Motor Sizing & VFD Calculator — Frame (IEC 60072-1), Torque & Energy Savings (IEC 60034-1)

The design power is the service factor times the load torque times the larger of the load speed and the motor's rated speed. A motor's rating is a torque at its rated speed, so a load that runs slower than that (a VFD in its constant-torque region, or a direct drive entered at a lower speed) is sized on its torque, not on its smaller shaft power; above rated speed it is sized on power. The tool then selects the smallest standard IEC frame — from the IEC 60072-1 frame numbers and standard output series, covering 0.25 kW to 250 kW — whose nameplate rating covers that design power. If the design power exceeds the top of that series, no frame is returned at all: the calculation is refused with an explanation rather than reporting the largest frame in the table as though it were adequate. The rated torque of the selected motor is back-calculated from the frame rating and the synchronous speed reduced by the assumed 3% full-load slip. The full-load line current is found from the motor nameplate power, the supply voltage, the power factor and the motor efficiency.

Three torque checks then feed the verdict. The load torque times the service factor must not exceed the continuous torque the frame delivers at the load speed. The locked-rotor torque of the selected motor (approximately 1.5 times rated for a standard squirrel-cage machine) must exceed the load torque, or the motor cannot start the load; hoists must additionally clear a 1.6× breakaway margin on the peak run-up demand — rated load torque plus the acceleration torque derived from the total moment of inertia and the required acceleration time. A stall check compares that peak demand against the pull-out (breakdown) torque (approximately 2.5 times rated); above the rated speed a VFD runs in field weakening, so the continuous torque is reduced in proportion to rated speed over load speed and the pull-out torque with its square. If the peak exceeds the available pull-out, the motor stalls during run-up. The frame comparison table reports breakaway and run-up separately for every candidate size, because a motor can break the load away and still stall while accelerating it.

The motor torque as a function of slip is described by the Kloss formula, which passes through the rated point and the breakdown (pull-out) torque at the critical slip. MechanixCalc blends the Kloss curve toward the user-specified locked-rotor torque at standstill, giving a realistic full-range characteristic from zero to synchronous speed. The load curve overlay (constant, quadratic or linear) is plotted on the same axes, and the steady-state operating point is found by interpolating the crossing on the stable running branch above half-synchronous speed.

For centrifugal fans and pumps, the shaft power scales with the cube of the speed ratio (the fan/pump affinity laws). Running a motor at reduced speed through a variable-frequency drive therefore cuts the shaft power — and the electrical input power — as the cube of the fractional speed. Conveyors are treated as a constant-torque load, where power falls linearly with speed and the saving is correspondingly smaller. MechanixCalc computes both the direct-on-line (DOL) electrical input power and the VFD electrical input power at each operating speed band, summing across all bands to give annual energy saved, annual cost saved, CO₂ avoided and the simple VFD payback period.

Thermal derating uses common manufacturer practice built on the IEC 60034-1 reference site conditions (1000 m altitude, 40 °C ambient): altitude above 1000 m reduces available power by 1% per 100 m of additional altitude; ambient temperature above 40 °C reduces available power by 1% per °C above 40 °C. These factors are applied multiplicatively to the rated nameplate power, and the nameplate service factor is then applied to give the available power.

The winding temperature beside that result is evaluated at the design's own load: the sized design power (application service factor kept) divided by the selected frame rating gives the load ratio L. At or above rated load the rated-load temperature rise is multiplied by L² (all losses scaled with load squared — the conservative member of the family above rated); below rated load it is multiplied by 0.5 + 0.5·L², which keeps a conservative 50 % share of the losses constant, since iron and windage losses do not fall with load. At L = 1 it is exactly the rated-load rise. Without a design load (or when the load runs below the motor's base speed on a VFD, where a self-ventilated motor cools less), the winding is evaluated at the nameplate service-factor load instead: the rated-load rise times the square of the service factor, never below the rated-load rise itself. The altitude and ambient factors are not folded into that ratio: they exist to hold the winding at the same rise despite degraded cooling, so including them would double-count and would report a cooler winding at altitude.