Frequency, Pole Count and Speed Calculator
Professionally analyze the relationship between frequency, pole count, and synchronous speed in AC motors. Also, calculate the slip ratio for asynchronous motors and send the result to ONX Control engineers to request a quote for suitable motor/drive selection.
Synchronous Speed
ns = 120f / PPole Count
P = 120f / nsFrequency
f = nsP / 120Slip Analysis
s = (ns - n) / nsReference Synchronous Speed Table
50 / 60 Hz| Pole Count | 50 Hz | 60 Hz |
|---|---|---|
| 2 poles | 3000 rpm | 3600 rpm |
| 4 poles | 1500 rpm | 1800 rpm |
| 6 poles | 1000 rpm | 1200 rpm |
| 8 poles | 750 rpm | 900 rpm |
| 10 poles | 600 rpm | 720 rpm |
| 12 poles | 500 rpm | 600 rpm |
The values in the table are theoretical synchronous speeds. The nameplate speed of asynchronous motors is usually slightly below this value due to slip.
Engineering Expert Window
Synchronous speed and actual speed are different: In asynchronous motors, the rotor speed remains below the synchronous speed. This difference is called slip and generally increases as the load increases. Therefore, the speed seen on the motor nameplate is rarely the exact synchronous speed.
Pole count selection changes the application: Lower pole counts produce higher speed, while higher pole counts produce lower speed. Gearbox requirements, fan/pump characteristics, and mechanical resonance risks are directly affected by this selection.
Frequency, Pole Count, and Speed Relationship in AC Motors
In AC motors, the theoretical synchronous speed is determined by the relationship between the supply frequency and the number of stator poles. The most common formula is ns = 120 × f / P. Here ns is the synchronous speed (rpm), f is the frequency (Hz), and P is the total number of poles.
On a 50 Hz grid, the synchronous speed of a 4-pole motor is 1500 rpm, while in 2-pole motors this value rises to 3000 rpm. In 60 Hz applications, the synchronous speed is higher for the same number of poles. However, asynchronous motors operate slightly below the synchronous speed under load; this difference is evaluated as slip.
This page prepared by ONX Control helps you quickly calculate synchronous speed, frequency, pole count, and slip ratio. You can request suitable motor, drive, and, if any, gearbox recommendations for your application by forwarding the result directly to our technical team.
Frequently Asked Questions
Why is the synchronous speed not the same as the motor nameplate speed?
Because in asynchronous motors, the rotor rotates slightly below the synchronous speed while following the magnetic field. This difference is the slip and generally grows as the load increases.
Why are 50 Hz and 60 Hz motor speeds different?
Synchronous speed is directly proportional to frequency. For the same number of poles, the theoretical synchronous speed of a motor in a 60 Hz system is higher than in a 50 Hz system.
What happens as the pole count increases?
As the pole count increases, the synchronous speed decreases. Therefore, motors with higher pole counts may be preferred in applications requiring low speed.
Why is a standard pole result shown in the pole count calculation?
The theoretical result can be fractional, but in practice, motor pole counts are standard and even numbers. Therefore, the nearest standard pole count is also recommended.
ONX Tech Summary
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