A statement such as “20% torque margin” is ambiguous unless it identifies the speed, supply condition, load case and source curves. The motor may have ample running torque yet little accelerating torque at an intermediate speed. A defensible record keeps the curves and assumptions visible.
Use accelerating torque as the core quantity
At each speed point:
Accelerating torque = available motor torque − driven-load torque
Positive accelerating torque is required to increase speed, but the acceptable separation depends on acceleration time, supply limits, thermal capability and project risk. Do not create a universal minimum percentage. Use the agreed design criterion for the application.
For a line-start motor, available torque must reflect the terminal voltage during starting, not only the catalogue curve at ideal supply. For a VFD system, use the motor-drive torque envelope and configured current or torque limits.
Record more than the rated point
| Checkpoint | Motor-side evidence | Load-side evidence | Decision recorded |
|---|---|---|---|
| Breakaway | Locked-rotor or zero-speed capability under stated supply | Static friction and initial process condition | Starts moving or remains stalled |
| Early acceleration | Motor torque-speed curve | Load torque-speed curve and gearing | Minimum accelerating-torque region |
| Near operating speed | Pull-up/breakdown or drive limit as applicable | Fan, pump, conveyor or machine curve | Stable acceleration to target speed |
| Normal operation | Continuous torque and thermal rating | Maximum expected process demand | Running margin and load variation |
| Upset/restart | Approved short-duration capability | Jam, loaded restart or process accumulation | Allowed, time-limited or prohibited |
Place every curve on the same speed and torque basis. State whether values are at the motor shaft or driven shaft and include gearbox ratio and efficiency where relevant.
Connect margin to acceleration time
The area between motor and load torque determines how the combined inertia accelerates. A narrow positive margin may produce an acceleration time that exceeds the motor thermal or starting limit. Conversely, a large peak margin at one speed does not compensate for a near-zero margin elsewhere.
Retain the inertia source, calculation method, voltage assumption, number of starts and hot/cold condition. If the load curve is estimated, label the uncertainty and assign confirmation to the driven-equipment owner.
Define what protection will do
The design file should distinguish a required operating case from a fault. If a jammed conveyor or blocked machine must trip, state the detection method and allowed time. Do not increase motor size or protection settings merely to hide an undefined process condition.
Package the curves with the electric motor RFQ checklist. For reading supplier data, use the motor performance-curve guide.