Motor starting time calculator
Estimate motor acceleration time from entered inertia, final speed and net accelerating torque for Australian motor-start review records.
omega = 2 x pi x rpm / 60; Tacc = Tavailable - Tload; t_s = J x omega / Tacc; E_kJ = 0.5 x J x omega^2 / 1000- Total inertia should be entered on the motor-shaft basis used by the project record.
- Available torque and load torque are average values for this simplified worksheet.
- Available torque must be greater than load torque.
- The review time is an entered comparison value, not a supplied threshold.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| J | Total inertia | kg.m2 | Entered reflected inertia for motor and driven load. |
| rpm | Final speed | rpm | Entered speed reached at the end of the reviewed acceleration period. |
| omega | Final angular speed | rad/s | Final speed converted from rpm. |
| Tavailable | Available torque | N.m | Entered average motor torque after starter or drive assumptions. |
| Tload | Load torque | N.m | Entered average driven-load torque during acceleration. |
| Tacc | Accelerating torque | N.m | Available torque minus load torque. |
| t_s | Estimated starting time | s | Time estimate from inertia, angular speed and accelerating torque. |
| E_kJ | Acceleration energy | kJ | Rotational energy associated with the entered inertia and final speed. |
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Motor starting time calculator technical guide
Estimate motor acceleration time from entered inertia, final speed and net accelerating torque for Australian motor-start review records.
Use this page when a motor-start record has an entered inertia, speed and torque basis and the project needs a transparent starting-time estimate. The calculator converts final speed to angular speed, subtracts load torque from available torque, then estimates acceleration time from inertia and net accelerating torque.
The output is a simplified worksheet value. It does not replace a torque-speed study, starter or drive data, thermal review, voltage-dip calculation, generator review or manufacturer instructions.
Field use cases
| Work setting | Real question | Useful action from this page |
|---|---|---|
| Fan package review | What starting-time estimate follows from entered inertia and torque data? | Enter total inertia, final speed, available torque and load torque. |
| Pump schedule note | Is there a starting-time record before a detailed product review? | Create a transparent estimate and mark whether a review time has been entered. |
| Conveyor replacement | Does the simplified estimate sit above an entered project time? | Compare the estimated time with the user-entered review value. |
| Generator or supply discussion | How long might the starting event last under the entered assumptions? | Export the time record before separate supply, voltage-dip or generator checks. |
This calculator is most useful when the project has a defensible inertia basis and a documented torque basis. If those values are only placeholders, keep that limitation with the record.
Torque basis
| Input | What it should represent | Common source | Watch point |
|---|---|---|---|
| Available torque | Average torque available to accelerate the load under the reviewed starting method. | Motor data, starter data, drive data or engineering estimate. | A single average value is a simplification of the torque-speed curve. |
| Load torque | Average driven-load torque during acceleration. | Pump, fan, conveyor or process equipment data. | Load torque can change with speed and process condition. |
| Accelerating torque | Available torque minus load torque. | Calculated by the worksheet. | Must stay positive for the estimate to be meaningful. |
| Review time | Optional project comparison value. | Manufacturer note, process limit or project criterion. | The calculator compares against the entered value only. |
Starting-time interpretation
| Result state | What it means | Practical response |
|---|---|---|
| Time record | No review time was entered. | Treat the result as an estimate and add a comparison value only when one exists. |
| Within entered review time | The estimate is not above the entered comparison time. | Keep the inertia and torque basis with the record before downstream use. |
| Above entered review time | The estimate is above the entered comparison time. | Recheck inertia, torque basis, starter or drive assumptions and process data. |
| Low accelerating torque | Available torque is only slightly above load torque. | Expect the estimate to be sensitive to small changes in torque data. |
| High acceleration energy | The entered inertia and speed create a larger energy record. | Review thermal, starting frequency and manufacturer data outside this calculator. |
Formula basis
The final speed is converted to angular speed:
text omega = 2 x pi x rpm / 60
The net accelerating torque and starting time are:
text Tacc = Tavailable - Tload t_s = J x omega / Tacc
The calculator also reports a rotational energy handoff:
text E_kJ = 0.5 x J x omega^2 / 1000
The formula uses average torque values. It is useful as a worksheet estimate, but detailed motor-start work may need torque-speed curves, starter or drive settings, supply modelling, process conditions and manufacturer thermal data.
Worked records
| Recorded situation | Calculation basis | Example result | Follow-up supported |
|---|---|---|---|
| Fan acceleration record | 18 kg.m2, 1475 rpm, 420 N.m available torque and 150 N.m load torque | Estimated starting time about 10.30 s and acceleration energy about 214.7 kJ. | Carry the record into starter, drive or supply review with the torque basis attached. |
| Pump estimate without review time | 6.5 kg.m2, 2900 rpm, 180 N.m available torque and 60 N.m load torque | Estimated starting time about 16.45 s. | Add a real comparison time before treating the result as a project comparison. |
| Conveyor time review | 12 kg.m2, 1450 rpm, 190 N.m available torque and 70 N.m load torque | Estimated starting time about 15.18 s, above the entered 10 s review time. | Recheck torque-speed data, starter settings and driven-load assumptions. |
Boundary with related calculations
| Related task | Use this page? | Why |
|---|---|---|
| Starting current | Not directly | Starting current depends on full-load current and starting method, not inertia alone. |
| Voltage dip | No | Voltage dip needs starting current and source impedance or transformer data. |
| Generator motor start | Sometimes before a wider review | Starting duration may be one input, but generator capability needs supplier data and starting kVA. |
| Running load | No | Use the motor efficiency load calculator when the question is measured running load. |
| Starter or drive settings | Supporting record only | Product settings need manufacturer data and project criteria outside this worksheet. |
Australian context
The page uses Australian 230/400 V, 50 Hz motor-start context, metric units and local project terminology. The formula is an engineering worksheet relationship, not a reproduced controlled standards table.
Motor-starting work can be affected by current standards, state or territory obligations, DNSP requirements, local authority requirements, project specifications, equipment ratings and manufacturer instructions. For high-inertia loads, frequent starts, weak supplies, generator-backed supplies or sensitive processes, treat this estimate as one entry in a wider review.
Stop points
- The total inertia is not on a consistent shaft basis.
- Available torque is not greater than load torque.
- The torque values are placeholders but are being treated as product data.
- A full torque-speed curve is needed for the driven load.
- The result is being used as a voltage-dip, generator or starter-setting answer.
- Starts per hour, thermal limits or process limits matter to the project decision.
- The entered review time has no documented project source.
Fan acceleration record
A fan package has an entered inertia and torque basis for a starting-time review before starter data is finalised.
- Motor reference
- FAN-4
- Total inertia
- 18 kg.m2
- Final speed
- 1475 rpm
- Available torque
- 420 N.m
- Load torque
- 150 N.m
- Review time
- 12 s
- Accelerating torque270 N.m
- Estimated starting time10.3 s
- Acceleration energy214.7 kJ
Carry the result forward with the inertia and torque basis.
The estimated time is within the entered review time, with the inertia and torque basis still visible for product review.
- Total inertia is entered at the motor shaft basis.
- Available torque and load torque are average values for this simplified worksheet.
- Review time is entered by the user.
Pump estimate without review time
A pump schedule line has inertia and torque data, but no project review time has been entered yet.
- Motor reference
- PMP-2
- Total inertia
- 6.5 kg.m2
- Final speed
- 2900 rpm
- Available torque
- 180 N.m
- Load torque
- 60 N.m
- Review time
- Not entered
- Accelerating torque120 N.m
- Estimated starting time16.45 s
- Acceleration energy299.7 kJ
Carry the result forward with the inertia and torque basis.
The result is a starting-time record only. Add a project or manufacturer review time before comparing the estimate.
- Final speed is entered in rpm.
- Net accelerating torque is positive.
- No review time was entered.
Conveyor time review
A conveyor replacement record checks whether a simplified acceleration estimate is above an entered review time.
- Motor reference
- CV-1
- Total inertia
- 12 kg.m2
- Final speed
- 1450 rpm
- Available torque
- 190 N.m
- Load torque
- 70 N.m
- Review time
- 10 s
- Accelerating torque120 N.m
- Estimated starting time15.18 s
- Acceleration energy138.3 kJ
The estimate is above the entered review time.
The estimate is above the entered review time, so the torque-speed data, starter settings and driven-load basis need review before handoff.
- Average available torque is entered by the user.
- Average load torque is entered by the user.
- The review time is a project comparison value.