Mixer duty is defined by the process sequence, not one viscosity. The difficult point may occur before normal production: cold material, settled solids, a partly immersed impeller or restart after a trip.
Convert the recipe into a torque envelope
List each stage with fill level, temperature, viscosity or rheological model, density, solids content, speed and duration. For non-Newtonian products, viscosity without shear rate may be misleading.
| Batch event | Motor-system question | Evidence |
|---|---|---|
| Low-level start | Is operation hydraulically stable? | Mixer operating limit |
| Cold start | Can breakaway torque be delivered? | Torque-speed calculation |
| Solids addition | Is overload permitted and timed? | Recipe and drive limit |
| Restart after trip | Has settling changed load? | Defined restart scenario |
Translate required shaft torque through gearbox ratio and efficiency. Keep continuous torque, short overload and prohibited operation separate.
Check low-speed heat and current
Self-ventilation falls with speed. Compare continuous low-speed torque with the manufacturer’s approved VFD envelope. An independent fan adds airflow but also needs a supply, alarm and maintenance plan. Check drive current limit and duration: motor overload capability is useless if the drive limits first.
Protect the mechanical train
Confirm gearbox service factor, coupling and maximum shaft torque. Define the response to a jam; do not assume automatic reversal is safe.
Provide the full recipe, starts per hour, speed range, acceleration, hazardous-area classification and cleaning exposure with the RFQ. During commissioning, trend speed, current and process state through the worst planned batch.
Related reading: matching a VFD to a motor and motor service factor.