiency | % | Gearbox efficiency. |
| Motor rotor inertia | kg·m² | Motor rotor inertia (from the motor data; estimate first and re-check). |
| Service factor | Service factor applied to the running power. | |
| Motor starting torque ÷ rated torque | Locked-rotor (starting) torque as a multiple of rated torque, from the motor data. |
Method and formulas
- Motor speed = synchronous speed × (1 − slip); ratio i = motor speed ÷ load speed.
- Running torque at the motor = T_L ÷ (i η); power = T_L ω_L ÷ η.
- Reflected inertia J_L ÷ (i² η); acceleration torque = (J_m + J_r) α_m; starting torque needed = running + acceleration torque.
- Motor rating = next standard IEC output ≥ SF × power; starting torque available = multiple × rated torque.
- Example: 200 N·m at 150 rpm with a 4-pole motor needs a 9.7:1 gearbox and a 5.5 kW motor.
Limitations
- Constant load torque during start; fans and pumps (torque rising with speed) start more easily.
- Supply frequency of 50 Hz assumed for the pole speeds.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a gearmotor for a conveyor, mixer or turntable
- Check that a motor can start a high-inertia load
- Compare 4-pole and 6-pole motor options
Who it is for
- Design engineers
- Electrical and mechanical engineers
- Maintenance engineers
What you get
- Gear ratio and motor speed
- Running power and chosen motor rating
- Starting torque check and inertia ratio
Interpreting the result
- A high inertia ratio is acceptable for induction motors on simple drives, but servo drives usually need it below about 10:1.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2569–2580, 3408–3412. Topic reference; no source tables or recommended settings are reproduced.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
