AC Motor RPM Calculation

AC Motor RPM Formula:

\[ RPM = \frac{120 \times f}{p} \]

Motor Speed (RPM):

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1. What is AC Motor RPM Calculation?

The AC Motor RPM calculation determines the rotational speed of an alternating current (AC) motor based on the electrical supply frequency and the number of magnetic poles in the motor. This is fundamental for motor selection and application in various industrial and commercial settings.

2. How Does the Calculator Work?

The calculator uses the AC motor RPM formula:

\[ RPM = \frac{120 \times f}{p} \]

Where:

\( RPM \) — Revolutions per minute (motor speed)
\( f \) — Frequency of the AC power supply (Hz)
\( p \) — Number of magnetic poles in the motor

Explanation: The 120 factor comes from converting seconds to minutes (60) and accounting for the fact that each AC cycle produces two poles (60 × 2 = 120). The number of poles must always be an even number in AC motors.

3. Importance of RPM Calculation

Details: Knowing a motor's RPM is crucial for proper equipment selection, ensuring compatibility with driven machinery, and maintaining optimal performance in applications like pumps, fans, conveyors, and compressors.

4. Using the Calculator

Tips: Enter the power supply frequency in Hertz (typically 50 or 60 Hz) and the number of poles in the motor (must be an even number like 2, 4, 6, etc.). All values must be valid (frequency > 0, poles ≥ 2 and even).

5. Frequently Asked Questions (FAQ)

Q1: Why is the number of poles always even?
A: AC motors require pairs of north and south magnetic poles to function, so the pole count is always an even number.

Q2: What are typical RPM values for AC motors?
A: Common values include ~3600 RPM (2-pole at 60Hz), ~1800 RPM (4-pole), ~1200 RPM (6-pole), and ~900 RPM (8-pole).

Q3: Why is actual motor speed slightly less than calculated?
A: The calculated value is synchronous speed. Actual speed is slightly lower due to "slip" needed to induce current in the rotor.

Q4: Can this 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 affect motor speed?
A: RPM changes proportionally with frequency, which is how variable frequency drives (VFDs) control motor speed.