Motor Speed Calculator Formula
Understand the math behind the motor speed calculator. Each variable explained with a worked example.
Formulas Used
Synchronous Speed
synchronous_speed = sync_speedActual Rotor Speed
actual_speed = sync_speed * (1 - slip / 100)Variables
| Variable | Description | Default |
|---|---|---|
frequency | Supply Frequency(Hz) | 50 |
poles | Number of Poles | 4 |
slip | Slip at Full Load(%) | 3 |
sync_speed | Derived value= 120 * frequency / poles | calculated |
How It Works
AC Induction Motor Speed
The speed of an induction motor depends on the supply frequency, number of poles, and load-dependent slip.
Formulas
N_sync = 120 f / P (synchronous speed)
N_actual = N_sync (1 - s) (actual speed)
where f is the supply frequency in Hz, P is the number of poles, and s is the slip (as a decimal). Synchronous speed is the rotating magnetic field speed. The rotor always turns slightly slower.
Worked Example
A 4-pole motor on 50 Hz supply with 3% slip at full load.
- 01Synchronous speed = 120 x 50 / 4 = 1500 RPM
- 02Actual speed = 1500 x (1 - 0.03) = 1455 RPM
Frequently Asked Questions
What is slip in an induction motor?
Slip is the difference between synchronous speed and actual rotor speed, expressed as a percentage. Typical full-load slip is 2-5% for standard motors. Without slip, no torque would be produced because no relative motion would exist between rotor and stator fields.
How many poles for common motor speeds?
At 50 Hz: 2-pole = 3000 RPM, 4-pole = 1500 RPM, 6-pole = 1000 RPM, 8-pole = 750 RPM. At 60 Hz: 2-pole = 3600, 4-pole = 1800, 6-pole = 1200.
Can I change motor speed?
Yes. Variable frequency drives (VFDs) change the supply frequency to control speed continuously. Multiple-speed motors have switchable pole configurations. Mechanical speed changes use gearboxes or pulleys.
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