1. What is the RPM Formula for Electric Motors?

The RPM (Revolutions Per Minute) formula calculates the rotational speed of an AC electric motor based on the power supply frequency and the number of magnetic poles in the motor. This relationship is fundamental to understanding synchronous motor speed.

2. How Does the Calculator Work?

The calculator uses the RPM formula:

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 factor 120 comes from converting seconds to minutes (60) and accounting for the fact that each AC cycle creates two poles (60 × 2 = 120).

3. Importance of RPM Calculation

Details: Knowing a motor's RPM is essential for proper motor selection, speed control applications, gear ratio calculations, and ensuring compatibility with driven equipment.

4. Using the Calculator

Tips: Enter frequency in Hz (typically 50 or 60 Hz depending on your region) and the number of poles (always 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 magnetic poles (north and south) to function, so the pole count is always an even number.

Q2: What is the RPM for a standard 4-pole motor at 60Hz?
A: 1800 RPM (120 × 60 / 4 = 1800). This is the synchronous speed for most common industrial motors in North America.

Q3: Why is actual motor speed slightly less than calculated?
A: The formula gives synchronous speed. Actual speed is less due to "slip" needed to produce torque in induction motors.

Q4: How does this apply to variable frequency drives (VFDs)?
A: VFDs change motor speed by varying the frequency while keeping the same pole count.

Q5: What's the relationship between RPM and torque?
A: Generally, motors with lower RPM (more poles) produce higher torque at the same power rating.