Enter the motor nameplate, drive frequency and sheaves to get motor and machine speed, slip, and the frequency to set on the drive to reach the speed you need.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Motor and machine speed at the load entered; the lines mark the present frequency and the one needed for the desired speed.
As a percentage of rated values, with V/Hz control: constant torque up to base frequency and constant power above it.
ns = 120 × f ÷ poles, at 50 and 60 Hz, and the speed estimated with this motor slip.
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Synchronous speed is 120 × frequency ÷ poles: 120 × 60 ÷ 4 = 1800 rpm for a 4-pole motor on 60 Hz. The rotor turns slightly slower because of slip: the example nameplate says 1770 rpm at full load. At 80% load, line-power speed is about 1776 rpm.
The difference between magnetic field speed and rotor speed, divided by synchronous speed. Without it an induction motor produces no torque. In the example, (1800 − 1770) ÷ 1800 = 1.7%. Large premium-efficiency motors have less slip; small motors more.
Almost proportional to frequency, minus slip in rpm. In the example, at 45 Hz the field turns at 1350 rpm and the motor, at 80% load, at about 1326 rpm. Each hertz changes a 4-pole motor speed by 30 rpm.
Enter the desired machine speed: the calculator solves the frequency including sheaves and slips. In the example, for 800 rpm at the machine with 6 in and 12 in sheaves, the drive runs at about 54.7 Hz. If the result is far below 30% or above 120% of base frequency, change the sheaves instead.
Yes, within the mechanical limits of motor and machine, but above base frequency voltage cannot rise and available torque drops: at 75 Hz, to 80% of rated. It suits light loads; with constant-torque loads the motor will slow down.
Torque is still available, but the shaft-mounted fan turns slowly and cools less. Pumps and fans, which need little torque at low speed, usually have no issue; for conveyors and other constant-torque loads, follow the manufacturer derating curve (NEMA MG 1 Part 30 or 31) or add a separate blower.
Multiply motor speed by the pitch diameter ratio: n_machine = n_motor × d_motor ÷ d_machine, minus 1% to 2% for V-belt slip. In the example, 1326 rpm × 6 ÷ 12 × 0.99 ≈ 656 rpm at the machine.
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