A requested speed range such as “10–60 Hz” is not yet a motor-duty specification. It says nothing about torque at the low end, power above the rated flux point, time spent in each region or whether the motor can cool itself. Base speed gives the review a useful dividing line, but the complete system envelope still has to be agreed.
Read the two sides of base speed differently
| Operating region | Main question | Evidence to request |
|---|---|---|
| Zero to minimum controlled speed | Can the drive control the load and provide the required breakaway torque? | Load torque, control mode, feedback and starting sequence |
| Minimum speed to base speed | Can required torque be carried without excessive heating? | Continuous/intermittent torque, cooling method and thermal assessment |
| At base speed | Do motor rated voltage, frequency, connection and drive output agree? | Nameplate, winding data and drive configuration |
| Above base speed | Is falling available torque compatible with the load? | Motor-drive torque envelope and driven-load curve |
| Maximum commanded speed | Are mechanical speed limits respected? | Rotor, bearing, fan, balance and driven-equipment approvals |
This table should be completed for the offered motor and drive together. A motor-only curve cannot confirm drive current capability, while a drive setting cannot establish the motor’s mechanical speed limit.
Low-speed torque is also a cooling question
For a shaft-cooled motor, fan airflow changes with speed. Loads such as conveyors and extruders may ask for substantial torque exactly where self-cooling is weakest. Record the duration of low-speed operation and whether an independently powered fan is required. Include the fan’s supply, airflow direction, alarm and interlock in the system design.
Short low-speed events and continuous crawling duty are not equivalent. The proposal should state which is approved rather than presenting one undifferentiated minimum frequency.
Above base speed, check the load curve
Fans and pumps, constant-power machine tools and constant-torque conveyors do not demand the same torque-speed relationship. Overlay the driven-load requirement on the supplier’s usable motor-drive envelope. Do not infer high-speed capacity by extending the nameplate point with a straight line.
Also verify overspeed margin, bearing arrangement, lubrication, external fan speed and coupling limits. The lowest mechanical limit in the train governs the setting.
Settings that should match the approved envelope
Before commissioning, compare the engineering record with drive parameters: motor rated voltage and frequency, current, base frequency, minimum and maximum speed, acceleration/deceleration ramps, torque/current limits and cooling-fan logic. Protect password-controlled parameters where the project requires it.
Cable length and waveform stress remain separate checks. See VFD cable length and motor insulation stress and dV/dt filter versus sine-wave filter for those interfaces.