How to Use the VFD Motor Speed Calculator
Frequency alone does not tell you how fast the shaft actually turns. An induction motor always runs slightly slower than its rotating field, and that difference, the slip, is exactly what produces torque.
The speed of the rotating field is Ns = 120 ร f รท poles. At 60 Hz a four-pole motor has a synchronous speed of 1,800 rpm, and with 3 percent slip the shaft settles near 1,746 rpm, which is why nameplates in the United States typically read 1,750 rpm. If you know the nameplate speed you can work slip backwards as (synchronous speed โ nameplate speed) รท synchronous speed.
Below base frequency the drive holds the volts-per-hertz ratio constant so full torque stays available. Above base frequency the voltage cannot rise any further, so the motor enters the constant power region and torque falls roughly in proportion to the frequency ratio. At very low speeds the shaft-mounted cooling fan slows down as well, so continuous low-speed duty needs separate blower cooling or an inverter-duty motor. If a belt or gearbox follows the motor, divide the shaft speed by the reduction ratio to get the driven speed.
Frequently Asked Questions
Read the nameplate speed. At 60 Hz the synchronous speeds are 3,600 rpm for two poles, 1,800 rpm for four and 1,200 rpm for six, so a nameplate showing 1,750 rpm is a four-pole motor.
The motor will turn faster, but torque drops and bearings and cooling fans are stressed. Check the mechanical speed limit and the load torque curve, and stay within the maximum frequency the manufacturer publishes.
General purpose induction motors commonly run 2 to 5 percent slip at full load. For a precise figure, work it back from nameplate speed against synchronous speed, and remember that lighter loading produces proportionally less slip.