Example 1: 50 Hz, 4-pole machine
For a 50 Hz supply and a 4-pole machine:
Ns = (120 x 50) / 4 = 1500 RPM
This is a common synchronous speed for 50 Hz motor systems.
Synchronous Speed Calculator
Compute synchronous speed using Ns = 120 x f / P in RPM.
Introduction
The synchronous speed calculator determines the speed of the rotating magnetic field in an AC machine. This is a core calculation for induction motors, synchronous motors, and alternators. Engineers use it when classifying motor speed, estimating slip, checking machine ratings, and understanding how supply frequency and pole count affect rotational speed.
Synchronous speed depends only on supply frequency and the number of poles. It does not depend on the mechanical load directly. That makes it a reliable reference point when comparing actual machine speed with theoretical operating speed.
This makes the calculator useful for quick motor checks, machine comparisons, and understanding how frequency or pole changes affect expected operating speed.
Formula
Ns = (120 x f) / P
Variable Definitions
Ns = synchronous speed in revolutions per minute
f = supply frequency in hertz
P = total number of poles
Units
Frequency is entered in hertz and pole count is entered as a whole number. The result is displayed in RPM, which is the standard unit for machine rotational speed.
Worked Examples
Example 2: 60 Hz, 6-pole machine
For a 60 Hz supply and a 6-pole machine:
Ns = (120 x 60) / 6 = 1200 RPM
This shows how adding poles reduces synchronous speed for the same frequency.
Practical Notes
Induction motors operate slightly below synchronous speed because of slip, while synchronous motors run at synchronous speed once locked in step with the supply. When selecting or analyzing AC machines, synchronous speed is often the first check before looking at slip, torque, efficiency, and operating current.
Why Synchronous Speed Matters
Knowing synchronous speed helps engineers classify motors correctly and match machine behavior to process needs. It is especially useful when reviewing motor nameplates, comparing 50 Hz and 60 Hz systems, or checking whether a selected machine will operate in the expected speed range before belt ratios or gearbox reductions are applied.
Common Speed Values
In 50 Hz systems, common synchronous speeds include 3000 RPM for 2 poles, 1500 RPM for 4 poles, 1000 RPM for 6 poles, and 750 RPM for 8 poles. In 60 Hz systems, common values include 3600 RPM, 1800 RPM, 1200 RPM, and 900 RPM. These standard speeds make it easier to compare machine types quickly during design and maintenance.