EngineeringMechanical EngineeringPart 09 of 15
Electric Motor Selection
A squirrel cage motor sizes itself to the load by drawing more current. That convenience is exactly why oversizing and undersizing both go unnoticed until something overheats or the power bill arrives.
- Pole count and speed
- Part-load efficiency
- Radial and axial limits
- IP protection
Executive summary
Two motor families cover the large majority of mechanical design work: the three phase squirrel cage motor, which should be the default wherever three phase supply exists, and the single phase squirrel cage motor for domestic and light commercial situations where it does not.
Frame size is a dimensional designation — the distance in millimetres from the base of the motor feet to the rotor centreline — used consistently across manufacturers. Power rises with frame size, so the frame number is a useful shorthand for physical envelope during layout.
Speed, poles and slip
Synchronous speed is fixed by the number of poles and the supply frequency. Actual speed sits slightly below it, and the gap — slip — grows with load.
| Poles | Synchronous speed | Typical full load speed | Common use |
|---|---|---|---|
| 2 pole | 3000 rev/min | Around 2880 rev/min | Pumps, fans, compressors, high-speed drives. |
| 4 pole | 1500 rev/min | Around 1450 rev/min | The general-purpose default across industry. |
| 6 pole | 1000 rev/min | Around 960 rev/min | Conveyors, mixers, where lower input speed reduces reduction ratio. |
| 8 pole | 750 rev/min | Around 720 rev/min | Heavy, slow-speed machinery. |
Under no load the actual speed is close enough to synchronous speed to be treated as equal. At full load it drops to the value listed in the performance data. Between the two the speed–load relationship is very nearly linear, so linear interpolation gives the speed at any intermediate design load with little error.
Manufacturers use "full load" to mean the maximum load for continuous operation of the motor. The full load requirement of the driven machine is usually less than that. Call the latter the design load and keep the two apart in every calculation — particularly when interpolating speed and efficiency.
Sizing: the two-sided error
Undersizing
A squirrel cage motor self-adjusts to the torque demanded by increasing current draw. Overload therefore does not stall the motor immediately — it heats it. Sustained overload means sustained overheating and eventual insulation failure.
Oversizing
A grossly oversized motor will last indefinitely, but it costs more, occupies more space, needs larger starting equipment and cabling, and runs at lower efficiency. Like most prime movers, an induction motor is less efficient at part load than at full load.
Performance tables typically list efficiency at full load, seventy-five per cent load and fifty per cent load. Linear interpolation between those points is sufficiently accurate for intermediate values. Reading the part-load efficiency — rather than the headline figure — is what turns a motor selection into an energy decision.
A motor selected two frame sizes above the duty may sit permanently at forty per cent load. The efficiency penalty at that point is not trivial, and it is paid every hour the machine runs for the whole of its life. Size to the duty, and add margin deliberately rather than by habit.
Enclosure, protection and mounting
Radial and axial shaft loads
Anything mounted directly on the motor shaft — a pulley, sprocket, gear or flywheel — imposes a radial overhung load on the motor bearings. Published radial and axial load capacities are normally quoted against a nominated bearing life, commonly in the order of 30 000 hours.
- Calculate the overhung loadFrom belt or chain tensions, or gear tooth forces, or the approximate expression used for gearbox output shafts.
- Compare against the radial capacityFor the frame size and speed. Exceeding it does not cause immediate failure — it shortens bearing life, quietly.
- Reduce the load if necessaryUse a larger pulley or gear to lower the tension, or accept a larger motor frame.
- Check the axial loadThrust capacity is normally published for thrust acting toward the motor; capacity in the opposite direction may not be quoted at all.
- Derate for combined loadingWhere radial and axial loads occur together, the allowable axial load must be reduced. Manufacturers publish interaction charts for this.
A belt drive taken directly off a motor shaft puts the whole belt tension into a bearing sized for the motor, not for the drive. Where the overhung load is marginal, a jackshaft in its own plummer blocks removes the problem entirely and often costs less than the next motor frame up.
Selection procedure
- Establish the mechanical dataRequired torque, power and speed at the motor shaft, including transmission efficiency losses downstream.
- Choose the pole countMatching motor speed to the drive so the downstream reduction ratio is sensible.
- Select a motor from the performance dataTorque and power output at least equal to the design requirement at the chosen synchronous speed.
- Determine the speed at design loadBy linear interpolation between synchronous speed at no load and the tabulated full load speed.
- Check the overhung loadAgainst the radial capacity for that frame.
- Check axial load and combined loadingDerating the axial capacity where both act together.
- Confirm enclosure and protectionEnclosure type and IP rating appropriate to the installed environment.
- Confirm mounting and terminal boxArrangement code, shaft orientation, and cable entry direction.
- Record the part-load efficiencyAt the actual design load, not at full load.
Precise motor performance varies from unit to unit and can only be established by testing the actual motor. Catalogue values are typical values. Where performance is genuinely critical — a synchronised line, a metering pump — specify tested performance rather than relying on the table.
Selection checklist
- Design load distinguished from motor full load throughout the calculation.
- Transmission efficiency losses included in the required motor power.
- Pole count chosen to give a sensible downstream reduction ratio.
- Speed at design load interpolated rather than assumed equal to full load speed.
- Part-load efficiency read at the actual operating point.
- Overhung load calculated and within the radial capacity for the frame.
- Axial load direction confirmed and combined loading derated.
- Enclosure type and IP rating matched to the installed environment.
- Mounting arrangement code and shaft orientation specified.
- Starting method considered and its effect on downstream service factors accounted for.
- Single phase alternative justified only where three phase supply is genuinely unavailable.