Turning a Supply Frequency into Shaft Speed
Standing at a drive panel with a keypad showing an output frequency, the question is always the same one: how fast is the coupling actually turning? Two different pieces of arithmetic answer it, and mixing them up is the most common mistake on a motor commissioning sheet. The first is pure unit conversion — a rate counted per second, restated per minute. The second is the induction machine's own rule, which brings pole count into it.
Two Layers Stacked on One Another
One Cycle Per Revolution — Two Poles Only
Where the 120 in the Machine Formula Comes From
Slip Is What Produces Torque
The Keypad Figure Is Not the Mains Figure
Checking a Drive Setting Against the Coupling
The sequence below is the one used when a pump or fan is being set up and the expected speed has to be confirmed before the guard goes back on.
Enter the drive output frequency
Type whatever the inverter is reporting into the hertz field. It starts at 1 and recalculates on every keystroke, so a partial figure such as 3 on the way to 37.5 is harmless. A comma works as the decimal mark if that is what the keypad shows, and stray spaces are ignored.
Divide the result by the pole pairs
The number that appears is the field speed a two-pole machine would reach. Halve it for a four-pole motor, take a third for six poles, a quarter for eight. That gives synchronous speed — the ceiling the rotor is chasing but never reaches while it is loaded.
Take off the slip before comparing with a tacho
Rated slip on a general-purpose machine sits between roughly 1 % on a large frame and 5 % on a small one. Subtract it and you have the speed a hand tachometer should read at full load; a reading far below that points at overload rather than at a wrong drive setting.
Work backwards from a measured speed
If the tachometer reading is the starting point instead, press the swap arrows or simply type the RPM straight into the second field — both boxes accept input and drive the other one. The copy button beside each field lifts the bare digits, with no unit attached, ready to drop into a commissioning record.
Synchronous Speed by Pole Count on 50 Hz and 60 Hz Mains
Every standard induction machine sits on one row of this table. It is worth knowing by heart, because it explains why a pump ordered for a North American plant arrives turning twenty percent faster than the identical frame in Europe, and why a fan curve quoted at one supply frequency cannot be read straight across to the other.
| Stator poles (pole pairs) | Field speed on 50 Hz mains | Field speed on 60 Hz mains | Shaft rate at 50 Hz, in Hz |
|---|---|---|---|
| 2 (1) | 3 000 RPM | 3 600 RPM | 50 Hz |
| 4 (2) | 1 500 RPM | 1 800 RPM | 25 Hz |
| 6 (3) | 1 000 RPM | 1 200 RPM | 16.67 Hz |
| 8 (4) | 750 RPM | 900 RPM | 12.5 Hz |
| 10 (5) | 600 RPM | 720 RPM | 10 Hz |
| 12 (6) | 500 RPM | 600 RPM | 8.33 Hz |
That last column applies the plain factor to the shaft rather than to the supply: a four-pole machine fed at 50 Hz has its rotor turning 25 times a second, exactly half the electrical frequency arriving at the terminals. Keeping those two hertz figures apart in your head — supply frequency on one side, shaft rate on the other — removes most of the confusion around this pair of units. A drive told to output 12.5 Hz on that same four-pole motor produces a 375 RPM field, one quarter of nameplate speed, which is the sort of turndown a mixer or a conveyor is normally asked for.
Walk a Ramp Profile Point by Point
Both boxes stay live while you edit, so stepping through 10, 20, 30 and 40 Hz to sketch an acceleration ramp needs nothing more than the keyboard and a glance at the second field.
Tachometer Reading Back to a Keypad Value
The swap arrows flip the pair round, which is what you want when a measured shaft speed has to be turned back into the frequency that would produce it.
Clean Figures for a Handover Record
Copying takes the number on its own, without unit or thousands spacing, so a speed lands in a spreadsheet cell as a value rather than as text that has to be tidied afterwards.
Other Rotational Units for the Same Shaft
Either dropdown can be searched and switched, so a motor speed can be restated in revolutions per second or in angular terms without leaving this page.
Motor Speed Questions from the Panel
Why does one 50 Hz motor turn at 3 000 RPM and the next at 1 000?
Because the stator windings are arranged into a different number of magnetic poles. The supply frequency tells the field how often to alternate; the pole count decides how far round the bore each alternation moves it. Two poles means a full revolution per cycle, six poles a third of a revolution, so identical 50 Hz terminals produce 3 000 or 1 000 RPM. Nothing about the supply differs — only the winding does. It is also why slow applications such as agitators and cooling towers are often served by an eight- or twelve-pole machine instead of by a gearbox.
What happens if a 60 Hz motor is connected to a 50 Hz supply?
Speed drops in proportion, to five sixths of the original, so an 1 800 RPM field becomes 1 500 RPM. The real problem is not the speed but the magnetic flux, which follows the volts-per-hertz ratio. A 460 V, 60 Hz machine is designed around 7.67 V/Hz; leave 460 V on it at 50 Hz and the ratio jumps to 9.2 V/Hz, saturating the iron and cooking the windings. The voltage has to come down with the frequency, to roughly 383 V, and even then the shaft power available falls along with the reduced speed.
Why is the nameplate speed lower than the figure this page gives?
An induction rotor makes torque only while the field is overtaking it. If it ever caught up, the relative motion would vanish, no current would be induced in the cage and no torque would exist. The plate therefore records the loaded speed — typically 1 440 or 1 450 RPM against a 1 500 RPM field, which is 4.0 % and 3.3 % slip respectively. Off load the same machine coasts up to within a few revolutions of the field. Only a synchronous or permanent-magnet design genuinely locks onto it.
Can a drive be pushed past the motor's base frequency?
It can, but the character of the machine changes there. Below base frequency the drive raises voltage in step with frequency and torque stays roughly constant. Above it the inverter has no voltage left to give, the volts-per-hertz ratio starts falling and the flux weakens — available torque drops away in rough inverse proportion to frequency while output power stays about level. Run a four-pole motor at 100 Hz and the field reaches 3 000 RPM, but only about half the rated torque comes with it. Bearing speed limits, the effectiveness of a shaft-mounted cooling fan and rotor balance all need checking before that region is used routinely.
Why do so many catalogue entries cluster at 1 500 and 1 800 RPM?
Four poles is the compromise nearly every general-purpose design settles on. Two-pole machines spin fast enough that bearing life, noise and balancing all get harder; six-pole and slower machines need more iron and copper for the same output, so they cost more and weigh more. Four poles delivers a usable speed for pumps, fans and gearbox inputs in the smallest frame, which is why those two columns dominate the catalogues.
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