1. What is the Motor Speed Formula?

The motor speed formula calculates the rotational speed (RPM) of an AC induction motor based on the frequency of the power supply and the number of magnetic poles in the motor. This fundamental relationship helps in motor selection and performance analysis.

2. How Does the Calculator Work?

The calculator uses the motor speed equation:

Where:

• \( RPM \) — Revolutions per minute
• \( f \) — Frequency of power supply in Hertz (Hz)
• \( p \) — Number of magnetic poles in the motor (always an even number)

Explanation: The formula shows that motor speed is directly proportional to frequency and inversely proportional to the number of poles.

3. Importance of RPM Calculation

Details: Knowing a motor's speed is crucial for proper application matching, gearbox selection, and ensuring the motor meets the requirements of the driven equipment.

4. Using the Calculator

Tips: Enter frequency in Hz (typically 50 or 60 Hz for mains power) and the number of poles (common values are 2, 4, 6, or 8 poles). The number of poles must be an even number.

5. Frequently Asked Questions (FAQ)

Q1: Why is the number 120 used in the formula?
A: The 120 comes from converting seconds to minutes (60 seconds) and accounting for the fact that each AC cycle produces two poles (×2).

Q2: What are typical RPM values for common motors?
A: For 60Hz systems: 2-pole ≈ 3600 RPM, 4-pole ≈ 1800 RPM, 6-pole ≈ 1200 RPM. For 50Hz systems: 2-pole ≈ 3000 RPM, 4-pole ≈ 1500 RPM.

Q3: Why do actual motor speeds differ from calculated synchronous speed?
A: The formula calculates synchronous speed, but induction motors operate slightly slower (slip speed) due to load requirements.

Q4: Can this formula be used for DC motors?
A: No, DC motor speed depends on voltage and magnetic field strength, not frequency and poles.

Q5: How does variable frequency drive affect motor speed?
A: VFDs change the frequency to control motor speed according to this formula, enabling variable speed operation.